How Does an Induction Motor Start?

You might remember clipping cards to your bicycle forks so they would make a motor-like sound flapping against the wheel spokes.

Now, instead of radial spokes, imagine a circular array of "spokes" as in a squirrel cage. Outside of this imagine a rotating drum with cards attached to it, pointing inward to engage the squirrel cage. This in turn makes the inner cage rotate. Until the cage catches up to the speed of the drum, the cards go floppity-flop, like the bike and also like slip in an induction motor. If the cage matches the speed of the drum, the cards no longer flop; this is like a synchronous motor.

In an electric motor, the three phases set up a rotating magnetic field in the stator (= outer drum). The poles of this field are like the cards, reaching into the rotor cage to turn it.

How's that? or ?

http://www.edumedia-sciences.com/en/a500-rotating-magnetic-field

Check the "vector addition" box for a good understanding

Good site! It's too bad they didn't animate the solenoids to follow the arrows; that would have helped to give a better mechanical flavor that the OP hoped for. (Maybe the $6 download would activate that feature?) And maybe Yesyen can fix that.

They do in a way actually ! Just check the three "Coil" boxes. A cute way to show the increasing, decreasing and reversing fields

Thank you Tornado and kvsridhar.

Tornado, you gave me a very good explanation and I can finally understand what is meant by a "rotating magnetic field", I didn't realize before that it's created by the 3 phases of the AC current but you brought it to life for me with your clever description.

Thank you so much!

I love the internet! It's the best!

Here's one of the basic "squirrel cage" concept. Unfortunately it doesn't include a graphic of the magnetic fields. I know I've seen one with that too, I'll look some more later in the bookmark files of my work PC.

But looking at this, think of each of those coils being energized by a rising and falling AC sine wave of energy. When energized, they "induce" a current on the cage as well, and the bars in that cage then exhibit their own magnetic field, and it opposes the field of the adjacent coil, so the cage is repelled from that coil towards the next one, and so on and so forth.

Single phase motors are a bit more complex, because for them the coil fields are not "rotating" per se since there are only two, not three. So additional "tricks" are used in the magnetics to BEGIN rotation in the proper direction.

Thank you JRaef for your animation and your description which I'm starting to understand with your help here.

I would appreciate it if you could help a bit more and explain what you mean by "additional "tricks" are used in the magnetics to BEGIN rotation in the proper direction."

If the rotating magnetic field can only rotate in one direction, does it not follow that the rotor can then only be "induced" in that same direction?

Unfortunately, this is one that doesn't have a really good mechanical analogue. However, if you understand the concept of a rotating magnetic field in the motor stator, you're half-way there. The induction motor works by interaction of the rotating stator field and separate magnetic field created in the rotor.

A "squirrel cage" rotor consists of a series of heavy copper bars which are connected together on the ends by shorting rings. As the stator field rotates, it generates a voltage in the rotor bars. Because the rotor bars are connected together on the ends as path exists for current to flow in the bars. The current creates a magnetic field on the rotor. The interaction between the stator and rotor fields causes the rotor to be pulled along by the stator field. The best mechanical analogue would be friction, which is usually viewed from the opposite perspective of a drag force imposed on a moving object by a surface. There is an equal and opposite motive force imposed on the stationary object which tries to pull it along with the moving object.

The rotor voltage is proportional to the strength of the stator field and the speed differential between the stator field and the rotor bars. When the motor is first energized the rotor is not moving, so the speed differential (and consequently the rotor voltage and current) is at maximum. The rotor field strength is proportional to the rotor current. Thus, the rotor field at startup is huge. The rotor field is strongly attracted to the rotating stator field (just like a pair of permanent magnets). The stator field simply "drags" the rotor field around with it. provides excellent starting torque to get the rotor moving, even under load. As the rotor picks up speed, the speed difference between the rotating stator field and the rotor bars decreases, reducing the rotor voltage, current and field until a happy medium is reached where the generated torque exactly equals the counter-torque of the load. If the rotor were to reach the same speed as the stator field, the differential would be zero, so there would be no voltage/current in the rotor to generate torque. In order to operate, the rotor of an induction motor must be slower than the rotating stator field.

There are additional, more complex interactions between the stator & rotor, but these are not applicable to answering the question you asked and are really only of concern for motor design or in-depth analysis of motor problems.

Actually there is sort of a mechanical analog, with a little imagination.

If you think of the cylinders as the coil windings, the magnetic fields as the power strokes and the distributor firing as the phase sequence.

I agree that's a very good description for the stator field, but I can't see how it describes the rotor field and its relationship to the stator field. Friction was the only mechanical principle I could think of where one object exerts a direct force on a second object, and the strength of the applied force is proportional to the speed differential between the objects.

OK, a LOT of imagination...

But I see your points. Electromagnetism is a unique physics phenomenon that is not directly translatable to other mechanical forces. I was just trying to get close.

Thank you pwr2thepeople.

That too has helped me a lot so thank you for taking the time to expand on it, I'm really very grateful

Can I ask you all, is this what Nikola Tesla invented way back in the 1890s?

I'm reading the story of his life just now and in the book they say he invented the first AC motor with a "rotating magnetic field" so hence my original question, and if that is one and the same invention that we are speaking of here, that was a pretty remarkable achievement for the time, wasn't it?

I'm the OP and I want to thank you all sincerely for helping me understand what for me was a difficult concept to grasp but thanks to all your very good explanations I've now got a good idea of how it works and I must say I think it's a marvelous concept and it's made me more appreciative of what I don't know of the many things that make up our fascinating natural and man-made world.

Thank you, thank you, thank you all.

On the contrary, OP, we thank you. You are one of a small percentage of good people who give us much happiness when we do our bit to help. May your tribe increase.

May i request you to register and become a member of this excellent community of experts? Thanks.