Author Topic: Motors running hot with modified sine wave  (Read 3110 times)

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domwild

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Motors running hot with modified sine wave
« on: April 05, 2008, 08:28:27 AM »
Hi,


From this forum I learned that modified sine wave inverters will lead to motors running hot and noisya and may burnout. Does this mean it is dangerous and therefore one has to go the more expensive way of pure sine wave inverters to drive any AC motor? I am asuming the inverter can handle the starting Amp surge.


Thanks.

« Last Edit: April 05, 2008, 08:28:27 AM by (unknown) »

Flux

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Re: Motors running hot with modified sine wave
« Reply #1 on: April 05, 2008, 03:02:22 AM »
I don't think there is a general answer to this.


Motors are designed for sine wave. On square wave you have many problems and the MSW is a compromised square wave with the peak of a sine and the rms of a sine, achieved by reducing the width to 50% of the square.


For motors the peak flux is higher ( it spends longer at peak). There are lots of harmonics about that affect some motors very badly and hardly affect others. The worst may hit a state where they crawl at the 5th or 7 th harmonic speed and will not get through it. Many motors run hotter but can still be within rating. Some will not sustain full load indefinitely within temperature rating but are ok intermittently.


Some get in a real mess with the starting circuits but if you can start them they may be ok when running.


All series universal motors ( power tools etc will be ok unless there is an electronic speed control).


Motors should be happy on a pure sine inverter as long as there is sufficient starting capability. The average inverter claims a surge rating of perhaps 200% for a few hundred milliseconds. A motor may draw 500% overlaod during starting for several seconds on a large inertia load. Fridges and freezers can be a real pig.


You may need an inverter far bigger than you suspect for some motors, others may present little problem.


In general small pure sine inverters seem able to cope with motor starting better than crappy little engine driven alternators.


I suspect the inverter is more likely to die than the motor if you use large motors on cheap msw inverters.


Flux

« Last Edit: April 05, 2008, 03:02:22 AM by Flux »

PaulJ

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Re: Motors running hot with modified sine wave
« Reply #2 on: April 05, 2008, 05:16:24 AM »
   My only experience running ac motors off MSW is with a couple of old fridges, they both ran fine but when I changed to a full sine inverter they both used about 20% less power (measured before the inverters as dc out of the batteries, and subtracting the standby power which was about the same for both inverters anyway).


  That 20% must be appearing as heat in the inverter and/or the motor - if either is marginal you could be in strife. I wouldn't try running an expensive new fridge off MSW...


   Paul.


   

« Last Edit: April 05, 2008, 05:16:24 AM by PaulJ »

domwild

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Re: Motors running hot with modified sine wave
« Reply #3 on: April 06, 2008, 12:49:46 AM »
Thanks for the answers. Much appreciated.


dom

« Last Edit: April 06, 2008, 12:49:46 AM by domwild »

ghurd

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Re: Motors running hot with modified sine wave
« Reply #4 on: April 06, 2008, 08:39:09 AM »
I never had a problem.

Then again, the motors don't run that long.  Drills, sanders, saws...

Popped more than my share of inverter fuses.  :-)

Popped a few inverters too.  :-(


I never had a problem with the corded 'fancy speed controlled' tools either.

Like those that make a high pitch squeal when they are partly powered, but not yet moving.

The first inverter test (for me) is a 1/4HP variable speed scroll saw, then a big variable speed 1/2" hammer drill, then a medium sized circular saw.

G-

« Last Edit: April 06, 2008, 08:39:09 AM by ghurd »
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Gog

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Re: Motors running hot with modified sine wave
« Reply #5 on: April 08, 2008, 04:14:01 AM »
With my cheapo msw inverters, I havent had any problems with motors.

But I have have done in a few thermal cut outs on little transfomer power supplies (wall warts)


cheers

« Last Edit: April 08, 2008, 04:14:01 AM by Gog »