As you may know, electric motors are devices that convert electrical energy into mechanical energy, and they currently dominate the modern industry. They are easy to use, basic in design, and come in many forms, allowing them to succeed in almost every situation. Electric motors can be powered via DC or AC. In this article, Linquip will investigate a specific AC motor known as the squirrel cage motor which is a specific kind of induction motor that uses the electromagnetic induction effect to transform electrical current into rotational energy. Read on to understand the principles of squirrel cage motors, how they operate, and what kinds of applications they are used for.
What is a squirrel cage motor?
These motors are a type of induction motors, which harness electromagnetism to generate motion. They are so-called squirrel cage motors because the shape of their rotor looks like a cage. Two circular end caps are joined by rotor bars, which are acted upon by the electromagnetic field generated by the stator or the outer housing composed of laminated metal sheets and coiling of wire.
The stator and the rotor are the two fundamental parts of any induction motor, and the squirrel cage is simply one method of leveraging the electromagnetic induction effect. The AC passed through the stator creates an electromagnetic field that fluctuates with the AC frequency which rotates around the rotor inducing opposing magnetic fields in the rotor bars that result in motion.
How does a Squirrel Cage Motor work?
Squirrel cage motors work the same as the most other induction motors and the only difference between them is in the specific interaction between rotor and stator. These motors maximize electromagnetic induction by utilizing rotor bars to interact with the stator’s electromagnetic field. The stator usually contains windings of wire which carry an AC; this current, changes in sync with a sinusoidal alternate, which then changes the current direction in the wire windings.
When the current oscillates, the generated electromagnetic field will follow suit, and in certain arrangements will cause it to rotate with a frequency similar to the AC frequency. This rotating electromagnetic field produces an opposing voltage and electromagnetic field in the rotor bars, thus pushing the rotor around, generating rotational motion.
This rotor does not spin at the exact frequency of the AC and that is why squirrel cage motors as well as other induction motors are considered asynchronous. There is always some loss between the AC frequency and the rotational frequency of the shaft, and this is a result of the rotor rotates in the first place. If the rotor were to spin at the same frequency, then the magnitude of the force on the rotor bars would equal zero, thus creating no motion.
Squirrel Cage Induction Motor Construction
Parts that are required for the construction of a squirrel cage motor are stator, rotor, fan, bearings. The stator consists of mechanically and electrically 120 degrees apart three-phase winding with metal housing and core. To provide the path of low reluctance for flux generated by AC, the winding is mounted on the laminated iron core.
Rotor gives electrical energy into mechanical output. The shaft, a core, short-circuited copper bars are the parts of the rotor. To avoid hysteresis and eddy currents that are leading to power loss, the rotor is laminated. And To prevent cogging, conductors are skewed which also helps to give a good transformation ratio. A fan attached at the back of the rotor for heat exchange helps in maintaining under a limit of the temperature of the motor. For the smooth rotation, bearings are provided in the motor.
Classification of Squirrel Cage Induction Motor
National Electrical Manufacturer’s Association in the United States and IEC in Europe have classified the design of the squirrel cage induction motors based on their speed-torque characteristics into some classes. These classes are Class A, Class B, Class C, Class D, Class E, and Class F.
Class-A Design
- A normal starting torque.
- A normal starting current.
- Low slip.
- In this Class, pullout torque is always of 200 to 300 percent of the full-load torque and it occurs at a low slip (it is less than 20 percent).
- For this Class, the starting torque is equal to the rated torque for larger motors and is about 200 percent or more of the rated torque for the smaller motors.
Class-B Design
- Normal starting torque
- Lower starting current
- Low slip
- The Induction Motor of this class produces about the same starting torque as the class-A induction motor.
- Pullout torque is always greater than or equal to 200 percent of the rated load torque. But it is less than that of the class A design because it has increased rotor reactance.
Class-C Design
- High starting torque.
- Low starting currents.
- Low slip at the full load (less than 5 %).
- Up to 250 percent of the full-load torque, the starting torque is in this class of design.
- The pullout torque is lower than that for Class A induction motors.
Class-D Design
- This Design of Class motors has a very high starting torque (275 percent or more of the rated torque).
- A low starting current.
- A high slip at full load.
- Again in this class of design, the high rotor resistance shifts the peak torque to a very low speed.
- It is even possible at zero speed (100 percent slip) for the highest torque to occur in this class of design.
Class-E Design
- Very Low Starting Torque.
- Normal Starting Current.
- Low Slip.
- A compensator or resistance starter is used to control the starting current.
Class-F Design
- Low Starting Torque, 1.25 times of full load torque when full voltage is applied.
- Low Starting Current
- Normal Slip
Squirrel Cage Motor Application
Squirrel cage induction motors are commonly used in many industrial applications. They are particularly suited for applications where the motor must maintain a constant speed, be self-starting, or there is a desire for low maintenance.
These motors are commonly used in:
- Centrifugal pumps
- Industrial drives (e.g. to run conveyor belts)
- Large blowers and fans
- Machine tools
- Lathes and other turning equipment
- Centrifugal pumps, fans, blowers, etc.
- In driving air compressors, conveyors, reciprocating pumps, crushers, mixers, large refrigerating machines, etc.
- Punch presses, shears, bulldozers, small hoists, etc.
Squirrel Cage Motor Advantages
Some advantages of squirrel cage induction motors are:
- Simple and rugged construction
- The low initial as well as the maintenance cost
- Maintains constant speed
- The overload capacity is high
- Simple starting arrangement
- High power factor
- Low rotor copper loss
- High efficiency in converting electrical energy to mechanical energy (while running, not during startup)
- Small and lightweight
- Have better heat regulation
- Explosion-proof
Squirrel Cage Motor Disadvantages
The disadvantages of a squirrel cage induction motor are as follows.
- Very poor speed control
- Although they are energy efficient while running at full load current, they consume a lot of energy on startup
- They are more sensitive to fluctuations in the supply voltage. When the supply voltage is reduced, the induction motor draws more current. During voltage surges, increase in voltage saturates the magnetic components of the squirrel cage induction motor
- They have high starting current and poor starting torque
So this is all you need to know about squirrel cage motors and their application within a system. If you enjoy the article in Linquip, feel free to share your experience in the comment section. Is there any question we can help you with? Sign up on our website and get some professional advice from our experts.
