Home Repair forum - It sorta worked...

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It sorta worked...

Toller

2007-06-14, 3:25 am

"Dave Martindale" <davem@cs.ubc.ca> wrote in message
news:f4q9lt$3d4$1@swain.cs.ubc.ca...
> "Toller" <Toller@Yahoo.com> writes:
>
>
>
> What matters is how the different appliances react to low voltage.
>
> The shop vac has a universal motor, which will draw less current on
> lower voltage and run slower. It's driving a fan, which is almost zero
> load at startup from zero speed. So if the generator's voltage droops
> under load, the shop vac will just take a while longer to get up to
> speed.
>
> The refrigerator uses an induction motor, which draws more current when
> voltage drops. It's probably driving a piston pump, which should be
> easy to start if the fridge has been off for a while and pressures have
> equalized, but not as easy as a fan. So you may find that drooping
> voltage under overload means the refrigerator won't start at all, and
> either the motor or the generator will eventually trip on overload.
>
Since people were actually interested, I tried it; leaving 5 minutes between
trys.
(Yamaha claims 3500w for 3 seconds, and then 1000w for 20 minutes, finally
900w indefinitely)
Wattages are per the meter on the transfer switch, so they are approximate.
When I tested on commercial power the refrigerator drew 13a for about a
second, and then 1a to run.

The first time it went to 1500w for about 5 seconds and the generator
tripped out. I reset the generator.
The second time it went to 1500w for a second and then dropped to 100w.
Normal operation.
The third time it went to 1500w for 3 seconds and I flipped the transfer
switch off, since it seemed to be like the first time and I didn't want to
go upstairs to reset the generator.
The fourth time was like the second; all normal.

I have have tried it maybe 3 times on commerical power and it has never held
at 13a like 1 & 3.
So the question is:
Does the generator somehow cause it to sometimes take longer to start up, or
is that a refrigerator issue that I just never happened to see before. I
don't know enough about refrigerators to know.
But, as I understand it, inverter generators hold the 120v; when they can't
do it, they trip out. So, they only give 120v or 0v, never low voltage.

I hope never to use this on the refrigerator; I have a Honda EU2000 that I
have used in 3 power outages. I bought this for another purpose where the
EU2000 was too heavy, but liked the idea of having it as a backup for the
backup.
Hey, I hope never to use the EU2000 either; in the 2 years since I put a
transfer switch in we haven't had an outage.

Dave Martindale

2007-06-14, 9:25 pm

"Toller" <Toller@Yahoo.com> writes:

>I have have tried it maybe 3 times on commerical power and it has never held
>at 13a like 1 & 3.
>So the question is:
>Does the generator somehow cause it to sometimes take longer to start up, or
>is that a refrigerator issue that I just never happened to see before. I
>don't know enough about refrigerators to know.

The induction motor in the refrigerator will take longer to start if the
voltage is low.

>But, as I understand it, inverter generators hold the 120v; when they can't
>do it, they trip out. So, they only give 120v or 0v, never low voltage.

That's probably true over a long time, but not necessarily true for
short periods. If you overload the generator, the voltage almost
certainly *will* drop somewhat. This may cause the generator to trip in
a short period (and it did on a couple of your tests), but if the
refrigerator comes up near operating speed before that the overload
will be gone, the voltage will recover, and the generator will decide
not to trip after all.

To be sure of what's going on, monitor the generator output waveform
with an oscilloscope. Then you'll know. A meter may not respond fast
enough to tell you anything about a transient voltage drop.

Dave

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