Benefits And Drawbacks Of Wound Rotor Induction Motors

Wound rotor induction motors are also known as slip-ring induction motors. The reason is that they have high starting torque. Also, they apply in loads that need high starting torque or excellent speed control. Its applications include elevators, hoists, printing presses, large ventilating fans and compressors. In addition, it’s common in applications where loads need speed control, such as driving lifts and pumps. So, what are the benefits and cons of these motors?

What Is a Wound Rotor/Slip Ring Induction Motor?

It is a unique 3-phase AC induction motor that provides high starting torque. It happens by connecting an external resistance to the rotor resistance. This motor’s rotor is a type of wound rotor. That’s why it’s recognized as a phase-wound induction motor or simply a wound rotor. The wound rotor induction motor operates at a speed that’s not equal to the synchronous speed of the rotor. Because of this, it’s also known as an asynchronous motor. In addition, you can control the torque performance of the motor by utilizing these variable resistors or liquid rheostats.  

Benefits Of Wound or Slip Ring Induction Motor

• It has a high starting torque and uses lesser current. It’s easy to attain maximum starting by inserting external resistance in each motor phase and cutting it during the start. 
• Speeds are adjustable by inserting a resistance, which makes these motors considered variable speed motors.
• Provide high overloading capacity, smooth acceleration under heavy loads and no unusual heating during start.
• The initial cost is moderate for high horsepower units.
• The make of the motor is simple and doesn’t need high-end maintenance. So, you can save a lot on the maintenance cost.

Although it has several benefits, there are some drawbacks to these motors, and they are as follows.

Drawbacks Of Wound Induction Motor

• Although its initial and maintenance cost is moderate, when compared to squirrel-cage motors, it’s expensive. The reason is the presence of slip rings, short-circuiting devices, brushes, e.t.c.
• Its speed regulation is poor when operated with external resistances in the rotor circuit.
• The lower power factor in light loads
• Sensitive to fluctuations in voltage supply.
• The power factor and efficiency are low compared to squirrel-cage motors.

Common Applications of Wound Rotor Induction Motor

As mentioned, you can use these motors in applications that require high starting torque, such as driving. The most common uses are in industrial drives, which include:

• Conveyors
• Contour or line shafts
• winding machines
• Hoists 
• Refineries
• Pumps
• Elevators
• Curving machines 
• Generators
• Cranes, lifts and mills.

These are some of the primary uses of these motors. Now it is time to learn how to start a wound induction motor.

Starting A Wound Rotor

The motor starting is through rotor resistance or rheostat starting. When starting it, motor manufacturers recommend the application of full line voltage across the stator terminals. Then the value of the starting current is adjusted through variable resistance in the rotor circuit.

The resistance that controls it is in the form of a star-connected rheostat. It means a gradual cut-out of resistance as the motor’s speed increases. When the rotor resistance increases, the rotor current reduces at starting. This reduces the stator current, but the torque increases due to improvement in power factor.

The extra resistance in the rotor circuit allows the wound motor to develop high starting torque with lesser starting current. So, due to this, you can start the motor with some load. Now that you understand the motor starts, it’s time to know how to control the speed.

Regulating The Speed of a Wound-Rotor Motor

Regulating a wound rotor’s speed is achieved by altering the rotor circuit’s resistance. The speed reduces when the motor runs, and the maximum resistance is connected to the rotor circuit. As the motor speed lessens, it generates more voltage in the rotor circuit to yield the required torque. Similarly, if the rotor resistance reduces, the speed of the motor increases.

The high starting torque of this motor makes it great for loads that require excellent speed control. Therefore, people use it in applications such as driving machines in industries. Also, the capability to alter the speed makes them favourable and efficient. Lastly, the motor is simple and doesn’t require complicated maintenance.

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