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Author Topic: Air Conditioner amperage draw at low voltages  (Read 1370 times)

Mike Sokol

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Air Conditioner amperage draw at low voltages
« on: September 22, 2018, 07:21:37 pm »

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? 
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Tom Bourke

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Re: Air Conditioner amperage draw at low voltages
« Reply #1 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.

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Mike Sokol

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Re: Air Conditioner amperage draw at low voltages
« Reply #2 on: September 22, 2018, 08:17:41 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.

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.   
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Stephen Swaffer

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Re: Air Conditioner amperage draw at low voltages
« Reply #3 on: September 22, 2018, 09:49:29 pm »

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.
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Mike Sokol

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Re: Air Conditioner amperage draw at low voltages
« Reply #4 on: September 23, 2018, 10:07:52 am »

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.


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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Ron Hebbard

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Re: Air Conditioner amperage draw at low voltages
« Reply #5 on: September 23, 2018, 01:57:09 pm »

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?
  If your "expert" were correct then an applied voltage of 1 volt would draw an unbelievable Amperage and the current consumption at zero volts would be truly unbelievable.  I suspect there's a flaw in his reasoning. 
Toodleoo! 
Ron Hebbard
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Tom Bourke

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Re: Air Conditioner amperage draw at low voltages
« Reply #6 on: September 23, 2018, 02:37:59 pm »

f
  If your "expert" were correct then an applied voltage of 1 volt would draw an unbelievable Amperage and the current consumption at zero volts would be truly unbelievable.  I suspect there's a flaw in his reasoning. 
Toodleoo! 
Ron Hebbard
At some point the motor no longer has the torque to maintain rotation and will slow to a stop.  Depending on the motor dc resistance the current draw will drop as voltage drops. At some point in the voltage X current curve the heating effect will be safe for the motor and wire.
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Stephen Swaffer

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Re: Air Conditioner amperage draw at low voltages
« Reply #7 on: September 23, 2018, 03:28:33 pm »

Indeed.  Some motors are actually designed as "impedance protected"-if the bearings lock up even full voltage won't overheat it-it just basically turns into a heater.  Usually seen on small blower motors or cooling fans.

The exact relationship depends on the motor design-more efficient motors are more efficient because they have less slip, etc.  VFD's actualy vary foltage aand frequency to keep current within design limits.  Honestly, I have only read enough on motor theory to make me dangerouos beyond this point-enough to understand the practical aspects so I can set up VFD's for relatively simple applications-but I am thinking the volts/amps/speed relationship is more complex than a linear relationship and really requires specific motor design criteria. 

We usually look at voltage/phase/HP/amps/frame on a motor-but a nameplate has a lot more information for a reason-AC compressors will also spec LRA-"locked rotor amps"-the reason all that info is on a nameplate is that itells the reader about the motor-if the reader is smart enough to understand the information.
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Mike Sokol

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Re: Air Conditioner amperage draw at low voltages
« Reply #8 on: September 23, 2018, 06:35:24 pm »

I am thinking the volts/amps/speed relationship is more complex than a linear relationship and really requires specific motor design criteria.

The reason for this exercise is the knee-jerk response I get from many RV technicians who say that low voltage will increase current draw and cause an RV's TT-30 plug to burn up. Think you have problems with low voltage at a show? I'm getting field reports showing static voltage down around 93 volts for extended periods in the afternoon when hundreds of RV owners turn on their air conditioners.

I just don't believe the current draw will go UP when the voltage goes DOWN. My WAG is that is that the current may not go down linearly, but I don't think it will go UP (unless the air conditioner starter winding kicks in). In any event, if the current draw does go UP, there's still an upstream 30-amp circuit breaker to protect the 30-amp receptacle and cord-set wiring. That's why I believe there must be some other reason for the overheated connectors.
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Marc Sibilia

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Re: Air Conditioner amperage draw at low voltages
« Reply #9 on: September 23, 2018, 09:00:28 pm »

I just don't believe the current draw will go UP when the voltage goes DOWN.

The speed of the motor (before accounting for slip) is determined by the frequency, not the voltage of the AC.  The mechanical power to the load doesn't change much because the speed doesn't change much.


More here:
Autotransformers, anyone?

Marc
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