Voltage regulator

[Greg Hermann]

>David A. Cooley wrote:
>>
>> At 04:55 AM 1/2/99 -0600, you wrote:
>> >How about using a big pump and cutting the voltage when all that
>> >capacity is not required? Will the pump last longer, etc.???
>> >
>>
>> That would be almost as bad as overvoltage...  Under voltage can kill them
>> as well.
>> ===========================================================
>>            David Cooley N5XMT           Internet: N5XMT at bellsouth.net
>>      Packet: N5XMT at KQ4LO.#INT.NC.USA.NA   T.A.P.R. Member #7068
>>        I am Pentium of Borg...division is futile...you will be approximated.
>> ===========================================================
>Actually, no. An AC motor is definitely sensitive to low voltage, for
>various reasons.

An AC motor will make full torque below its rated speed if the
voltage/frequency ratio given to it is the same as at its rated speed. At
any given frequency and speed, the voltage which an AC motor needs to
produce a given fraction of its rated torque is the same proportion of its
rated voltage as the torque load is of the motor's rated torque. Feeding
the motor more than the required voltage when it is loaded to less than its
rated torque (as is commonly done) results in an efficiency loss due to an
increase in the IR loss in the windings.

A DC motor does NOT suffer this way. A 24 volt motor
>will run on 12 volts virtually forever, as long as the load placed on it
>does not cause it to overload and overheat, or most dangerous, stall.

The current/load/speed/torque characteristics for a shunt wound DC motor
were outlined in an earlier post.

A series wound DC motor's characteristic is a hyperbolic curve: very high
torque at low speed, very low torque at very high speed. As long as your
particular torque/speed requirement is not above this curve, it will do the
job. If you overload it with too much torque, it will slow down, or even
stall, trying to meet the load. Stalling IS NOT detrimental to a series
wound motor so long as it has enough cooling to dissipate the heat produced
by the IR loss in its windings by the applied current (which, of course, is
a function of the applied voltage). Said cooling can be produced by any
combination of the following, even if the motor is stalled: adequate frame
size, submersion in a liquid, or a separately driven cooling fan.
Naturally, a cooling fan driven by the motor shaft will not give any
cooling if the motor is stalled.

Consider the following: diesel-electric railroad locomotives use DC
traction motors. they have switchable windings, but in starting to move a
train from rest, the traction motors are put in a series wound
configuration. And they are most certainly asked to produce a VERY high
torque while stalled, and then while running at a VERY low speed!! (At
higher speeds, locomotive motors are switched through various combinations
of series/shunt and into full shunt at high speeds.)

The moral of this story is that ADEQUATE HEAT DISSIPATION of IR losses in
the windings and eddy current losses in the core is the key to making any
electric motor live a long, happy life. (As long as you do not act like a
racer, and spin it fast enough to make its windings fly apart!!)  8-)

>Using PWM control is better yet, as it does NOT reduce the peak
>voltage/torque at all - it just modulates it. PWM control on a DC type
>motor reduces speed without APPRECIABLY reducing torque.

To claim that there is not enough inductance in the motor windings to
smooth out the highs and lows of the PWM voltage notches is living in
dreamland or a sign of having ingested too much egg-nog!!

Regards, Greg