3-Phase Induction Motor 4

[kingtutley]

Ok, here is another newbie question.

I understand the physics behind the induction motor and the electromechanics, but what I don't get is the slip.

I know the math behind it, but I don't understand how it the rotor can be running slower than the rotating field.

Here's what I mean: I have a motor starting under no-load condition. The 3-phase rotating field induces the voltage on the rotor and the rotor begins to turn from bring push/pulled, right? Ok. Now when the rotor reaches full speed, it is said to be running slower than the stator field rotation. How is this possible? If the rotor is running slower than the field it would eventually stall wouldn't it -- like a synchronous motor being dragged beyond the torque limit? I understand that there should be a phase angle difference, but it seems to me that it has to be running at the same speed as the field in order to keep running.

So, what am I missing?

 

[electricpete]

So, the behvior of the induction motor mimics the synchronous motor in that beyond a certain point of loading, the motror stalls quickly.

I assume the stalled induction motor rotor will experience very large currents due to the continuing stator field (assuming motor protection failed for whatever reason) and would eventually -- probably quickly -- exceed its thermal limit and begin breaking down the conductor insulation.

I agree with that.

For a wound rotor motor, there is a set of windings per phase, right? If it is a 4-pole motor (2-pole pair) there would be 2 sets of windings (2 per phase) on the Stator, right? Does the number of windings increase on the rotor also when increasing the number of poles?

A wound rotor winding resembles the stator winding. It is wound for the same number of poles, and typically has three phases. I believe that strictly speaking the rotor could be wound for any number of phases (2,3,4,5,6 7, etc) as long as the number of poles is correct, although I have never heard anything other than three phase rotor winding.

For a squirrel cage motor, is there a similar change in the number of rods used in the rotor or is it a fixed number? Since they are all shorted at the ends, I would tend to think it is some fixed quantity.

Number of rotor bars is chosen based on a number of factors, including avoiding rotor/stator slot combinations which produce unfavorable harmonic effects. It would never be the same as the number of stator slots - that would produce tremendous harmonioc torque variation as the bars went into and out of alignment all at the same time.


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[electricpete]

Perhaps worthwhile to mention again that induction motor breakdown and sync motor pole slip are two very different phenomena. Exceeding breakdown torque might cause thermal damage if for too long. Pole slip will cause mechanical damage from very high transient currents and magnetic forces.

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[electricpete]

Number of rotor bars is chosen based on a number of factors, including avoiding rotor/stator slot combinations which produce unfavorable harmonic effects. It would never be the same as the number of stator slots - that would produce tremendous harmonioc torque variation as the bars went into and out of alignment all at the same time.

As a further clarification, the number of rotor bars is unrelated to the number of poles, stator phases, and stator slots (except that unfavorable slot/bar combinations are avoided).

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[kingtutley]

Thank you all. I have gained a much better understanding.

 

[Sparweb]

the number of rotor bars is unrelated to the number of poles, stator phases, and stator slots (except that unfavorable slot/bar combinations are avoided).

Would they use something like prime numbers, or favour a number of bars with higher common denominators with the number of stator slots?

Steven Fahey, CET

 

[sreid]

It turns out to be prime numbers. You don't want the number of rotor bars to be related to the number of stator poles and since there are always an even number of poles, even numbers of rotor bars are out. 6 pole motors you don't want three, 10 pole motors you don't want five. You also want a "Large" number of bars so the additive magnetic field from the bars is smooth and sine shaped but you don't want to many bars for practical mechanical and electrical reasons so the number of bars tends to be 13, 17, 19, 23, etc.