SOLUTION

For electric traction, DC series motors are best suited. DC series motors and AC series motors are very much recommended, as they provide a high starting torque Nowadays three-phase induction motors are also being used.

• The main drawback of dc motors is the presence of commutators and brushes, which require frequent maintenance and make them unsuitable for the explosive and dirty environment.
• On the other hand, induction motors, particularly squirrel-cage are rugged, cheaper, lighter smaller, more efficient, require lower maintenance, and can operate in dirty and explosive environments.
• Although variable speed induction motor drives are generally expensive than d drives, they are used in a number of applications such as fans, blowers, mill run-out tables, crane conveyers, traction, etc. because of the advantages of induction motors.
• Other dominant applications are underground and underwater installations and explosive and dirty environments.
• The modern electric traction system uses the pulse width modulation (PWM) principle.
• The PWM inverter produces a symmetrical three-phase output voltage. The amplitude and frequency of the three-phase output voltage can be controlled continuously.
• Thus the speed and torque of the squirrel cage induction motor used for traction can be adjusted easily in motoring and braking modes as well as in both directions of rotation with a fully static device, with no operational contacts.
• The PWM inverter allows the construction of propulsion drives for a wide range of applications, with slight modification, to the basic modules, such as the traction inverter and the traction motor.
• The PWM AC drives cover trolley-buses, subways railcars, electric locomotives, and diesel-electric locomotives. For the application of electric drives, the AC traction motor staler winding insulation and needs special bearings. The rotor of the AC traction motor is to be rigid and robust and must withstand high rotating speed.