Motors Employed in Traction : Earlier, dc series motor was widely used in Traction Motors. It has high starting torque and capability for high torque overloads. With an increase in torque, the flux also increases; therefore, for the same increase in torque, the increase in motor current is less compared to other motors. Thus during heavy torque overloads, power overload on the source and thermal overloading of the motor are kept limited to reasonable values. The motor speed-torque characteristic is also suitable for better sharing of loads between motors. Further due to a large inductance in the field, sharp fluctuations in supply voltage do not produce sharp peaks in armature current. Thus the motor commutation remains satisfactory, which does not happen in a separately excited motor, unless an additional inductance is connected in the armature circuit.
With the availability of semiconductor converters, separately excited motor is now preferred over series motor. With independent control of armature and field, the speed-torque characteristic of separately excited motor can be shaped to satisfy the traction requirements in the optimum manner. Further, because of low regulation of its speed-torque characteristics, the coefficient of adhesion has higher value. On the other hand, series motor has a number of limitations. The field of a series motor cannot be easily controlled by semiconductor switches. If field control is not employed, the series motor must be designed with its speed equal to the highest desired speed of the drive. The higher base speeds are obtained by using fewer turns in the field winding This, however, reduces the torque per ampere at start and therefore, acceleration. Further, there are a number of problems with regenerative and dynamic brakings of a series motor. On the other hand, regenerative and dynamic brakings of a separately excited motor are fairly simple and efficient, and can be carried out down to very low speeds.
Currently compound motor is being preferred for traction applications as it incorporates the advantages of both series and separately excited motor.
Due to the availability of reliable variable frequency semiconductor inverters, squirrel-cage induction motor and synchronous motor are now finding applications in traction. Because of a number of advantages associated with these motors, they are likely to replace dc motors for traction applications.
Some of the important advantages of squirrel-cage induction motors over dc are: ruggedness; lower maintenance; better reliability; lower cost, weight, volume and inertia; higher efficiency; and ability to operate satisfactorily with sharp supply voltage fluctuations and in dirty environment. The major drawback of dc motor is the presence of commutator and brushes, which require frequent maintenance, particularly when the flashovers at the commutator occur due to sharp voltage fluctuations. In terms of advantages mentioned for squirrel-cage motor in comparison with dc motors, the synchronous motor lies in-between the two and has one important advantage over squirrel-cage induction motor, that it can be operated at leading power factor. Thus permitting the use of load-commutated thyristor inverter which is cheaper and occupies less volume and weight compared to forced commutated thyristor inverter required by induction motors. The weight and volume of an induction motor drive can also be kept low by using GTO (gate turnoff thyristor) inverter, but is more expensive than a load commutated thyristor inverter.
Traction Motor Control:
Operation of a dc separately excited motor for traction applications can be divided into three regions, First two are identical. i.e. constant torque and power regions. In constant torque region, from zero to base speed, the field current is maintained constant at the rated value and the armature voltage is controlled. In constant power region, which is carried out above base speed, the armature voltage is maintained constant at the rated value and field current is controlled. In both these regions, the armature current is allowed to reach rated value on continuous basis. The limit of constant power operation is reached when a decrease in field current to increase motor speed leads to sparking at the brushes at the rated armature current. The motor is said to reach the commutation limit. Operation at higher speeds (and lower field currents) can now be carried out by progressively decreasing maximum allowable armature current. This is the third region of operation in which available output power of the. motor progressively decreases with the increase in speed. Traction Motors can be operated in third region because the torque required at high speeds is much less compared to the accelerating torque. The form of third region is determined by whether or not the motor is compensated and the type of power modulator. For a non-compensated motor, the ratio of maximum allowable armature current to field current is maintained constant. In a compensated motor, the maximum allowable armature current is varied inversely with speed. A compensated machine is always preferred because it allows greater degree of field weakening and therefore, higher maximum speed.
The variable frequency controlled squirrel-cage induction motors are also operated in three identical regions. Constant torque region from standstill to base speed with a constant V/f ratio and a constant maximum allowable stator current; constant power region from base speed to the speed at which breakdown torque limit is reached, here V and maximum allowable stator current are constant; for higher speeds the motor operates in the third region where maximum allowable current is reduced inversely with speed, thus ensuring that the motor torque does not exceed its breakdown value.