TRACTION MOTOR : CONTROL AND APPLICATIONS

Content :•Introduction•Requirements of a traction system•Control of DC motors•Electronic Speed Control Methods for DC

Motors• Pulse Width Modulation•Braking operation in DC motors•Conclusion •References

INTRODUCTION

•Traction motor refers to an electric motor providing the primary rotational torque of a machine, usually for conversion into linear motion (traction).

•Traction motors are used in electrically powered rail vehicles such as electric multiple units and electric locomotives

Requirements of an Ideal Traction System

• High starting pulling effort in order to have rapid acceleration.

• Equipment capable of withstanding large temporary overloads for short periods.

• The locomotive should be self contained and able to run on any route.

• Capability of withstanding voltage fluctuations and temporary interruption of supply.

• Parallel running usually more than one motor (two or four motors ) should be possible.

COMPONENTS OF AN A.C LOCOMOTIVE

SEPEX

Short form of SEParate EXcitement of traction motors where the armature and field coils of an electric motor are fed with independently controlled current. This has been made much more useful since the introduction of thyristor control where motor control can be much more precise.  SEPEX control also allows a degree of automatic wheel slip control during acceleration

The DC Traction Motor: How it Drives the Axle

How a DC motor drives the axle through a pinion and gearwheel

Speed control of dc motor :

•The purpose of a motor speed controller is to take a signal representing the required speed, and to drive a motor at that speed.

• Motor speed can be controlled by controlling- - Armature voltage (Va)- Armature current (Ia)

Cont………

•It is obvious that the speed can be controlled by varying

flux/pole, (Flux Control)

resistance Ra of armature circuit (Rheostatic Control)

applied voltage V (Voltage Control)

Cont…•The above methods have some demerits i.e

A large amount of power is wasted in the controller resistance. Hence, efficiency is decreased.

It needs expensive arrangement for dissipation of heat produced in the controller resistance.

It gives speeds below the normal speed.

Electronic Speed Control Methods for DC Motors :•compared to the electric and

electromechanical systems of speed control, the electronic methods have

higher accuracy

greater reliability

quick response

higher efficiency

Principle :•The main principle is control of power by varying

the duty cycle.•Here the conduction time to the load is controlled.•Let

for a time t1, the input voltage appears across the load ie ON state.

for t2 time the voltage across the load is zero.The average voltage at output is given by

Va = 1/T vodt = t1/T Vs = ft1 Vs = kVsthe average load current

Ia = Va/R = KVs/R where T is the total time period =t1+t2

k = t1/T is the duty cycle

Cont……• The rms value of output voltage is

V0 = ( i/T V0

2 dt ) ½ = k Vs• The output power and is given by

Pi = 1/T v0idt = 1/T v02/R dt = kVs2/R

• The duty cycle can be varied from 0 to 1 by varying t1, T or f. Therefore, the output voltage V0 can be varied from 0 to Vs by controlling k, and the power flow can be controlled.

• As the time t1 changes the width of pulse is varied and this type of control is called pulse width modulation (PWM) control.

Pulse Width Modulation :

Pictures of the Waveform :

• The motor is on for most of the time and only off a short while, so the speed is near maximum

• The switch is on 50% and off 50%.

• The motor will only rotate slowly.

Power Supply :

Circuit Description:•The transformer is a center tap 12-0-12V 500mA

and supply 12v to the circuit.•The i/p sine wave is rectified by using full wave

rectifier which convert it to one constant polarity.

•A 1000Fcapacitor is used for filtration purpose.

•The three terminal voltage regulators 7812 provides regulated DC 12 v outputs for the operation of the circuit.

Circuit Diagram : (Motor Controller Sections)

Circuit Description :

Cont…..•As output of IC1a goes low, whereupon C1 is

discharged via D2, R2, and P1.•The discharge time could be set between

0.2ms and 25ms.•the duty factor of the output signal may be

varied between 5% and 90%.•The signal is inverted again and then applied

to the input of transistor BC557 through 4k7.•The transistor Bc557 act as a buffer which

transfer electrical impedance from one circuit to another circuit i.e. it act as a pre amplifier.

Cont…•The o/p of Bc557 is given to the power

transistor 2N3055 which is used in a circuit as an amplifier, detector, or switch.

•The diode connected across the motor is for freewheeling purpose i.e. the diode used to eliminate flyback, the sudden voltage spike seen across an inductive load when its supply voltage is suddenly reduced or removed.

•The resistance of P1(Potentiometer) is at a minimum, the rotary speed of the motor is at a maximum.

Advantage :

•PWM duty cycle control techniques enable greater efficiency of the DC motor .

•PWM switching control methods improve speed control and reduce the power losses in the system.

•The pulses reach the full supply voltage and will produce more torque in a motor by being able to overcome the internal motor resistances more easily.

Disadvantages :

•The main Disadvantages of PWM circuits are the added complexity and the possibility of generating radio frequency interference .

• It can give speed below the full speed, not above.

• It cannot be used for fast controlling of speed.

THYRISTOR CONTROL

Schematic of AC Electric Locomotive Power System with Thyristor Control and Separately Excited DC Motors

DC Choppers

Thyristor Control Circuit for DC Supply to DC Motors

AC Locomotives with DC Drives

Schematic of AC locomotive with Tap Changer Control of Transformer Output

Braking in traction motors

• Dynamic Braking: where the motors become generators and feed the resulting current back.

• It classified into two categories: 1 Plugging2. Rheostatic Braking 3. Regenerative Braking

• PLUGGING: Generally plugging is applied by changing the phase sequence of the three phase induction and synchronous motor

Rheostatic Braking

• The motors become generators and feed the resulting current into an on-board resistance.

• When the driver calls for brake, the power circuit

connections to the motors are changed from their power configuration to a brake configuration and the resistors inserted into the motor circuit.  As the motor generated energy is dispersed in the resistors and the train speed slows, the resistors are switched out in steps, just as they are during acceleration.

Regenerative Braking

• the motors become generators and feed the resulting current back into the supply system

• a train could use its motors to act as generators and that this would provide some braking effect if a suitable way could be found to dispose of the energy.

• Trains were designed therefore, which could return current, generated during braking, to the supply system for use by other trains.

REGENERATIVE BRAKING

CONCLUSION

• no single motor is ideal for traction purposes. But; DC Series Motors are most suitable for this work

• Power Electronics have been resorted to in a major way as in case of thyristor control.

• Different types of braking for the traction system was studied.

REFERENCES

WEBSITES• http://www.railway-technical.com• http://en.wikipedia.org/wiki/Traction_motor• http://www.engineeringexpert.net/Engineering-Expert-Witness-Blog/?

tag=traction-motor

BOOKS• Ned Mohan, Tore M. Undeland, Power electronics: converters,

applications, and design , Third Edition , page 377-395• Singh, K. B. Khanchandani,Power Electronics, Second Edition ,

pages 846-890 

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