windows-1256''motor branch circuit

7/29/2019 Windows-1256''Motor Branch Circuit

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Motor Branch Circuit

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Good friends are hard to find, harder to leave, and impossible to forget

4. Motor Branch CircuitBe careful, it may be dangerous


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4.1. Motors overheating Protection

4.1.1. Overheating reasons

Thermal overload with slow variation

Overload condition or loss of cooling that produces a rise of temperature that is sufficiently slow that the

temperature of the thermal protector or detector follows it without appreciable delay. Some of the ways in which a thermal overload with slow variation may be caused are:

Defects in ventilation or the ventilation system due to excessive dust in the ventilation ducts, or dirt

on windings or frame cooling ribs, etc

An excessive rise in ambient temperature or the temperature of the cooling medium

Gradual increasing mechanical overload

Prolonged voltage drop, over-voltage or unbalance in the machine supply

Excessive duty on a motor rated for intermittent duty

Frequency deviations

Thermal overload with rapid variation

Overload condition or loss of cooling that produces a rise of temperature that is too rapid for the

temperature of the thermal protector or detector to follow without appreciable delay resulting in a

significant temperature difference between the thermal device and the part to be protected.

Some of the ways in which a thermal overload with rapid variation may be caused are:

Stalling the motor

Phase failure

Starting under abnormal conditions, for example, inertia too great, voltage too low, load torque

abnormally high

Sudden and significant increase in load

Starting repeatedly during a short time

4.1.2. Motor protection requirements

The motor can withstand Stalling, starting current and Overload for a limited periods.

A thermal limit curve is a plot of the maximum permissible safe time versus line current in the windings of

the machine under conditions other than normal operation. It represents the following three situations:

a) Locked rotor

b) Starting and acceleration

c)

Running overload

The thermal limit curve is intended to be used in conjunction with the machine time-current curve for a

normal start to set the machine protective devices for the thermal protection of the machine during starting

and running conditions.


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Figure 4-1 Typical thermal limit curves per IEEE 620-1996


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4.1.3. Motor Overheating protection devices

IEC 60204

7.3 Protection of motors against overheating Protection of motors against overheating shall be provided for each motor rated at more than 0,5 kW.

Protection of motors against overheating can be achieved by:

Overload protection (7.3.2),

NOTE 1 Overload protective devices detect the time and current relationships (I2t ) in a circuit that

are in excess of the rated full load of the circuit and initiate appropriate control responses.

Over-temperature protection (7.3.3), or

NOTE 2 Temperature detection devices sense over-temperature and initiate appropriate control

responses.

Current-limiting protection (7.3.4)

Automatic restarting of any motor after the operation of protection against overheating shall be prevented

where this can cause a hazardous situation or damage to the machine or to the work in progress.

7.3.2 Overload protection

Where overload protection is provided, detection of overload(s) shall be provided in each live conductor

except for the neutral conductor. However, where the motor overload detection is not used for cable

overload protection (see also Clause D.2), the number of overload detection devices may be reduced at the

request of the user (see also Annex B).

For motors having single-phase or DC power supplies, detection in only one unearthed live conductor is

permitted. Where overload protection is achieved by switching off, the switching device shall switch off all live

conductors. The switching of the neutral conductor is not necessary for overload protection.

Where motors with special duty ratings are required to start or to brake frequently (for example, motors for

rapid traverse, locking, rapid reversal, sensitive drilling) it can be difficult to provide overload protection with

a time constant comparable with that of the winding to be protected. Appropriate protective devices

designed to accommodate special duty motors or over-temperature protection (see 7.3.3) can be necessary.

For motors that cannot be overloaded (for example torque motors, motion drives that either are protected

by mechanical overload protection devices or are adequately dimensioned), overload protection is not

required.

7.3.3 Over-temperature protection

The provision of motors with over-temperature protection (see IEC 60034-11) is recommended in situations

where the cooling can be impaired (for example dusty environments).

Depending upon the type of motor, protection under stalled rotor or loss of phase conditions is not always

ensured by over-temperature protection, and additional protection should then be provided.

Over-temperature protection is also recommended for motors that cannot be overloaded (for example

torque motors, motion drives that are either protected by mechanical overload protection devices or are

adequately dimensioned), where the possibility of over-temperature exists (for example due to reducedcooling).


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4.1.4. Current dependent devices

Figure 4-2 Typical Overload Relay

Bimetal Overload Relays

Consists of a small heater element wired in series with the motor and a bimetal strip that can be used as a

trip lever. The bimetal strip is made of two dissimilar metals bonded together. The two metals have different thermal

expansion characteristics, so the bimetal strip bends at a given rate when heated.

Under normal operating conditions, the heat generated by the heater element will be insufficient to cause

the bimetal strip to bend enough to trip the overload relay.

As current rises, heat also rises. The hotter the bimetal strip becomes, the more it bends. In an overload

condition, the heat generated from the heater will cause the bimetal strip to bend until the mechanism is

tripped, stopping the motor.

Figure 4-3

Ambient Compensated Bimetal Overload Relay

In certain applications (such as a submersible pump), the motor may be installed in a location having aconstant ambient temperature. However, the motor control and overload relay may be installed in a location

with a varying ambient temperature. In such cases, the trip point of the overload relay will vary with the

temperature of the surrounding air as well as current lowing through the motor, which can lead to

premature and nuisance tripping.

Ambient compensated bimetal overload relays are designed to overcome this problem. A compensated

bimetal strip is used along with a primary bimetal strip. As the ambient temperature changes, both bimetal

strips will bend equally and the overload relay will not trip the motor. However, current low through the

motor and the heater element will affect only the primary bimetal strip. In the event of an overload

condition, the primary bimetal strip will engage the trip unit.


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Figure 4-4

Overload relay set value

For continuous run motor, the overload relay must be selected such that it has a set value equal to motor

nameplate full load current.

This ensures that the motor doesnt run overloaded for any time and no increase of temperature.

Condition:

Overload relays applied must be able to carry full starting current for the entire

acceleration period without nuisance trip.

The previous condition can be accomplished by using higher class overload relay which preclude the need for

selection of a higher trip current.

A Class 20 or Class 30 overload relay will provide a longer motor acceleration time than a Class 10 or Class 20,

respectively.

Overload relays trip class

Overload relays are rated by a trip class which defines the length of time it will take for the relay to trip in an

overload condition.

IEC 947-4-1 (Abstract)

8.2.1.5.1 Limits of operation of time-delay overload relays when all poles are energized

Trip time from the following state

Cold Hot Hot Cold

at 1.05 Ir at 1.2 Ir at 1.5 Ir at 7.2 Ir

Class

10A > 2 hrs < 2 hrs < 2 mins 2 s < tp < 10 s10 > 2 hrs < 2 hrs < 4 mins 4 s < tp < 10 s

20 > 2 hrs < 2 hrs < 8 mins 6 s < tp < 20 s

30 > 2 hrs < 2 hrs < 12 mins 9 s < tp < 30 s

Cold state: initial state without previous load

Hot state: thermal equilibrium reached at Ir

Ir: setting current of the overload relay

Example

A motor has an accelerating time of

= 7, a full-load current of

=1.1and a locked-rotor current

of = 6 . Tesys LRD-08 (From Schneider Electric) will be used as overload relay. Figure 4-5 shows the time-current characteristic of the overload used, Will the overload relay nuisance trip?


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Figure 4-5 Trip curve for LRD overload relay

The trip time for different rms current must be calculated, the rms value of the current is given by

= + + + + + + (4.1)

From equation (4) with n = 2 and solving for time , we get= + = 1 + (4.2)

For =1.5 . = 1 + 6 1.51.11.51.1 1.1 7 =1 6 1 . 0 1 2 (4.3)

Table 4-1 was devolved by calculating different trip times using equation (4.2)

Table 4-1 . Time Period (s)6 5.75

5 8.394 13.42

3 25.16

2 67.09

1.5 161.01

Values from Table 4-1 are plotted on the overload relay time current characteristic shown in Figure 4-5

We can conclude that Class 10A overload relay will cause a nuisance trip while Class 20 will not.

NEC requirements

For the previous condition, if the selection of higher trip class is not sufficient for starting, then the use of an

overload relay of higher trip current become necessary.

In this case use the NEC to detect whether your selection meets the minimum safety requirements or not.


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NEC ARTICLE 430 Motors, Motor Circuits, and Controllers

III. Motor and Branch-Circuit Overload Protection

430.32 Continuous-Duty Motors

More Than 1 Horsepower.

Each motor used in a continuous duty application and rated more than 1 hp shall be protected against

overload by one of the means in 430.32(A)(1) through (A)(4).

Separate Overload Device

A separate overload device that is responsive to motor current. This device shall be selected to trip or shall

be rated at no more than the following percent of the motor nameplate full-load current rating:

Motors with a marked service factor 1.15 or greater 125%

Motors with a marked temperature rise 40C or less 125%

All other motors 115%

Modification of this value shall be permitted as provided in430.32(C)

Phase Loss

Table 4-2 shows the approximate effect upon line currents resulting from possible phase loss conditions.

With one thermal unit per phase, if the motor is running at full load current, normal overload protection

produces a trip for all phase loss conditions.

If a motor is running lightly loaded at the time of the phase loss, however, the increases line current could

still fall within the tripping current of the overload relay, this could cause serious problem

If an application were such that the motor continuously run lightly loaded, overload protection could be

selected based upon the actual current that removing this problem

If the load varies, the only way to ensure protection against loss of phase is to provide a separate device

designed for that purpose.

Table 4-2 Line currents in case of phase loss

Status of power supply Phase Loss During Reaction of Motor Resultant Line current

Primary phase loss

(Before Transformer)

Start attempt Dont start 0.5 LRA, 0.5 LRA, LRA

Running Continuous to run 1.15 FLA, 1.15 FLA, 2.3 FLA

Secondary phase loss

(After Transformer)

Start attempt Dont start 0.87 LRA, 0.87 LRA, 0

Running Continuous to run 1.73 FLA, 1.73 FLA, 0

Overload relay with intermittent duty cycle

Overload relays are not suitable for high starting duty or large numbers of switching operations. Differences

in the thermal time constants for the overload relay and the motor results in unnecessary early tripping

when the protection switch is set to rated current.

Example

A motor has a full-load current of 6 A and a locked-rotor current of 36 A. its operating cycle is:

5 sec accelerating time, 30 sec run time, and 90 sec off time

Will a properly selected thermal overload relay trip

The RMS value of the current is given by


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= 36 5 + 6 305 + 3 0 + 9 0 =7.8 Since 7.8 is about 130% of 6, the overload should eventually trip because a properly selected overload should

produce a protection level within 115% (According to NEC)

4.1.5. Temperature dependent devices

Thermistor protection relay for use with PTC thermistor probes

Thermistor protection units continuously monitor the temperature of the machines to be protected by

means of PTC thermistor probes embedded in the machine windings.

Thermistor probes

Thermistors are solid state temperature sensors that behave like temperature-sensitive resistors; hence their

name is a contraction of "thermal" and "resistor".

There are two types : Positive Temperature Coefficient PTC (resistance increase with temperature) and

Negative Temperature Coefficient NTC (resistance decrease with temperature). PTC thermistors are the most common in motor protection.

If the nominal operating temperature (NOT) of the thermistor probes is reached, rapid change in resistance

will result.

Figure 4-6 Thermistor probes Figure 4-7 Thermistor resistance-temperature curve

Commercial available PTC resistor sensor has a cold state resistor between 50 150 at 20 and warmstate resistor (NOT) of10000

The PTC thermistor temperature sensors must be selected depending on:

The motor insulation class IEC Publication 60034-11

The motor utilization category

The special characteristics of the motor, such as conductor cross-sections of the windings,

permissible overload factor etc

Special conditions prescribed by the user, such as permissible ambient temperature, risks resulting

from locked rotor, extent of permitted overloading etc


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Table 4-3 Possible selection of OT temperature corresponding to different insulation classes

Insulation Class Possible Selection of NOT Temperature

B 110

F 140

H 160

In the case of three- phase squirrel cage motors for instance, three sensors are embedded in the stator

winding. For pole-changing motors with one winding (Dahlander connection), 3 sensors are also sufficient.

However, pole-changing motors with two windings require 6 sensors.

Thermistor protection relay

Thermistor protection relay convert the rapid increase in resistance into a switching function (Usually done

by heating a bimetal strip) which can be used to switch off the motor or as a signal.

Figure 4-8 Thermistor protection relay

4.1.6. Comparison

Current-dependent protection is particularly effective in the case of a locked rotor.

For standard duty with short start-up times and starting currents that are not excessive and for low numbers

of switching operations, Current-dependent protection provides adequate protection.

For high starting duty, large numbers of switching operations and Impaired cooling Current-dependent

protection is not suitable

Table 4-4 Current vs. Temperature dependent protection devices

Protection of the motor under the following conditions Using bimetal Using thermistor

Overload in continuous operation + +Extended starting and stopping (+) +

Switching to stalled rotor (stator-critical motor) + +

Switching on stalled rotor (rotor-critical motor) (+) (+)

Single-phasing + +

Intermittent operation +

Excessive frequency of operation +

Voltage and frequency fluctuations + +

Increased coolant temperature +

Impaired cooling +

+ Full protection

(+) Partial protection

No protection


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4.2. Control element

4.2.1. Contactor

Most motor applications require the use of remote control devices to start and stop the motor. Magnetic

contactors (similar to the ones shown below) are commonly used to provide this function.

IEC 60947-12.2.12 Contactor (mechanical)

Mechanical switching device having only one position of rest, operated otherwise than by hand, capable of

making, carrying and breaking currents under normal circuit conditions including operating overload

conditions.

NOTE Contactors may be designated according to the method by which the force for closing the main

contacts is provided.

2.2.14 Contactor relay

Contactor used as a control switch.

4.2.2. Basic Electromechanical Contactor Operation

The following illustration shows the interior of a basic contactor. There are two circuits involved in the

operation of a contactor: the control circuit and the power circuit.

The control circuit is connected to the coil of an electromagnet, and the power circuit is connected to the

stationary contacts.

Figure 4-9

The operation of this electromagnet is similar to the operation of the electromagnet we made by wrapping

wire around a soft iron core.

When power is supplied to the coil from the control circuit, a magnetic field is produced, magnetizing the

electromagnet.

Figure 4-10

The magnetic field attracts the armature to the magnet, which in turn closes the contacts. With the contacts

closed, current flows through the power circuit from the line to the load.


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When current no longer flows through the power circuit, the electromagnets coil is de-energized, the

magnetic field collapses and the movable contacts open under spring pressure.

4.2.3. Contactor in control circuit

The following schematic shows the electromagnetic coil of a contactor connected to the control circuit

through a switch (SW1).

The contacts of the contactor are connected in the power circuit to the AC line and a three-phase motor.

When SW1 is closed the electromagnetic coil is energized, closing the M contacts and applying power to

the motor.

Opening SW1 de-energizes the coil, opening the M contacts and removing power from the motor.

Figure 4-11

4.2.4. Contactors utilization categories

The utilization category determines the operating frequency and endurance of a contactor. The category

depends on the type of load. If the load is a motor; the category also depends on the service classification.

Table 4-5

Category lc /le cos Type of load Contactor usage Typical applications

AC1 1 0.8 non-inductive energisation Heating, distribution

AC2 2 0.65 slip-ring

motors

starting , switching off during

running, regenerative

braking inching

wire drawing machines

AC3 2 0.45 for le < 100A

0.35 for le > 100A

squirrel-cage

motors

Starting, switching off during

running

compressors, lifts, mixing,

pumps, escalators, fans,

conveyers, air-conditioning

AC4 6 0.45 for le < 100A

0.35 for le > 100A

squirrel-cage

motors

starting, switching off during

running, regenerative

braking, plugging, inching

printing machines, wire

lc : Current made or broken

le: rated operational current

AC3 utilization category

This category covers asynchronous squirrel-cage motors that are switched off during running. This is the

most common situation (85 % of all cases).

The control device establishes the starting current and interrupts the rated current at a voltage equal toapproximately one-sixth of the rated value. Current interruption is carried out with no difficulty.


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Figure 4-12 AC3 utilization category, the contactor interrupts the rated current of the motor.

AC4 utilization category

This category covers asynchronous squirrel-cage or slip-ring motors capable of operating under regenerative-

braking or inching (jogging) conditions.

The control device establishes the starting current and is capable of interrupting the starting current at a

voltage that may be equal to that of the mains.

Such difficult conditions require oversizing of the control and protective devices with respect to category

AC3.

Figure 4-13 AC4 utilization category, the contactor must be capable of interrupting the starting current Id.


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4.3. Starter

IEC 60947-1

2.2.15 Starter

Combination of all the switching means necessary to start and stop a motor, in combination with suitable overload

protection

Figure 4-14 Starter

4.3.1. Starting methods

The most common starting methods for asynchronous squirrel-cage motors are detailed below

Direct starting

With direct starting, the DOL (Direct On Line) starter, with the closing of line contactor KM1, the line voltage

is applied to the motor terminals in a single operation. Hence a squirrel-cage motor develops a high starting

torque with a relatively reduced acceleration time.

This method is generally used with small and medium power motors which reach full working speed in a

short time.

Figure 4-15 D.O.L. starter with contactor and O/L relay Figure 4-16 Torque/speed curve at Star-Delta start


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These advantages are, however, accompanied by a series of drawbacks, including, for example:

high current consumption and associated voltage drop which may cause damages to the other parts

of the system connected to the network

violent acceleration which has negative effects on mechanical transmission components (belts,

chains and mechanical joints), reducing working life

Other types of starting for squirrel-cage motors are accomplished by reducing the supply voltage of the

motor: this leads to a reduction in the starting current and of the motor torque, and an increase in the

acceleration time.

Star-Delta starter

The most common reduced voltage starter is the Star-Delta starter (Y-), in which

The motor is designed to operate in delta connection winding.

on starting, the stator windings are star-connected, thus achieving the reduction of peak inrush

current;

once the normal speed of the motor is nearly reached, the switchover to delta is carried out

After the switchover, the current and the torque follow the progress of the curves associated withnormal service connections (delta).

As can be easily checked, starting the motor with star-connection gives a voltage reduction of3, and thecurrent absorbed from the line is reduced by 1/3 compared with that absorbed with delta-connection.

The start-up torque, proportional to the square of the voltage, is reduced by 3 times, compared with the

torque that the same motor would supply when delta-connected.

This method is generally applied to motors with power from 15 to 355 kW, but intended to start with a low

initial resistant torque.

Figure 4-17 Star-delta starter with contactors and O/L relay - Torque/speed curve

4.3.2. Motor Starter in a Control Circuit

The following diagram shows the electrical relationship of the contactor and overload relay. The contactor

(highlighted with the darker grey) includes the electromagnetic coil, the main motor contacts, and the

auxiliary contacts. The overload relay, highlighted by the lighter grey, includes the OL heaters and overload

contacts.

The contactor and the overload relay have additional contacts (known as auxiliary contacts) for use in thecontrol circuit.


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In this circuit, a normally closed OL contact has been placed in series with the M contactor coil and L2.

A normally open M auxiliary contact (Ma) has been placed in parallel with the Start pushbutton.

Figure 4-18


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4.4. Starter short Circuit Protection

4.4.1. Development of the short-circuit current (far from generator)

A simplified network comprises a source of constant AC power, reactance X and resistance R is shown below

Here, X and R replace all components such as cables, conductors, transformers and motors.

Figure 4-19

When a fault occurs between A and B, the negligible impedance between these points results in a very high

short-circuit current that is limited only be impedance .

The following differential equation can be used to describe the short circuit process:

Where is the phase angle at the point in time of the short circuit.

Equation (4.5) is Inhomogeneous first order differential equation can be solved by determining the

homogenous solution

i and a particular solution

i

For particular solution, we obtain:

The homogenous solution yields:

The total short circuit current is composed of both components:

For far-from-generator short circuit, the short circuit current is therefore made up o a constant AC periodic

component and a decaying DC periodic component.

Figure 4-20 Shows the time behavior of the short circuit current for the occurrence of far-from-generator

The peak value used to determine the making capacity of the required circuit breakers and to define theelectrodynamic forces that the installation as a whole must be capable of withstanding can be calculated

from

Where = 1.02 + 0.98 e

= + (4.4)

+ = 2 sin + (4.5)

= + = + (4.6)

= 2 sin + = 2 sin + (4.7)

= 2 sin (4.8)

= 2 sin + sin (4.9)

= (4.10)


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Figure 4-20 Time behavior of the short circuit current for the occurrence of far-from-generator

4.4.2. Short Circuit current calculations (Impedance Method)

Calculation of =

Where

U: (phase-to-phase voltage) corresponds to the transformer no-load voltage

: as given in (4.4)

: Sum of all series resistances upstream of the fault: Sum of all series resistances upstream of the fault

Transformer no-load voltage is 3 to5% greater than the on-load voltage across the terminals. For example, in380 V networks, the phase-to-phase voltage adopted is U = 399 V


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Determining the various short-circuit impedances

Upstream network impedance referred to secondary in m

Where

: The no-load phase-to-phase voltage of the network in V

: Short-circuit power in KVA (Data supplied by the power distributor) Resistance: negligible

Reactance:

Internal transformer impedance referred to secondary in m

where

: No-load phase-to-phase voltage of the transformer (V)

: Transformer KVA rating; : Voltage that must be applied to the primary winding of the transformer for the rated current to

flow through the secondary winding, when the LV secondary terminals are short circuited.

For public distribution MV / LV transformers, the values ofuhave been set by the European Harmonizationdocument HD 428-1S1 issued in October 1992

Resistance in m

Where:

: Windings losses in (KW) : Transformer rated current (A)

Reactance in m

Line impedance

Resistance in m

Where

: Resistivity in mmm2/m

: Length in m

: Cross section area in mm2

The resistivity of copper is = 21 m mm/m while for aluminum is = 33 m mm/m

= (4.11)

= (4.12)

=

100

(4.13)

Rating (kVA) of the MV / LV transformer 630 800 1,000 1,250 1,600 2,000

Short-circuit voltage (%) 4 4.5 5 5.5 6 7

= 3 (4.14)

= (4.15)

= (4.16)


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Reactance in m

Table 4-6 Cables reactance values depending on the wiring system

Wiring system Busbars Three-

phase

Spaced single-

core

Touching

single core

cables

(triangle)

3 touching

cables (flat)

3 d spaced

cables (flat)

d = 2r d = 4r

Diagram

Reactance per

unit length

values

recommended in

UTE C 15-105

(m/m)

0.08 0.13 0.08 0.09 0.13 0.13

Average

reactance

per unit length

values (m/m)

0.15 0.08 0.15 0.085 0.095 0.145 0.19

Extreme

reactance

per unit length

values (m/m)

0.12-0.18 0.06-0.1 0.1-0.2 0.08-0.09 0.09-0.1 0.14-

0.15

0.18-

0.20

LV circuit breakers Resistance: negligible

Reactance: Value of 0.15 m is typical

4.4.3. How do short circuit damage motor starter?

Since we are concerned with the damage to the motor starter caused by the short circuit, lets look briefly at

what causes that damage before we address how Type 2 coordination is achieved.

There are two things that cause short circuit damage; one is magnetic forces and the second is excessive

heat. Both of these are a function of the current.

Magnetic forces are a function of the instantaneous peak let-through current (Ip) passed by the SCPD.

This is the current that flows in the circuit before the SCPD can respond to the current and open the

circuit.

Magnetic forces put physical stress on the starter. The forces cause the contacts of the starter to

blow apart and can cause the housing of the starter to fracture, both of which can render the

starter inoperable.

Excessive heat is a function of let-through energy (I2t), the current squared over a period of time, during the

clearing of the short circuit fault current by the SCPD. Excessive heat generally causes the contacts of the

motor starter to weld.


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4.4.4. How can short circuit damage to motor starter be minimized?

There are a number of ways that the damage can be minimized including: using physically larger, more

rugged starters and using better protecting short circuit protective devices.

In general, the ability of a starter to withstand a short circuit fault current (and not be damaged) is a function

of its physical size and construction. Typically, starters of approximately the same physical size will be

capable of withstanding the same amount of short circuit fault current.

Damage to a starter caused by magnetic forces can be minimized by increasing the electrical

clearances between current paths and supporting the current carrying parts with more insulating

material (robust moldings with large cross sectional areas).

Damage caused by excessive heat can be minimized by incorporating larger current carrying parts

with higher contact mass in the starter.

Larger parts can absorb more heat, reducing the possibility of the parts welding. The disadvantage of trying

to minimize damage with this approach is that it increases the size of starter, which in turn increases the size

of the control panel and ultimately the facility. In addition, more material equates to higher costs.

A second way to limit damage to a motor starter is to use better protecting (faster acting, more current

limiting) short circuit protective devices.

Since short circuit damage is a function of the current passed by the SCPD, limiting that current will minimize

the damage to the starter.

4.4.5. Current Limiting MCCB as SCPD

IEC 60947-2

2.3 Current-limiting circuit-breaker

A circuit-breaker with a break-time short enough to prevent the short-circuit current reaching its otherwise

attainable peak value.IEC 60947-1

2.5.5 prospective current (of a circuit and with respect to a switching device or a fuse)

current that would flow in the circuit if each pole of the switching device or the fuse were replaced by a

conductor of negligible impedance

2.5.6 prospective peak current

peak value of a prospective current during the transient period following initiation

Figure 4-21 Figure 4-22 shows the limit curve for Tmax T2L160, In160 circuit-breaker.


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The x-axis shows the effective values of the symmetrical prospective short-circuit current, while the y-axis

shows the relative peak value. The limiting effect can be evaluated by comparing, at equal values of

symmetrical fault current, the peak value corresponding to the prospective short-circuit current (curve A)

with the limited peak value (curve B).

Circuit-breaker T2L160 with thermomagnetic release In160 at 400 V, for a fault current of 40 kA, limits the

short-circuit peak to 16.2 kA only, with a reduction of about 68 kA compared with the peak value in the

absence of limitation 84 kA).

Figure 4-22

4.4.6. Types of co-ordination

Achieving proper co-ordination means matching the characteristics of SCPD and the downstream equipment

including cables to ensure that the let-through energy and peak cut-off current do not rise above the levels

that the starter can withstand

4.4.7. How is certified Type 2 co-ordination achieved?

The only way to ensure Type 2 co-ordination is to carry out exhaustive tests (found in IEC 60947-4) on

particular MCCB with the starter. Usually Starter and MCCB manufacturers do this test on different combination of them, so all we have to do

is select a prober one from the manufacturers coordination tables

IEC 60947-4-1

8.2.5 Co-ordination with short circuit protective devices

Two types of co-ordination are permissible

Type 1 co-ordination requires that, under short-circuit conditions, the contactor or starter shall cause no

danger to persons or installation and may not be suitable for further service without repair and replacement

of parts.

Type 2 co-ordination requires that, under short-circuit conditions; the contactor or starter shall cause no

danger to persons or installations and shall be suitable for further use.

The risk of contact welding is recognized, in which case the manufacture shall indicate the measures to

be taken as regards the maintenance of the equipment


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4.4.8. Example of Coordination Tables

Table 4-7 ABB


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Table 4-8 Schneider Electric


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4.5. Cables

4.5.1. Select the cable type

The following parameters are used to select the cable type:

Conductive material (copper or aluminum)

The choice depends on cost, dimension and weight requirements, resistance to corrosive environments

(chemical reagents or oxidizing elements).

In general, the carrying capacity of a copper conductor is about 30% greater than the carrying capacity of an

aluminum conductor of the same cross section.

An aluminum conductor of the same cross section has an electrical resistance about 60% higher and a weight

half to one third lower than a copper conductor.

Insulation material (none, PVC, XLPE-EPR)

The insulation material affects the maximum temperature under normal and short-circuit conditions and

therefore the exploitation of the conductor cross section

Table 4-9 - Maximum operating temperatures for types of insulationType of insulation Temperature limit ( C )

Polyvinyl-chloride (PVC) 70 at the conductor

Cross-linked polyethylene (XLPE) and ethylene propylene rubber (EPR) 90 at the conductor

Mineral (PVC covered or bare exposed to touch) 70 at the sheath

Mineral (bare not exposed to touch and not in contact with combustible material) 105 at the sheath

Conductor type

The type of conductor (bare conductor, single-core cable without sheath, single- core cable with sheath,

multi-core cable) is selected according to mechanical resistance, degree of insulation and difficulty of

installation (bends, joints along the route, barriers...) required by the method of installation.

4.5.2. Cable Sizing

Step 1: Load current calculation

Load current in A for a three-phase system is calculated by the following formula:

Where

: rated load input power (W): Operating line voltage (V)

: Power factor

Step 2: Methods of installation

To define the current carrying capacity of the conductor and therefore to identify the correct cross section

for the load current, the standardized method of installation that better suits the actual installation situation

must be identified among those described in Appendix B

Step 3: Correction factor for temperature The current carrying capacity of the cables that are not buried in the ground refers to 30 C ambienttemperature. If the ambient temperature of the place of installation is different from this reference

temperature, the correction factor k1 on Table 4 shall be used, according to the insulation material.

= 3 (4.17)


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Table 4-10 Correction factor for ambient air temperature other than 30 C

Step 4: Correction factor for grouping The cable current carrying capacity is influenced by the presence of other cables installed nearby. The heat

dissipation of a single cable is different from that of the same cable when installed next to the other ones.

The factor is tabled according to the installation of cables laid close together in layers or bunchesTable 4-11 Groupping reduction factor

Step 5: Apparent load current

Calculate the value of current by dividing the load current (or the rated current of the protective device)by the product of the correction factors calculated:

Step 6: Determine the cross sectional area

From Table 4-12 or from Table 4-13 , depending on the method of installation, on insulation and conductive

material and on the number of live conductors, determine the cross section of the cable with capacity

The actual cable current carrying capacity is calculated by

=

(4.18)

= (4.19)


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Table 4-12 Current carrying capacity of cables with PVC or EPR/XLPE insulation (method A-B-C)

Table 4-13 Current carrying capacity of cables with PVC or EPR/XLPE insulation (method E-F-G)


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4.5.3. Example of Cable sizing in a balanced three- phase circuit

load current: 100 A

conductor material: copper

insulation material: PVC

type of cable: multi-core

installation: cables bunched on horizontal perforated tray

Installation conditions:

ambient temperature: 40 C

adjacent circuits with three-phase circuit consisting of 4 single-core cables, 4x50 mm2

three-phase circuit consisting of one multi-core cable, 1x(3x50) mm2

three-phase circuit consisting of 9 single-core (3 per phase) cables, 9x95

mm2

single-phase circuit consisting of 2 single-core cables, 2x70 mm2

Type of installation

In Appendix A, it is possible to find the reference number of the installation and the method of installation to

be used for the calculations. In this example, the reference number is 31, which corresponds to method E

(multi-core cable on tray).

Correction factor of temperature

From Table 4-10, for a temperature of 40 C and PVC insulation material,

=0.87 Correction factor of grouping

For the multi-core cables grouped on the perforated tray see Table 5.

As a first step, the number of circuits or multi-core cables present shall be determined; given that:

each circuit a), b) and d) constitute a separate circuit;

circuit c) consists of three circuits, since it is composed by three cables in parallel per phase;

The cable to be dimensioned is a multi-core cable and therefore constitutes a single circuit;

The total number of circuits is 7.

Referring to the row for the arrangement (cables bunched) and to the column for the number of circuits (7)

After and have been determined, is calculated by:

From Table 4-13, for a multi-core copper cable with PVC insulation, method of installation E, with three

loaded conductors, a cross section with current carrying capacity of =212.85 , is obtained. A 95 mm2 cross section cable can carry, under Standard reference conditions, 238 A.

The current carrying capacity, according to the actual conditions of installation, is

=0.54 = = 1000.870.54 =212.85

=2380.870.54=111.81


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4.5.4. Cables Protection against overload

The Standard IEC 60364-4-43 Electrical installation of buildings - Protection against overcurrent specifies

coordination between conductors and overload protective devices (normally placed at the beginning of the

conductor to be protected)

IEC 60364-4-43

433.1 (433.2) Co-ordination between conductors and overload protective devices

The operating characteristics of a device protecting a cable against overload shall satisfy the two following

conditions:

Where

the current for which the circuit is designed

the continuous current-carrying capacity of the cable (see clause 523)

the nominal current of the protective deviceNOTE For adjustable protective devices, the nominal current , is the current setting selected The current ensuring effective operation in the conventional time of the protective device.

The current ensuring effective operation of the protective device is given in the product standardor may be provided by the manufacturer.

NOTE Protection in accordance with this clause does not ensure complete protection in certain cases, for

example sustained overcurrent less than , nor will it necessarily result in an economical solution. Thereforeit is against assumed that the circuit is so designed that small overloads of long duration will not frequently

occur

(4.20) 1 . 4 5 (4.21)

According to first condition to correctly choose the protective device, it is necessary to check that the circuit-

breaker has a rated (or set) current that is:

Slightly higher than the load current, to prevent unwanted tripping;

Lower than the current carrying capacity of the cable, to prevent cable overload.

The Standard allows an overload current that may be up to 45% greater than the current carrying capacity of

the cable but only for a limited period (conventional trip time of the protective device).

The verification of second condition is not necessary in the case of circuit-breakers because the protective

device is automatically tripped if:

= 1 . 3 for circuit-breakers complying with IEC 60947-2 (circuit-breakers for industrial use);

=1 . 4 2 for circuit-breakers complying with IEC 60898 (circuit-breakers for household andsimilar installations).

Therefore, for circuit-breakers, if , the formula 1 . 4 5 will also be verified.


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4.5.5. Protection against short-circuit

A cable is protected against short-circuit if the specific let-through energy of the protective device (I2t) is

lower or equal to the withstood energy of the cable (k2S

2):

Where

the effective short-circuit current expressed as r.m.s. value (A)the short circuit duration (s)

factor taking account of the resistivity, temperature coefficient and heat capacity of the

conductor material, and the appropriate initial and final temperatures.

the cable cross section (mm2)

in the case of conductors in parallel it is the cross section of the single conductor

is the specific let-through energy of the protective device which can be read on the curves supplied bythe manufacturer or from a direct calculation in the case of devices that are not limiting and delaying;

depends on the cable insulating and conducting material. The values of the most common installations are

shown in Table 1.Table 4-14 Values of k for phase conductor

Conductor insulation

PVC300

mm2

PVC>300

mm2

EPR

XLPE

Rubber

60 C

Mineral

PVC Bare

Initial temperature C 70 70 90 60 70 105

Final temperature C 160 140 20 200 160 250

Material of conductor:

Copper 115 103 143 140 115 135/115

a

Aluminum 76 68 94 93 - -

in-soldered joints in copper

conductors

115 - - - - -

aThis value shall be used for bare cables exposed to touch.

NOTE 1 Other values of k are under consideration for.

small conductors (particularly for cross section less than 10 mm2);

duration of short-circuit exceeding 5 s;

other types of joints in conductors;

Bare conductors.

NOTE 2 The nominal current of the short-circuit protective device may be greater than the current carrying

capacity of the cable

NOTE 3 The above factors are based on IEC 60724.

Example

For the simplified network shown in Figure 4-23

Figure 4-23

If a fault occur at point F, with short circuit current available = 40 , T2S160MA 100 MCCB is used forcable protection against short circuit, is it capable to do so?

(4.22)


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A current limiting MCCB must break the fault in less than half cycle 0.01 sec, the current that 16 mm2 copper

cable can withstand for this time is given by:

From the circuit breaker limiting curve, it can be shown that it limit the 40 KA prospective fault current to 10

KA only, so the circuit breaker is able to protect the cable under this fault

Figure 4-24 T2 160 Limiting curve

= = 115 160.01 =18.400(4.23)


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References1. IEEE Std C37.96-2000 IEEE Guide for AC Motor Protection

2. IEEE Std 620-1996 IEEE Guide for the Presentation of Thermal Limit Curves for Squirrel Cage Induction

Machines

3. IEC 60204-1 Safety of machinery - Electrical equipment of machines Part 1: General requirements

4. IEC 60947 Low-voltage switchgear and control gear Part 1: General rules

Part 2: Circuit-breakers

Part 3: Switches, disconnectors, switch-disconnectors and fuse-combination units

Part 4-1: Electromechanical contactors and motor-starters

5. National Electrical Code NEC 2008 Edition

6. Motor and Branch-Circuit overload protection: A Guide in solving problems

7. Siemens Course: Control Components

8. IEC 60909-2 Short-circuit currents in three-phase AC systems Part 2: Data for short-circuit current

calculations

9. Schneider-electric Cahier technique no. 158: Calculation of short-circuit currents

10.PAUL E. ALWIN: IEC Type 2 Coordination for Short Circuit Protection of Motor Starters, IEEE 1993 Annual

Textile, Fiber and Film Industry Technical Conference, 1993.

11. IEC 60364-4-4 Electrical installations of buildings

Part 4-43: Protection for safety - Protection against overcurrent

Part 5-52: Selection and erection of electrical equipment - Wiring systems

12.Electrical installation handbook -ABB

13.Electrical Installation Guide according to IEC International Standards- Schneider Electric

windows-1256''motor branch circuit

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