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From: Amesbury, Kurt (no email)
Date: Tue Aug 24 1999 - 10:53:09 EDT
> I finally tracked down why my refrigerator was pulling too much current
>lasts summer - low voltage, the same as mentioned above. After some
>studying I finally understood that the motor was actually driven by ac run
>from a small inverter (Adlar Baurbor). If thedc voltage drops, it has to
>pull more amps to put out the same ac power. Cleaning up connections and
>increasing wire size has helped it a lot.
Gene,
You may have correctly identified the cause of the problem as low voltage,
but it might help to have a better understanding of reasons why the current
may increase.
There are two components to resistance: DC resistance and reactance.
Reactance is an apparent resistance caused by a varying flow of current
through a coil or capacitor. E=IR is pretty much immutable, but when
dealing with dynamic (AC) systems, you have to consider that the reactance
varies with frequency and treat R as being made up of three components: R
(DC resistance) + Xc (capacitive reactance) + Xl (inductive reactance). To
further complicate things, the physical dynamics of a motor may cause
changes in the load with time and the motor may have its own internal
frequency which is dependent quite a number of complicated factors including
magnetic coupling and reactance.
Without the specifics of the motor, and while avoiding the calculus that
goes along with a full analysis of what's happening in the circuit, suffice
to say that inductive reactance is low at low frequencies and high at high
frequencies. If the frequency of the inverter is reduced due to overloading
(which is entirely dependent upon the design of the inverter) or the motor
is running at less than the design speed, the motor may draw more current
than the design anticipated. You can in fact burn out a motor by giving it
too little power!
On a different, but related subject, the most efficient use of power in
applications that use large inductors (usually electric motors) requires
analysis of the load's Power Factor. For those of you who are familiar with
phasor analysis, (ooops! Just lost half my audience!) the reactive elements
lag and lead the DC resistance phasor. Since apparent resistance is the
vector sum the three, the shortest resultant vector occurs when inductive
and capactivie reactance cancel each other's phase angles leaving only their
DC resistive components. This would be MUCH easier to understand with
pictures. Sorry, but I don't think the list is set up to handle that!
So what does all this mean in PRACTICAL terms? Just this: If done correctly
(and I stress this) you can actually reduce the power required by inductive
loads by adding capacitance. The amount of power saved will be proportional
to the size of the inductive load.
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