In an electric traction system, electric motors are used to develop the necessary propelling torque. The electric motors which are used in the electric traction system are called Traction Motors.
The type of electric motor used for a traction system depends upon the characteristics and performance of the motor. The following are the mechanical and electrical features that a traction motor is expected to have as far as possible.
Mechanical Features :
- As the motor has to withstand the vibrations continuously due to the severe service conditions, therefore motor should be robust.
- The size of the motor should be as small as possible in terms of overall dimensions (especially in overall diameter) since traction motors are placed beneath the motorcoach.
- The motor must be totally enclosed type to protect from ingress of dirt, dust, water, mud, etc.
- The weight of the traction motor should be less so that the load-carrying capacity of the vehicle may be increased.
Electrical Features :
- A traction motor must have high starting torque especially when the train is to be accelerated at a high rate.
- The speed control methods for the motor should be simple, since a locomotive has to start and stop very often.
- The motor should withstand high voltage fluctuations in supply voltage since the traction motor is subjected to rapid voltage fluctuations owing to heavy starting current.
- Their should be possibility of applying rheostatic or regenerative braking of the motor and it should be simple and efficient.
- The motor should have capability of taking excessive load as it is subjected to very arduous duty.
- While crossing over the crossovers and section insulators, there can be chances of disconnection of supply to the locomotive, thus traction motor should have the capability of withstanding temporary interruptions of supply.
- The motor should have better commutation to avoid sparking at the brushes and commutator segments.
- In a traction system, a number of motors will be operated in parallel to carry the load, thus the motors should possess such speed-torque and current-torque characteristics, and motors should share the load almost equally.
- The motor should withstand high temperatures during abnormal conditions.
- The motor should possess self relieving property i.e., With an increase in load on the motor the speed should fall so that the motor will be self-protective against overloading as the power output of the motor is equal to the product of torque and speed.
From the above-discussed features of traction motor, none of the electric motor can have all the features. But there are some electric motors that meet the requirements discussed above as far as possible. The following are the electric motors that are employed for the traction system.- DC series motors,
- DC compound motors,
- Single-phase ac series motors,
- Three-phase induction motors,
- Linear induciton motors.
DC series motors are best suited for traction systems because of their high starting torque and ability to handle high torque overloads. Also, a better load sharing between the motors is possible by their speed-torque characteristics. But the speed control of dc series motor is not easy using power semiconductor devices.
This drawback can be overcome by designing the motor with speeds equal to the highest desired speed of the load by reducing the number of field winding turns.
However, the reduction in field turns decreases the torque developed per ampere at starting period thereby reducing the acceleration. Also, the regenerative and dynamic braking of a dc series motor is complicated.
In the case of a dc separately excited motor when compared with a series motor, the regenerative and dynamic braking is simple and efficient, and also the speed of the motor can be controlled by power semiconductor devices easily. However, dc compound motors are being preferred due to the combined features of dc series motor and dc separately excited motor.
Nowadays, due to the availability of variable frequency inverters, squirrel cage induction motors and synchronous motors are preferred over dc motors for traction purposes.
Compared to dc motors, squirrel cage induction motors are robust in construction, highly efficient, have less maintenance, low cost, and are highly reliable. The advantages of synchronous motor lie between dc motor and squirrel cage induction motor.