electric motor 7

Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramco’semployees. Any material contained in this document which is not alreadyin the public domain may not be copied, reproduced, sold, given, ordisclosed to third parties, or otherwise used in whole, or in part, withoutthe written permission of the Vice President, Engineering Services, SaudiAramco.

Chapter : Electrical For additional information on this subject, contactFile Reference: EEX21607 W.A. Roussel on 874-1320

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Motor Protection Requirements

Engineering Encyclopedia Electrical


Saudi Aramco DeskTop Standards

CONTENTS PAGE

TYPICAL FACTORS THAT ARE SPECIFIED ON A MOTORNAMEPLATE .......................................................................................................1

Rated Volts .................................................................................................2

Full-Load Amperes.....................................................................................2

Service Factor (S.F.)...................................................................................3

Horsepower ................................................................................................3

Temperature Factors...................................................................................4

Temperature Rise ............................................................................4

Insulation Class and Ambient Temperature ....................................4

Time (Duty)................................................................................................5

Locked-Rotor Codes...................................................................................5

Miscellaneous Information.........................................................................6

Maker’s Name.................................................................................6

Frequency and Number of Phases ..................................................6

Speed ..............................................................................................6

ANSI/IEEE DEVICES AND FUNCTION NUMBERS THAT RELATETO AC INDUCTION MOTOR PROTECTION ....................................................7

Purpose.......................................................................................................7

Standard Device Function Numbers...........................................................7

Device 2RS .....................................................................................7

Device 27 ........................................................................................7

Device 46 ........................................................................................9

Device 47 ........................................................................................9

Device 49 ........................................................................................9

Devices 50/50G/50GS ..................................................................10

Device 51LR.................................................................................10

Device 86M ..................................................................................10

Device 87M ..................................................................................10




T/C CHARACTERISTIC CURVES OF AC INDUCTION MOTORS ...............11

Thermal Capability Curve ........................................................................11

Stall Time Vs Locked Rotor Current ............................................11

Motor Starting Curve................................................................................13

Locked-Rotor Current ...................................................................13

Starting Time ................................................................................13

Full-Load Current .........................................................................13

THERMAL PROTECTION FUNDAMENTALS OF AC INDUCTIONMOTORS.............................................................................................................15

Thermal Overload Protection ...................................................................15

Replica-Type Relays.....................................................................15

Resistance Temperature Detectors (RTDs)...................................17

Protection Versus Stall Time ........................................................18

Thermal Locked-Rotor Protection............................................................18

Induction Disc Relays ...................................................................19

Protection Versus Stall Time ........................................................22

Combined Protection................................................................................23

Underprotection - Device 49.........................................................23

Overprotection - Device 51...........................................................23

FUNDAMENTALS OF FAULT PROTECTION FOR LOW ANDMEDIUM VOLTAGE AC INDUCTION MOTORS...........................................24

Introduction ..............................................................................................24

Phase Faults..............................................................................................24

Current Limiting Fuses .................................................................25

Circuit Breakers ............................................................................26

Ground Faults ...........................................................................................31

Residual Connection .....................................................................31

Zero Sequence Connection ...........................................................32

OTHER TYPES OF MOTOR PROTECTION FUNDAMENTALS FORAC INDUCTION MOTORS................................................................................36




Undervoltage Protection...........................................................................36

Purpose and Thermal Effects ........................................................36

Time-Delay Relays - Device 27....................................................37

Coordination .................................................................................37

Phase Unbalance Protection .....................................................................39

Purpose and Thermal Effects ........................................................39

Voltage Unbalance Relays - Device 47 ........................................39

Current Unbalance Relays - Device 46.........................................40

Voltage Unbalance (Low Voltage Motors)...................................46

Miscellaneous Protection..........................................................................47

High Speed Reclosing...................................................................47

Repetitive Starting - Device 2RS ..................................................47

Protection Scheme One-Line Diagrams ...................................................48

Low Voltage Motors .....................................................................48

Medium Voltage Motors...............................................................51

SOLID-STATE MOTOR PROTECTION PACKAGE (MPP)FEATURES .........................................................................................................54

General Description..................................................................................54

Features and Capabilities ..............................................................54

Benefits .........................................................................................54

Multilin MMR 269 Plus ...........................................................................55

Single-Line Drawing.....................................................................56

Protection Features .......................................................................58

Communication Features ..............................................................59

Diagnostic Features.......................................................................60

Other Features...............................................................................62

Westinghouse IQ-1000II ..........................................................................63

Block Diagram..............................................................................63

Protection Features .......................................................................65

Communication Features ..............................................................67




Diagnostic Features.......................................................................68

Other Features...............................................................................73

GLOSSARY ........................................................................................................74

LIST OF FIGURES

Figure 1. Typical Ac Motor Nameplate ................................................................1

Figure 2. Ac Motor Voltages ................................................................................2

Figure 3. Nema Temperature Ratings ...................................................................4

Figure 4. Locked-Rotor Kva Codes ......................................................................5

Figure 5. Ac Motor Protection One-Line Diagram ...............................................8

Figure 6. Motor Curves.......................................................................................12

Figure 7. Motor Starting Current ........................................................................14

Figure 8. Bl-1 T/C Curves...................................................................................16

Figure 9. Dt-3 Relay ...........................................................................................17

Figure 10. O/L Relay Protection .........................................................................18

Figure 11. Starting Time Ts < 20 Seconds..........................................................19

Figure 12. Starting Time 20 < Ts < 70 Seconds .................................................20

Figure 13. Starting Time Ts > Tlr .......................................................................21

Figure 14. L/R Relay Protection .........................................................................22

Figure 15. Combined Protection .........................................................................23

Figure 16. Current Limiting Fuses (R-Rated) .....................................................25

Figure 17. Fuse Protection ..................................................................................26

Figure 18. Mcp Protection ..................................................................................27

Figure 19. Phase Faults: Device 50....................................................................28

Figure 20. Partial Differential Protection.............................................................29

Figure 21. Full Differential Protection................................................................30

Figure 22. Residual Connection..........................................................................31

Figure 23. Zero Sequence Feeder Breaker..........................................................32

Figure 24. Three-Wire Circuit.............................................................................33

Figure 25. Four-Wire Circuit ..............................................................................33




Figure 26. Zero-Sequence Connection................................................................34

Figure 27. Ground Fault Protection - Mv System...............................................35

Figure 28. Effects Of Voltage Variation .............................................................36

Figure 29. Time Curves - Undervoltage Relay ...................................................38

Figure 30. Cvq Relay ..........................................................................................39

Figure 31. Cm Relay ...........................................................................................40

Figure 32. Cm Relay Operating Characteristics..................................................41

Figure 33. Primary Open (Three-Line Diagram) ................................................42

Figure 34. Phasor Diagram (Primary Open) .......................................................43

Figure 35. Secondary Open (Three-Line Diagram) ............................................44

Figure 36. Phasor Diagrams (Secondary Open)..................................................45

Figure 37. Voltage Unbalance Derating Factors.................................................46

Figure 38. Protection: 0.75 Kw (1.0 Hp) Or Less ...............................................48

Figure 39. Protection: Greater Than 0.75 Kw To 75 Kw (1.0 To 100Hp) .......................................................................................................49

Figure 40. Protection: Greater Than 75 Kw (100 Hp) .......................................50

Figure 41. Protection: Class E2 Controllers (<1125 Kw) ..................................51

Figure 42. Power Circuit Breaker (<7500 Kw)...................................................52

Figure 43. Protection: Power Circuit Breaker (>7500 Kw) ...............................53

Figure 44. Multilin 269 Plus: Faceplate.............................................................55

Figure 45. Multilin 269 Plus: Legend ................................................................56

Figure 46. Multilin 269 Plus: Single-Line Drawing...........................................57

Figure 47. Multilin 269 Plus: Protection Features .............................................58

Figure 48. Iq-1000ii: Faceplate..........................................................................63

Figure 49. Iq-1000ii: Block Diagram.................................................................64

Figure 50a. Iq-1000ii: Protection Features.........................................................65

Figure 50b. Iq-1000ii: Protection Features (Cont’d)..........................................66

Figure 51a. Iq-1000ii: Monitor Data..................................................................68

Figure 51b. Iq-1000ii: Monitor Data (Cont’d) ...................................................68

Figure 51c. Iq-1000ii: Monitor Data (Cont’d) ...................................................69

Figure 51d. Iq-1000ii: Monitor Data (Cont’d) ...................................................69




Figure 52. Iq-1000ii: Alarm Conditions.............................................................70

Figure 53a. Iq-1000ii: Trip Conditions ..............................................................71

Figure 53b. Iq-1000ii: Trip Conditions (Cont’d) ...............................................72

Figure 54. Iq-1000ii: Internal Diagnostics .........................................................73



Saudi Aramco DeskTop Standards 1

TYPICAL FACTORS THAT ARE SPECIFIED ON A MOTOR NAMEPLATE

NEC Article 430-7 states a motor nameplate must be marked with the following information:

• volts, full-load amperes, service factor, horsepower

• temperature factors, time (duty), locked-rotor codes

• maker’s name, frequency, number of phases, speed

Figure 1 is an example nameplate that contains the NEC minimum required nameplateinformation; 16-SAMSS-503 also requires the nameplate to contain additional informationpertaining to insulation class, winding temperature rise, type of bearings, rotor Wk2, types ofenclosure, etcetera.

Figure 1. Typical AC Motor Nameplate




Rated Volts

The voltage marked on the motor nameplate is the rated motor terminal voltage per NEMAMG-1. The nominal three-phase system voltage that matches the rated three-phase voltage islisted in Figure 2.

Figure 2. AC Motor Voltages

Full-Load Amperes

The full-load amperes marked on the motor nameplate are based on the rated voltage,horsepower, and frequency. Overload protection, as specified by NEC Art 430-32, is basedon the marked full-load amperes.




Service Factor (S.F.)

When the voltage and frequency are maintained as per the nameplate markings, the motormay be overloaded up to the hp obtained by multiplying the rated hp by the service factorshown on the nameplate. When the motor is operated at the higher service factor, efficiency,power factor, and speed may be different than at rated load, but locked rotor torque andcurrent, and breakdown torque remain unchanged. For example, a 100 hp, 1.15 S.F. motormay be safely loaded to 115 hp.

Horsepower

Horsepower is the rated output mechanical power that may be applied to the motor shaft. IECmotors are rated on output kW vice output hp, where 1hp equals 0.746 kW. For example, aNEMA MG-1 rated 1000 hp motor is equivalent to a nominal 750 kW (746 kW actual) IECrated motor.




Temperature Factors

The temperature rise or the insulation class and ambient temperature must be marked on themotor (See Figure 3). Note: Saudi Aramco motor specifications (17-SAMSS-502 and 503)require Class F insulation. However, for fractional horsepower motors, SAES-P-113 permitsa minimum Class B insulation.

Figure 3. NEMA Temperature Ratings

Temperature Rise

The temperature rise shown in the above Figure is based on motor operation at altitudes of1000 meters (3300 ft) or less, ambient temperatures of 40oC, and rated horsepower for 1.0S.F. motors or 1.15 times rated horsepower for 1.15 S.F. motors.

Insulation Class and Ambient Temperature

The insulation class as shown above (Figure 3) is based on a 40oC ambient, but if the motor isoperated at higher ambients, the motor temperature rise must be calculated in accordance withNEMA MG 1-12.43.




Time (Duty)

The time ratings for motors, per NEMA MG1-10.36 are 5, 15, 30 and 60 minutes, andcontinuous. Note: Saudi Aramco specifications call for continuous duty motors only.

Locked-Rotor Codes

Both NEMA MG 1-10.37 and the NEC require the locked-rotor indicating code letters to bemarked on the motor nameplate. The letter designations are based on full voltage and ratedfrequency (See Figure 4).

Figure 4. Locked-Rotor kVA Codes




Miscellaneous Information

Maker’s Name

The NEC requires the motor manufacturer’s name to be marked on the nameplate. Mostmanufacturers also include additional markings such as serial numbers, model numbers,bearing numbers, etcetera.

Frequency and Number of Phases

The motor frequency (50 or 60 hertz) as well as the number of phases (1 or 3) are requiredmarkings on the motor nameplate. Virtually all other ratings are based on loadings at ratedfrequency. All AC motors are required by NEMA MG1-12.44 to operate successfully underrunning conditions at rated load and voltage and at plus or minus 5 percent frequency.

Speed

NEMA MG1-10 lists the synchronous speed of motors (Nrpm = 120f/p), whereas thenameplate speed for induction rotors includes slip (rotor speed).




ANSI/IEEE DEVICES AND FUNCTION NUMBERS THAT RELATE TO ACINDUCTION MOTOR PROTECTION

Purpose

The devices in switching equipment are referred to by numbers, with appropriate suffix letterswhen necessary, according to the functions they perform. Exercise caution when interpretingthe letter suffix: There are often dual meanings. For example, the suffix G can mean ground(50G) or generator (87G).

The numbers are based on a system, and adopted as standard for automatic switchgear byANSI/IEEE Std.C37.2. The system is used in connection diagrams, one-line diagrams,instruction books, and in specifications. Figure 5 is a one-line diagram showing applicationof standard ANSI/IEEE device numbers.

Standard Device Function Numbers

Device 2RS

A time-delay starting, or closing relay is a device that functions to give a desired amount oftime delay before or after any point of operation in a switching sequence or protective relaysystem, except as specifically provided by device functions 48, 62, and 79. Saudi Aramcouses this device to block repetitive starting (RS) of large motors rated at 3750 kW(5000 hp) orlarger.

Device 27

An undervoltage relay is a device that functions on a given value of undervoltage. SaudiAramco uses this device to detect undervoltage on a motor bus or individual motors.




Figure 5. AC Motor Protection One-Line Diagram




Device 46

A reverse-phase, or phase-balance current relay is a relay that functions when the polyphasecurrents are of reverse-phase sequence, when they are unbalanced, or when they containnegative phase-sequence components above a given amount. This relay is primarily used toprotect motors against single-phasing (primary or secondary opens).

Device 47

A phase-sequence voltage relay is a relay that functions on a predetermined value ofpolyphase voltage in the desired phase sequence. This relay, in conjunction with a Device 27relay, is used to detect undervoltage, reverse phasing, and single- phasing of a motor.

Device 49

A machine, or transformer thermal relay, is a relay that functions when the temperature of aparticular element exceeds a predetermined value. These elements consist of a machinearmature, or other load-carrying winding or element of a machine, or a power rectifier orpower transformer (including a power rectifier transformer). Thermal relays are used tooverload protect all Saudi Aramco motors; however larger motors require more sophisticated(capable) Device 49 relays. For example, a small 7.5 kW (10 hp) motor may be protected bya simple solder-pot overload device, whereas a large 3750 kW (5000 hp) motor would requireuse of a much more sophisticated ABB type BL-1 thermal overload relay.




Devices 50/50G/50GS

An instantaneous overcurrent, or rate-of-rise relay, is a relay that functions instantaneously onan excessive value of current or on an excessive rate of current rise, thus indicating a fault inthe apparatus of the circuit being protected. Saudi Aramco uses this device for both phaseand fault protection of motors. The suffix G is the abbreviation for ground and GS is theabbreviation for ground sensor. Module EEX216.04 will describe the different applicationsof Device 50.

Device 51LR

An AC time overcurrent relay is a relay with either a definite or inverse time characteristicthat functions when the current in an AC circuit exceeds a predetermined value. SaudiAramco uses this device to provide thermal locked-rotor (LR) protection for medium voltagemotors.

Device 86M

A locking-out relay is an electrically-operated hand or electrically reset, relay that functions toshut down and hold a piece of equipment out of service on the occurrence of abnormalconditions. Saudi Aramco uses this device to lock out large motors (M) after occurrence of afault. This device is activated by Device 87. Note: Device 86 requires manual reset.

Device 87M

A differential protective relay is a protective relay that functions on a percentage, phase angle,or other quantitative difference of two currents or other electrical quantities. Saudi Aramcouses this device for fault protection of motors (M) rated greater than 4 kV.




T/C CHARACTERISTIC CURVES OF AC INDUCTION MOTORS

Thermal Capability Curve

Heating characteristics of motors are very difficult to obtain and vary considerably with motorsize and design. These heating characteristics are modeled as curves, and are an approximateaverage of an imprecise thermal zone, where varying degrees of damage or shortenedinsulation life may occur.

Figure 6 shows a typical motor capability curve, which is the motor designer’s estimate of theamount of load current that may flow in the motor without exceeding permissibletemperatures.

Stall Time Vs Locked Rotor Current

Cold Start - The locked-rotor time (tLR) shown in Figure 6 depicts the time (capability) of themotor (current versus time), which is based on starting the motor cold (the motor windings,rotor, etcetera are at ambient temperature).

Hot Start - If the motor’s duty cycle permits hot starts - the motor windings, etcetera are at anelevated temperature, the manufacturer must be consulted to determine a permissible startingtime (ts) to prevent motor damage.




Figure 6. Motor Curves




Motor Starting Curve

Locked-Rotor Current

The starting current is represented by the curve (solid line) as previously described in Figure 6and the current (labeled LRAa and LRAs) shown inFigure 7.

Asymmetrical (DC Transient) -The asymmetrical starting current (LRAa) exceeds thesymmetrical locked rotor current (LRAs) during the first few cycles because of the transientdirect current. This transient current appears, as it does under fault conditions, because theseries reactance (inductance) prevents an instantaneous change in the magnitude of thealternating current. The magnitude of the asymmetrical starting current is approximately 1.5LRAs for low voltage motors and 1.6 LRAs for medium voltage motors.

Symmetrical - After the transient current decays, the starting current hovers near thesymmetrical starting current (LRAs). The magnitude of this starting current is typically 4 to 6times the motor’s full-load amperes (FLA). The exact amount is based on the subtransientreactance (X”d) of the motor, which ranges from 16.7 to 25 percent.

Starting Time

The starting time (ts) of the motor is the approximate time it takes the motor to approach ratedrunning speed. For purposes of this course, it is assumed that the starting time (ts) is less thanthe locked-rotor (stall) time (tLR).

Full-Load Current

After the motor reaches rated speed, it acquires its normal rated value (full-load amperes),assuming rated load, voltage and frequency.




Figure 7. Motor Starting Current




THERMAL PROTECTION FUNDAMENTALS OF AC INDUCTION MOTORS

Thermal Overload Protection

Overload (O/L) protection is always applied to motors to protect them from overheating. NECArticle 430-38 requires an O/L device in each phase except “where protected by othermeans.” This requirement (one-per-phase) is necessary because single phasing of the primaryin a delta-wye configuration results in a 2:1:1 three-phase motor current relationship. Thisprotection is provided by replica-type relays for small kW-rated motors and by resistancetemperature detectors (RTDs) for larger motors.

Replica-Type Relays

Replica-type relays operate directly from motor circuit current. They receive their name“replica” because they tend to “replicate” the heating characteristics of the motor. For verysmall motors, this type of relay is simply a bimetallic element that operates within a heaterunit. For large kW-rated motors, they are truly a type of overcurrent relay. For instance,Saudi Aramco specifies an ABB-type BL-1 O/L relay for motors rated 4 kV and larger andless than 7500 kW. Figure 8 is a T/C characteristic curve of a BL-1 relay.




This relay is a temperature-compensated relay, which means it has different T/C curvesdepending on the motors’s temperature. For instance, if the motor is at room ambient (justturned on) when the O/L occurs, the relay responds to the 0 percent curve. If the motor hasbeen running continuously, the relay would respond to the 100 percent curve. Becausereplica-type relays only respond to current, they will not typically protect for blockedventilation.

Figure 8. BL-1 T/C Curves




Resistance Temperature Detectors (RTDs)

RTD-type relays operate from exploring coils embedded by the manufacturer directly in themotor windings. They are commonly used in industrial applications in motors rated above1125 kW (1500 hp). Note: Saudi Aramco (17-SAMSS-502) requires RTD applications inmotors rated above 150 kW (200 hp). RTDs respond to temperature alone, and they willprotect against blocked ventilation. Figure 9 is an ABB DT-3 type relay used to detectovertemperature (overloads) with RTDs in a large motor.

Figure 9. DT-3 Relay




Protection Versus StallTime

Thermal-type relaysoffer very goodprotection for lightoverloads as shown inFigure 10, but provideinadequate protection(shaded area) for heavyoverloads or duringstarting.

Figure 10. O/L RelayProtection

Thermal Locked-RotorProtection

Thermal locked-rotor(L/R) protection, similarto O/L protection,

involves the matching of a relay to the motor’s thermal capability curve, and at the same timeremembering that the capability curve is at best an approximation.

A motor with a locked-rotor condition is particularly vulnerable to damage because of thelarge amount of heat generated (I2R). Also, remember that a motor at standstill cannotdissipate the heat as well as a rotating motor.




Induction DiscRelays

The type of L/Rprotectiondepends oncomparison of thestarting time (ts)of the motor to itspermissiblelocked-rotor time(tLR).

If the starting time(ts) is less than orequal to 20seconds, and lessthan L/R time(tLR), it is best touse an extremelyinverse relaysimilar to typesABB CO-11 orGE IFC 77 (seeFigure 11).

Figure 11.Starting Time ts

< 20 Seconds




If ts is between 20 and 70 seconds and less than tLR, it is best to use a relatively flat relaysimilar to types ABB CO-5, CO-6 or GE IFC-95 (see Figure 12).

Figure 12. Starting Time 20 < ts < 70 Seconds




If ts is greater than tLR (see Figure 13), a mechanical zero-speed switch may be used. Thisdevice supervises an overcurrent unit (Device 51) and prevents its operating a timer whenrotation is detected. Note: This scheme will not detect a failure to accelerate to full speed norpullout with continued rotation.

Figure 13. Starting Time ts > tLR




Protection Versus Stall Time

The overcurrent relay offers excellent protection for heavy overloads as shown in Figure 14,but overprotects (shaded area) for light overloads.

Figure 14. L/R Relay Protection




Combined Protection

The best (recommended) thermal protection for large motors is to combine both O/L and L/Rprotection as shown in Figure 15.

Figure 15. Combined Protection

Underprotection - Device 49

A typical scheme is to provide two overload protective devices (i.e. BL-1 relays) in phases Aand C, which underprotects (thermally) for heavy overloads (i.e. locked-rotor conditions), butadequately protects for light overloads.

Overprotection - Device 51

To complement Device 49 thermal protection, one locked-rotor device (i.e. CO relay) isapplied to phase B, which overprotects for light overloads, but adequately protects for heavy(i.e. locked-rotor conditions).




FUNDAMENTALS OF FAULT PROTECTION FOR LOW AND MEDIUMVOLTAGE AC INDUCTION MOTORS

Introduction

As with thermal protection, the size of the motor and the type of service will influence thetype of fault protection required to protect the motor.

Although NEC Article 430-52 and Table 430-152 dictate phase and ground fault protectionfor low voltage motor circuits, the type of protective device is a designer’s choice. There arefive types, each having different benefits depending on the size of the motor, cost ofprotection, etcetera. The five types used for low voltage motor protection are:

• non-time delay fuse (non-current limiting).

• time delay fuses (current limiting).

• inverse time circuit breaker.

• magnetic only circuit breaker.

• motor circuit protector (MCP).

Medium voltage motors, typically large and expensive, are fault protected by NEMA Type Rcurrent limiting fuses or differential relays. Ground fault protection can be provided by aresidual scheme, but zero sequence protection is the preferred scheme.

Phase Faults

Although the NEC permits current limiting fuses for low voltage motor phase fault protection,Saudi Aramco SAES-R-114 specifies magnetic-only molded case circuit breakers or MCPsfor protection of motors rated below 75 kW (100 hp), and devices 50 or 87 for all othermotors (low and medium voltage).




Current Limiting Fuses

R-rated (NEMA Type R) current limiting fuses are used in NEMA Class E2 controllers toprovide short circuit fault protection up to 350,000 kVA. Note: Class E2 controllers will bediscussed in detail in EEX216.05. Figure 16 lists the continuous current ratings of NEMAType R fuses, while Figure 17 shows a typical T/C coordination scheme for protecting amedium voltage motor. Note: SAES-P-114 permits Class E2 controllers with current limitingfuses for motors rated 4.0 kV, 1125 kW (1500 hp) or smaller sized motors.

Figure 16. Current Limiting Fuses (R-Rated)




Figure 17. Fuse Protection

Circuit Breakers

SAES-P-114 requires: a) magnetic-only or MCP fault protection for low voltage motors ratedless than or equal to 75 kW (100 hp); b) low voltage power circuit breakers (LVPCBs) for lowvoltage motors rated above 75 kW; and c) medium voltage power circuit breakers for motorsrated greater than 1125 kW (1500 hp).




Low Voltage Motors -The T/C characteristics of an MCP (or magnetic-only molded casecircuit breaker) are shown in Figure 18.

Figure 18. MCP Protection




Medium Voltage Motors - Phase fault protection for medium voltage motors rated above1125 kW (1500 hp) is provided by a power circuit breaker controlled by relays.

• Instantaneous trip units (device 50) are recommended where the ratio I3φ/LRAsis greater than 5 and the kVA rating of the motor is less than 50 percent of thekVA rating of the transformer (see Figure 19).

Figure 19. Phase Faults: Device 50




• Partial differential (Device 87M) is the preferred phase fault protection forlarge motors, and is recommended when I3φ is approximately equal to LRAs,which varies from 4-6 times the full load, three-phase current. The advantageof this scheme is that it has excellent sensitivity, the starting currents cancel,and only three current transformers (CT) are required. The biggest problemwith this protection scheme is a “physical limitation” based on the CT size (seeFigure 20). Note: Saudi Aramco specifies differential protection (87M) only formedium voltage motors.

Figure 20. Partial Differential Protection




• Full differential (Device 87M) is recommended whenever I3φ isapproximately equal to LRAs, which varies from 4-6 times the full load three-phase current, and a partial differential scheme does not work. The onlyadvantage of a full differential scheme over the partial differential scheme isthat it offers cable protection. Obvious disadvantages are that six CTs arerequired, and the scheme is often oversensitive (nuisance trips) to high startingcurrents because of unequal CT saturation. (See Figure 21). Note: SaudiAramco specifies differential protection (87M) only for medium voltage motors.

Figure 21. Full Differential Protection




Ground Faults

SAES-P-114 requires ground fault protection for all motors rated 22.5 kW (30 hp) and larger.Residual protection is permitted only on induction motors rated above 7500 kW (10,000 hp),where high cable charging currents would cause false operation of zero sequence (50GS)protection.

Residual Connection

The residual connection is not very sensitive because it “sees” current through the “eyes” ofthe phase CTs. This connection often causes nuisance trips as well because of the unequalsaturation of the three CTs (see Figure 22).

Figure 22. Residual Connection




Zero Sequence Connection

The zero sequence connection (Device 50GS) is the preferred ground fault protection scheme.Low voltage motors use a static trip (solid-state) device to trip the breaker, whereas a relay(ABB Type SC or GE Type PJC) trips the breaker via a lockout relay (Device 86M) formedium voltage motors. Note: Saudi Aramco typically specifies zero sequence CTs forground fault protection.

Low Voltage Motors - Figure 23 shows the zero sequence connection for protecting a lowvoltage motor. Figures 24 and 25 show alternate connection schemes with the zero sequenceCT connection being the preferred Saudi Aramco connection.

Figure 23. Zero Sequence Feeder Breaker




Figure 24. Three-Wire Circuit

Figure 25. Four-Wire Circuit




Medium Voltage Motors - Figure 26 shows the ground fault protection scheme for mediumvoltage motors. The primary advantages of this type of system are increased sensitivity (nocurrent flows under normal conditions), which eliminates false tripping during motor startingand the lowest CT cost (only one required). The primary disadvantage is CT saturation,especially when induction disc (Device 51) relays and/or solidly-grounded systems are used.

Figure 26. Zero-Sequence Connection




Figure 27 is a typical one-line diagram and accompanying coordination scheme using zerosequence ground fault protection schemes.

Figure 27. Ground Fault Protection - MV System




OTHER TYPES OF MOTOR PROTECTION FUNDAMENTALS FOR ACINDUCTION MOTORS

Undervoltage Protection

A low voltage condition will prevent motors from reaching their rated speed on starting, orcause them to lose speed and draw heavy overload current. While overload relays (Device49) will eventually detect this condition, the motor should be quickly disconnected whensevere low voltage conditions exist. Where continuous operation is essential, such as stationauxiliary service or continuous manufacturing processes, an undervoltage relay is used foralarm purposes only.

Purpose and Thermal Effects

The primary purpose of undervoltage relay protection (Device 27) in Saudi Aramcoapplications is as a backup device for locked rotor protection (Device 51). Device 51 isapplied to phase B, while Device 27 is applied to phases A and C. Because power (I2R) isdirectly proportional to the current squared and any decrease in voltage (see Figure 28) resultsin an increase in current, Device 27 will eventually remove the motor if Device 51 fails,although some damage may occur as a result of the increased temperature (approximately 17percent for just 10 percent low voltage).

Figure 28. Effects of Voltage Variation




Time-Delay Relays - Device 27

Time-delay voltage relays, similar to time delay overcurrent relays (Device 51), use inductiondisc relays for their time-undervoltage characteristics (see Figure 29).

Coordination

Device 27 relays must be coordinated with upstream fault relays to prevent tripping the motorfor any upstream faults that cause voltage dips on the system. Additionally, caution must beexercised to ensure the relay does not trip due to voltage sags as a result of the motor, oradjacent large motors starting on the same bus.




Figure 29. Time Curves - Undervoltage Relay




Phase Unbalance Protection

Purpose and Thermal Effects

The purpose of phase unbalance protection is to prevent motor overheating damage. Motoroverheating occurs because increased phase currents flow in order that the motor can continueto deliver the same kW (hp) as it did with balanced voltages. Negative-sequence voltagesalso appear and cause abnormal currents to flow in the rotor. Because a motor’s negativesequence impedance (Z2) approximates a motor’s locked rotor impedance, a small negativesequence voltage produces a much larger negative sequence current.

Voltage Unbalance Relays - Device 47

SAES-P-114 recommends use of an ABB Type CVQ relay (see Figure 30) for voltageunbalance protection. This relay protects against system undervoltage (a Device 27 function),single-phasing of the supply, and reversal of phase rotation of the supply (100 percentnegative sequence). No settings are required for the CVQ relay.

Figure 30. CVQ Relay




Phase Reversal protection is primarily protection for the process instead of protection for themotor. Imagine reversing the phases for a pump. The motor begins “sucking” the fluidinstead of pumping the fluid.

Current Unbalance Relays - Device 46

SAES-P-114 recommends use of an ABB Type CM current unbalance relay for motors ratedabove 1125 kW (1500 hp). This relay is used to detect phase unbalance or open phase. Itconsists of two mechanically independent disc units. Phase A and B currents energize theupper electromagnets, while phase B and C currents energize the lower electromagnets.When phase currents are balanced, the electromagnets create equal and opposing torques oneach of the discs (see Figure 31).

• The relay contacts are electrically common and connected in parallel. Closingof any one contact on either the upper or lower disc completes the trip circuit.

• Because the CM relay is calibrated for one ampere sensitivity and is set tooperate on an unbalance, no setting of this relay is required.

• Note: If this relay is applied on a multi-motor bus, an unbalance on any motorcould trip the entire bus. The best recommendation is to apply one CM relayper motor.

Figure 31. CM Relay




Figure 32 describes the CM relay’s operating characteristics.

Figure 32. CM Relay Operating Characteristics




Single-Phasing is caused by the opening of either a primary or secondary conductor feeding amotor. Figures 33 through 36 describe the three-wire and phasor diagrams for theseconditions.

Figure 33. Primary Open (Three-Line Diagram)

Primary open phasor diagrams and equations:

• = + = ∠ °+ ∠ °=

• = + = ∠ °+ ∠ ° = ∠ °

• = + = ∠ °+ ∠ °= ∠ ° = −

I I I

I I I p u

I I I p u I

A A A

B B B

C C C B

1 2

1 2

1 2

1 120 1 300 0

1 0 1 60 3 30

1 240 1 180 3 210

. .

. .

• = + = ∠ °+ ∠ °= ∠ °• = + = ∠ °+ ∠ ° = ∠ ° =• = + = ∠ °+ ∠ ° = ∠ °

I I I p uI I I p u II I I p u

a a a

b b b a

c c c

1 2

1 2

1 2

1 90 1 330 1 301 330 1 90 1 301 240 1 240 2 240

. .. .

. .




Figure 34. Phasor Diagram (Primary Open)




Figure 35. Secondary Open (Three-Line Diagram)

Secondary open phasor diagrams and equations:

• ∠ ° ∠ ° ∠ °• ∠ °+ ∠ ° ∠ °• ∠ ° ∠ ° ∠ °• ∠ ° ∠ °

•

I = I + I = 1 120 + 1 240 = 1 180 = -1.0 p.u.I = I + I = 1 0 1 0 = 2 0 = 2.0 p.u.I = I + I = 1 240 + 1 120 = 1 180 = -1.0 p.u. = II = I + I = 1 90 + 1 270 = 0

I

A A1 A 2

B B1 B2

C C1 C2 A

a a1 a 2

b = I + I = 1 330 + 1 30 = 3 0 = 3 p.u.

I = I + I = 1 210 + 1 150 = 3 180 - 3 p.u. = -Ib1 b 2

c c 1 c2 b

∠ ° ∠ ° ∠ °

• ∠ ° ∠ ° ∠ °




Figure 36. Phasor Diagrams (Secondary Open)




Voltage Unbalance (Low Voltage Motors)

Protection of low voltage motors using voltage unbalance relays is usually not cost effective.As previously discussed, increased heating occurs as a result of the voltage unbalance, and theonly other practical means to reduce the thermal effects is to reduce the shaft kW (hp) loadingin accordance with the following formula and Figure 37.

Percent NEMA unbalance =

Max deviation from average voltageaverage voltage

Figure 37. Voltage Unbalance Derating Factors




EXAMPLE A: Given the following data, what is the maximum safe connected shaft kW(hp) to avoid thermal overheating of the motor?

Motor Ratings: 150 kW (200 hp), 3-phase, 460V

Voltages: Vab = 449V, Vbc = 459V, Vca = 421V

ANSWER Vavg = (449 + 459 + 421)/3 = 1329/3 = 443V

Maximum voltage deviation from average = 443 - 421 = 22V

Percent NEMA unbalance = (22/443) X 100 = 4.97%

Per Figure 37, the motor should be derated approximately 75% to 112.5kW (150 hp) for a 5% voltage unbalance. Note: Derating the motor isnot the preferred method to avoid overheating. The preferred method isto correct the causes of the voltage unbalance. For example, removingsingle-phase loads from the motor bus, balancing the single-phase loadson the bus, etcetera.

Miscellaneous Protection

High Speed Reclosing

If a motor is reenergized before it has stopped rotating, high transient torques can develop (Tα V2), and possible damage (e.g. broken shafts) can occur. The most probable cause ofreenergization is utility high speed reclosing (10-36 cycles) after a fault. The simplestprotection schemes are a timing relay that allows the motor to coast to a stop before restarting,or delaying restart using an undervoltage permissive relay in the starting control circuit set at25-33% of normal voltage.

Repetitive Starting - Device 2RS

Restarting motors with insufficient cooling time, or operating with extreme load variations(jogging) can result in dangerously high motor temperatures. Timing circuit protectionschemes based on manufacturer-recommended starting cycles (e.g., 2 hot/1 cold per hour), ortemperature sensitive relays, such as the CT relay just previously discussed, are also used toprotect the motor against repetitive starting. Use of this type of relay requires very carefulanalysis of the motor and its projected operating cycles.




Protection Scheme One-Line Diagrams

SAES-P-114 (Chapter 6) very clearly lists the preferred protection scheme for the varioustypes of induction motors used in Saudi Aramco industrial applications. Figures 38 through43 are one-line diagrams developed to describe the SAES-P-114 motor protectionrequirements.

Low Voltage Motors

Voltage motor protection is separated based on the following motor rating categories:

• 0.75 kW (100 hp) or less

• Greater than .75 kW to 75 kW (1.0 to 100 hp)

• Greater than 75 kW (100 hp)

0.75 kW (1.0 hp) or Less - This category of low voltage motor is protected by thermalmagnetic molded case circuit breakers (MCCB) with three-pole thermal magnetic trips(Figure 38a), or combination controllers with overloads, a contactor, and a magnetic-onlyMCCB or thermal-magnetic MCCB as shown in Figure 38b.

Figure 38. Protection: 0.75 kW (1.0 hp) or Less




Greater than 0.75 kW to 75 kW (1.0 to 100 hp) -This category of motor protection permitsuse of motor circuit protectors (MCP), and requires window-type CT ground fault protectionfor motors rated 22.5 kW (30 hp) and larger. Overload and contactor requirements are thesame as the less than 0.75 kW (1.0 hp) category. The one-line diagram for this category isdescribed in Figure 39.

Figure 39. Protection: Greater Than 0.75 kW to 75 kW (1.0 to 100 hp)




Greater Than 75 kW (100 hp) to a maximum of 185 kW (250 hp) - This category of motorrequires a low voltage power circuit breaker (LVPCB), drawout type, electrically-operated,with shunt-trip device. Undervoltage protection (Device 27), in addition to ground faultprotection (Device 50GS), is required for the larger, low voltage motors. SAES-P-114permits individual or common bus undervoltage protection (see Figure 40).

Figure 40. Protection: Greater Than 75 kW (100 hp)




Medium Voltage Motors

Medium voltage motor protection is separated into the following two motor rating categories:

• 150 kW (200 hp) through 7500 kW (10,000 hp)

• 7500 kW (10,000 hp) or greater

150 kW (200 hp) through 7500 kW (10,000 hp) - SAES-P-114 further breaks this categoryof motor protection into two sub-categories. Power circuit breakers are the typical protectivedevices with Class E2 controllers permitted for motors rated 1125 kW (1500 hp or less).Figure 41 is the recommended protection scheme using Class E2 controllers.

Figure 41. Protection: Class E2 Controllers (<1125 kW)




Figure 42 is the recommended protection scheme using a power circuit breaker for motorsrated 150 kW (200 hp) through 7500 kW (10,000 hp) ranges.

Figure 42. Power Circuit Breaker (<7500 kW)




7500 kW (10,000 hp) or Greater - This category of motor requires differential (Device 87M)protection versus Device 50 short circuit protection, and temperature (Device 49T) protectionas opposed to thermal overload protection using a BL-1 relay (Device 49). Additionaloverload protection for this motor category is also provided by using an ABB COM relay (seeFigure 43).

Figure 43. Protection: Power Circuit Breaker (>7500 kW)




SOLID-STATE MOTOR PROTECTION PACKAGE (MPP) FEATURES

General Description

Solid-state motor protection packages (MPPs) are typically self-contained, door-mounted,motor protection devices. Saudi Aramco (SAES-P-114) permits use of MPPs for motors 4.0kV or greater in any kW (hp) rating.

Features and Capabilities

Solid-state MPPs (latest generation), such as the Multilin 269 Plus or Westinghouse IQ1000-II, are the best or preferred method of protecting medium voltage motors in today’s industrialenvironment. These MPPs develop very accurate thermal models of the motor, and,therefore, the protection set points (for example, locked-rotor and thermal protection) canbetter match the thermal characteristics of the motor. In contrast, conventional relays are setto protect based on an estimate of the motor’s thermal capabilities. Algorithms, used in theMPPs for the motor’s I2t thermal characteristics, are calculated based on the motor’s actualload amps. Most MPPs also continuously calculate positive and negative sequence currentsas well. The primary features of a typical MPP are:

• Protection

• Communication

• Diagnostics

Benefits

The key benefit of an MPP is that these types of relays offer, for all practical purposes,unlimited motor protection. Another and often overlooked benefit is that they extend amotor’s life, because the protection set points are based on much more accurate thermalmodels of the motor. Note: Because all of the motor protection is in a single, self-containedpackage, a designer must consider backup electromechanical relays when fail-safe tripping isnot allowable for an MPP failure.




Multilin MMR 269 Plus

This relay is primarily a current (CT secondary) sensing relay. Voltage functions (meteringand/or relaying) are optional. Figure 44 is a description of the MMR 269 Plus faceplate.

Figure 44. Multilin 269 Plus: Faceplate




Single-Line Drawing

Figure 45 is the legend describing the relays shown on the single-line drawing (Figure 46).

Figure 45. Multilin 269 Plus: Legend




Figure 46. Multilin 269 Plus: Single-Line Drawing




Protection Features

The protection features (relay functions) listed in Figure 47 are no different than theirelectromechanical counterparts. The differences are that all of the functions are self-contained in one case, and the user enters the set points via a keypad. Note: SAES-P-114requires a separate undervoltage relay (Device 27) if the meter option is not selected.

Figure 47. Multilin 269 Plus: Protection Features




Communication Features

The communication features of the relay provide motor data/status to a remote device such asa computer. The following list describes the communication features of the Multilin MMR269 Plus.

• Overload alarm

• Stator RTD alarm

• Ground fault alarm

• Undercurrent alarm

• Unbalance alarm

• Bearing RTD alarm

• Broken RTD alarm

• Undervoltage alarm (meter option only)

• Power factor alarm (meter option only)

• Self test alarm

• Alphanumeric display

• Actual motor values displayed

• Status indication

• RS485 port

• Analog output load amps

• Analog output motor thermal capacity

• Analog output stator temperature

• Analog output (average RMS amps) (meter option)

• Analog output kW (meter option)

• Analog output kVAR (meter option)

• Analog output p.f. (meter option)




After the relay has been programmed and the motor is running, operations personnel, usingthe keypad, can demand the following list of actual values.

• Three-phase average current

• Individual phase currents

• Hottest stator RTD temperature

• Individual stator RTD temperature

• Maximum stator RTD temperature since last access

• Unbalance ratio (% In/lp)

• Ground leakage current

• Individual motor bearing RTD temperatures

• Individual drive bearing RTD temperatures

• Individual maximum bearing temperatures since last access

• Thermal capacity remaining/ Estimated time to trip at present overload level

• Motor load as a % of full load

• Phase-to-phase voltage

• kW, kVAR, MWH, p.f., frequency

Diagnostic Features

The diagnostic features of the relay include the following:

• Learned motor parameters

• Pre-trip values

• Motor operation historical data

• Latched fault indications

Several of the learned motor parameters include cool down time from run to stop, cool downtime from run-overload to run-normal, learned negative sequence contribution, andacceleration time.

To assist in fault diagnosis, the relay will identify the cause of trip and the following valuescan be recalled for rapid fault diagnosis.




• Average motor current

• Unbalance ratio

• Ground fault current

• Maximum stator RTD temperature

• Phase voltage

• kW

• Power factor

• Frequency

To assist in fault diagnosis, maintenance and operations monitoring, the relay will display thefollowing list of statistical values.

• Running hours since last commissioning

• Number of starts since last commissioning

• Number of trips since last commissioning

• Number of overload trips

• Number of unbalance trips

• Number of ground fault trips

• Number of RTD trips

• Number of short circuit trips

• Number of start trips

• Total watt-hours




Other Features

Numerous other features as shown in the following list are also available on the relay.

• Emergency restart

• Learned acceleration time

• Start inhibit

• Single shot restart

• Output relays

• Draw-out case option (extra)

• Optional DC control supply (extra)

• Exponential running cool down

• Anti backspin timer




Westinghouse IQ-1000II

The IQ-1000II is also a current (CT secondary) sensing relay. Figure 48 is a description ofthe IQ-1000II faceplate.

Figure 48. IQ-1000II: Faceplate

Block Diagram




The IQ-1000II receives motor current sensing derived from 3 separate current transformers,each of which monitors one phase of an AC line to the motor (see Figure 49). If an optionalzero sequence ground fault transformer is used, the IQ-1000II monitors ground fault currentlevels and compares them to a user-selected setpoint. If optional RTDs are used, the IQ-1000II gathers winding temperature data from six RTDs embedded in the stator windings ofthe motor. Four RTDs associated with the motor and load bearings can also be monitored fortemperature levels. Additionally, one auxiliary RTD, such as motor case temperature, can bemonitored.

Figure 49. IQ-1000II: Block Diagram




Protection Features

The IQ-1000II protection features are current sensitive only (see Figures 50a and b). SAES-P-114 would, therefore, require a separate undervoltage relay (Device 27) to protect themotor.

Figure 50a. IQ-1000II: Protection Features




Figure 50b. IQ-1000II: Protection Features (Cont’d)




Communication Features

IMPACC, a Westinghouse Local Area Network, can be used to communicate with one ormultiple IMPACC-compatible devices. These devices include the following: IQ-500,IQ1000, IQ-1000 II, IQ Data Plus, IQ Data PlusII, IQ Data, IQ Generator, Digitrip,Addressable Relay II Advantage Motor Starters, and IQ Energy Sentinels. Up to 1000devices can be connected on a network via shielded twisted pair wire. Four differentcommunication levels are available and are described below:

• IMPACC Series I: Standardized software package that runs on a 100% IBM-compatible computer. The software is packaged with a computer interface card(CONI). Series I offers the following features: System Monitoring, DataLogging, Event Logging, Remote On/Off Control, Dialup Capabilities andGateway Interface.

• IMPACC Series II: Customer-written software for special applications.Custom software is required in situations where (1) Westinghouse softwaredoes not provide feature(s) desired by the customer or (2) the customer wantsto communicate to a non-IBM compatible computer or a programmablecontroller. A MINT translator module converts device-messages into 10-byteASCII RS-232 signal. An RS-232 Protocol Manual is included with eachMINT.

• IMPACC Series III: Standardized software package that runs on most 100%IBM-compatible computers. Series III requires a CONI or a MINT to operate.Series III runs in the Microsoft Windows environment and includes thefollowing features: System Monitoring, Data Trending, Event Logging,Spreadsheet-compatible Trend and Log files, Remote On/Off Control, GatewayInterface, Device/System Alarming, Analog Alarming, Security (passwordprotection) and Enhanced Graphics.

• IMPACC Driver Software (Third Party Vendors): Data acquisition softwarewritten by third party vendors. Software drivers are available to gather datafrom systems such as IMPACC, Programmable Controllers and/or EnergyManagement Systems. The IMPACC Driver for ICONICS’ Genesis (real-timegraphics interface program) offers the following features: System Monitoring,Data Trending, Event Logging, Remote On/Off Control Device/SystemAlarming, Customized Graphics and Communications to other Genesis-compatible systems (PLC’s), Energy Management Systems




Diagnostic Features

The IQ-1000II has 52 set points, most of which can be used for diagnostic purposes(maintenance, troubleshooting, etcetera). Figures 51a, b, c, and d list the monitor data.

Figure 51a. IQ-1000II: Monitor Data

Figure 51b. IQ-1000II: Monitor Data (Cont’d)




Figure 51c. IQ-1000II: Monitor Data (Cont’d)

Figure 51d. IQ-1000II: Monitor Data (Cont’d)




Figure 52 lists the alarm conditions data.

Figure 52. IQ-1000II: Alarm Conditions




Figures 53a and b list the trip conditions.

Figure 53a. IQ-1000II: Trip Conditions




Figure 53b. IQ-1000II: Trip Conditions (Cont’d)




Other Features

In addition to load-associated protection, the IQ-1000II relay also, through the use of specialalgorithms, provides rotor temperature protection. The relay continuously measures/monitorsboth the positive and negative sequence currents, and incorporates their combined effect intoan algorithm that effectively tracks rotor temperature.

Another unique feature of the relay is the internal diagnostics failure message capability asshown in Figure 54.

Figure 54. IQ-1000II: Internal Diagnostics




GLOSSARY

American National An organization whose members approve various standardsStandards Institute (ANSI) for use in American industries.

analog (output) One type of continuously variable quantity used to representanother; for example, in temperature measurement, an electriccurrent output represents temperature input.

asymmetrical (current) The combination of the symmetrical component and thedirect-current component of the current.

current-limiting (fuse) A fuse that, when it is melted by a current within its specifiedcurrent-limiting range, abruptly introduces a high arc voltageto reduce the current magnitude and duration. Note: thevalues specified in standards for the threshold ratio, peak let-through current, and I2t characteristic are used as themeasures of current-limiting ability.

diagnostic Pertaining to the detection and isolation of either amalfunction or mistake.

duty A variation of load with time, which may or may not be(rotating machinery) repeated, and in which the cycle time is too short for thermal

equilibrium to be attained.

horsepower (shaft) (hp) The mechanical output (shaft) rating of a motor. One(1) hp equals 746 watts. See kilowatt (shaft).

hottest-spot A conventional value selected to approximate the degreestemperature allowance of temperature by which the limiting insulation temperaturerise

exceeds the limiting observable temperature rise.

induction motor An alternating-current motor in which a primary winding onone member (usually the stator) is connected to the powersource and in which a polyphase secondary winding or asquirrel-cage secondary winding on the other member (usuallythe rotor) carries induced current.

instantaneous (relay) A qualifying term applied to a relay indicating that no delay ispurposely introduced in its action.

Institute of Electrical and A worldwide society of electrical and electronics engineers.Electronics Engineers (IEEE)




jogging The quickly repeated closure of the circuit to start a motorfrom rest for the purpose of accomplishing small movementsof the driven machine.

kilowatt (shaft) (kw) The mechanical output (shaft) rating of a motor. Seehorsepower (hp).

locked-rotor The condition existing when the circuits of a motor are(rotating machinery) energized, but the rotor is not turning.

locked-rotor current The steady-state current taken from the line with the rotorlocked and with rated voltage (and rated frequency in the caseof alternating-current motors) applied to the motor.

locked-rotor indicating Code letters marked on a motor nameplate to show motorkVAcode letter per hp under locked-rotor conditions.

low voltage Voltage levels below 1000 volts usually called utilizationlevel outages.

medium voltage Voltage levels greater than or equal to 1000 volts and lessthan 100,000 volts.

motor protection A solid-state, self-contained motor protection relay, such asthepackage (MPP) Multilin 269 Plus or the Westinghouse IQ-1000II.

National Electric Code An electrical safety code developed and approved every three(NEC) years by the National Fire Protection Association (NFPA).

National Electrical A nonprofit trade association of manufacturers of electricalManufacturers Association apparatus and supplies, whose members are engaged in(NEMA) standardization to facilitate understanding between users and

manufacturers of electrical products.

negative sequence Three balanced current phasors equal in magnitude, displacedcurrent components from each other by 1200 in phase, and having the phasesequence

opposite to that of the original set of unbalanced phasors.




positive sequence Three balanced current phasors equal in magnitude, displacedcurrent components from each other by 1200 in phase, and having the same phase

sequence as the original unbalanced phasors.

relay An electrically controlled, usually two-state, device that opensand closes electrical contacts to effect the operation of otherdevices in the same or another electric circuit.

replica temperature A thermal relay whose internal temperature rise isproportionalrelay to that of the protected apparatus or conductor, over a range of

values and durations of overloads.

residual (current) The sum of the three-phase currents on a three-phase circuit.The current that flows in the neutral return circuit of threewye-connected current transformers is residual current.

rotor (rotating machinery) The rotating member of a machine with shaft.

service factor (S.F.) A multiplier that, when applied to the rated power, indicates apermissible power loading that may be carried under theconditions specified for the service factor.

single-phasing (motor) An abnormal operation of a polyphase machine when itssupply is effectively single-phase.

starter (motor) An electric controller for accelerating a motor from rest tonormal speed and for stopping the motor.

starting current The current drawn by the motor during the starting period.(rotating machinery) It is a function of speed or slip.

stator The portion that includes and supports the stationary(rotating machinery) active parts. The stator includes the stationary portions of themagnetic circuit and the associated winding and leads. It may, depending on the design,

include a frame or shell, winding supports, ventilation circuits, coolers, and temperature detectors. A base, if

provided, is not ordinarily considered to be part of the stator.

symmetrical (current) A periodic alternating current in which points that are one-halfa period apart are equal and have opposite signs.

synchronous speed The speed of the rotation of the magnetic flux, produced by orlinking the winding.




temperature rise A test undertaken to determine the temperature rise(rotating machinery) above ambient of one or more parts of a machine underspecified operating conditions. Note: The specified conditions may refer to current, load,

etcetera.

time-current The correlated values of time and current thatcharacteristics designate the performance of all or a stated portion of thefunctions of a protective device. Note: The time- current characteristics of a protectivedevice are usually shown as a curve.

time-overcurrent relay An overcurrent relay in which the input current and operatingtime are inversely related throughout a substantial portion ofthe performance range.

total current See asymmetrical current.

zero sequence Three balanced current phasors equal in magnitudecurrent components and with zero displacement from each other.

electric motor 7

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