Design and Mode of Operation

Closed-loop functions in armature circuit

Speed setpoint

The source for the speed setpoint and additional setpoints can be freely selected through parameter settings, I.e. the setpoint source can be programmed as:

Analog values 0 to +10 V, 0 to +20 mA, 4 to 20 mA Integrated motorized potentiometer Binectors with functions:

Fixed setpoint, inch, crawl Serial interfaces on basic unit Supplementary boards

The normalization is such that 100 % setpoint (product of main setpoint and additional setpoints) corresponds to the maximum motor speed.

The speed setpoint can be limited to a minimum or maximum value by means of a parameter setting or connector. Furthermore, "adding points" are included in the software to allow, for example, additional setpoints to be injected before or after the ramp-function generator. The "Setpoint enable" function can be selected with a binector. After smoothing by a parameterizable filter (PT1 element), the total setpoint is transferred to the setpoint input of the speed controller. The ramp-function generator is effective at the same time.

Actual speed value

One of four sources can be selected as the actual speed signal.

Analog tachometerThe voltage of the tacho-generator at maximum speed can be between 8 and 270 V. The voltage/maximum speed normalization is set in a parameter.

Pulse encoderThe type of pulse encoder, the number of marks per revolution and the maximum speed are set via parameters. The evaluation electronics are capable of processing encoder signals (symmetrical: With additional inverted track or asymmetrical: Referred to ground) up to a maximum differential voltage of 27 V.The rated voltage range (5 V or 15 V) for the encoder is set in a parameter. With a rated voltage of 15 V, the SIMOREG converter can supply the voltage for the pulse encoder. 5 V encoders require an external supply. The pulse encoder is evaluated on the basis of three tracks, i.e. track 1, track 2 and zero marker. Pulse encoders without a zero marker may also be installed. The zero marker allows an actual position to be acquired. The maximum frequency of the encoder signals must not exceed 300 kHz. Pulse encoders with at least 1 024 pulses per revolution are recommended (to ensure smooth running at low speeds).

Operation without tachometer and with closed-loop EMF control No actual-value sensor is needed if the closed-loop EMF control function is employed. Instead, the converter output voltage is measured in the SIMOREG. The measured armature voltage is compensated by the internal voltage drop in the motor (I*R compensation). The degree of compensation is automatically determined during the current controller optimization run. The accuracy of this control method is determined by the temperature-dependent change in resistance in the motor armature circuit and equals approximately 5 %. In order to achieve greater accuracy, it is advisable to repeat the current controller optimization run when the motor is warm. Closed-loop EMF control can be employed if the accuracy requirements are not particularly high, if there is no possibility of installing an encoder and if the motor is operated in the armature voltage control range. Caution: The drive cannot be operated in EMF-dependent field- weakening mode when this control method is employed.

Freely selectable actual speed signalAny connector number can be selected as the actual speed signal for this operating mode. This setting is selected in most cases if the actual speed sensor is implemented on a technological supplementary board. Before the actual speed value is transferred to the speed controller, it can be smoothed by means of a parameterizable smoothing (PT1 element) and two adjustable band filters. The band filters are mostly used in order to filter out resonant frequencies caused by mechanical resonance. The resonant frequency and filter quality can be selected.

Ramp-function generator

The ramp-function generator converts the specified setpoint after a step change into a setpoint signal that changes constantly over time. Ramp-up and ramp-down times can be set independently of one another. The ramp-function generator also features a lower and upper transition rounding (jerk limitation) which take effect at the beginning and end of the ramp time respectively.

All time settings for the ramp-function generator are mutually independent.

3 parameter sets are provided for the ramp-function generator times. These can be selected via binary selectable inputs or a serial interface (via binectors). The generator parameters can be switched over while the drive is in operation. The value of parameter set 1 can also be weighted multiplicatively via a connector (in order to change generator data by means of a connector). When ramp-function generator time settings of zero are entered, the speed setpoint is applied directly to the speed controller.

Speed controller

The speed controller compares the speed setpoint and actual value and, if these two quantities deviate, applies a corresponding current set point to the current controller (operating principle: Closed-loop speed control with subordinate current controller). The speed controller is a PI controller with additional selectable 0 component. A switchable speed droop can also be parameterized. All controller characteristics can be set independently of one another. The value of Kp (gain) can be adapted as the function of a connector signal (external or internal).

The P gain of the speed controller can be adapted as a function of actual speed, actual current, setpoint/actual value deviation or winding diameter. To achieve a better dynamic response in the speed control loop, a feedforward control function can be applied by, for example, adding a torque setpoint quantity after the controller as a function of friction or drive moment of inertia. The friction and moment of inertia compensation values can be calculated in an automatic optimization run.

The output quantity of the speed controller directly after enabling can be set via a parameter.

Depending on how parameters are set, the speed controller can be bypassed and the converter operated under torque or current control. Furthermore, it is possible to switch between closed-loop speed control/closed-loop torque control in operation by means of selection function "Master/slave switch-over". The function can be selected as a binary assignable-function terminal or a serial interface. The torque setpoint is applied by means of a selectable connector and can thus be supplied by an analog assignable-function terminal or a serial interface.

In "slave drive" operation (under torque or current control), a limiting controller is active. Here, the limiting controller can intervene on the basis of an adjustable, parameterized speed limit in order to prevent the drive from accelerating too far. In this case, the drive is limited to an adjustable speed deviation.

Torque limitation

Dependlng on parameterization, the speed controller output acts as either the torque setpoint or current setpoint. In closed-loop torque control mode, the speed controller output is weighted with machine flux F and then transferred as a current setpolnt to the current limitation. Torque-control mode is mostly used in conjunction field weakening so that the maximum motor torque can be limited Independently of speed.

The following functions are available:

Independent setting of positlve and negative torque limits via parameters. Switchover of torque limit via binector as a function of a parameterizable changeover speed. Free input of torque limit by means of a connector, e.g. via analog input or serial interface.

The lowest input quantity is always applied as the current torque limit. Additional torque setpoints can be added after the torque limit.

Current limitation

The purpose of the current limitation set after the torque limit is to protect the converter and motor. The lowest input quantity is always applied as the current limit.

The following current limit values can be set:

Independent setting of positive and negative current limits via parameters (setting of maximum motor current). Free input of current limit via a connector, e.g. from an analog input or serial interface. Separate setting of current limit via parameters for shutdown and fast stop. Speed-dependent current limitation: Parameters can be set to implement an automatically triggered, speed-

dependent reduction in the current limitation at high speeds (commutation limit curve of motor). I2t monitoring of power section: The temperature of the thyristors is calculated for all current values. When the

thyristor limit temperature is reached, the converter current is either reduced to rated DC current or the converter shut down with fault message, depending on how the appropriate response parameter is set. This function is provided to protect the thyristors.

Current controller

The current controller is a PI controller with mutually independent P gain and reset time settings. The P or I component can also be deactivated (to obtain pure P controller or pure I controller). The actual current is acquired on the three-phase AC side by means of current transformers and applied to the current controller after A/D conversion via a burden and rectifying circuit. The resolution is 10 bits for converter rated current. The current limiting output is applied as the current setpoint.

The current controller output transfers the firing angle to the gating unit, the feedforward control function acts in parallel.

Feedforward control

The feedforward control function in the current control loop improves the dynamic response of the control, allowing rise times of between 6 and 9 ms to be achieved in the current controlloop. The feedforward control operates as a function of the current setpoint and motor EMF and ensures that the necessary firing angle is transferred speedily to the gating unit, in both intermittent and continuous DC operation or when the torque direction is reversed.

Auto-reversing module

The auto-reversing module (only on converters for fourquadrant drives) acts in conjunction with the current control loop to define the logical sequence of all processes required to reverse the torque direction. One torque direction can be disabled by a parameter setting if necessary.

Gating unit

The gating unit generates the gate pulses for the power section thyristors in synchronism with the line voltage. Synchronization is implemented independently of the rotating field and electronics supply and is measured on the power section. The gating pulse position timing is determined by the output values of the current controller and feedforward control. The firing angle setting limit can be set in a parameter.

The gating unit is automatically adjusted to the connected line frequency within a frequency range of 45 to 65 Hz.

Design and Mode of Operation

Closed-loop functions in field circuit

EMFcontroller

The EMF controller compares the EMF (induced motor voltage) setpoint and actual value and specifies the setpoint for the field current controller, providing an EMF-dependent closed-loop field-weakening control. The EMF controller operates as a PI controller, the P and I components can be set independently of one another. The controller can also be operated as a pure P or pure I controller. A feedforward control operates in parallel to the EMF controller. This applies feedforward control as a function of speed to the field current setpoint by means of an automatically recorded field characteristic (see optimization runs). An adding point is located after the EMF controller, at which additional field current setpoints can be

entered via a connector, e.g. analog input or serial interface. The limitation for the field current setpoint is then applied (maximum and minimum setpoint limits can be set independently of one another). The limitation is implemented via a parameter or connector, in which case the minimum is applied as the upper limit and the maximum for the lower limit.

Field-current controller

The current controller for the field is a PI controller with independent settings for Kp and Tn. It can also be operated as a pure P or pure I controller. A feedforward control operates in parallel to the field current controller. This calculates and sets the firing angle for the field circuit as a function of current setpoint and line voltage. The feedforward control supports the current controller and ensures a good dynamic response in the field circuit.

Gating unit

The gating unit generates the gate pulses for the power section thyristors in synchronism with the line voltage in the field circuit. Synchronization is measured on the power section and is not therefore dependent on the electronics supply. The gate pulse position timing is determined by the output values of the current controller and feedforward control. The firing angle setting limit can be set in a parameter. The gating unit is automatically adjusted to the connected line frequency within a frequency range of 45 to 55 Hz.

Design and Mode of Operation

Optimization run

6RA70 converters are supplied with parameters set to the factory settings. Automatic optimization runs can be selected by means of special key numbers to support setting of the controllers.

The following controller functions can be set in an automatic automatization run:

Current controller optimization run for setting current controllers and feedforward controls (armature and field circuit).

Speed controller optimization run for setting characteristic data for speed controller. Automatic recording of friction and moment of inertia compensation for feedforward control of speed controller. Automatic recording of field characteristic for an EMF-dependent closed-loop field-weakening control and

automatic optimization of EMF controller in field-weakening operation.

Furthermore, all parameters set automatically during optimization runs can be altered afterwards on the operator panel.

Parameterization devices

PMU simple operator panel

All units feature a PMU panel mounted in the converter door. The PMU consists of a five-digit, seven-segment display, three LEOs as status indicators and three parameterization keys.

The PMU also features connector X300 with a USS interface in compliance with the RS232 or RS485 standard.

The panel provides all the facilities required during start-up for making adjustments or settings and displaying measured values. The following functions are assigned to the three panel keys:

P (select) keySwitches over between parameter number and parameter value and vice versa, acknowledges fault messages.

UP keySelects a higher parameter number in parameter mode or raises the set and displayed parameter value in value mode. Also selects a higher index on indexed parameters.

DOWN keySelects a lower parameter number in parameter mode or reduces the set and displayed parameter value in value mode. Also selects a lower index on indexed parameters.

LED functions

Ready: Ready to operate, lights up in "Wait for opera tion enable" state. Run: In operation, lights up when operation is enabled. Fault: Disturbance, lights up in "Active fault" status, flash es when "Alarm" is active.

The quantities output on the five-digit, seven-segment display are easy to understand, e.g.

percentage of rated value, - servo gain factor, seconds, amperes or volts.

Through the X300 connector on the PMU communication can be established via the DriveMonitor program for parameterization, monitoring, troubleshooting, and control of the converter by a PC.

OP1S Extended operator panel

The OP1 S optional extended operator panel can be mounted either in the converter door or externally, e.g. in the cubicle door. For this purpose, it can be connected up by means of a 5 m long cable. Cables of up to 200 m in length can be used if a separate 5 V supply is available. The OP1S is connected to the SIMOREG via connector X300. The OP1S can be installed as an economic alternative to control cubicle measuring instruments which display physical measured quantities.

The OP1S features an LCD with 4 x 16 characters for displaying parameter names in plaintext. German, English, French, Spanish and Italian can be selected as the display languages. The OP1 S can store parameter sets for easy downloading to other devices.

Keys on OP1S:

P (Select) key UP key DOWN key Reversing key (not functional on SIMOREG) ON key OFF key Inching key Numeric keys (0 to 9)

LEDs on OP1S:

Green: Lights up in "Run", flashes in "Ready" Red: Lights up with "Fault", flashes with "Alarm"

Software structure

Two powerful microprocessors (C163 and C167) perform all closed-loop and drive control functions for the armature and field circuit. Closed-loop control functions are implemented in the software as program modules that are "wired up" via parameters.

Connectors

All important quantities in the closed-loop control system can be accessed via connectors. They correspond to measuring points and can be accessed as digital values. 14 bits (16 384 steps) correspond to 100 % in the standard normalization. These values can be used for other purposes in the converters, e.g. to control a setpoint or change a limit. They can also be output via the operator panel, analog outputs and serial interfaces.

The following quantities are available via connectors:

Analog inputs and outputs Inputs of actual-value sensing circuit Inputs and outputs of rampfunction generator, limitations, gating unit, controllers, freely available software modules Digital fixed setpoints General quantities such as operating status, motor temperature, thyristor temperature, alarm memory, fault

memory, hours run meter, processor capacity utilization

Binectors

Binectors are digital control signals which can assume a value of "0" or "1". They are employed, for example, to inject a setpoint or execute a control function. Binectors can also be output via the operator panel, binary outputs or serial interfaces.

The following states can be accessed via binectors:

Status of binary inputs Fixed control bits Status of controllers, limitations, faults, ramp-function generator, control words, status words

Intervention points

The inputs of software modules are defined at intervention points using the associated parameters. At the intervention point for connector signals, the connector number of the desired signal is entered in the relevant parameter so as to define which signal must act as the input quantity. It is therefore possible to use both analog inputs and signals from interfaces as well as internal variables to specify setpoints, additional setpoints, limitations, etc.

The number of the binector to act as the input quantity is entered atthe intervention point for binector signals. A control function can therefore be executed or a control bit output by means of either binary inputs, controls bits of the serial interfaces or control bits generated in the closed-loop control.

Switchover of parameter sets

4 copies of parameters with numbers ranging from P100 to P599 as well as some others are stored in the memory. Binectors can be used to select the active parameter set. This function allows, for example, up to four different motors to be operated alternately or four different gear changes to be implemented on one converter. The setting values for the following functions can be switched over:

Definition of motor and pulse encoder Optimization of closed-loop control Current and torque limitation Conditioning of speed controller actual value Speed controller Closed-loop field current control Closed-loop EMF control Ramp-function generator Speed limitation Monitors and limit values Digital setpoints Technology controller Motorized potentiometer Friction compensation Flywheel effect compensation Speed controller adaptation

Switchover of BICO data sets

The BICa data set can be switched over by the control word (binector input). It is possible to select which connector or binector quantity must be applied at the intervention point. The control structure or control quantities can therefore be flexibly adapted.

Motorized potentiometer

The motorized potentiometer features control functions "Raise", "Lower", "Clockwise! Counterclockwise" and "Manual/Auto" and has its own rampfunction generator with mutually independent ramp time settings and a selectable rounding factor. The setting range (minimum and maximum output quantities) can be set by means of parameters. Control functions are specified via binectors.

In Automatic mode ("Auto" setting), the motorized potentiometer input is determined by a freely selectable quantity (connector number). It is possible to select whether the ramping times are effective or whether the input is switched directly through to the output.

In the "Manual" setting, the setpoint is adjusted with the "Raise setpoint" and "Lower setpoint" functions. It is also possible to define whether the output must be set to zero or the last value stored in the event of a power failure. The output quantity is freely available at a connector, e.g. for use as a main setpoint, additional setpoint or limitation.

DC DRIVES

DC drives consist of an SCR (Silicon Controlled Rectifier) bridge, which converts incoming three or single-phase AC volts to DC volts. During this conversion process DC drives then can regulate speed, torque, voltage and current conditions of the DC motor. This is ideal for industrial processes such as tube mills, extruders, mixers, paper machines and various other controlled applications. Joliet Technologies can provide several DC Drives from different reputable manufactures. Packages can vary from Onsite Retrofits to custom multi drive cabinets.

Engineering and Integration

Joliet Technologies with its highly experienced engineers and Technical staff can provide solutions for existing drive problems or new applications. Solutions that may be helpful consist of:

Sizing a drive to a target horsepower, current and voltage required.

Power and harmonics issues can be addressed and corrected through the use of isolation transformers, reactors and filter packages.

Customizing control and analog circuitry to be integrated in with existing engineered specifications.

Operator consoles / Door mounted pilot and metering

devices MCC and Switchgear installation type custom

packaging for growing demands of the industry. Peer to Peer and Master/Slave drive

configurations for Follower and High Horsepower applications.

Engineered cabinet cooling system for any environment. Today's drives are more compact and can be placed in smaller enclosures only if the correct cooling is applied.

Communication Systems for advanced Modbus, Profibus and Data HighWay linking multiple devices such as power management and HMI/PLC systems.

Service and Parts

Joliet Technologies can be reached 24/7 for any emergency or non-emergency service and parts request. To make this service work better for anyone interested, please pre-qualify your company using this simple form. Click here, fill out the form and someone from Joliet Technologies will contact you.

DC DRIVES - PRINCIPLES OF OPERATION

DC drives, because of their simplicity, ease of application, reliability and favorable cost have long been a backbone of industrial applications. A typical adjustable speed drive using a silicon controller rectifier (SCR) power conversion' section, common for this type unit, is shown in Figure 2. The SCR, (also termed a thyristor) converts the fixed voltage alternating current (AC) of the power source to an adjustable voltage, controlled direct current (DC) output which is applied to the armature of a DC motor.

SCR's provide a controllable power output by "phase angle control", so called because the firing angle (a point in time where the SCR is triggered into conduction) is synchronized with the phase rotation of the AC power source. If the device is triggered early in half cycle, maximum power is delivered to the motor; late triggering in the half cycle provides minimum power, as illustrated by Figure 3. The effect is similar to a very high speed

switch, capable of being turned on and "conducted" off at an infinite number of points within each half cycle. This occurs at a rate of 60 times a second on a 60 Hz line, to deliver a precise amount of power to the motor. The efficiency of this form of power control is extremely high since a very small amount of triggering energy can enable the SCR (Silicon Controlled Rectifier) to control a great deal of output power.

DC DRIVE TYPES

Nonregenerative DC Drives - Nonregenerative DC drives are the most conventional type in common usage. In their most basic form they are able to control motor speed and torque in one direction only as shown by Quadrant I in Figure 4. The addition of an electromechanical (magnetic) armature reversing contactor or manual switch (units rated 2 HP or less) permits reversing the controller output polarity and therefore the direction of rotation of the motor armature as illustrated in Quadrant III. In both cases torque and rotational direction are the same.

Regenerative DC Drives - Regenerative adjustable speed drives, also known as four-quadrant drives, are capable of controlling not only the speed and direction of motor rotation, but also the direction of motor torque. This is illustrated by Figure 4.

The term regenerative describes the ability of the drive under braking conditions to convert the mechanical energy of the motor and connected load into electrical energy which is returned (or regenerated) to the AC power source.

When the drive is operating in Quadrants I and III, both motor rotation and torque are in the same direction and it functions as a conventional nonregenerative unit. The unique characteristics of a regenerative drive are apparent only in Quadrants II and IV. In these quadrants, the motor torque opposes the direction of motor rotation which provides a controlled braking or retarding force. A high performance regenerative drive, is able to switch rapidly from motoring to braking modes while simultaneously controlling the direction of motor rotation.

A regenerative DC drive is essentially two coordinated DC drives integrated within a common package. One drive operates in Quadrants I and IV, the other operates in Quadrants II and III. Sophisticated electronic control circuits provide interlocking between the two opposing drive sections for reliable control of the direction of motor torque and/or direction of rotation.

Converter Types - The power conversion or rectified power section of a DC drive is commonly called the converter. The individual characteristics of the various converter types used in standard industrial applications have had a definite influence in the design of compatible DC motors as shown in Table 2.

TABLE 1. COMPARISON OF NON-REGENERATIVE VS. REGENERATIVE DC DRIVE CAPABILITIES

Nonregenerative Regenerative

Braking

No inherent braking capability. Requires the addition of a dynamic braking circuit which dissipates the braking energy as heat in a resistor. Braking effort is exponential with initial high torque which reduces to zero at zero speed. Braking circuits are rated for stopping only, not continuous hold back, or as a holding brake.

Inherent electronically by regeneration whereby the knetic energy of the motor and driven machine is restored to the AC power source. Can be regulated to control the braking torque down to, and at zero speed. Typically capable of contonuous braking torque for hold back applications.

ReversingNo inherent reversing capability. Requires the addition of reversing contacts or a switch to reverse the polarity of DC voltage applied to the motor. Normally rated for occasional reversing.

An inherent capability. Motor polarity is reversed electronically with no contacts to arc, burn or wear. Desirable for applications requiring frequent reversals.

Simplicity The least complex and least expensive form of electronic adjustable speed motor control.

More complex since it includes double the nonregenerative circuitry.

Efficiency and Speed Range

Controller efficiency up to 99%, complete drive with motor 87%. Speed range up to 50:1 without a feedback tachometer, 200:1 and greater with a tachometer or encoder.

TABLE 2.

Recertified Power Source Motor RatingConverter

TypeNEMACode

Form(2)

FactorRipple(2)

HzSource

VACHP

RangeArmature

VDCFieldVDC

Full Converter6 SCRNonregenerative12 SCRRegenerative

C 1.01 360

230 1-250 240 150

460 1-1000 500 300

Semiconverter3 SCR, 4 Diode D 1.05 180

230 1-150 240 150460 1-150 500 300

Half WaveConverter

E 1.10 180 230(3) 1-250 240(3) 300(3)

460 1-250 240(3) 300

3 SCRNonregenerativeSemiconverter2 SCR, 3 Diode(1)

K 1.35 120 115230

1/6-11/2-5

90180

100200

Full Converter4 SCRNonregenerative8 SCRRegenerative

- - 120 115230

1/6-11/2-5

90180

100200

NOTES:

1. Single-phase: others are three-phase 2. Ripple frequency shown for 60 Hz power source. 50 Hz power sources result in ripple currents 20%, higher than

those for a 60 Hz source under the same operating conditions. The higher ripple produces additional heating which may be compensated by reducing the continuous load capability below base speed by approximately 5%. Form factor is at base speed, full load. Form factor of the current is the ratio of the rms current to the average current. For pure DC, such as a battery, the form factor is 1.0. For motors operated on rectified power the AC ripple content of the rectified current causes additional heating which increases as the square of the form factor. A motor is suitable for continuous operation of the form factor stamped on the data plate at rated load and rated speed. Actual motor heating when run from a half-wave converter should be determined by test, and is the responsibility of the purchaser.

3. Center tap step-up isolation transformer used on primary to increase converter voltage to 480V.

DC motorFrom Wikipedia, the free encyclopedia

Jump to: navigation, search

A DC motor is a mechanically commutated electric motor powered from direct current (DC). The stator is stationary in space by definition and therefore so is its current. The current in the rotor is switched by the commutator to also be stationary in space. This is how the relative angle between the stator and rotor magnetic flux is maintained near 90 degrees, which generates the maximum torque.

DC motors have a rotating armature winding but non-rotating armature magnetic field and a static field winding or permanent magnet. Different connections of the field and armature winding provide different inherent speed/torque regulation characteristics. The speed of a DC motor can be controlled by changing the voltage applied to the armature or by changing the field current. The introduction of variable resistance in the armature circuit or field circuit allowed speed control. Modern DC motors are often controlled by power electronics systems called DC drives.

The introduction of DC motors to run machinery eliminated the need for local steam or internal combustion engines, and line shaft drive systems. DC motors can operate directly from rechargeable batteries, providing the motive power for the first electric vehicles. Today DC motors are still found in applications as small as toys and disk drives, or in large sizes to operate steel rolling mills and paper machines.

Contents[hide]

1 Brush 2 Brushless 3 Uncommutated 4 Connection types

o 4.1 Series connection o 4.2 Shunt connection o 4.3 Compound connection

5 See also 6 External links 7 References

[edit] BrushMain article: Brushed DC electric motor

A brushed DC electric motor generating torque from DC power supply by using internal mechanical commutation, space stationary permanent magnets form the stator field. Torque is produced by the principle of Lorentz force, which states that any current-carrying conductor placed within an external magnetic field experiences a force known as Lorentz force. The actual (Lorentz) force ( and also torque since torque is F x l where l is rotor radius) is a function for rotor angle and so the green arrow/vector actually changes length/magnitude with angle known as torque ripple) Since this is a single phase two pole motor the commutator consists of a split ring, so that the current reverses each half turn ( 180 degrees).

The brushed DC electric motor generates torque directly from DC power supplied to the motor by using internal commutation, stationary magnets (permanent or electromagnets), and rotating electrical magnets.

Like all electric motors or generators, torque is produced by the principle of Lorentz force, which states that any current-carrying conductor placed within an external magnetic field experiences a torque or force known as Lorentz force. Advantages of a brushed DC motor include low initial cost, high reliability, and simple control of motor speed. Disadvantages are high maintenance and low life-span for high intensity uses. Maintenance involves regularly replacing the brushes and springs which carry the electric current, as well as cleaning or replacing the commutator. These components are necessary for transferring electrical power from outside the motor to the spinning wire windings of the rotor inside the motor.Brushes are made of condectors.

[edit] BrushlessMain articles: Brushless DC electric motor and Switched reluctance motor

Typical brushless DC motors use a rotating permanent magnet in the rotor, and stationary electrical current/coil magnets on the motor housing for the rotor, but the symmetrical opposite is also possible. A motor controller converts DC to AC. This design is simpler than that of brushed motors because it eliminates the complication of transferring power from outside the motor to the spinning rotor. Advantages of brushless motors include long life span, little or no maintenance, and high efficiency. Disadvantages include high initial cost, and more complicated motor speed controllers. Some such brushless motors are sometimes referred to as "synchronous motors" although they have no external power supply to be synchronized with, as would be the case with normal AC synchronous motors.

[edit] Uncommutated

Other types of DC motors require no commutation.

Homopolar motor – A homopolar motor has a magnetic field along the axis of rotation and an electric current that at some point is not parallel to the magnetic field. The name homopolar refers to the absence of polarity change.

Homopolar motors necessarily have a single-turn coil, which limits them to very low voltages. This has restricted the practical application of this type of motor.

Ball bearing motor – A ball bearing motor is an unusual electric motor that consists of two ball bearing-type bearings, with the inner races mounted on a common conductive shaft, and the outer races connected to a high current, low voltage power supply. An alternative construction fits the outer races inside a metal tube, while the inner races are mounted on a shaft with a non-conductive section (e.g. two sleeves on an insulating rod). This method has the advantage that the tube will act as a flywheel. The direction of rotation is determined by the initial spin which is usually required to get it going.

[edit] Connection typesSee also: Excitation (magnetic)

There are three types of electrical connections between the stator and rotor possible for DC electric motors: series, shunt/parallel and compound ( various blends of series and shunt/parallel) and each has unique speed/torque characteristics appropriate for diffent loading torque profiles/signatures.[1]

[edit] Series connection

A series DC motor connects the armature and field windings in series with a common D.C. power source. The motor speed varies as a non-linear function of load torque and armature current; current is common to both the stator and rotor yielding I^2 (current) squared behavior[citation needed]. A series motor has very high starting torque and is commonly used for starting high inertia loads, such as trains, elevators or hoists.[2] This speed/torque characteristic is useful in applications such as dragline excavators, where the digging tool moves rapidly when unloaded but slowly when carrying a heavy load.

With no mechanical load on the series motor, the current is low, the counter-EMF produced by the field winding is weak, and so the armature must turn faster to produce sufficient counter-EMF to balance the supply voltage. The motor can be damaged by over speed. This is called a runaway condition.

Series motors called "universal motors" can be used on alternating current. Since the armature voltage and the field direction reverse at (substantially) the same time, torque continues to be produced in the same direction. Since the speed is not related to the line frequency, universal motors can develop higher-than-synchronous speeds, making them lighter than induction motors of the same rated mechanical output. This is a valuable characteristic for hand-held power tools. Universal motors for commercial power frequency are usually small, not more than about 1 kW output. However, much larger universal motors were used for electric locomotives, fed by special low-frequency traction power networks to avoid problems with commutation under heavy and varying loads.

[edit] Shunt connection

A shunt DC motor connects the armature and field windings in parallel or shunt with a common D.C. power source. This type of motor has good speed regulation even as the load varies, but does not have as high of starting torque as a series DC motor.[3] It is typically used for industrial, adjustable speed applications, such as machine tools, winding/unwinding machines and tensioners.

[edit] Compound connection

A compound DC motor connects the armature and fields windings in a shunt and a series combination to give it characteristics of both a shunt and a series DC motor.[4] This motor is used when both a high starting torque and good speed regulation is needed. The motor can be connected in two arrangements: cumulatively or differentially. Cumulative compound motors connect the series field to aid the shunt field, which provides higher starting torque but less speed regulation. Differential compound DC motors have good speed regulation and are typically operated at constant speed.

DC MOTOR CONTROL CHARACTERISTICSA shunt-wound motor is a direct-current motor in which the field windings and the armature may be connected in parallel across a constant-voltage supply. In adjustable speed applications, the field is connected across a constant-voltage supply

and the armature is connected across an independent adjustable-voltage supply. Permanent magnet motors have similar control characteristics but differ primarily by their integral permanent magnet field excitation.

The speed (N) of a DC motor is proportional to its armature voltage; the torque (T) is proportional to armature current, and the two quantities are independent, as illustrated in Figure 5.

CONSTANT TORQUE APPLICATIONSArmature voltage controlled DC drives are constant torque drives. They are capable of providing rated torque at any speed between

zero and the base (rated) speed of the motor as shown by Figure 6. Horsepower varies in direct proportion to speed, and 100% rated horsepower is developed only at 100% rated motor speed with rated torque.

CONSTANT HORSEPOWER APPLICATIONSArmature Controlled DC Drives - Certain applications require constant horsepower over a specified speed range. The screened area, under the horsepower curve in Figure 6, illustrates the limits of constant horsepower operation for armature controlled DC drives. As an example, the motor could provide constant horsepower between 50% speed and 100% speed, or a 2:1 range. However, the 50% speed point coincides with the 50% horsepower point. Any constant horsepower application may be easily calculated by multiplying the desired horsepower by the ratio of the speed range over which horsepower must remain constant. If 5 HP is required over a 2:1 range, an armature only controlled drive rated for 10 (5 x 2) horsepower would be required.Table 3 provides a convenient listing of horsepower output at various operating speeds for constant torque drives.

Field Controlled DC Drives - Another characteristic of a shuntwound DC motor is that a reduction in field voltage to less than the design rating will result in an increase in speed for a given anmature voltage. It is important to note, however, that this results in a higher armature current for a given motor load. A simple method of accomplishing this is by inserting a resistor in series with the field voltage source. This may be useful for trimming to an ideal motor speed for the application. An optional, more sophisticated method uses a variable voltage field source as shown by Figure 6. This provides coordinated automatic armature and field voltage control for extended speed range and constant HP applications. The motor is armature voltage controlled for constant torque-variable HP operation to base speed where it is transferred to field control for constant HP-variable torque operation to motor maximum speed.

AC VS. DC DRIVE COMPARISON

AC and DC drives both continue to offer unique benefits and features that may make one type or other better suited for certain applications.

AC DRIVES MAY BE BETTER BECAUSE. . .

They use conventional, low cost, 3-phase AC induction motors formost applications.

AC motors require virtually no maintenance and are preferred forapplications where the motor is mounted in an area not easily reached for servicing or replacement.

AC motors are smaller, lighter, more commonly available, and less expensive than DC motors. AC motors are better suited for high speed operation (over 2500 rpm) since there are no brushes, and

commutation is not a problem. Whenever the operating environment is wet, corrosive or explosive and special motor enclosures are required.

Special AC motor enclosure types are more readily available at lower prices. Multiple motors in a system must operate simultaneously at a common frequency/speed. It is desirable to use an existing constant speed AC motor already mounted and wired on a machine. When the application load varies greatly and light loads may be encountered for prolonged periods. DC motor

commutators and brushes may wear rapidly under this condition. Low cost electronic motor reversing is required. It is important to have a back up (constant speed) if the controller should fail.

DC DRIVES MAY BE BETTER BECAUSE. . .

DC drives are less complex with a single power conversion from AC to DC. DC drives are normally less expensive for most horsepower ratings.

DC motors have a long tradition of use as adjustable speed machines and a wide range of options have evolved for this purpose:

Cooling blowers and inlet air flanges provide cooling air for a wide speed range at constant torque. Accessory mounting flanges and kits for mounting feedback tachometers and encoders. DC regenerative drives are available for applications requiring continuous regeneration for overhauling loads. AC

drives with this capability would be more complex and expensive. Properly applied brush and commutator maintenance is minimal. DC motors are capable of providing starting and accelerating torques in excess of 400% of rated. Some AC drives may produce audible motor noise which is undesirable in some applications. OTHER APPLICATION FACTORS Constant Torque Speed Range - On large motors, minimum speed limitations may be necessary for self-

ventilated motors, since their cooling is entirely dependent upon motor speed and, there fore, diminishes as speed is reduced. Where rated torque operation is required continuously at lower speeds, either a higher rated drive motor or supplemental motor ventilation, such as a motor mounted cooling blower or external air duct, is required.

Torque Limitations - Most adjustable speed drives feature a torque limiter to protect the drive and the machine from torque overloads. The torque limiter (current limit) is normally adjusted to 150% of rated torque to allow extra momentary torque for breakaway, acceleration or cyclic overloads. Most drive systems are capable of sustaining the 150% torque overload for one minute or less.

Duty Cycle - Certain applications may require continuous reversals, long acceleration times at high torque due to inertia loads, frequent high rate acceleration, or cyclic overloads which may result in severe motor heating if not considered in the selection of the drive. Most drives with 150% overload capability will operate successfully if there are compensating periods of operation where motor temperatures can be normalized.

MEASURING MACHINE TORQUE To measure the torque required to drive a machine, fasten a pulley securely to the shaft which the motor is to

drive. Fasten one end of a cord to the outer surface of the pulley and wrap a few turns of the cord around the pulley. Tie the other end of the cord to a spring scale. See Figure 22.

Pull on scale until the shaft turns. The force in pounds or ounces, indicated on the scale, multiplied by the radius of

the pulley (measured from the centerline of the machine shaft) in inches gives the torque value in Ib-inches or oz-inches. On some machines, this torque may vary as the shaft rotates. The highest value of torque must be used when selecting a motor.

The running torque required by a machine will be approximately equal to the starting torque if the load is composed almost entirely of friction. If the load is primarily inertia or windage, the producing elements must be determined.

The running torque of a machine can be accurately determined by making a test run with an armature controlled DC drive (with a shunt wound or permanent magnet DC motor) of known horsepower rating. The DC drive should have an ammeter in the armature circuit so significant current readings can be observed and recorded throughout the speed range of the machine. Since armature current and torque are directly proportional within very close limits, the current readings will provide accurate information for selecting the drive rating required by the machine.

Most machines require a higher torque value to break it away, but once running, the torque requirement will decrease. Many drives have 150% load capability for one minute, which may allow the required additional breakaway torque to be obtained without increasing the drive horsepower rating.

If the running torque is equal to or less than the breakaway torque divided by 1.5, use the breakaway torque divided by 1.5 as the fullload torque required to determine the motor horsepower.

If the running torque is greater than the breakaway torque divided by 1.5, but less than the breakaway torque, use the running torque as thefull load rated torque required to determine the motor horsepower.