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Re-activating compressors after power outage.

Thread starter #1
I was told by an electrician who works nearby that if an appliance which comprises a compressor (like refigerator, air-conditioner) is shut down (due to a sudden power outage for example), then it must not be re-activated during a ceratin period of time that follows the power outage.

He said that it would mainly damage the timer which the appliance comprises.

Why is that?
Does the compressor contains that much energy which takes that much time to dissipate?

Thanks fellas.
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Most Helpful Member
Basically the compressor has built up head pressure on its outlet that has to balance out with the inlet first.

The starting torque of typical compressor motors is rather low. They normally start up with the inlet and outlet pressures equal which takes very little torque to get them up to speed. Once up to speed the momentum of the rotation parts works as a flywheel and balances out the high torque needed during the compression stroke so the overall running torque is more uniform.

I am not sure what he means by damaging the timer though. The whole purpose of the timer is to reset to a delayed on state every time the thermostat shuts off the compressor or power is interupted and thus gives it a few minutes of off time so that the pressures can reballance balance (if there is any) before applying power to the compressor motor again.

Many older or cheaper units just use a thermal circuit breaker that trips if too much starting current is drawn for too long. By the time the thermal circuit breaker cools off and resets itself the compressor pressures have equalized and the motor can restart.


Well-Known Member
The torque required to drive a positive displacement compressor is generally proportional to the pressure that it pumping against. Some induction motors, especially capacitor-run single phase ones, have a much lower start torque than maximum running torque.

What that means is that some compressors can't restart until the pressure drops. On air-conditioning compressors, it will take some time for the pressures to equalise from the hot (high pressure) and cold (low pressure) sides. If the compressor is turned on before the pressure has equalised, the compressor doesn't start and the motor will stall. In small fridges and some aircon systems, the motor overheat cutout operates. By the time the motor has cooled down. the pressure has equalised and the compressor starts OK.

Some electronically controlled systems have timers so that the motor won't overheat.

Small cooling systems have a fixed throttle, and it takes a few minutes to equalise. Larger systems have an expansion valve, and I don't know whether an expansion valve would speed up or slow down equalisation.

On air compressors, there is a similar problem. Air in the reservoir can give to much back pressure. That can be solved by having a small volume between the compressor and the reservoir, and releasing the pressure in that when the compressor stops. The compressor then has no back pressure as it starts. However, some small compressors have mechanical pressure switches that also operate the decompressor. If power is lost, the compressor isn't decompressed so it can't restart immediately. Eventually, the pressure in the small volume leaks away and the compressor can start.

I can't see how a timer could be damaged.

PS I was doing other things when writing this and Tcmtech beat me to it.
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Yup, the previous posters have nailed it.

Basically a compressor normally cycles on and off, with fairly long "off" periods before a restart.

During these long off periods, the pressure at the compressor outlet bleeds away, meaning that when the compressor next starts up, it will (initially) not be pumping against any back pressure.

If you switch off the power while the compressor is running, than switch the power back on quickly, the compressor is forced to start up against the full discharge pressure that has built up. This creates a massive load at start-up, and can overload the motor, and what switches power to the motor.
Thread starter #5
Wow, thank you so much guys. :)

You really explained it well!

How much time does it usually take a compressor to discharge the built-up pressure?
Like, what magnitude of order are we talking about?
Seconds or minutes?

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