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Desirable Characteristics of Traction Motors
The electric motors that generate power to rotate the wheels of the train are known as traction motors. The turning force produced by the traction motor is transmitted to the wheels of the train through the driving gear unit and axle. Traction motors should have high efficiency.
Desirable Characteristics of Traction Motors
The desirable characteristics and features of the electric motors used for traction purpose are described below.
Suitable Speed-Torque Characteristics
The traction motor should have suitable speed-torque characteristics. In a traction system, the torque required at start is very high, while during the constant speed, the torque requirement is not high because kinetic energy is developed and the tractive effort required is only for overcoming the track resistance and gravity component.
Therefore, the requirement is that the traction motor should develop very high starting torque which should fall off at high speeds.
High Overload Capacity
Traction motors should have high overload capacity. Traction motors are subjected to heavy loads that cause large rush of current. This high current may produce large armature reaction and bad commutation. The arcing produced on commutator surface may exceed over the whole periphery and flashover may occur, which is to be avoided at all costs.
Therefore, the traction motor should be capable of taking heavy loads without flashover.
Operate in Parallel
Traction motors should be capable of operating in parallel. In traction work, several motors operate at the same time. Therefore, the traction motors should be capable of operating in parallel.
However, there occurs a small difference in rotational speed of various motors because of uneven wear and tear of wheels. This should not produce wide variations in torques developed and current drawn by various motors.
Robust Construction
A traction motor must be robust in construction, so that it is capable to withstand continuous vibrations since these motors are subjected to severe conditions. Traction motors should be further provided with mechanical protection to prevent dirt, water, mud, etc.
Withstand Voltage Fluctuations
In traction work, on account of heavy current in rush at starting, considerable voltage fluctuation of supply line is a normal feature. Therefore, the traction motor should be capable of withstanding these voltage fluctuations without adverse effect on their performance.
Weight of Traction Motor
The weight of the traction motor should be minimum in order to increase the payload capacity of the vehicle. Also, the traction motor should have high power to weight ratio.
Small Dimension
Generally, the physical size of the motor depends on the type of insulation used. The traction motors are wound with class-H insulation. Also, the traction motor is located underneath a motor coach and the space underneath the motor coach is limited by the size of driving wheels and the track gauge. Therefore, the traction motor must be small in overall dimensions.
Simple Speed Control
Traction motors should have simple speed control. As the electric trains have to be started and stopped very often, the traction motor should be amenable to simple speed control methods.
Self-Relieving Property
Traction motor should have self-relieving property. The speed-torque characteristics of the traction motor should be such that the speed may reduce with the increase in load, i.e.,
$$\mathrm{\mathit{T}\propto \frac{\mathrm{1}}{\mathit{N}}}$$
Where, T and N be the toque and speed, respectively.
The motors having such speed-torque characteristics are self-protective against excessive overloading as the power output of the motor is proportional to product of torque and speed, i.e.,
$$\mathrm{\mathrm{Power\: output}\propto \mathit{T}\:\times \mathit{N}}$$
Hence, this gives a self-relieving property to a traction motor.
Withstand Temporary Interruption of Supply
There can be temporary interruption of supply when section insulators and cross-overs are crossed with the controller ON. Hence, the traction motor should withstand these fluctuations without heavy inrush of current.
Dynamic or Regenerative Braking
A traction motor should be amenable to easy and simple methods of dynamic or regenerative braking.
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