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Generator won't run a 1/2hp split phase motor... WHY?


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  #1  
Old 11-08-2014, 02:09:03 PM
Burt Lade Burt Lade is offline
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Default Generator won't run a 1/2hp split phase motor... WHY?

Hello all,

I rebuilt a small single cylinder air compressor and it works just fine on
grid power. It will not operate on any generator I have tried. It turns and
will not quite disengage the starter winding and then begins to slow down
at which point I turn it off. The start capacitor says it is 375 microfarads
and my tester says it is about 330 or so. I'm running the generators at good
speed. Maybe someone out there has had similar experience and could
shed some light on the subject. I would try another start cap but am tending
to think that is not the problem. Any ideas??
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Old 11-08-2014, 03:37:03 PM
Power Power is offline
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Default Re: Generator won't run a 1/2hp split phase motor....WHY?

What size generator?
What is the voltage at the motor terminals when you try to start it?
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  #3  
Old 11-08-2014, 06:11:34 PM
Burt Lade Burt Lade is offline
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Default Re: Generator won't run a 1/2hp split phase motor....WHY?

I have tried a 2000 watt Honda, a Miller Generweld 130 that has 1000 watt AC outlet,
a 4000 watt diesel Generator and a 3000 watt inverter. Voltage starts out at 120 or so
and drops a bit as it slows down and fails. I don't think the starter winding has kicked out
leaving both the main winding and the starter winding connected and drawing lots of current.
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Old 11-08-2014, 11:27:01 PM
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Jim McIntyre Jim McIntyre is offline
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Default Re: Generator won't run a 1/2hp split phase motor....WHY?

Couple of things.

First off - minor technicality - if your motor has a 375 uF starting capacitor, the motor isn't a split-phase motor. It's a capacitor start motor.

I have a 1.5 HP motor on my air compressor. And a 7.5 kW Onan genset wouldn't reliably start the motor. Like you, the air compressor worked fine on mains power. I change to a smaller capacitor and it worked. My 1.5 HP motor had a 900 uF capacitor and I changed it to a 400 uF. If the same ratio were applied to your 1/2 HP motor, you'd want about 130 uF.

http://www.smokstak.com/forum/showth...t=97509&page=2

Other more obvious things that might help are keeping the compressor warm, using lighter-weight oil, and making sure the pressure unloader is working...
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Old 11-09-2014, 07:22:10 AM
Burt Lade Burt Lade is offline
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Default Re: Generator won't run a 1/2hp split phase motor....WHY?

Quote:
Originally Posted by Jim McIntyre View Post
Couple of things.

First off - minor technicality - if your motor has a 375 uF starting capacitor, the motor isn't a split-phase motor. It's a capacitor start motor.

I have a 1.5 HP motor on my air compressor. And a 7.5 kW Onan genset wouldn't reliably start the motor. Like you, the air compressor worked fine on mains power. I change to a smaller capacitor and it worked. My 1.5 HP motor had a 900 uF capacitor and I changed it to a 400 uF. If the same ratio were applied to your 1/2 HP motor, you'd want about 130 uF.

http://www.smokstak.com/forum/showth...t=97509&page=2

Other more obvious things that might help are keeping the compressor warm, using lighter-weight oil, and making sure the pressure unloader is working...
Jim,

Thanks for setting me straight on a capacitor start motor. Your comments are
very informative and interesting. I will look in my spare parts boxes and see
if I have any capacitors in that size range and give it a try. Also I have a 240
volt band saw that is capacitor start and basically the same scenario. Apparently
the generated AC in these small generators has some different characteristics,
what that might be I am not sure of. I will try to post the results after I have done
my experimentation.

---------- Post added at 07:22 AM ---------- Previous post was at 06:57 AM ----------

Quote:
Originally Posted by Jim McIntyre View Post
Couple of things.

First off - minor technicality - if your motor has a 375 uF starting capacitor, the motor isn't a split-phase motor. It's a capacitor start motor.

I have a 1.5 HP motor on my air compressor. And a 7.5 kW Onan genset wouldn't reliably start the motor. Like you, the air compressor worked fine on mains power. I change to a smaller capacitor and it worked. My 1.5 HP motor had a 900 uF capacitor and I changed it to a 400 uF. If the same ratio were applied to your 1/2 HP motor, you'd want about 130 uF.

http://www.smokstak.com/forum/showth...t=97509&page=2

Other more obvious things that might help are keeping the compressor warm, using lighter-weight oil, and making sure the pressure unloader is working...
Also I did some reading on the internet and some interesting information showed up.
I was reading on the Kohler site and found out that motors of this type draw up to
six times the running current in starting. That would explain why if the generator just
couldn't supply the needed starting current and failed to spool up. Also, the
motor has to get to about 75 percent of running speed for the start winding to kick out.
That's good information for anyone using electric motors of this type with a generator.
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Old 11-09-2014, 10:11:51 AM
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Jim McIntyre Jim McIntyre is offline
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Default Re: Generator won't run a 1/2hp split phase motor....WHY?

I just realized that I forgot one thing. My motor example was with a 240V motor. I don't know if yours is 240V. If it's not, you probably need more capacitance than the 130 uF I recommended.

Anyway, be glad it's NOT a true split-phase motor. They draw the highest levels of starting current...
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Old 11-09-2014, 10:45:34 AM
Burt Lade Burt Lade is offline
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Default Re: Generator won't run a 1/2hp split phase motor....WHY?

Thanks Jim! More good information. It's a 120 volt unit so will experiment around
and see what happens, probably 200mfd or so to start. It's getting cold up here so
may have to put this off a while. We had snow yesterday.....and now the big cool off.......
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Old 10-13-2015, 09:40:09 PM
Flange Flange is offline
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Default Re: Generator won't run a 1/2hp split phase motor....WHY?

Your little motor might be about 10 amps @120 VAC or 5 amps @ 240 VAC running so you can see if the start current is 6 times at start you get 60 amps @120 And only 30 amps @240. This large pull only lasts about 1 to 2 seconds and then drops back down to run current. The start current can be lower on a better motor.
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Old 10-13-2015, 10:42:09 PM
beezerbill beezerbill is offline
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Default Re: Generator won't run a 1/2hp split phase motor....WHY?

If you decouple the motor from the compressor, does the motor start OK? If so, does the compressor turn smoothly or is it binding? What do you have for an unloader? I have seen some compressor setups where there is a small, half liter to liter sized vessel on the line between the compressor and check valve - this vessel unloads and presumably presents minimal load to the motor as it pumps up, while the motor is on its start cycle.

I have put rope cup starters on electric motors to get them to start under load - kind of weird and you need to be careful, but it actually works. Wrap the rope around the cup, throw the switch on, and without delay pull on the rope.
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Old 11-12-2015, 10:56:50 PM
miro miro is offline
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Default Re: Generator won't run a 1/2hp split phase motor....WHY?

One of the things to remember about generator ratings is that most of them show TOTAL power output.
Generators that are rated above 1000Watts usually have a 220V output as well as 120V outputs.

Since the generator is a 2 phase machine, the amount of power available at the 120V output is half of the total output.
So a 3000 W generator (220V) has only 1500 W at 120V

It sounds like your starting current requires way more than 1500 W.

M
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Old 11-12-2015, 11:37:05 PM
Bill Hazzard Bill Hazzard is offline
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Default Re: Generator won't run a 1/2hp split phase motor....WHY?

Quote:
Originally Posted by miro View Post
One of the things to remember about generator ratings is that most of them show TOTAL power output.
Generators that are rated above 1000Watts usually have a 220V output as well as 120V outputs.

Since the generator is a 2 phase machine, the amount of power available at the 120V output is half of the total output.
So a 3000 W generator (220V) has only 1500 W at 120V

It sounds like your starting current requires way more than 1500 W.

M
Any generator that he would have used would not be a two phase machine. He would be using a split single phase generator. A 3000 watt generator will give 3000 watts of power on both 120V or 240V, not half.
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Old 11-13-2015, 10:21:41 PM
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Default Re: Generator won't run a 1/2hp split phase motor....WHY?

I don't know how often you need it "off grid" but it would probably be easier to just install a gas engine on it with a pressure unloader valve.
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