In this article we will discuss about the general features and rating of traction motor.
General Features of Traction Motors:
The chief requirements of electric motors used for traction work are:
1. Mechanical Features:
(i) A traction motor must be robust and capable to withstand continuous vibrations since service conditions are extremely severe.
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(ii) The weight of the traction motor should be minimum in order to increase the payload capacity of the vehicle. This is achieved by using high speed motors, upper limit being fixed by excessive centrifugal stresses.
(iii) The traction motor is located underneath a motor coach (except in few electric locomotives). The space underneath a motor coach is limited by the size of driving wheels and the track gauge. The traction motor, therefore, must be small in overall dimensions especially in its overall diameter.
(iv) The traction motor must be totally enclosed type, particularly when mounted beneath the locomotive or motor coach, to provide protection against ingress of dirt, dust, water, mud etc.
Thus, for magnetic circuit of traction motor cast iron, which cannot withstand continuous vibrations, is not suitable. Use of cast steel or fabricated steel, which gives more mechanical strength, is made in place of cast iron. Those parts of the motor, which are not highly stressed; must be made of pressed or fabricated steel plates and light alloys.
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2. Electrical Characteristics:
(i) High Starting Torque:
A traction motor must be capable of developing high starting torque, especially when the train is to be accelerated at a reasonably high rate such as in case of urban or suburban services.
(ii) Simple Speed Control:
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The traction motor should be amenable to simple speed control methods as an electric train has to be started and stopped very often.
(iii) Self-Relieving Property:
The speed-torque characteristics of the motor should be such that the speed may fall with the increase in load. The motors having such speed- torque characteristics are self-protective against excessive overloading as power output of a motor is proportional to the product of torque and speed.
(iv) Possibility of Dynamic or Regenerative Braking:
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The traction motor should be amenable to easy and simple methods of rheostatic and/or regenerative braking.
(v) Capability of Withstanding Voltage Fluctuations:
Traction motor should be capable of withstanding rapid fluctuations in supply voltage without undue effect on its performance since in traction work rapid voltage fluctuation owing to heavy current inrush at start, is common feature.
(vi) Capability of Withstanding Temporary Interruption of Supply:
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Traction motor should be capable of withstanding temporary interruptions of supply without undue rush of current, since it occurs at the instant of crossing over the cross-overs and section insulators.
(vii) Overload Capacity:
Traction motor should be capable of taking excessive load as it is subjected to very arduous duty.
(viii) Parallel Running:
In traction work, usually more than one motor (two or four motors per motor car) are required. Traction motors, therefore, should be of such speed-torque and current-torque characteristics that when they are operated in parallel and mechanically coupled, they share the load almost equally.
No motor meets all the requirements mentioned above. Most suitable motors for dc systems are the series and compound motors whereas for ac systems the single phase series motors and 3-phase induction motors are employed.
Rating and Ventilation of Traction Motor:
The motors are designed for intermittent loading as well as continuous loading. The motors employed in traction work are subject to intermittent loading particularly in case of tramway and suburban services, therefore, traction motors are given one-hour rating as well as continuous rating. The ratings have been fully defined in Indian Standard Specifications (ISS). The continuous rating of an ordinary self-ventilated motor is approximately one-third to one-fourth of its one hour rating while the continuous rating of a forced ventilated motor is about four-fifth of its one hour rating.
The motors employed in traction work are usually totally enclosed so that protection against the ingress of dust, dirt, mud and water may be provided. Since the weight and overall dimensions of the motor are to be kept to the minimum, efficient cooling is, therefore, essential. For cooling purpose either self-ventilation (by using a fan mounted on the motor shaft which blows air through suitable ducts in the motor) or forced ventilation (by employing an external fan) is used. In either case care must be taken in designing the ducts to prevent their becoming choked with dirt and axial rather than radial ducts are always used.
Forced ventilation is invariably employed in case of locomotives, as it is operative even when the vehicle is stationary but for motor coaches and road vehicles, self-ventilation is usually employed as it does not cause noise due to fan employed for this purpose.