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All of the air conditioners I've worked with have the following words on them:

Before restarting, wait three minutes.

In the event the air conditioner's compressor is switched off and back on too quickly, the compressor motor stalls with a characteristic humming noise instead of running, and either a PTC trips to stop the compressor or the circuit breaker trips altogether. The same thing happens when the same is done on a refrigerator (and by extension, any device which uses vapor-compression refrigeration).

Why do refrigeration compressors stall when switched off and on quickly?

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Can someone explain the downvote? This question is asking for a technical explanation of this phenomenon. I don't have 250 rep here, so I can't see any close votes. –  DragonLord Jun 18 at 21:41
    
This question does not seem to belong to this stack exchange, that is all about electronic design and such. –  Vladimir Cravero Jun 18 at 22:16
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@VladimirCravero: It's rather borderline, and Home Improvement seemed to be the wrong place for this question, so I'm seeking further community input. See meta.electronics.stackexchange.com/questions/3733/… –  DragonLord Jun 18 at 22:24
    
    
+1 just for those close votes and that downvote. –  hkBattousai Jun 19 at 18:29
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3 Answers 3

up vote 18 down vote accepted

The compressor compresses coolant on one side of a closed loop. If you shut off the compressor, you still have the load side of the closed loop full of pressurized coolant. That pressurized coolant makes it much more difficult to start the motor. A motor starting at 0 RPMs will want to draw large amounts of current. With an added load to the motor (pressurized coolant) the motor will draw excessive current and won't turn over.

Compressors are likely leaky and therefore will allow the pressurized side to slowly decrease pressure until it's equal pressure between tho two sides. If you wait 3 minutes, it's expected that the pressures balance out and you have virtually no load when you try and start the motor again.

A compressor running at speed has one side of the closed loop pressurized and so is under load, but in that case, it already has momentum to keep it going. Also, at speed the motor doesn't need as much current to continue spinning.

Here's a graph depicting induction motor torque and current vs speed to help illustrate why this happens.

enter image description here

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The "leak" is built into the system -- it's the expansion valve through which the coolant expands in order to reduce its pressure and temperature. –  Dave Tweed Jun 18 at 22:34
    
@DaveTweed Interesting, I thought the expansion valve required a certain pressure differential to let anything through. Are you saying it's designed different than that? –  horta Jun 19 at 1:06
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Yes. It's just a simple fixed aperture. Nothing fancy, ultra reliable. –  Dave Tweed Jun 19 at 2:03
    
The speed/current graph is for a DC motor. Refrigerator compressors use induction motors, and the graph looks different for those. (A DC motor actually produces the most torque at standstill, so a compressor driven by a DC motor would not stall like this.) –  ntoskrnl Jun 19 at 14:26
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@ntoskrnl Swapped the graph out for an induction motor graph. It definitely makes the problem even more obvious. –  horta Jun 19 at 18:22
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The answers regarding built-up pressure are correct, but there's another aspect which hasn't yet been mentioned. In order for an induction motor to produce torque, it must have within it a magnetic field which is rotating at a particular speed (called the synchronous speed). Assume a particular motor is set up to run at a synchronous speed of 600rpm from 60Hz current. The magnetic field will then have six north poles and six south poles in a circle. When the "hot" wire is positive, the coils will try to drive the magnetic field so that the north poles are at the 12, 2, 4, 6, 8, and 10 o'clock positions, while the south poles are at 1, 3, 5, 7, 9, and 11 o'clock. When the "hot" wire is negative, the coils will try to drive the field so the poles are the opposite. If the motor is turning clockwise at slightly under 600rpm and a particular pole was at the 3 o'clock position at some point of time, then 1/120 second later that pole will be almost to the 4 o'clock position and the motor coils will try to pull it the rest of the way. If the motor was spinning counter-clockwise, then a pole which was at 3 o'clock at some point would be almost to the 2 o'clock position when the coils try to pull it the rest of the way. Note that the coils don't care which way the motor is turning--they rely upon its momentum for that.

To start such a motor, it's necessary to arrange things so that rather than simply going between two active positions, it goes between three or four. Typically this may be done by adding a capacitor and additional coils, so that on one line phase the motor will initially be pulled toward 12:00, 2:00, etc. but then soon thereafter to 12:10, 2:10, etc. Then on the next phase it will be pulled toward 1:00, 3:00, etc. followed by 1:10, 3:10, etc. Since 12:10 is a little closer to 1:00 than 11:00, the phase which tries to pull toward even numbers will apply a little clockwise torque. This amount of torque will be much smaller, however, than what could be produced if the motor were already spinning at a significant speed.

DC brush motors driven with a given voltage will produce maximum torque when they are starting or stalled. Likewise with AC induction motors which are driven with multiple "strong" phases. Most compressor motors powered by house current, however, produce near-zero torque at near-zero speeds. When there is no back pressure, the motors don't need to produce much torque to start moving; once they're moving, back pressure will increase, but so will their ability to produce torque. Shortly after a compressor is stopped, however, it will be unable to produce significant torque (since it's not turning) but will be unable to move without producing significant torque (because of the pre-existing back pressure).

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You need the time delay for the load to die away sufficiently so that the starting torque on the motor is reduced. This doesn't happen if the motor is 3 phase as per some big trucks. Neither does it happen on diesel engine driven compressors.

It tends to happen on single phase induction motors that use a capacitor starter - if the motor armature doesn't immediately start making progress thru an angle of 90 degrees (to match the capacitor/2nd coil phase angle), the motor armature will return to the rest position and then try again and fail to reach the 90 degree point. This repeats until you turn the motor off and wait for 3 minutes (or longer) for the compression force/load to die away somewhat.

If the compressor retains its pressure indefinitely then the motor won't restart but, I believe compressors will be a bit leaky.

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Pressure coming out of the compressor does not die away immediately because it is a sealed system. Low pressure converts to high pressure out. While ever a differential exists it is the electric motor that at start up, will see the biggeststalling force. –  Andy aka Jun 18 at 22:07
    
This does happen with three phase induction motors as well to an extent, but the effect is less pronounced than with single phase motors that are barely adequately sized. Diesel engines produce zero torque at standstill, so they usually have a DC starter motor. :) –  ntoskrnl Jun 19 at 14:42
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