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Air Conditioner amperage draw at low voltages
Mike Sokol:
So I just had another "expert" tell me that the amperage draw of an air conditioner compressor in an RV goes UP if the voltage is too low. Now it will certainly draw a lot more current at start up via the starter capacitor since it won't open the relay quickly. And I certainly understand about back EMF and how a low voltage on a AC-DC motor can lead to excessive current draw. But this is for a sealed air conditioner compressor with an induction motor that has a starter winding and capacitor. I think the current draw will go do as a direct function of reduced voltage, and the air conditioner will need to have a higher duty cycle to keep up with the demand. But what physics could explain current draw on an induction motor going UP when the voltage goes DOWN?
Tom Bourke:
If I recall, the lower voltage creates less torque. This leads to lower RPMs. Lower RPM's means the slip, difference between real RPM and ideal RPM at line frequency, is high. High slip creates less counter EMF. Less counter EMF leads to higher current.
The continuation of this is higher current leads to more heat and that burns out the motor.
Mike Sokol:
--- Quote from: Tom Bourke on September 22, 2018, 07:48:25 PM ---If I recall, the lower voltage creates less torque. This leads to lower RPMs. Lower RPM's means the slip, difference between real RPM and ideal RPM at line frequency, is high. High slip creates less counter EMF. Less counter EMF leads to higher current.
The continuation of this is higher current leads to more heat and that burns out the motor.
--- End quote ---
Sounds plausible, but does low voltage create an actual increase in current draw, or perhaps the current doesn't drop linearly as it would do for a resistive load. I need to see a voltage/amperage curve for an air conditioner compressor. The premise is that too low of a voltage will increase current to where it burns up extension cords. That is, a long extension cord will have enough voltage drop that the compressor will draw more current which will burn up the receptacle.
Stephen Swaffer:
Tom is on the mark. Think about it-the reason for the high start op surge is that a stopped motor presents a near short circuit as far as a load is concerned because the only thing impeding current flow is the DC resistance of the windings. At rated speed, the counter EMF liimits the current to the rated ampo draw. The counter EMF is the main impedance to current flow-you really aren't dealing with Ohm's law in this situation.
Loading a motor slows it down-leading to a greater amp draw. Lower voltage tends to make a motor stall or slow down even more.
The AC motor will have a relatively constant load-to get the same power at a lower voltage requires more amps-which leads to a greater voltage drop in the cord-so a snow ball effect.
People have a tendency to look at the ampacity of a wire to determine the size of the cord they need-but as is often discussed on here in audio applications, voltage drop is far more relevant at any distance-and the distance of concern is usually the distance from the transformer.
Mike Sokol:
--- Quote from: Stephen Swaffer on September 22, 2018, 09:49:29 PM ---Loading a motor slows it down-leading to a greater amp draw. Lower voltage tends to make a motor stall or slow down even more.
The AC motor will have a relatively constant load-to get the same power at a lower voltage requires more amps-which leads to a greater voltage drop in the cord-so a snow ball effect.
--- End quote ---
OK, so I've got that in my head right now. So I understand how an induction more draws more current as it's loaded more. Basically it's the flux slippage angle. As the rotor falls behind the rotating flux field it draws more current. If the motor was spun up externally to the same rotational speed and phase of the flux it should draw almost no current. Sound right?
So lets take the same motor with the same load (air conditioner compressor) and put a big Variac on it. As I change the motor voltage from 120 to 110 to 100 to 90 volts, what happens? Is there a linear curve of the motor drawing more and more current as the voltage drops? That is, will it draw maybe 10 amps at 120 volts, and 12 amps at 110 volts and 14 amps at 100 volts and 16 amps at 90 volts (or something to that effect?). I don't expect it to act like a resitive load, but is there some inverse function that will predict extra current draw at lower voltages if the motor is attempting to spin the same compressor load? This is independent of any starter capacitor effects....
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