Wound-Rotor Motor Startup
For a high-starting torque, the motor is started with the rotor circuit starter resistance in the circuit.
As the motor gains speed, the rotor resistance is reduced gradually. This will shift the synchronous speed and maximize the torque curve from the wound-rotor motor to an induction motor curve. Finally, the rotor or slip rings will be short-circuited.
For some motors, the rotor resistance is introduced in small steps.
Merely interchanging the two supply voltage leads can change the direction of the motor.
The following are the characteristics of a three-phase wound-rotor motor:
• Achieve high-starting torque with low-starting current
• Generally used in applications where on load starting is required
• Is self-starting
• Speed adjustment is possible up to a good extent
• Speed varies a lot when used with the rotor resistance in picture.
The high starting torque of the wound rotor motor and the capability to control the speed by varying the resistance has made this form of motor popular for lifting applications such as hoists and cranes. Also, the relatively lower starting current and the high torque makes it a popular choice for large capacity drives on weak electrical systems and for high inertia loads such as rotary kilns and blowers.