7.5 HP motor on a compressor

[Tom Godley]

I have been keeping my eye out for a nice compressor -- saw one that is bigger than I need -- but it may be a good buy.

But it has a 7.5 HP Baldor motor on it:eek:

Looks like a Baldor motor of that size uses 32.0 amps/leg at 220v


My default compressor has always been the IR 2 stage 5HP unit that many recommend - goes for around $1200.00. But with the economy the way it is I see a lot of larger units on the market for great prices. Plus I would like to do some sandblasting and that can take more air than the IR


My house has 200amp service feeding my shop with 100amp -- So I have the power - but I am wondering if everything in the house is going to flash when that motor kicks on. Anyone with experience with a 7.5 motor in the shop?

[Dan Hintz]

Does anything flash when your HVAC heater kicks in, your oven turns on, or your water heater starts running? Probably not. 30A is not a whole lot in the grand scheme of things...

[Mark Leu]

30 Amps is only the running load of the motor, you could see current as high as 200 Amps on startup. I work for a power company and we just recently dealt with a customer who tried the exact same thing. He experienced lights flickering on startup. I personally didn't think the flicker was that bad, but the customer didn't like it. We upgraded the wire size feeding the house and the transformer which helped quite a bit. He still complained about the flicker, but that was as much as we could do on our end.

I think you'll definitely see noticeable flicker with a 7.5 hp motor, but all motors are not created equal. Some have lower starting currents than others. Also, everyone has a different tolerance level to light flicker, so you may find that it doesn't bother you while your better half might find it extremely bothersome.

[Paul Murphy]

I have a 4 cylinder 2-stage compressor with the 7.5 HP single phase Baldor motor. I've had this compressor for about 8 years, and have not had any problems with it at all. That said, you should wire it to code because it does take plenty of current at startup which is why it is the largest size single phase motor commonly sold.

I'm happy with mine because the pump is also used in higher HP units turning at higher pump speeds. Mine is turning so slowly it doesn't get very warm even when using high volumes of air. It is also pretty quiet for a large compressor, which I appreciate.

[Paul Murphy]

Mark brought up some good points, so I looked up the breaker size for the 7.5 HP unit, and it is 80A. For the 5 HP motor it is 60A. As Mark pointed out the startup current can be pretty high. If the pump has an unloader valve it will help with the startup load.

[Rob Damon]

I have a 7.5 hp IR air compressor (32amp FLA) in my home shop and runs fine on 50A/2P circuit breaker and the lights do not flicker.

Rob

[Tom Godley]

Thanks for the feedback -- The start current was something like 168 !

At my house in NJ -- both my big pump and my larger A/C condenser (5 ton) give me a little flicker - thats why I was concerned about this motor. They are both quite far from the panel on oversized lines.

I have had it looked at and everybody tells me its normal. That house has a 200 amp service also. Not sure of the motor size in a 5 ton unit - but it's not 7.5hp.

[Paul Canaris]

I have a similar issue with my Thickness planer which has a 12 hp motor for the cutter head. The fluorescents will actually go out when I start it up, they then come back on.

[Stephen Cherry]

I have the IR 7.5hp compressor also, no problems with blacking out the neighborhood. As I understand it, this is basically a 5 hp pump, turning faster with a 7.5 hp motor. The one I have is approx 3600 rpm motor, rather than the 1800 rpm 5 hp motor, so the motor torque is actually lower.

[Scott T Smith]

I have a 27.5 CFM 7.5 hp Campbell Hausfeld compressor, and it will momentarily (and lightly) dim the lights when it comes on. No problems feeding a glass bead blasting machine with it though.

[Brian Runau]

The only thing I would suggest is make sure the motor has the correct Nema rating for the load draw during start up for a compressor. I know a standard nema B would not operate correctly.

I believe you want a nema C for the high starting torque.

http://www.reliance.com/prodserv/motgen/b9652_d.htm

Thanks.

Brian