What is a Squirrel Cage Motor and How Does it Work?

Electric motors are machines that convert electrical energy into mechanical energy, and they currently dominate 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 direct current (DC) or alternating current (AC), and this article will investigate a specific AC motor known as the squirrel cage motor. These motors are a specific kind of induction motor, which use the electromagnetic induction effect to transform electrical current into rotational energy (more information can be found in our article all about induction motors). This article will explain the principles of squirrel cage motors, how they operate, and what kinds of applications they are used for. This way, designers can make informed choices when choosing the right motor.

What are Squirrel Cage Motors?

Squirrel cage motors are a subclass of induction motors, which harness electromagnetism to generate motion. They are so-called “squirrel cage” motors because the shape of their rotor – the inner component connected to the output shaft – looks like a cage. Two circular end caps are joined by rotor bars, which are acted upon by the electromagnetic field (EMF) 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 current passed through the stator creates an EMF that fluctuates with the AC frequency, which “rotates” around the rotor, inducing opposing magnetic fields in the rotor bars, thus causing motion.

How do Squirrel Cage Motors Work?

In essence, squirrel cage motors work no differently than most other induction motors and only differ in the specific interaction between rotor and stator. Our article all about induction motors contains a discussion of the principal laws behind all induction motors and gives an understanding of how motion is created from magnetism.

Squirrel cage motors maximize electromagnetic induction by utilizing rotor bars to interact with the stator’s EMF. The stator usually contains windings of wire which carry an AC current; this current changes in sync with a sinusoidal curve (or “alternates”), which changes the current direction in the wire windings. When the current oscillates, the generated EMF will follow suit, and in certain arrangements will cause it to “rotate” with a frequency similar to the AC frequency.  This rotating EMF produces an opposing voltage and EMF in the rotor bars, thus pushing the rotor around, generating rotational motion.

This rotor does not spin at the exact frequency of the AC current and is why squirrel cage motors (as well as other induction motors) are considered asynchronous. There is always some loss, or “slip”, between the AC frequency and the rotational frequency of the shaft, and this is a consequence of why 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. The rotor must always be slower to feel the electromagnetic induction effect as if the rotor is playing a constant game of magnetic “catch-up”. To learn more, feel free to visit our article on the types of AC motors.

Squirrel Cage Motor Specifications

Our article all about induction motors explains the specifications for all types of induction motors and is a good place to see all the different induction motor characteristics. This article will focus on what needs to be specified for squirrel cage induction motors, which involves phase, speed, torque, and current. Since these motors are massively popular, NEMA and the IEC have made standardized classes of squirrel cage motors based on their speed-torque characteristics. This allows for interchangeable motors between manufacturers and simplifies motor replacement. These principles, as well as the various classes of standard squirrel cage motors, will be briefly explained below.

Phase type

Induction motors can be driven by a single-phase (one AC frequency) or poly phases (multiple AC frequencies) depending upon the input power supply. Some of the most common types of squirrel cage motor use three phases, meaning the input current is three identical AC frequencies, split by 120 degrees of phase. Three-phase motors are self-starting, meaning the only necessary input is a starting voltage, and makes these motors essentially plug-and-play. Single-phase motors are also common, but they are not self-starting and require some initial “shove”. This is because one AC frequency is not enough to create a truly “rotating” EMF, and some compensation must be done to simulate the rotating field. This can be done with starters, which can be capacitors, split phases, or other components. More information on starters can be read in our article on the types of motor starters.

Motor torque & the torque-speed curve

While squirrel cage motors operate at base speeds and torques, they need to reach this steady state through some transient start-up. This start-up, usually visualized through a torque-speed curve, is vital to know because it defines what kinds of operating conditions the motor can handle. Figure 1 below shows the important regions of the torque-speed curve for any induction motor.

Figure 1: Torque-speed curve for asynchronous motors, with important regions designated.

The starting torque is the torque upon starting the motor. The pullout or break-down torque is the peak torque achieved before maximum speed. The rated torque is the steady-state torque output and is what is usually provided on the motor’s nameplate. The difference between the synchronous speed and the speed reached at the rated torque defines the slip of the motor.

NEMA Classes for polyphase squirrel cage induction motors

Figure 2: Torque-speed curves for standard NEMA motor classes.

Table 1: Summarized characteristics of standard NEMA squirrel cage motors.

Figure 2 shows the curves for different NEMA classes of squirrel cage motors. There are four main classes (A, B, C, and D), though there are more depending upon specificity. These four classes are summarized in Table 1 in terms of their starting torque, current, and amount of slip. Other, non-standard squirrel cage motors exist but are usually built as-per buyer specifications.

Class A motors are the most popular type of squirrel cage motor. They have a normal starting torque and current, as well as a slip less than 5% of the synchronous speed. Common applications are fans, compressors, conveyors, or anything with low inertial loads that allows quick motor acceleration.

Class B motors can be started at full load, making them useful for high-inertia uses (large fans, centrifugal pumps, etc.). They have normal starting torque, lower starting current than class A motors, and have less than 5% slip at full load. These motors are sometimes interchangeable with Class A motors, especially when a reduced starting voltage is needed.

Class C motors have high starting torque and low starting current, thanks to their double-cage rotor design. They are more expensive than class A and B motors because of this improvement, but also possess the ability to handle high-starting torques such as those found in loaded pumps, compressors, crushers, etc. Their slip is also usually less than 5%.

Class D motors sport the highest starting torques, low starting current, and a large amount of slip at full load (between 5%-20% depending on application). Their pullout torque occurs at a much lower speed than the other motor classes, as can be seen by comparing the locations of each curve’s peak in Figure 2. The high rotor resistance which makes class D motors so strong is also responsible for its lower peak torque speed, sometimes causing the peak torque to occur at zero speed (100% slip). Common applications for Class D motors include bulldozers, foundry machines, punch presses, etc.

Applications and Selection Criteria

Squirrel cage induction motors are popular choices in industry, in part due to their low cost, ease of maintenance, high efficiency, good heat regulation, and safety. Their biggest downside is their lack of speed control, which is why other motors (wound rotor motors) have been developed to address these applications. NEMA’s standard frames make it easy to choose the right motor, only requiring the operating characteristic of the project.

So, for example, if a forging business is creating a new power hammer which must deliver fast and hard strikes, they should investigate class D motors as they provide extremely high starting torque. Similarly, if a motor is needed for a simple HVAC fan, class A and B motors will work great. Determine the necessary torques, speeds, and voltages for the job, and there is bound to be the right squirrel cage on the market.

Summary

This article presented an understanding of what squirrel cage induction motors are and how they work. For more information on related products, consult our other guides or visit the Thomas Supplier Discovery Platform to locate potential sources of supply or view details on specific products.

Sources:

1. https://geosci.uchicago.edu
2. http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/indmot.html
3. http://www.egr.unlv.edu/~eebag/Induction%20Motors.pdf
4. https://www.controleng.com/articles/what-to-consider-when-choosing-an-ac-induction-motor/
5. http://ocw.uniovi.es
6. http://people.ece.umn.edu/users/riaz/animations/sqmovies.html

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