# AC Motor Question



## Voltswagen (Nov 13, 2008)

Hey Guys - I need to pick your collective brains......
This is not a question about my EV Electric Motor but rather about my
Compressor Motor. 
My old compressor head gave up the ghost so I purchased the $99 - 
2 Cylinder one from Harbor Fright. (I had a 20% off coupon and just couldn't resist.)
Anyway, I used my old compressor motor (15 amp, 6hp peak, 3450 rpms )
and tanks. I go from compressor to 21 gal to 40 gal. I set my tanks at 120psi. 
The new compressor will go 140 psi , wants 3hp or more and is belt driven by a 10 1/4 " pully. The manufacturer limits it to 1200 rpms. So I install a 3 " pully on the motor figuering at 3450 rpms it will convert to less than 1200 rpms on the compressor. And for the first few days it runs fine. 

Then it begins to trip the 20 amp circuit breaker which feeds the outlets and lights over my work bench. So I turn off the 4 - 100 watt lights.
Then a while later the motor trips it's onboard circuit breaker (red button pops out).
I let it cool down for a few hours and I plug my Kil-A-Watt into the outlet
and the compressor plug into the Kil-A-Watt setting it to read amps.
I empty the tanks and start the compressor....reads 13 amps...then climbs
within 5 minutes to 20.5 amps...I shut it down knowing the motor breaker will eventually pop.
The belt has approx. half inch play thanks to the temporary block of wood wedged between the compressor & motor.....God made it that way .
Any thoughts on what I'm doing wrong or how I can correct this?
Thanks - Roy


----------



## ngrimm (Oct 19, 2007)

What voltage does the motor use? If it is 120 volts the motor will never make 3 hp. 120v x 15a / 746 = 2.41 hp. With normal losses it is probably closer to 1.8 hp. When they claim 5 or 6 hp peak, they are using the startup surge amps to calculate the horsepower which is really misleading. That is what I discovered a few years ago after buying a 120 volt compressor that was supposedly 5 hp to replace my older 220 volt 3 hp unit. I wound up selling the 120 volt compressor and am still using the old one since it pumps way more air.


----------



## Voltswagen (Nov 13, 2008)

NG
Yes, It's a 120v motor........hmmmmmmmm.....good point about the horse power. I didn't think of that.
Why did you use a divisor of 746 ?....just wondering.


----------



## ngrimm (Oct 19, 2007)

A horsepower is a unit of power equal to 746 watts. Jim Hustead of HiTorque says to use 1000 for the divisor to account for motor losses. Here are some formulas I found doing a Google search. 
http://www.elec-toolbox.com/Formulas/Motor/mtrform.htm

You may need to gear it a little slower so it won't load the motor down so much.


----------



## Voltswagen (Nov 13, 2008)

Thanks NG
Thats a good formula page.
So you suggest a smaller pully on the motor?


----------



## piotrsko (Dec 9, 2007)

My guess:

this only happens when the compressor is above 30 psi? then you need an unloader to give the motor some room to come up to speed before compressing gasses. two ways to fix: get an unloader, Mcmaster carr has them. 2: add a check valve, and loosen the fitting slightly on the pump side of the check valve so that it leaks a wee bit.

you may also want to use say 50 wt oil, them harbor frieght things are made really cheap, so you probably ate the shaft bearings by now.


----------



## Voltswagen (Nov 13, 2008)

Thanks Pio
I'm using 40wt oil....I doubt the compressor bearings have worn at all.
It has less than 40 hours on it.
The problem wasn't with the compressor but rather the AC Motor.
I'm going to try a smaller pully first....maybe 2 1/2 " to reduce the load on the motor. It will take a little longer to get up to pressure as this will drop the compressor rpms to about 850.


----------



## Dennis (Feb 25, 2008)

> What voltage does the motor use? If it is 120 volts the motor will never make 3 hp. 120v x 15a / 746 = 2.41 hp. With normal losses it is probably closer to 1.8 hp. When they claim 5 or 6 hp peak, they are using the startup surge amps to calculate the horsepower which is really misleading. That is what I discovered a few years ago after buying a 120 volt compressor that was supposedly 5 hp to replace my older 220 volt 3 hp unit. I wound up selling the 120 volt compressor and am still using the old one since it pumps way more air.


Actually they get that peak HP rating from the breakdown torque curve (which is not at zero RPM!!) of AC induction motors which just so happens to be the peak HP too unlike a DC motor which is peak HP at 50% efficiency. This rating is only guaranteed for short duty cycles where adequate cool off time is allowed (compressor cuts off when pressure is reached). If one needs lots of air then a continuous rated duty air compressor will be needed for the job(s).


----------

