Compressor motor overheating

[Mojo353]

So I bought a 60 Gal compressor with a bad pump. It has a 5hp motor 240v.
Replaced the pump with a 2 stage Princess Auto pump rated for between a 5 and 7.5 HP motor, it's Rolair pump Model # 8771792,
Max rpm of the pump is 1400 I believe, I'm running it @ 1280 rpm with the 5hp motor.
I use it for sand blasting and it's fine pressure and volume wise but the motor gets really hot after 2 cycles.
So much so that I've tripped the thermal cut off twice and fried the start capacitor once.
The motor has new bearings also.
So is it that the motor is too small and if so what are my options ? Do I absolutely need to go to a 7.5hp motor ?

 

[Mojo353]

Pic of the motor data plate below.

 

[PCustoms]

Delete

 

[micromind]

The only way that I know of is to check the current while the motor is running.

A bad run capacitor will cause excessive current draw and overheat the motor fairly quickly.

 

[larry_g]

So you have a pump rating of 5-7.5 HP and a speed rating of 900-1400 RPM. This tells me that you should run 900 rpm with the 5 hp motor and up to 1400 rpm with a 7.5 hp motor. So you have a 14" flywheel and a 3450 RPM motor. That tell me that you need a 3.5-4" drive pulley to get the pump to run at ~900 RPM.

That said have you checked the current that the motor is drawing? Are you pulling to much?

What is your cutout pressure? Dropping the cutout pressure will lower your max current draw.

Those two knobs, pressure and speed are what you have to adjust if you want to keep the 5HP motor.

lg
no neat sig line

 

[Dagny]

2 pole motors **** on air compressors. But its the way of the world.

 

[Mojo353]

The only way that I know of is to check the current while the motor is running.

A bad run capacitor will cause excessive current draw and overheat the motor fairly quickly.

The run capacitor is new also, I'm thinking Larry_g may be correct - smaller pully won't be as hard on the motor.

 

[Mojo353]

So you have a pump rating of 5-7.5 HP and a speed rating of 900-1400 RPM. This tells me that you should run 900 rpm with the 5 hp motor and up to 1400 rpm with a 7.5 hp motor. So you have a 14" flywheel and a 3450 RPM motor. That tell me that you need a 3.5-4" drive pulley to get the pump to run at ~900 RPM.

That said have you checked the current that the motor is drawing? Are you pulling to much?

What is your cutout pressure? Dropping the cutout pressure will lower your max current draw.

Those two knobs, pressure and speed are what you have to adjust if you want to keep the 5HP motor.

lg
no neat sig line

I'm running a 5 3/4" pully ( rough measurement- didn't take the cage off)
It's been a while but from what i remember when I did the calculations that gave me around 1200 to 1300 rpm - confirmed by the pully supplier.
I had it cut out at 130psi and back on at 105psi, haven't done a current draw yet.
I'm thinking you are correct - the smaller pully will help. I assume the pump will run longer but not as labored?
And if I adjust the pressure lower that too would help save the motor ?

 

[American Locomotive]

Lowering the pressure will help, but realistically you are spinning the pump too fast for your motor. You are presenting it with a 6.5-7hp load. You gotta' slow it down to 900 by putting a smaller motor pulley on it. 900 RPM is the "5HP Speed"

 

[Mojo353]

Yeah I have the original pulley from the original bad motor kicking around somewhere- I'll stick that on it once it's the correct size and go from there.