traction motor : control and applications
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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.
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.
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.
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.
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
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.
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