I just bought a small freezer and tested the current requirements so I would
know how to use it with my generator, should it come to that.
I unplugged it and let it sit for a while. Then plugged it in. It drew
4.9a for a second and dropped to 1.2a; about what I expected. Just to be
sure, I unplugged it and plugged it back in. It spiked at 19a, then dropped
to 4.9a for a second and rand at 1.2a. The 19a spike alarmed me, especially
after I recalled that air conditioners have a built in delay to prevent them
from being damaged when turned off and then on immediately. I waited a
couple hours and then tested it again, more carefully. It was like the
first time, no spike.
It seems to be running normally. What was the spike about? Why are A/C
damaged by being turned off and on?
Is this anything to be concerned about? We have a few second or two outages
every year; if they are going to damage the freezer I ought to pick up the
extended warranty before it is too late.
Not a big deal and it won't happen often in the scheme of life. If you start
a refrigeration system the motor starts the compressor and the compressor
starts building pressure on one side of the system. If you stop it and
don't let the pressure equalize,, the motor has a much harder time turning
the compressor against the high pressure in the system.
AC's have a built in method to prevent this because a lot of idiots are
going to turn them off and on either by the switch or thermostat. Most
people, including the idiots, just let refrigerators and freezers cycle
normally and this spike does not happen.
You car AC is subject to the same thing, but you have a 200 HP engine to get
it started again so you don't see the lights dim.
I'm not an expert, but it could be because you didn't allow enough time for
the pressure on the high side to come down which causes the compressor to
work harder when starting. Same reason why you need to wait a few minutes
before turning your central A/C back on.
If a freezer, refrigerator or air conditioner is turned off when the
compressor is running and then turned back on, the motor tries to start
pushing against the very high pressure of the compressed refrigerant.
If you start the motor a few minutes later, the pressure has a chance to go
down as the refrigerant flows into the evaporator. As you say, compressor
motors are usually protected against stalling and overheating in the
high-pressure restart situation by thermal overload devices. If you pull
the plug of your freezer or fridge while the motor is running and then plug
it back in, that overload will probably be triggered and you will hear the
clicking of the relay and the humming of the motor as it tries to restart.
The current spike is due to the "locked rotor" current of the compressor
motor as it either tries to start or the overload device cycles.
Locked rotor situations should be avoided because of wear and tear on the
system, but they are normal and do happen. You could protect your emergency
generator from the current surges such things cause by building in a delayed
start in the case of a power failure, but that kind of defeats one of the
reasons for the generator.
Is it possible that this could open the high pressure switch, or some
other protective switch, when it happens. I have a friend whose home
AC wouldn't work, at the very end of last summer. I pushed the red
button in, in the condenser cabinet, and it started to work again, but
it was too late in the year to really test it. (He ran it for 10
minutes or a couple hours and that was the last hot day.)
Regardless, I'm wondering what could have made the high pressure
It's starting against head pressure. If the pressure is too great the
compressor clicks off on overload until the head pressure bleeds off
enough for it to start. You can see why this could eventually cause damage.
#1 Offishul Ruiner of Usenet, March 2007
#1 Usenet Asshole, March 2007
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