# three phase induction motor

Discussion in 'Electronic Basics' started by jimmy, May 12, 2004.

1. ### jimmyGuest

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

2. ### John PopelishGuest

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.

3. ### Bill VajkGuest

Lowering the voltage allows for greater magnetic slippage, but
also generates a lot more heat and inefficiency. RPM is not
effectively/efficiently controlled in 3 phase ac motors by
reducing the voltage. Frequency control is the accepted
state of the art.

4. ### Don KellyGuest

Look at the speed torque curve of an induction motor- Normal operation is
between the peak torque and synchronous speed (slip =0). The operating point
is where the load speed torque crosses this curve. If the voltage is reduced
the peak torque is reduced approximately as the square of the voltage. The
slope of the speed torque curve becomes lower (slip increases at any given
torque) so that the intersection between this curve and the load curve
occurs at a lower speed. This approach provides very limited speed control
and isn't really very desirable. It is easy to stall if the voltage is too
low. It is often used with a small single phase fan motor such as on a stove
vent hood or a forced air flow for a fireplace because it is cheap.
"Dimmers" designed for this purpose allow setting of a minimum speed limit
so it is not possible to accidentally stall the motor (they also start the
motor at full voltage).

cheers

jimmyg

6. ### cornytheclownGuest

evidently you didnt type the search phrase "variable frequency drive" into google.

7. ### Rob PaisleyGuest

---------------------

This method is not a variable voltage although it could be controlled by one.

From Google: wound rotor motor "3 phase"

http://www.ecmweb.com/mag/electric_woundrotor_motors_high/

Rob.