Pump slows at low pressure

by Vince
(Mt. Albert, Ontario, Canada)

Older air compressor

Older air compressor

Trying to solve a slowing pump at low pressure.

Work Done:
-Replaced the electric motor with a new 2hp motor
-Replaced pump with a new piston pump
-Checked the pulley ratio to confirm the correct RPM to the pump

Tried 2 check valves (original and new) and both hold pressure when I fill the tank from an alternate air source.

Pump wheel rotates in the designated direction
Power source is not otherwise used or loaded

When the pump begins the pressure climbs until the pressure reaches about 20psi. At that point the pump and motor slow until it stops resulting in a tripped breaker.

As everything is new, except the tank, I'm not certain the cause.

Would appreciate any insights

Regards & Thanks


Comments for Pump slows at low pressure

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May 01, 2018
Power issue to compressor motor?
by: Doug in s.d.ca.

I'll take your word that that's a new motor, although it doesn't look it.

I'd be checking the voltage across the motor - maybe at the pressure switch. You're using 230V, I presume - how does that hold up as the pump slows? Should stay within about 4%, or ~220. If not, check for bad connections.

May 01, 2018
by: Anonymous

The tank is empty of air and any moisture.
When the line is line is disconnected from the tank, either with or without the check valve the pump and motor run as you would expect it to and merrily runs without any apparent strain or ever tripping the breaker.

May 01, 2018
20 PSI...
by: Bill

Vince, sorry, misread your post.

OK, so it's slowing at 20 PSI until it pops the breaker?

Of course, the issue is... is the motor slowing because the pump's failing and overloading the motor, or is the motor failing and can no longer drive the pump?

Assuming so, and all connections are secure, and assuming that the new motor works fine and is large enough to drive the new pump, that the air flow to the tank is unimpeded by the check valve since it's new, then that's pointing to the pump as being an issue.. yet it's a new pump.

If you disconnect the line from the pump head WHERE IT ENTERS THE TANK, and then run the compressor, does it run OK?

Oh, and one more thing to check. You have ensured that the tank is completely empty of air and water before you run the new compressor, yes?

Apr 30, 2018
20 psi
by: Anonymous

No, it slows at 20 psi not 200 psi

Apr 30, 2018
Compressor pump slows
by: Bill

I've moved this to the compressor motor forum as I surmise that this is likely the issue.

You say it slows around 200 PSI?

What's the normal cut out pressure please.

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