# Explanation: Something missing??

#### nsaspook

Joined Aug 27, 2009
11,290
I don't see much of a problem as they didn't try to actually explain how a AC motor works. Always think, energy.

Opposed magnetic fields (at least one has to be electromagnetic) drive the motor. At unloaded startup we have a static condition of a rotating (a phase shift between fields by some means electrical or mechanical) magnetic field but non-rotating magnets. Energy flows into the mass of the motor shift via the Lorentz Force. The motor shaft follows the path of least action, that means it wants to move in a way that minimizes the energy on that path.

Last edited:

Joined Jul 18, 2013
27,043
At the point of switch the squirrel cage induction motor is essentially a transformer with a shorted turns secondary's, this effect of the rotor was not really covered or discussed at all, IMO.
Re: "why is the motor start up current so high" !!

#### nsaspook

Joined Aug 27, 2009
11,290
At the point of switch the squirrel cage induction motor is essentially a transformer with a shorted turns secondary's, this effect of the rotor was not really covered or discussed at all, IMO.
Re: "why is the motor start up current so high" !!
The shorted secondary transformer is an imprecise analogy IMO. The electrical response (high current) is essentially the same but the EM reasons why are different with a AC motor vs a transformer.

Joined Jul 18, 2013
27,043
Just compare what happens when the rotor bars become disconnected, result, a 1ph or 3ph very low current inductance/choke . i.e. no motion.
It happened fairly frequently when motors had soldered copper squirrel cage.
I never saw any reference to the rotor effect at switch on, in spite of the title.
I assumed it was aimed at those with limited knowledge/experience with such motors?

Last edited:

#### MisterBill2

Joined Jan 23, 2018
14,722
The article is certainly well written but quite technically deficient . Was it written by that AI program, I wonder. Certainly the author was not familiar with induction motor operation.
A very simple explanation is that motor current is limited by the counter-EMF, which is generated by the rotation, and at startup there is no rotation, thus no counter EMF, thus no current limiting. (This explanation is correct but does not explain much.)