Turning Archive

for Doc Green and the payoff

GaryG in MD
>>Gary, I've got a shiny $10 bill that says you can't explain in fundamental terms why it is that a simple single-phase induction motor will run if you give it an initial push.<<

OK, Doc, I’ve learned a lot about ac motors from my friend who is a EE with wide-ranging interests. Here's your answer and again as much. This must be worth at least a Jackson. See if this makes sense to you...

A more precise question is, if an induction motor is already turning (since you nudged it), beyond a certain speed why does it accelerate until it is turning at full speed?

The question would be much easier if it were asked about a “synchronous” motor, which has a permanent magnet for a rotor and which cannot speed up in this way; it must be externally spun at the right speed before it will “catch” and continue running. While it is running, when the applied AC field is of the right polarity, it will attract the magnet and accelerate it. A half cycle later, the field reverses electrically, the magnet reverses mechanically, and there is still an attractive force. In between, when the applied sine wave is near zero, mechanical inertia keeps the rotor spinning.

And now your answer!
In an induction motor, when it is turning slowly, the magnetic field as seen by the rotor is not at the line frequency, but at a frequency equal to the difference between the AC power and rotation rates. This difference frequency has a rotary component that generates eddy currents in the rotor. The eddy currents in turn induce a (precessing) virtual magnet in the rotor, which is what gets “cranked” by the applied sine wave, thereby accelerating the rotor.

As the rotor approaches synchronous speed, the difference frequency approaches zero, the eddy currents become smaller, and the virtual magnet gets weaker. At synchronous speed, the available torque from the motor is zero. That is why in practical applications an induction motor is designed to “slip” by a few percent in order to keep the virtual magnet energized.

Comments not crucial to the answer:

Typical motor speeds, assuming 60 Hz AC supply (3600/minute) and 3% slip:
2 pole: 97% of 3600 = 3492 rpm; 4 pole: 97% of 1800 = 1746 rpm
Speed varies, depending on the load on the motor. Typical specs are 3450 or 1725 rpm.

The effect can be enhanced by using soft iron for the armature, separated into poles by slits and placing into the slits copper rods that have their ends shorted together. The copper structure resembles an exercise wheel for animals, and is called a “squirrel cage”. The iron conducts the magnetic flux and the squirrel cage conducts the eddy currents. See: https://en.wikipedia.org/wiki/Squirrel-cage_rotor

Almost any metallic cylinder can be made to rotate in place of an armature; iron is not necessary, only electrical conductivity.

There is no specific CW or CCW component mentioned above. Single-phase simple motors easily run in either direction. Indeed, in some reversible applications, the rotational direction is set only at the start.

Three-phase motors are easier to understand than single-phase, because the three phases allow the production by the stator of a rotating magnetic field which drags the rotor around. Large induction and synchronous motors usually run on three-phase electricity.
Single-phase motors provide only a reciprocating magnetic field, which must be converted to rotary.

Steam engines also convert reciprocating into rotary motion, but the phasing of the valve gear determines the direction of rotation. Note that steam locomotive wheels are 90-degrees out of phase on the left and right or the train wouldn’t be able to start moving. See https://en.wikipedia.org/wiki/Dead_centre_(engineering)

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