Air compressor pump HP requirement?

[Ryoko]

I replaced the pump on my air compressor with a Harbor Freight unit. The new pump says it requires 1050RPM and 5HP to drive it. The compressor has a 5HP motor which is supposed to be rated for continuous duty according to its spec plate. But it's unable to drive the new pump for more than a few minutes without tripping the overheat breaker on the motor. This is leading me to believe the pump is drawing more than 5HP. I've tried reducing the drive pulley size so the pump is now running around 800 RPM. But it still wants to trip the overheat breaker. The old pump had no issues until it croaked. So I'm wondering what's going on. Should the pump be undersized vs the motor to run reliably?

 

[Averagesupernova]

The 5 HP spec on your old motor is quite likely false. If the motor has a spec for continuous amperage draw that will give you an idea of what it actually is.

 

[Ryoko]

The 5 HP spec on your old motor is quite likely false. If the motor has a spec for continuous amperage draw that will give you an idea of what it actually is.

I think you're right. I looked up the specs again on the motor and it says 230v @ 15A and claims to be 5HP. The power specs work out to 4.6HP. So they rounded up. But that also assumed 100% efficiency. Since this is an induction motor, I'm guessing the actual HP is around 80% of the ideal figure or 3.7HP. This may well explain why the motor is struggling. As an added handicap, the line voltage is 220V rather than 230V which is probably contributing to the problem. So I guess I need to de-rate the motor accordingly and resize the pump.

 

[Averagesupernova]

They have creative ways of lying about the spec. Maybe the service factor is significantly reduced. This effectively says it will start a load typically suited for a 5 HP motor and run it for a short time. This is basically what you have experienced. When it's sold as a package the user is happy. It pumps and doesn't trip on overload. The company will say that you have modified it by replacing the pump so tough luck.
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My shop compressor is a very old Quincy pump with a rusted out tank. The rusted out tank simply serves as the mount for the pump and motor. There is 200 gallon tank along side that I pump into. It's slow because I had a spare 3 HP motor to use in place of the 5 that came on it so the pump is slowed down so the motor can take it.

 

[Baluncore]

What was the capacity of the old pump in cubic feet of free air per minute?
How does that compare with the replacement?

Did the original pump have an unloading valve in the head, does the replacement pump have the same? Or is the motor turning off when the set pressure is reached?

Consider also the immediate cooling of the compressed air. The outlet from the pump will be hot air, which should be cooled at the head, before it enters the reservoir. Cooling the pump head will lower the peak pressure reached during compression.

The outlet pressure setting on the system, or the pump RPM and capacity, should be reduced for an under-rated motor.

 

[Ryoko]

What was the capacity of the old pump in cubic feet of free air per minute?
How does that compare with the replacement?

Did the original pump have an unloading valve in the head, does the replacement pump have the same? Or is the motor turning off when the set pressure is reached?

Consider also the immediate cooling of the compressed air. The outlet from the pump will be hot air, which should be cooled at the head, before it enters the reservoir. Cooling the pump head will lower the peak pressure reached during compression.

The outlet pressure setting on the system, or the pump RPM and capacity, should be reduced for an under-rated motor.

I don't know the old pump's specs. It was extremely noisy. The unloading valve is built into the pressure switch. I already have a radiator on the pump outlet which works quite well.