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Traction Motor

By on May 12, 2013

Traction Motor is the most important part of any locomotive converting electrical energy into mechanical requiring sturdiness of electrical, mechanical, magnetic and insulation system for safe and reliable function.

The constraints, limitation and challenges of designing a suitable traction motor are

  • Envelope of Traction Motor
    • Limitation of space and volume for physical installation weight per axle
  • Electrical
    • Maximum thermal loading of conductors, thermal time constant; voltage and current levels
    • Limits of dielectric stress of motor insulation
    • Requirement of economy in operation i.e. efficiency and initial cost
  • Mechanical
    • Permissible speed
    • Harsh environment of vibration, shock, humidity, ambient temperature etc.
    • Mechanical design for fully suspended/axle hung-nose suspended
    • Load Characteristics
    • Torque Speed characteristics to match the load requiring high starting torque

Types of motor suitable for Traction application

DC series motor and single phase low-frequency AC motor are having inherent characteristics to match the traction load. These motors were in application for traction from the very inception of electric traction. Considering the problem associated with commutation,  the challenge had been ‘how to use three-phase induction motor for traction application because of simple, robust, efficiency, less maintenance features. This could become possible with development of power electronic devices and their control which opened up new opportunities of developments in electric traction.  Separately excited DC series motor, asynchronous and synchronous motors started finding space for traction application.

Status on Indian Railways

  • DC Series Motor widely used and about 80% motors in use are this motor
  • Separately excited DC series motor on Thyristor base WAG6A, B&C class of locomotive 18 in nos.
  • Three phase induction/asynchronous motor started with introduction of power controller with GTO/IGBT power devices.

Future Developments

Research and development in Permanent magnet synchronous motor is under progress and will find commercial application very soon for traction purpose. There are also developments for using wheel axle as permanent magnet mounted rotor to ease the mounting arrangements.

Maximum Power Rating of Traction Motor in Indian Conditions

  • Permitted Axle Load=20.5T
  • Maximum Adhesion achievable=40%
  • Maximum TE per TM= 20.5*0.4=8.2T
  • Rail HP per TM= 8.2*1000*40/270 = 1214HP
  • Assuming Maximum Tractive Effort upto 40kmph
  • TM Power = HP*.746/Efficiency(.95)=1.17MW

Maximum Axle load increased to 25T on selected route of IR and DFC route in totality

  • Maximum Power rating of TM can be increased by 20%

 Torque Density

Torque density is an important index for comparison of traction machines.  Consider the basic equation of force on a conductor carrying current I of active length l in a magnetic field B is given by

\small \fn_cs Force\left ( F \right )=B\times I\times l   ;

\small \fn_cs Torque\left ( \tau \right ) = B\times I\times l\times r

where r is rotor radius;

\small \fn_cs Current\left ( I \right )=J\times k\times r

 where J is the rotor surface current density , k depends on winding topology and number of poles

then we get

\small \inline \fn_cs Torque=B\times J\times \times k\times l\times r^{2}=B\times J\times k\times v_{m}

where v m is magnetic volume of the rotor and k is constant for a particular design of a machine.   

Now if actual volume is v then

\small \inline \fn_cs Torque Density= B\times J\times k\times \frac{v_{m}}{v}

The torque density of synchronous and induction motor is much better as compared to DC machine because of less achievable surface current density.

Power Density

Power is a product of torque and speed for a given torque density. Speed is constrained in DC machine due to commutation whereas it can go higher in synchronous and induction motor. This improves power density. Higher gear ratio is used to reduce speed at wheel end. Higher speed calls for designing of suspension arrangement to handle the issue of vibration and bearing lubrication. (Article: Selection of suspension Arrangement of Traction Motors: A Right Approach written by R N Lal Sr.Executive Director RDSO)

 HP/Weight Ration of Different Class of Locomotive over IR

TM

Loco

Voltage

Current

RPM

HP

Weight

HP/KG

HS15250

WAG7/P4

750

900

895

840

3650

0.2301

MT710

WAM1

1250

460

1020

710

2750

0.2582

MG 1420

WAG1

1250

920

630

1448

5600

0.2586

1580A1

WAG3

1270

1000

680

1580

5850

0.2701

TAO659

WAG5

750

840

1095

770

2800

0.2750

HS 15256

WAG6C

850

960

970

1020

3650

0.2795

EFCO-HKK

WAG2

1250

1020

710

1610

5300

0.3038

HS 15556

WAG6B

850

960

NA

1020

3200

0.3188

MB3045A

WAM2,3

725

780

1020

704

2200

0.3200

LM 450

WAG6A

850

960

NA

1020

3100

0.3290

FRA 6068

WAG9

2080

270

1283

1156

2150

0.5377

FXA 7059

WAP5

2180

370

1585

1563

1990

0.7854

Note: HS373-AR/EF531 type of TM used on WCM class of Locomotive was having HP/Weight(Kg) of the order of 0.12 to 0.18

Three phase motors has opened new possibilities of high HP/Kg. This could be possible for two reasons; higher speed and voltage.

Temperature versus Rating of Traction Motor

Traction Motor is designed to deliver varying traction duty as follows:

  • Continuous Operation where TM sustain indefinite duty
  • Starting Operation where TE is greater than continuous
  • Short term operation where power is greater than continuous operation

Thermal Time Constant of Motor Winding is much greater of the order of 20 minutes as compared to the acceleration and has been used for deriving short-term rating of traction motor

Standard permissible temperature rise in motor insulation provides for continuous and one hour rating over ambient

Class B Class F Class H Class C
Rotor Winding 120 140 160 180
Stator Winding 130 155 180 200
Commutator 105 120 120 120

Class C insulation is generally used in traction application.

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There Are 6 Comments

  1. Noopur says:

    Thank you so much sir for sharing such an immense collection of knowledge.

  2. Shayak says:

    One of the issues which often goes unnoticed is the fact that tap changer controlled locos such as WAP4 are very difficult to drive. Many times I have seen fully rated WAP4 performing poorly at acceleration because the loco pilot does not have a good technique. To ameliorate this problem I propose the following algorithm to be followed by WAP4 loco pilot for achieving quick acceleration :

    For 1100 A in traction motor :
    1. At start of acceleration run, keep shunt set to 0. Use a small current to bring the couplers to tension and then take notches quickly until ammeter reading becomes 1100 A.
    2. From this point on keep taking one additional notch at equal intervals of speed. This interval is 5 (km/hr) if OHE voltage is high, 4 if OHE voltage is low. Just before taking each notch, your ammeter should read about 1050 A.
    3. Stop taking notches when voltmeter reads 750 V. This should happen at speed around 75. Note the exact speed at which you have taken the last notch.
    4. Starting from the speed noted above, take the four shunts at speed intervals of 10, 10, 10 and 25 respectively. Just before taking 1st, 2nd and 3rd shunt your ammeter should read 1000 A or lower. Just before taking 4th shunt your ammeter should read 950 A or lower. However, a shunt transition is best avoided if you are close to the train MPS and the acceleration is still appreciable.
    5. Take additional notches after successive shunts to compensate voltage drop due to shunting.

    For 1250 A in traction motor :
    1. At start of acceleration run, keep shunt set to 0. Use a small current to bring the couplers to tension and then take notches quickly until ammeter reading becomes 1250 A.
    2. From this point on keep taking one additional notch at equal intervals of speed. This interval is 4.5 if OHE voltage is high, 3.5 if OHE voltage is low. Just before taking each notch, your ammeter should read 1200 A or lower.
    3. Stop taking notches when voltmeter reads 750 V. This should happen at speed around 60. Note the exact speed at which you have taken the last notch.
    4. Starting from the speed noted above, take the four shunts at speed intervals of 10, 10, 10 and 15 respectively. Just before taking 1st, 2nd and 3rd shunt your ammeter should read 1150 A or lower. Just before taking 4th shunt your ammeter should read 1100 A or lower. However, a shunt transition is best avoided if you are close to the train MPS and the acceleration is still appreciable.
    5. Take additional notches after successive shunts to compensate voltage drop due to shunting.

    This algorithm has been published in RDSO’s Indian Railway Technical Bulletin and I hope to see it being implemented across the driving schools.

    Shayak
    IIT Kanpur

    • Mahesh Kumar Jain says:

      Thanks and will request you to share your article for posting on the website in guest column. For this, send your article along with a brief on yourself.

      • Shayak says:

        Sir I have sent you the article over email along with my brief bio. For more details you can kindly see my homepage home.iitk.ac.in/~shayak which contains my full CV.

        Sincerely,
        Shayak

  3. […] These two equations derives all equations of DC Traction Motor […]

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