Till electronics came into the field of traction recently, the best suited drive for traction application was the dc series motor because of its inherent characteristics to give high torque at low speeds and vice-versa. The dc series motor was the best solution found for meeting the actual service as well as control requirements. On a fixed frequency, the asynchronous motor has a characteristic which is not well suited for traction application. However, this motor is of rugged construction when compared to a dc motor and requires much lower inputs for maintenance. Therefore its choice as a traction motor will always be preferred if control of the motor to suit the service requirements is possible. With the development of GTO thyristors, power electronics and micro processor control, controlling of any drive to suit specific requirements has become quite easy. In the circumstances, adoption of 3-phase induction motor for traction application has been tried for the past 10 years and has now become commercially viable proposition. The control is being achieved through variable voltage and variable frequency briefly referred to as WVF. Originally thyristors were used with commutation requirements for cut off. With the advent of GTO, the control has been made much simpler and more efficient thus use of 3-phase asynchronous motors for traction could also become financially viable.
A locomotive with 3-phase asynchronous motor drive has the following advantages :
- higher power capability due to high power/weight ratio,
- regeneration capability over a wide speed range,
- lesser maintenance,
- unity power factor at pantograph,
- higher adhesion.
Why 3-phase Drive is used?
The speed/torque characteristic of an induction motor supplied from a 3-phase voltage source of fixed frequency “f” is of the form shown in this figure.
It may be seen that at low speed, the torque is small and the stator current is high. The zone normally usable lies at the extreme from the point of maximum torque, in the vicinity of synchronous speed. For traction applications a high TE is necessary for starting and accelerating the train. Therefore, when constant torque is required, It is necessary to obtain whole series of characteristics curves such as shown in figure given below. This calls for change in synchronous speeds and therefore variable frequency.
The control of the ac induction motor drive in traction application is achieved in 3 stages
Constant torque mode:
The torque developed by a motor is proportional to the product of the magnetic flux in the air gap, and the rotor current. The applied voltage is proportional to the synchronous frequency and the magnetic flux in the air gap. To keep the magnetic flux constant, therefore, the applied voltage to the traction motor is to be made proportional to the synchronous speed.
The rotor current depends upon the slip frequency of the rotor. By keeping this constant, the rotor current is also kept constant. This is possible till the voltage is increased to the rated terminal voltage of the traction motor. Thus in this mode the control is achieved by increasing the voltage and frequency uniformly with respect to the actual speed of the rotation of the rotor and the required slip frequency.
Constant power mode:
In this mode the voltage is already reached to the rated voltage. By increasing the frequency, the magnetic flux in the air gap is reduced proportionately. By keeping the slip frequency constant, the motor current is kept constant. Thus the motor is made to give constant power output till the maximum service speed is reached.
Balancing speed stage:
Once the maximum pre-determined speed is achieved, the same power output from the traction motor may not be required and the output is to be matched to meet the resistance of the train for running at the balancing speed. This is achieved by suitably reducing the terminal voltage of the traction motor.
These functions are performed through micro processor control giving various inputs of parameters like voltage, current and speed.
In order to achieve the above control requirements, the converter/ inverters system with a dc link is adopted. The converter rectifies the ac voltage to dc and feeds it to the dc link. The dc link supplies power to the inverters. The converter output power is controlled by controlling the output current keeping the dc link voltage constant. The inverters output power is controlled by varying the terminal voltage initially till the full voltage is achieved. However the motor output current is kept constant.
The control of output current of the converter and output voltage of the inverters is achieved by pulse width modulation control.
The power circuit diagram of a typical 3-phase locomotive fitted with asynchronous motors is shown in figure given below.