A slip ring induction motor, also known as a wound rotor induction motor, is a type of three-phase induction motor that uses a wound rotor with insulated windings connected to slip rings. The slip rings allow external resistance to be connected in series with the rotor windings, allowing the motor to be started at high slip (low speed) and then adjusted to operate at lower slip as it accelerates to its rated speed.
This allows the motor to have a high starting torque, making it suitable for applications that require high starting torque, such as pumps, fans, and conveyors. The slip rings and brushes also allow the rotor resistance to be adjusted during operation, allowing the motor to be used for speed control.
What is a Squirrel cage induction motor?
A squirrel cage induction motor is a type of three-phase induction motor that uses a squirrel cage rotor, which is made up of a cylindrical core with slots and shorting bars (also called end rings) made of copper or aluminum. The rotor windings are created by the shorting bars, which are short-circuited at each end by the end rings.
The slots in the rotor core allow the magnetic field generated by the stator to induce currents in the rotor bars, which create a magnetic field that interacts with the stator field to produce torque. The rotor of a squirrel cage induction motor is simple and rugged, making it suitable for use in a variety of industrial and commercial applications. Unlike a slip ring induction motor, a squirrel cage motor does not have any external connections, so it is not possible to add external resistance to the rotor circuit.
The rotor slots are also typically skewed, which helps to reduce the harmonics in the rotor currents and improve the motor's efficiency.
Key Differences Between Slip Ring & Squirrel Cage Induction Motor
- Rotor construction: The main difference between these two types of motors is the construction of the rotor. A squirrel cage rotor consists of a cylindrical core with shorting bars (also called end rings) made of copper or aluminum, while a wound rotor has insulated windings that are connected to slip rings on the shaft.
- Starting torque: A wound rotor induction motor has a higher starting torque than a squirrel cage motor because the rotor resistance can be adjusted by changing the resistance in the rotor circuit. This allows the motor to be started at high slip (low speed) and then adjusted to operate at lower slip as it accelerates to its rated speed. In contrast, the rotor resistance in a squirrel cage motor is fixed, so it has a lower starting torque.
- Speed control: A wound rotor induction motor can be used for speed control by adjusting the rotor resistance, while a squirrel cage motor is not suitable for speed control because the rotor resistance is fixed.
- Maintenance: A wound rotor induction motor requires more maintenance than a squirrel cage motor because the slip rings and brushes need to be regularly replaced to ensure good electrical contact.
- Efficiency: In general, squirrel cage motors are more efficient than wound rotor motors because the rotor resistance is fixed, which reduces losses due to resistance in the rotor circuit.
- Power factor: A squirrel cage motor has a higher power factor than a wound rotor motor because the rotor resistance is fixed and does not vary with load. This means that the current drawn by a squirrel cage motor is in phase with the voltage applied to it, which leads to a higher power factor. In contrast, the power factor of a wound rotor motor can be lower because the rotor resistance can be adjusted, which can cause the current to be out of phase with the applied voltage.
- Cost: Squirrel cage motors are generally less expensive to manufacture than wound rotor motors because they have a simpler construction.
- Size and weight: Wound rotor motors are typically larger and heavier than squirrel cage motors because they have slip rings and brushes, which add to their size and weight.
- Noise: Wound rotor motors can produce more noise than squirrel cage motors because the slip rings and brushes can create electrical noise as they rotate.
- Applications: Squirrel cage motors are more widely used than wound rotor motors because they are simpler, more efficient, and less expensive. They are suitable for a wide range of applications, including pumps, fans, conveyors, and compressors. Wound rotor motors are typically used in applications that require high starting torque, such as cranes, hoists, and elevators. They are also used in some speed control applications where the rotor resistance can be adjusted.
Key Point Summary:
- Slip ring motors have a phase wound rotor, while squirrel cage motors have a cage-like rotor.
- Slip ring motors have a more complex construction with slip rings and brushes, while squirrel cage motors have a simpler construction.
- Slip ring motors can have an external resistance circuit, while squirrel cage motors cannot.
- Slip ring motors require a rotor resistance starter for starting, while squirrel cage motors do not.
- Slip ring motors have a higher starting torque and a lower efficiency compared to squirrel cage motors.
- Slip ring motors have brushes for transferring power, while squirrel cage motors are brushless.
- Slip ring motors have a higher copper loss and a lower power factor compared to squirrel cage motors.
- The speed of a slip ring motor can be controlled using an external resistance circuit, while the speed of a squirrel cage motor cannot be controlled.
- Slip ring motors are more expensive and have a higher maintenance cost than squirrel cage motors.
- Slip ring motors are typically used in applications that require high starting torque, such as hoists and cranes, while squirrel cage motors are used in applications such as drilling machines and lathe machines.