Slip Ring vs. Squirrel Cage in Induction Motors: Comprehensive Comparison

An induction motor is an electrical device that converts electrical energy into mechanical energy. In the field of electrical engineering, due to its self - starting and stable power features, it is widely used as a driving device for numerous machines and equipment. There are two types of rotors used in it: squirrel cage and slip ring. Today, ByTune will, together with you, understand what an induction motor is, how it operates, and compare the structures, performances, applications, etc. of the two types of rotors.

 

The induction motor, known as an asynchronous motor, is an AC motor. It mainly consists of two major parts: the stator and the rotor. In addition, it also includes auxiliary components such as end covers, bearings, and fans.
• Stator: It is the stationary part of the motor, composed of the stator core, stator winding, and frame. The stator winding is made up of many regularly arranged and connected coils. These coils are embedded in the slots of the stator core. By passing three - phase alternating current through them, a rotating magnetic field is generated.
• Rotor: The rotor is the rotating part of the motor, which can generate electromagnetic torque to drive the motor and machinery to rotate. The most common rotor is the "squirrel - cage" design, consisting of a laminated core and conductive bars that extend along its length, short - circuited at both ends by rings. However, the "wound rotor" with slip - rings and external resistors is also very popular.

 

An induction motor mainly consists of two parts: the stator and the rotor. When three - phase alternating current is applied to the stator winding, a rotating magnetic field is generated inside the stator. Its synchronous speed Ns=120f/p, where:
f is the power supply frequency (in Hertz, Hz).
p is the number of pole pairs of the motor.
Relative to the stationary rotor, the rotor conductors inside the motor can cut the magnetic induction lines to generate an induced electromotive force. Since the rotor conductors form a closed loop, the induced electromotive force will cause an induced current in the rotor conductors.
According to Lenz's law, the magnetic field generated by the induced current interacts with the rotating magnetic field, generating a torque that makes the rotor start to rotate. However, the rotor always rotates at a speed slightly lower than the synchronous speed. The slip rate s=Ns-Nr/Ns(Nr is the rotational speed of the induction motor). This value is generally between 1% and 5%.

 

Slip Ring Rotor

• What it is: The slip ring is a component of a wound - rotor induction motor. It consists of conductive rings, brushes, brush holders, insulating materials, etc. The rotor winding of a slip ring induction motor is a three - phase winding connected in a star shape, and the three outlet terminals are respectively connected to three conductive rings.
• Working principle: When the motor is running, an induced electromotive force and an induced current are generated in the rotor winding. Through the sliding contact between the brushes and the conductive rings, the current in the rotor winding is led out to the external circuit. This is used to control the starting, speed regulation, braking, and other operations of the motor.

Squirrel Cage Rotor

What it is: The squirrel - cage is the rotor used in a squirrel - cage induction motor. It gets its name because the overall shape of its winding resembles a squirrel cage. The squirrel - cage rotor is composed of multiple conductive bars inserted into the rotor slots and two circular end rings. Small - sized cage - type motors generally use cast - aluminum welding, while motors above 100kW are welded with copper bars and copper end rings.

Working principle: During operation, after three - phase alternating current is applied to the stator winding, a rotating magnetic field is generated inside the stator. This magnetic field rotates inside the stator at synchronous speed and cuts the squirrel - cage conductive bars on the rotor. Since the squirrel - cage conductive bars are closed conductive loops in the rotor slots, when the rotating magnetic field cuts the bars, an induced electromotive force and an induced current are generated in the bars. It drives the rotor to make the motor rotate in the direction of the rotating magnetic field.

Although the structure of the squirrel - cage is more robust, the slip - ring rotor also brings many benefits to the slip - ring induction motor, such as:
• Higher Starting Torque: The slip - ring can provide a high starting torque and power factor by inserting an appropriate resistor into the rotor circuit. Therefore, when slip - ring induction motors are used in cranes and winches that need to start under load, they can overcome the load resistance and start these devices smoothly.
• Lower Starting Current: As we all know, an excessive starting current can cause a significant impact on the power grid. When a resistor is inserted into the slip - ring rotor circuit, the starting current will decrease. This is because when the resistance of the rotor circuit increases, the rotor current decreases, and according to the transformer principle, the stator current will also decrease accordingly.
• Wideer Speed - regulation: The slip - ring induction motor can achieve speed regulation by changing the value of the resistor connected in the rotor circuit. This is very useful in some metal - processing equipment, such as lathes and milling machines. When the connected resistor increases, the motor speed decreases; conversely, when the connected resistor decreases, the motor speed increases. This is something that squirrel - cage induction motors cannot do.

From the above content, we can conclude that slip - rings and squirrel - cages each have their characteristics in induction motors. In simple terms, if you have high requirements for starting torque and speed - regulation ability, a slip - ring induction motor may be more suitable for you. However, if you have strict cost requirements and want to use it in constant - speed equipment, you can choose a squirrel - cage induction motor.

In practical applications, we need to select the appropriate motor type according to the specific industrial equipment and usage requirements. And if you wanna learn more, our article "What is the Uses of Slip Rings in an Alternator?" may also be helpful. As a slip ring supplier for over 26 years, ByTune is confident in providing reliable slip - rings for any of your induction motors.