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The WAP4 is more rugged and more reliable than its ac counterpart. For this reason, very long haul superfast trains such as Kerala Express are often allotted WAP4 even though the load characteristics make them ideal WAP7 candidates. Once in a while, Rajdhanis and Shatabdis are held up due to failure of WAP7 en route; then WAP4 generally comes to the rescue. The reliability issue is also due to the induction motors – while the machines themselves are extremely rugged, their control circuits are highly sophisticated and ipso facto somewhat delicate. The control algorithm used in the WAP7 is direct torque control (DTC)which uses a two level voltage source pulse width modulator (PWM) and achieves decoupled control of the flux and torque. The control is extremely precise but needs a lot of complex devices such as hysteresis controller, switching table and flux position sensors. The increased complexity decreases the reliability. The...
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simplest and most rugged form of induction motor control is Volts/Hz or scalar control. In this algorithm, the applied voltage and slip frequency are adjusted depending on the torque requirements. However, an ordinary Volts/Hz drive is not suitable for traction applications as very large amplitude transients are generated during acceleration.