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Transfers Switch Question
We have been traveling across Kansas today, 95*+. Ran generator all day with A/C on. When we pulled in for the day, I turn off the A/C, plug up the shore power and shut off the generator. Turn back on the A/C. Is this proper and acceptable? What happens if I just plug into shore power with the A/C running? Is it ok to turn on the shore power with the generator running, no load, just cooling down? Normally I just shut off the Air, cool generator for a few minutes, turn it off, then connect shore power and re-start the Air.
Thanks for your thoughts,
Roy
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5 Replies
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I don’t think you’re doing anything wrong Roy, but I always unload the A/C’s (push the temps up until they shut down) then kill the gen and connect shore power and turn A/C’s back on when shore power Is stable. Not saying that’s for everyone, just the way I do it.
I’ve seen shops just yank the power off the coach while everything is running and I think that’s careless.
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I agree with Tom. You are doing fine because the AC motors are not running when you switch. Unloading an AC before you turn it off is even better.
Pulling the plug or switching supplies while motors are running can lead to disaster. The reason is motors have inductance which cause large voltage spikes when the electric current abruptly changes. In essence, the coils in the motor are fighting any change in current. Look up “back EMF” if you are curious. The size of the voltage spike is proportional to how quickly the current changes (dI/dt). You can easily have spikes of up to 500-1000V.
Now, AC current surges back and forth and goes through zero amps 120 times a second but it also goes to max value 120 times a second. So, the question is, “Do you feel lucky?”. If you happen to turn the current off while it is close to zero, you’ll be ok; if you turn it off near its positive or negative max, you’ll most likely have damage. There was just a recent thread where an inverter blew because someone inadvertently switched from generator to shore power or vice versa while the ACs were running. In this case the inverter could not handle the voltage spike.
Turning off resistive loads is less of an issue. There is no voltage spike. You may create an arc but next time the current goes to zero the arc will extinguish. You’ll just be eroding the contact over time.
Probably too much info…
Gerald
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Excellent information from Gerald.
The best thing you can do is get in the habit of never throwing a switch, plugging in, or going from one source to another when there is a load. Period.
In that way you are protecting everything and even if some loads are less harmful than others you don’t have to figure out which are good loads or bad.
This leads into energy management issues and how failure to understand limits and what happens if they are exceeded can have serious consequences. Shore power is the usual limiting power supply and if it is exceeded the breaker is going to trip. Sometimes we think we are OK if our legs are sitting at 40 amps as an example, but it is easy to forget we have some hidden loads over which we have no control. A HW tank, water pump, or aux compressor are all examples of loads we cannot control and if we trip a shore power breaker the first thing we should do before we go out and flip the breaker back on is shut down our loads. If we just go out and flip the shore power breaker we can create issues Gerald describes. Autostarting a generator when shore power is disrupted can have the same effect. Power managment is pretty important.
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Like button ! Good info from the Guru’s ! Thanks
Safe travels,
Rocky -
Great information guys, thank you.
Roy
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