jimmy said:
hi
Does anyone know of a good resource on the internet where i can find
information to answer the following question.
and yes i have tried search Engines.
Describe how three phase induction motor speed is controlled with the
use of a variable voltage, constant frequency method
cheers
In any induction motor, the field coils produce some approximation of
a rotating magnetic field that magnetizes and drags around the rotor.
To achieve variable speed at fixed frequency (of rotation of that
field) the rotor must fall behind the field rotation by various
amounts (variable slip). This is an inefficient process, but usable
torque can be produced, especially if the shorting bars in the rotor
are deeply buried in the iron of the rotor, and/or are of higher than
normal resistance. An induction motor optimized for this service is
called class D.
Explanation of Class A,B,C, and D induction motors:
http://alperucar.f2o.org/induction_motor/
Simple phase control circuits have been used to reduce the effective
voltage applied to small induction motors to achieve variable slip
speed control, but their efficiencies are terrible.
Now that fractional horsepower 3 phase variable frequency drives are
approaching $100 each, the variable slip method is rarely used.