Understanding How Squirrel-Cage Rotor Bars Work

You may have heard of the different parts making up your typical electric motor – including the squirrel-cage rotor bars. However, wrapping your mind around these rotor bars entail a different matter.

First and foremost, the squirrel-cage rotor got its name from its structural similarity to a squirrel cage. But its functionality has no connection or whatsoever with how a squirrel cage works.

In fact, the functions of a squirrel cage rotor in the motor are limitless. From preventing power losses to providing motion – it boosts the efficiency of a motor just like the other essential components within.

And all these are possible because of the skewing of squirrel-cage rotor bars. So, let’s move on and broaden your horizon about how this particular part works in your motor.

What are squirrel-cage rotor bars?

Among many other motor types, the squirrel-cage induction motor is one of the most sought after motors because of its simple and rugged construction – boosting properties like low-cost and low maintenance.

A standard squirrel-cage is made up of a cylinder of steel laminations in the core, together with proportionately spaced bars of aluminium or copper placed axially all around the outer edge, shorted at the ends permanently by the end rings.

With a few adjustments made here and there to the shape of the rotor bars, the speed-torque characteristics can be altered – to maximize low-speed torque or minimize the starting current for instance.

Reasons why squirrel-cage rotor bars are skewed

Unlike some of the other motors out there, the squirrel-cage rotor’s assembly is slightly different. The rotor bars are either skewed or slanted and they are done so for many various reasons:

Prevent cogging from occurring

If the rotor conductors are straight, in other words, not parallel to each other – the possibility of strong coupling or magnetic locking between the stator and rotor is pretty high. Even if the full voltage is being applied to the motor, there’s no way of starting it.

Which is why skewing comes in handy as it prevents this phenomenon – cogging – from happening.

Improved starting torque

When skewing is done, the length of the rotor bars increases while the cross-sectional area reduces. And as the resistance of the conductor is dependent on both of these factors – an increased resistance will lead to an improved power factor of the rotor circuit. In turn, increase the starting torque of the motor.

Prevent crawling

When the induction motor runs at extremely low speed, it is known as crawling. And this happens as a result of harmonics, introducing oscillations in torque. Once the rotors are skewed, this negative outcome is avoided as they reduce the tooth harmonics effectively.

A reduction in magnetic hum

Skewed or slanted motors cause a drop in magnetic hum. When this happens, the chances of rotor locking decreases and the motor runs much quieter.

Signs of squirrel-cage rotor bars failure

Unfortunately, a failure in the squirrel-cage rotor bar is bound to happen. And at times, it would mean that its time for an electric motor overhauling to figure out what is the main reason behind the failure.

While it is an event that is unavoidable, there are still ways to restore the motor back to its original state or even prevent the malfunction from occurring.

Below, are some signs you should look out for to spot a failure:

Broken bars: Usually, the breaks happen within the slot section, where the end ring meets the bar, and when the bar goes into the core.

Excessive starting: A decrease in motor performance will happen sooner or later due to the number of uses and starts.

Ring rotors: Due to a welding failure, breaks within the rotor winding occur – resulting it to slip from its original position.

Long start-up time: Large inertial loads are usually the source for this problem, causing unnecessary strain on the motor – resulting in permanent damage.

Bearing failures: When an already taut belt is overloaded – bearing failure happens. Besides the mentioned issue, other reasons also include stresses and axial impacts.

In order for you to understand what is wrong with your motor, or how to improve it – you need to know each and every part of the equipment completely. While carbon brushes and commutators do play a part – so does the squirrel-cage rotor.

Know how it works and you’ll understand when it’s time to do an electric motor repair. But at the end of the day, these are just the basics of the squirrel-rotor bar. When none of the above-mentioned signs is spotted, it would be good to call for an electric motor service to find out the issue with your equipment.

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