At 10:12 PM 11/23/99 +0800, Alan Aldaba wrote:
>
>
>        In one of my readings, I came across an article about a circuit that
>they claim was originally developed by NASA . In this article, they
>stated that one way to keep power consumption down of electric motor
>(induction type) driven equipments is to try to improve their
>efficiency. Efficiency here means the ratio of the actual work performed
>by the motor to the applied power in the form of electricity.
>
This is a goal of all motor designers, manufacturers, and users.

>  The product of the voltage, current and the power factor represents the
>true power actually consumed by the motor to do the work, while the
>other power, i.e., the apparent power, is lost due to its conversion
>into heat by the windings of the motor. The closer the power factor to
>unity value, the more true power is delivered to the load and less is
>lost.
>
Clearly, unity power factor is better.
>        The mentioned circuit continuously monitors the power factor of the
>motor and adjusts the voltage accordingly down to a level where the
>power factor is nearly at unity level. This  is the highest power factor
>yielding the best results. Note that the voltage is being decreased, it
>is an accepted fact that when the voltage is decreased, the speed of the
>motor will slow down.

We have now departed from the facts.  In an induction motor, speed is
determined only by the frequency of the applied voltage, and not the
voltage.  Lowering the voltage will improve the power factor (somewhat),
but when lowered to the point that it would otherwise go above 1, the
motor will stop.  It is very touchy finding this point, since the load
may not be constant (or even linear).  From the users standpoint, the
gain in efficiency is small, and the cost relatively high.  For this
reason, induction motors rarely have their voltage changed.  If, on the
other hand, variable speed is needed, it is customary to provide a circuit
that varies the frequency *and the voltage at the same time*.  The
reason for this is to keep the current from getting out of hand.  (ie
keep the power factor within bounds.)

The issue is much more complex than appears on the surface.

Bob McClure