General Requirements of Traction Motor

A traction motor is basically an electric motor that produces the rotational torque of the vehicle. The rotating force produced by the traction motor is transmitted to the wheels of the vehicle through the pinion and gear wheel.

Before incorporating an electric motor in a traction system the motors should meet some electrical and mechanical characteristics that suit the nature of the operation. The below are the electrical and mechanical requirements of traction motor.

Mechanical Requirements :

Some of the important mechanical requirements of traction motors are,
  • The weight of the traction motor should be less to increase the payload (carrying) capacity.
  • A traction motor must be robust and capable to withstand continuous vibrations.
  • The traction motor must be small in size to fit even in less space.
  • The traction motor must be a totally enclosed type in order to protect it against dust, water, and mud.

Electrical Requirements :

The desired electrical requirements of a traction motor are,
  • Starting torque of the motor should be high to meet the train resistance and acceleration. The speed-torque curve should be such that starting torque is high and reduces as speed increases.
  • The motor should be self-governing. When an up gradient is encountered the speed can be reduced such that the kilowatt loading gets controlled.
  • The motor should protect itself from overload. If the speed-torque characteristic is such that speed is inversely proportional to torque, their product will always be constant. Therefore, hp which is proportional to the product of torque and speed is also constant. This gives motor self-protective property against overloads.
  • The motor should have simple speed control.
  • The motor should have a high power-to-weight ratio.
  • The motor should have high mechanical and electrical efficiency.
  • The motors must be able to share the load with the parallel operation. The locomotives may have 4 to 6 traction motors mounted on the axles.
  • The voltage supplied to the catenary will vary over 20% of the rated value. The motor should withstand without flashover or sudden high current.
  • If there are any small interruptions in the power supply, the motor should not take a sudden inrush of current.
  • The motor should be capable of dynamic or regenerative braking.
  • When the locomotive shakes when it passes over joints and crossings, these vibrations affect bearings, gears, and commutator. The motor should be in a position to tolerate these effects.
  • The motor should withstand voltage fluctuations and temporary interruption of supply.
Upon considering all the requirements mentioned above. No single electric motor can have all the requirements, but some of the traction motors which find applications in electric traction are,
  • DC series motors,
  • DC compound motors,
  • AC series motor,
  • Three-phase induction motors.


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