How do external resistors affect the starting torque and speed of a wound rotor induction motor?
1 Answer
External resistors play a crucial role in controlling the starting torque and speed of a wound rotor induction motor. This type of motor is also known as a slip ring induction motor. The rotor windings in a wound rotor motor are connected to external resistors through slip rings, allowing for external control of the rotor circuit.
Here's how external resistors affect the starting torque and speed of a wound rotor induction motor:
Starting Torque:
High resistance in the rotor circuit: When the external resistors are set to a high value, the rotor circuit's impedance increases. This results in a higher starting torque since the difference between the stator's rotating magnetic field and the rotor's induced magnetic field becomes significant. The rotor current lags behind the stator field, generating a larger torque to overcome the inertia of the motor during starting.
Low resistance in the rotor circuit: Lowering the resistance in the rotor circuit reduces the impedance, leading to a decrease in starting torque. The rotor current comes into phase with the stator field at a faster rate, which reduces the initial torque during motor startup.
Starting Speed:
High resistance in the rotor circuit: With higher external resistors, the rotor current is limited, resulting in a slower acceleration of the rotor. As a consequence, the starting speed of the motor decreases.
Low resistance in the rotor circuit: Lower external resistors allow a higher current to flow through the rotor circuit, leading to a faster acceleration of the rotor. Consequently, the motor reaches its starting speed more quickly.
In summary, by adjusting the external resistors, you can control the starting torque and speed of a wound rotor induction motor. High resistances provide higher starting torque but slower starting speed, while low resistances result in lower starting torque and faster starting speed. This feature is particularly useful in applications where precise control over the starting characteristics of the motor is required, such as in crane systems and high-inertia loads. After the motor reaches its running speed, the external resistors are often bypassed using centrifugal switches or other control methods to improve motor efficiency during normal operation.
Here's how external resistors affect the starting torque and speed of a wound rotor induction motor:
Starting Torque:
High resistance in the rotor circuit: When the external resistors are set to a high value, the rotor circuit's impedance increases. This results in a higher starting torque since the difference between the stator's rotating magnetic field and the rotor's induced magnetic field becomes significant. The rotor current lags behind the stator field, generating a larger torque to overcome the inertia of the motor during starting.
Low resistance in the rotor circuit: Lowering the resistance in the rotor circuit reduces the impedance, leading to a decrease in starting torque. The rotor current comes into phase with the stator field at a faster rate, which reduces the initial torque during motor startup.
Starting Speed:
High resistance in the rotor circuit: With higher external resistors, the rotor current is limited, resulting in a slower acceleration of the rotor. As a consequence, the starting speed of the motor decreases.
Low resistance in the rotor circuit: Lower external resistors allow a higher current to flow through the rotor circuit, leading to a faster acceleration of the rotor. Consequently, the motor reaches its starting speed more quickly.
In summary, by adjusting the external resistors, you can control the starting torque and speed of a wound rotor induction motor. High resistances provide higher starting torque but slower starting speed, while low resistances result in lower starting torque and faster starting speed. This feature is particularly useful in applications where precise control over the starting characteristics of the motor is required, such as in crane systems and high-inertia loads. After the motor reaches its running speed, the external resistors are often bypassed using centrifugal switches or other control methods to improve motor efficiency during normal operation.