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Old 06-05-2020, 03:58 PM   #1
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Inverter Characteristics

I had an interesting "problem" or limitation of one of my inverters that hopefully someone can explain. On my boat I have a 1000 watt true sign wave inverter and a 1400 watt modified sign wave inverter. I would prefer to use the TSW inverter all the time but my polishing pump is driven by a continuous duty, 1/3 hp AC motor and I often polish fuel while underway when the tanks are stirred up. I have good reasons for that setup as opposed to a DC pump/motor for fuel polishing but that's another story so I won't get into it here. The problem is the TSW inverter cannot handle the startup power spike of this motor and always trips out. For those who don't know, 1/3 hp is about 250 watts so the operational loads of this motor is well within the limits of the inverter but as I said, it just can't handle the startup spike. The interesting thing is that last year I forked out a lot of money to buy a 2000 watt TSW inverter thinking that would definitely have the beef to do the job but that one would trip out as well at startup so I returned it. However, the 1400 watt MSW inverter can start this motor with no problem. Does anybody know why this is?

Thanks, Jeff
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Old 06-05-2020, 04:14 PM   #2
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Yes and no.

First of all, small AC motors are horribly inefficient, sometimes as low as 50%. So that 1/3 hp could easily be 500 watts running and 3-4 times that at startup (well not exactly but three or four times the running current, not power).

Secondly MSW inverters are more tolerant of high inrush loads. I can't explain it electrically, but that has been my experience.

David
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Old 06-05-2020, 05:37 PM   #3
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Maybe just wire the polishing motor to the modified inverter and just use it for that only. Use the true sine wave for everything else.
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Old 06-05-2020, 06:21 PM   #4
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It is my understanding that AC motors starting under load (which yours mostly is) draw a lot of amps when trying to start without a full sine wave. It would have been interesting to have seen the amp draw with a clamp on amp meter. The other interesting question would be whether or not the inverter tripped from low voltage because of the amp draw.

Did you measure the voltage as it was tripping? A common error in inverter installations is undersizing supply cables. The issue is that while running amperage may be lower once the motor is running, voltage may drop below the trip level on startup amp surge.

Ted
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Old 06-06-2020, 08:17 AM   #5
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The sine wave we are all used to seeing is a nice rounded set of curves, but usually that is hardly what an inexpensive sine wave inverter puts out.

The peak voltage will be more like a pencil standing on end.

A motor sees the power under the voltage curve which may be tiny on some sine wave units.

The old chop chop, square wave inverter may not keep music systems from buzzing but it delivers way more power in its full square wave.

Motors can feel the difference and run hot with low power.

For iffy AC power the best motors are the old Universal style , with brushes.

Feed them low voltage and they simply become less powerful, but they work fine and do not overheat.
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Old 06-07-2020, 10:49 AM   #6
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Quote:
Originally Posted by DavidM View Post
Yes and no.

First of all, small AC motors are horribly inefficient, sometimes as low as 50%. So that 1/3 hp could easily be 500 watts running and 3-4 times that at startup (well not exactly but three or four times the running current, not power).

Secondly MSW inverters are more tolerant of high inrush loads. I can't explain it electrically, but that has been my experience.

David
I think you are right about the lack of tolerance of inrush loads. I'm guessing the reason is complicated. Thanks for your reply.
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Old 06-07-2020, 10:52 AM   #7
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Quote:
Originally Posted by FF View Post
The sine wave we are all used to seeing is a nice rounded set of curves, but usually that is hardly what an inexpensive sine wave inverter puts out.

The peak voltage will be more like a pencil standing on end.

A motor sees the power under the voltage curve which may be tiny on some sine wave units.

The old chop chop, square wave inverter may not keep music systems from buzzing but it delivers way more power in its full square wave.

Motors can feel the difference and run hot with low power.

For iffy AC power the best motors are the old Universal style , with brushes.

Feed them low voltage and they simply become less powerful, but they work fine and do not overheat.
This is good info, thanks.
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Old 06-07-2020, 10:54 AM   #8
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I wonder if you could install a soft start device ?
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Old 06-07-2020, 10:54 AM   #9
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Originally Posted by Comodave View Post
Maybe just wire the polishing motor to the modified inverter and just use it for that only. Use the true sine wave for everything else.
I might end up doing just that but that involves running a new dedicated AC circuit so it wasn't my first choice.
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Old 06-07-2020, 10:55 AM   #10
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I wonder if you could install a soft start device ?
I'm not aware of that option. Can you point me to something which would work in this way?
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