Desirable Characteristics of Traction Motors

This article describes the desirable characteristics of traction motors. The electric motor when powered produces torque to rotate the wheels of the train and it is called a traction motor. The torque produced by the motor is transmitted to the wheels through a gear unit and axle. It is desirable that the motor should produce high torque and it should have high efficiency.

Desirable Characteristics of Traction Motors

The following are the desirable characteristics and features of the traction motor.

  • Suitable Speed-Torque Characteristics
  • High Overload Capacity
  • Parallel Operation
  • Robust Construction
  • Capacity to Withstand Voltage Fluctuations
  • Low Weight of Traction Motor
  • Small Dimension
  • Simple Speed Control
  • Self-Relieving Property
  • Withstand Temporary Interruption of Supply
  • Compatible with Dynamic or Regenerative Braking

Now, we will discuss the above-given features of traction motors in detail.

Suitable Speed-Torque Characteristics

In a traction system, starting torque requirement is very high because the train has a large mass and high moment of inertia. Therefore, when we start a train, it requires a very high starting torque. Once speed picks up and trains run at a constant speed, there is no need for high torque because, in these conditions, the torque required is only for overcoming the track resistance and gravity component.

Therefore, we can say that the traction motor should produce high torque at the start. The speed-torque characteristic of the traction motor should meet the load requirement.

High Overload Capacity

The capacity of a drive to deliver more currents above its continuous rating is called the overload capacity of a motor. If the overload capacity of a motor is 120 % for 1 minute, it means the motor can withstand 120 % current of its FLC for one minute, without getting damage. The motor used for a traction application must have a high overload capacity.

The traction motor undergoes heavy stress during its operation and it causes the flowing of a large inrush current in its winding. The large current in the armature of a motor can cause a large armature reaction that causes demagnetization of the main flux. Moreover, a large armature current cause bad commutation over the entire periphery of the commutator that may cause flashover.

Therefore, the traction motor must have the capacity to sustain heavy loads.

Parallel Operation

A single motor may not be feasible for driving a heavy load because the motor can not deliver the required torque. In these conditions, the multidrive system is required to be installed for driving a single load. All the motors must have almost identical speed-torque characteristics to drive a single load. This type of operation is called the parallel operation of motors.

We know a traction system demands a large torque and a single motor can not meet the torque requirements. Therefore, many traction motors are installed to drive the single load(train). The speed-torque characteristics of installed traction motors are not exactly identical, but the speed-torque characteristics of all the motors must be within acceptable limits, and must not have much more speed variations.

Robust Construction

A traction motor has to experience severe vibrations because it works in severe conditions. It is desirable that a traction motor should be capable to withstand continuous vibration without damage. The traction motors are made to withstand vibration and mechanical shocks. Also, motors are provided protection against the ingress of dirt, water, and mud.

Capacity to Withstand Voltage Fluctuations

At the start, a traction motor draws a high current to deliver the maximum starting torque. The high current cause a voltage drop (V=IR) in the system, and thus the supply voltage drops. A large voltage drop in supply voltage may cause deterioration in the motor’s performance.

The traction motor is designed so that a large starting current does not affect the motor’s performance on account of voltage drop.

Therefore, a traction motor’s desirable feature is that it must withstand voltage fluctuations without getting affected by voltage dip.

Low Weight of Traction Motor

The weight of the motor should be less to have a good power transmission performance. The lesser weight of the motor increases the payload capacity of the train. The maximum amount of weight a vehicle can safely carry is known as payload capacity. Therefore, a traction motor should have a high power-to-weight ratio.

Small Dimension

The traction motor is mounted underneath a motor coach. The motor coach has very limited space because of the space occupied by the size of the driving wheels and the track gauge. Therefore, a traction motor should be compact in size so that it can be accommodated in a smaller space.

Simple Speed Control

The electric train has to be stopped at many stations and restarted after every stop. Therefore, speed control must be very simple and user-friendly.

Self-Relieving Property

When a motor delivers maximum torque and speed does not reduce, then a motor goes into heavy stress. The feature of a motor that reduces the speed on increasing torque is called the self-relieving property.

Therefore, the traction motor should have a self-relieving property in order to have the best motor performance. The speed of the traction motor decreases with an increase in the torque, thus the traction motor has excellent self-reliving properties.

According to the speed-torque characteristics of the traction motor, the speed reduces with the increase in load.

formula showing Self-Relieving Property of traction motor

A motor with a self-reliving feature has good protection against overloading because the output power decreases with the torque and the motor functions in a constant power mode.

A traction motor’s output power is proportional to the product of torque and speed.

power formula of traction motor

Withstand Temporary Interruption of Supply

The train should move on for a while because of its inertia if there happens a temporary interruption of the power supply. The temporary disconnection of the supply may happen during crossing over the crossovers and section insulators.

Therefore, the traction motor should have the capacity to sustain these fluctuations, without taking a heavy inrush current.

Compatible with Dynamic or Regenerative Braking

A traction motor should be compatible with effective dynamic or regenerative braking.

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