Unit 4 Traction Drives Motors employed in railway

Unit – 4 : Traction Drives • • Motors employed in railway traction and road vehicles Control of Railway traction by dc motors using ac- dc converters Control of Railway traction ac motors using ac -dc and dc-ac converters Power Electronic control circuits of electric vehicles and hybrid electric vehicles.

Motors employed in railway traction and road vehicles S. No. Name of the Motor Adv / Disadv Reason 1 DC Series Motor Adv High starting Torque High torque overload capability Speed torque characteristics give better sharing of motors As large inductance in the field never allow the sharp peaks of current even though the voltage is fluctuating. Dis adv Field is not easily controlled by the semiconductor switches. It reduces the acceleration 06 -02 -2022 Dr. P. Kishore Kumar 2

S. Name of the N Motor o. Adv / Disadv 2�� S. No. DC Adv Name of the Motor Separately � Adv / Disadv � Reason Excited Motor � 1 � DC Series Motor � Adv � High starting Torque Reason Availability of semiconductor devices and separate control for armature and field. Low regulation of speed torque characteristics, coefficient of adhesion is more. Speed torque characteristics give better sharing of motors � High torque overload capability � As large inductance in the field never allow Adv the sharp peaks of current even though Speed torque characteristics give better sharing of motors the voltage is fluctuating. � � Regenerative and dynamic braking are can carried at very low As large inductance in the field never allow the simple sharp peaksand of current evenbe though the voltageout is fluctuating. speeds Adv Dis adv Dis Adv Extra inductance is added in armature circuit not to allow the sharp peaks of current even though the voltage is 06 -02 -2022 Dr. P. Kishore Kumar fluctuating. 3

S. N Name of o. the Motor 3 4 Adv / Reason Disadv DC Adv Compound Motor Incorporates both series and separately excited motors. DC Motors Dis Adv Presence of commutator and brushes, increases maintenance, low life, less efficiency. Squirrel cage Induction Motor Adv Ruggedness, low maintenance, better over dc reliability, lower cost, weight, volume, inertia, higher efficiency, ability to work for fluctuated voltage supply and in dirty environments Dis Adv 5 Synchrono Adv us Motor Here forced commutated inverters are used It is in between DC and IM The main advantage over all is that it can work for leading power factors. Here load commutated thyristor inverters are used which are cheaper in cost 06 -02 -2022 Dr. P. Kishore Kumar 4

Advantage of Semiconductor converter controlled drives 06 -02 -2022 Dr. P. Kishore Kumar 5

Various Traction Drives � AC traction using semi conductor converter controlled dc motors (Control of DC motors by AC-DC converters) � DC traction employing AC motors (Control of Railway traction AC motors using DC-AC converters) ◦ PWM – VSI Squirrel cage Induction Motor Drive ◦ Load commutated inverter Synchronous Motor Drives � AC traction employing AC motors (Control of Railway traction ac motors using AC-DC converters) ◦ CSI Squirrel cage Induction motor Drive ◦ PWM VSI Squirrel cage Induction motor Drive ◦ Load commutated inverter Synchronous Motor Drives 06 -02 -2022 Dr. P. Kishore Kumar 6

AC traction using semiconductor converter controlled DC motors 06 -02 -2022 Dr. P. Kishore Kumar 7

Explanation �It is applicable to low power shunting locomotives. �Limitations : ◦ At low output voltages the converter power factor is low. ◦ Source current in square wave shape is rich in harmonics. �As it is used for low power applications, the power factor and harmonics are less effected. �This simplest arrangement is sufficient for these type of applications 06 -02 -2022 Dr. P. Kishore Kumar 8

Two stage converter 06 -02 -2022 Dr. P. Kishore Kumar 9

Voltage and Current Waveforms 06 -02 -2022 Dr. P. Kishore Kumar 10

Alternate Method 06 -02 -2022 Dr. P. Kishore Kumar 11

Reactive Power 06 -02 -2022 Dr. P. Kishore Kumar 12

AC Motors for Traction Drives �Adv of IM over DC ◦ Negligible Maintenance ◦ Ruggedness ◦ High Power per unit volume �Adv of SM over IM ◦ Higher Efficiency ◦ Simpler inverter ◦ Cheaper inverter 06 -02 -2022 Dr. P. Kishore Kumar 13

Modes of Operation in Variable Frequency Control for both IM and SM 06 -02 -2022 Dr. P. Kishore Kumar 14

Motoring �From zero to base speed ωmb, the motor is accelerated at constant Torque keeping v/f ratio constant. �Above the base speed the motor is accelerated in constant power mode with const v and variable f. �At critical speed ωmc 1 motor’s break down torque limit is reached �So, motor power is gradually reduced by operating it with lesser current. 06 -02 -2022 Dr. P. Kishore Kumar 15

Braking �From zero to ωmc 2 motor is braked at a constant torque. �Above ωmc 2, the motor is braked at reduced torque. �So as to avoid exceeding wheel to rail adhesion capability. 06 -02 -2022 Dr. P. Kishore Kumar 16

PWM – VSI Squirrel cage Induction Motor Drive 06 -02 -2022 Dr. P. Kishore Kumar 17

Explanation � PWM inverter convert the DC into variable frequency and variable AC � Switches S 1 to S 6 are self commutated devices such as GTO, IGBT or power transistors � Because of PWM the drive has smooth operation � It has inherent regenerative capacity. Just reducing the frequency, the synchronous speed is made less than motor speed. � If the source is not capable of absorbing all the energy by regenerative braking, dynamic braking is applied by incorporating RB, S and D � Excess energy is absorbed by the C to voltage Vc and if Vc exceeds a limit S is opened to isolate the inverter and SB is closed for dynamic braking 06 -02 -2022 Dr. P. Kishore Kumar 18

Advantages and Disadvantages of PWM Advantages ◦ Smooth acceleration due to absence of low speed torque pulsations ◦ Good adhesion due to fast response and absence of torque pulsations ◦ VSI is more suitable for multi motor drives ◦ Low weight, volume and cost ◦ Simpler control and efficient operation ◦ Regenerative braking capability Disadvantages ◦ Possibility of shoot through faults ◦ Expensive fuses are used to protect switches from shoot through faults 06 -02 -2022 Dr. P. Kishore Kumar 19

Load commutated inverter Synchronous Motor Drives 06 -02 -2022 Dr. P. Kishore Kumar 20

Explanation �It is a current source inverters employing thyristors �A chopper is used to obtain variable DC voltage. �During motoring the power flows from DC to motor �If the firing angle changed from 180⁰ to 0⁰ from the vdl is reversed �If chopper operation is also made vds negative but less than vdl, load will feed the supply. 06 -02 -2022 Dr. P. Kishore Kumar 21

Comparison with PWM � Because of an additional power stage (chopper) the converter efficiency is low and motor efficiency is high. � Due to presence of high inductance Ld the drive has slow dynamic response giving inferior adhesion � It has larger weight and volume � Each motor should have an inverter � Motors can be connected in series but not in parallel. � Series connection of motors will give adverse effects on adhesion � If large traction motors are involved the drive becomes expensive and complex � It is more reliable since absence of shoot through faults � Because of torque pulsations, the operation is not smooth gives adverse effects on adhesion. 06 -02 -2022 Dr. P. Kishore Kumar 22

CSI Squirrel cage Induction motor Drive 06 -02 -2022 Dr. P. Kishore Kumar 23

Explanation �Filter and Static VAR compensator are used to get power factor above 0. 8 and to keep harmonics in acceptable limits. �Each motor is fed by a separate CSI. �CSI are not suitable for parallel operation so connected in series. �Regenerative braking is inherent just to reduce the frequency. �The drive has following features �Bulky, Heavy and expensive. �Poor adhesion due to slow dynamic response and series connection. 06 -02 -2022 Dr. P. Kishore Kumar 24

PWM VSI Squirrel cage Induction motor Drive 06 -02 -2022 Dr. P. Kishore Kumar 25

Explanation �Because of diode rectifier, regenerative braking is not possible. �So, dynamic braking is employed �Regenerative can be obtained by removing the left of AB by Synchronous link converter (SLC) �SLC has IGBT or GTO switches �It has unity fundamental power factor and low harmonic content. 06 -02 -2022 Dr. P. Kishore Kumar 26

Single Phase Synchronous link converter 06 -02 -2022 Dr. P. Kishore Kumar 27

Explanation � Fig. Shows the single phase SLC � Inverter and Ls together form SLC. � To get Is inphase with Vs, V 1 is required magnitude and phase � Motoring: produced with the ◦ Is is in phase with Vs. ◦ Directions of Vd and Id are as in figure. � Braking: � Id reverses and Is is 180 degree out of phase with Vs � As power supplied to inverter is independent of power taken from it, a closed loop control is used to balance the two. � It has high power factor, low harmonics � Ride through capability against voltage sag and under voltages. 06 -02 -2022 Dr. P. Kishore Kumar 28

Load commutated inverter Synchronous Motor Drives �Advantages ◦ High Efficiency ◦ High Speed Capability ◦ Ability for regenerative braking �CSI is replaced by LCI and IM by SM in Fig. 10. 23 and LCI is in Fig. 10. 22 �Compared to PWM – VSI, ◦ Lower power factor ◦ High harmonic content 06 -02 -2022 Dr. P. Kishore Kumar 29

Power Electronic control circuits of electric vehicles and hybrid electric vehicles. � Lead acid battery is available at affordable cost. �Motors used in EVs ◦ ◦ DC Series and Separately Excited PMDC BLDC Induction Motor 06 -02 -2022 Dr. P. Kishore Kumar 30

Advantages of EVs over IC Vehicles �Less Pollution �Quieter Operation �Less Maintenance ◦ No water cooling ◦ No filter and belt to replace ◦ No oil to change �More parts ◦ ◦ ◦ Reliable because of absence of these Fuel Injectors Compressors Pumps and valves Water cooling system Filters 06 -02 -2022 Dr. P. Kishore Kumar 31

Dis - advantages of EVs over IC Vehicles �More Expensive �EV cann’t go for full fuel charge as IC Vehicles for full fuel. �It takes more time to recharge compared to fill petrol for IC Vehicles. 06 -02 -2022 Dr. P. Kishore Kumar 32

DC Drive for EV 06 -02 -2022 Dr. P. Kishore Kumar 33

Explanation �Voltage employed are 6 V, 12 V, 24 V, 48 V and 110 V �Higher the voltage, lesser weight, less volume and less cost the of motor but costlier battery. �Regenerative braking is used and it increases 7 to 15 % life of vehicle. It has no zero speed so mechanical brakes are also applied with this braking. �Lf and Cf filter out harmonics generated by the chopper. Ms and RS are the manual and reversing Switches. L is to keep the current low and to assist the regenerative braking. 06 -02 -2022 Dr. P. Kishore Kumar 34

Motoring �Ms is kept closed �T is operated at constant frequency with variable Ton to get variable DC voltage for starting and speed control. �T is on ; current flows through source, Lf, Ms, L, R, Armature, S, T. �T is off ; current flows through S, D 1, Ms, L and R 06 -02 -2022 Dr. P. Kishore Kumar 35

Regenerative Braking �Ms is opened �Motor armature is reversed by RS �T is on ; current flows through T, D 2, L �T is off ; current flows through D 1, Lf, Battery, D 2, L. Energy is fed back to supply. �It is operated in closed loop current control. 06 -02 -2022 Dr. P. Kishore Kumar 36

Induction Motor Drive for EV 06 -02 -2022 Dr. P. Kishore Kumar 37

Explanation �It employs squirrel cage induction motor. �PWM – VSI fed to SCIM �Motoring to regenerative braking is achieved by reducing the inverter frequency. �PWM gives efficient and ride smooth operation. 06 -02 -2022 Dr. P. Kishore Kumar 38