Re: How is Back EMF generated in a typical induction motor?

Hello Ian: Very good question. The induction motor operates in a like 
manner as the synchronous machine. The difference is that the rotor 
windings are short-circuits and the flux fields generated by the stator 
poles create their magnetism. Two major differences from DC or synchronous 
ac machines: induction motors normally operate with some degree of slippage 
(depending on the load), and also have relatively high starting currents 
(until the rotor spins fast enough to produce the counter emf and limit the 
stator current). Typical starting currents are five to seven times greater 
than that of normal operation. Also the initial torque is much lower than 
that of a synchronous machine. However, they are fairly cheap to 
manufacture, reliable, and can stand a lot of overload abuse (that creates 
slippage and reduces the counter emf) by an integral heat activated switch 
power disconnect. Induction motors can be operated from single or 
multi-phase electric supply. Small motors (less than 1 horsepower) are 
usually single phase and require a capacitor start circuit to initiate 
rotation. This capacitor creates a leading effect between voltage and 
current that permits rotation to start (the rotor begins chasing itself 
around). The capacitor may be left in circuit permanently or switched off 
by a centrifugal (mechanical) switch. A second trick in starting 
single-phase motors is to use "shaded pole" construction. Shaded pole 
machines have very low starting torque and therefore limited use. Ref 1 
below discusses the shaded pole construction. Small single-phase induction 
motors are commonly found in household items and light industry. Larger 
multiphase induction motors are typically called squirrel cage by their 
design. They are constructed to prevent large inflow of starting current. 
Refs 4, 5, and 6 give examples of this type of motor construction.

Ref 1: 

http://www.britannica.com/bcom/eb/article/4/0,5716,108542+1,00.html

Ref 2: 
< href="http://www.isa.org/journals/mc/feature/1,1773,160,00.html">http://www
.isa.org/journals/mc/feature/1,1773,160,00.html

Ref 3: 
http://powerlearn.ec
e.vt.edu/modules/index.html

Ref 4: 
http:
//www.postglover.com/ac_squirrel_cage_induction_motor.htm

Ref 5: 
http://www.eece.ksu.
edu/~starret/589/man/I.html

Ref 6: 
http://www.el
ec-toolbox.com/Formulas/Motor/mtrform.htm

Ref 7: 
 
http://newton.ruph.cornell.edu/p101-102/p102/13/concepts/index.html

If you have additional questions concerning the subject, please direct them 
to me through the MAD.SCI folks. Your MAD.SCI Micro.