How does the external resistance in the rotor circuit of a wound rotor motor affect its speed and starting torque?
1 Answer
The external resistance in the rotor circuit of a wound rotor motor can significantly impact its speed and starting torque characteristics. A wound rotor motor, also known as a slip ring motor, is a type of induction motor with a wound rotor (hence the name) instead of a short-circuited squirrel cage rotor found in most standard induction motors.
The external resistance in the rotor circuit of a wound rotor motor is typically used for a process called rotor resistance control or rotor rheostat control. By varying the amount of external resistance connected to the rotor circuit, you can control the motor's speed and starting torque in the following ways:
Starting Torque: When a wound rotor motor starts, it experiences a higher slip (the difference between synchronous speed and actual rotor speed) than when it's running at full speed. By adding external resistance in the rotor circuit during starting, you increase the rotor circuit impedance. This higher impedance reduces the current flowing through the rotor windings, which in turn reduces the torque developed by the rotor. This reduction in starting torque can be beneficial in applications where a gradual start is required to avoid mechanical shocks or high starting currents.
Speed Control: Adding external resistance to the rotor circuit increases the total resistance in the rotor circuit. This has a direct impact on the rotor's slip at a given load. The slip is given by the formula: Slip = (Ns - N) / Ns, where Ns is the synchronous speed and N is the actual rotor speed. By increasing the rotor resistance, you increase the slip, which means the motor will run at a slower speed than its synchronous speed. This type of speed control is often used in applications where variable speed operation is required, such as conveyor belts, cranes, and rolling mills.
It's important to note that while adding external resistance provides control over starting torque and speed, it also introduces energy losses due to the dissipation of power in the resistance. As the motor accelerates and reaches a higher speed, the external resistance can be gradually reduced to minimize these losses while maintaining the desired operating characteristics.
In summary, the external resistance in the rotor circuit of a wound rotor motor is a crucial parameter for controlling its starting torque and speed. By adjusting this resistance, you can achieve the desired balance between torque and speed characteristics to meet the specific requirements of different industrial applications.
The external resistance in the rotor circuit of a wound rotor motor is typically used for a process called rotor resistance control or rotor rheostat control. By varying the amount of external resistance connected to the rotor circuit, you can control the motor's speed and starting torque in the following ways:
Starting Torque: When a wound rotor motor starts, it experiences a higher slip (the difference between synchronous speed and actual rotor speed) than when it's running at full speed. By adding external resistance in the rotor circuit during starting, you increase the rotor circuit impedance. This higher impedance reduces the current flowing through the rotor windings, which in turn reduces the torque developed by the rotor. This reduction in starting torque can be beneficial in applications where a gradual start is required to avoid mechanical shocks or high starting currents.
Speed Control: Adding external resistance to the rotor circuit increases the total resistance in the rotor circuit. This has a direct impact on the rotor's slip at a given load. The slip is given by the formula: Slip = (Ns - N) / Ns, where Ns is the synchronous speed and N is the actual rotor speed. By increasing the rotor resistance, you increase the slip, which means the motor will run at a slower speed than its synchronous speed. This type of speed control is often used in applications where variable speed operation is required, such as conveyor belts, cranes, and rolling mills.
It's important to note that while adding external resistance provides control over starting torque and speed, it also introduces energy losses due to the dissipation of power in the resistance. As the motor accelerates and reaches a higher speed, the external resistance can be gradually reduced to minimize these losses while maintaining the desired operating characteristics.
In summary, the external resistance in the rotor circuit of a wound rotor motor is a crucial parameter for controlling its starting torque and speed. By adjusting this resistance, you can achieve the desired balance between torque and speed characteristics to meet the specific requirements of different industrial applications.