Compressor tripping thermal overload

[bochnak]

I have a 5hp compressor and the thermal overload trips on the mag starter after 10-15min of run time. After reset, it seems to run fine.

The motor is pulling 30-32A. I checked this since I installed a new replacement pump. Yes it is spinning at correct RPM. Also verified the current draw with pump MFG and he said it's normal.

I also have this unit hooked up with 30A breaker and #10 wire and it has never tripped the breaker.

The mag starter is a Furnas and may be original to the unit dated 89'. I've read that heaters get weak over time?

Matt

 

[mm08822]

Pulling 30+ - 32a on a 30a cb is going to trip the cb eventually.

What is tripping - cb or starter overload?

Post a pic of motor nameplate.
What size heaters are in the starter?
Is there a heater table in the starter spec'ing current range for that heater? Pic?

Did you tach the pump? How do you know correct rpm?
What are the specs of the new pump - hp, rpm, cfm?
What are the pulley sizes?

 

[Bert_]

30-32 amp is pretty high for a 5hp motor. Sometimes on air compressors the motor is overloaded a bit towards the end of the cycle but I doubt that much is normal. The heaters are doing their job. They are telling you there is a problem and also saving your motor. Most modern 5hp single phase motors are rated 22-24 amps.

 

[bochnak]

Pulling 30+ - 32a on a 30a cb is going to trip the cb eventually.

What is tripping - cb or starter overload?

Post a pic of motor nameplate.
What size heaters are in the starter?
Is there a heater table in the starter spec'ing current range for that heater? Pic?

Did you tach the pump? How do you know correct rpm?
What are the specs of the new pump - hp, rpm, cfm?
What are the pulley sizes?

It's been almost a year and CB has never tripped.

Starter overload is tripping.

I tached the pump RPM at 800 RPM, exactly where it should be. CFM is right on the money also.

I need to get some pics of nameplate and starter etc....don't have any at the moment.

 

[bochnak]

30-32 amp is pretty high for a 5hp motor. Sometimes on air compressors the motor is overloaded a bit towards the end of the cycle but I doubt that much is normal. The heaters are doing their job. They are telling you there is a problem and also saving your motor. Most modern 5hp single phase motors are rated 22-24 amps.

I recall name plate is 22A or so.

Here is the compressor video with it up and running:

 

[Dagny]

check the run capacitor if it has one.

 

[wyliesdiesels]

I recall name plate is 22A or so.

Here is the compressor video with it up and running:

NEC FLC table lists 5HP as 28a

So 30-32 is high.

If the nameplate lists 22a and its running 30-32a, then youre overloading the motor.

The heaters are doing their job to protect the motor from overheating.

 

[bochnak]

Nameplate pic attached

 

[Wrench97]

Could this be a low voltage condition?

 

[bochnak]

Just went out to the garage and did some blasting for 20min or so. Set meter to MAX and it hit 28.3 right before cut out at 175psi.

No tripping today????

 

[manwithtools]

Could this be a low voltage condition?

I'd like to know this answer as well. What is the voltage measured at the motor starter while it's running? Do you have voltage loss due to long wire runs?

 

[fitter30]

Name plate on motor max amps is full load amps x servicefactor 21x1.15= 24.15 max amps
Is the compressor not shutting off on pressure switch? This would overamp the motor.

 

[ClappedOutBport]

Sounds like you've got a bit too much pump/pulley there for the motor. IF you don't really need 175psi, I would cut it down, and extend the pump and motor life dramatically.

 

[mm08822]

It's been almost a year and CB has never tripped.

Starter overload is tripping.

I tached the pump RPM at 800 RPM, exactly where it should be. CFM is right on the money also.

I need to get some pics of nameplate and starter etc....don't have any at the moment.

Ok, wasn't sure it this was just converted and in initial start-up.

Check the caps and make sure cent switch is opening. Could be dirt/rust preventing free movement of switch.

You said trips after 10-15 mins. Is it running 100% of the time or cycling?

Can you plot a current vs. pressure curve?

Pull the belt and measure the no load current?

 

[Zeke]

Sounds like you've got a bit too much pump/pulley there for the motor. IF you don't really need 175psi, I would cut it down, and extend the pump and motor life dramatically.

I'd focus more on the 175 PSI. That's ridiculous. At that pressure I assume it's a 2-stage pump or that's even more ridiculous. If you need more air, volume is the answer, not higher pressure.

 

[mm08822]

You said speed and cfm were correct but never mentioned hp.
What is the required hp for the pump to deliver at that psi?

 

[rlitman]

Name plate on motor max amps is full load amps x servicefactor 21x1.15= 24.15 max amps
...

This! Anything over 24.15A is overloading the motor. The solution is to slow down the RPM. Is the motor sheave adjustable? If so, just turn it out (make the diameter smaller by moving the sides apart) a half turn, re-adjust the belt tension and check the current with your clamp meter. Repeat until it is under the max load. Forget the whole RPM BS.

 

[mm08822]

Just went out to the garage and did some blasting for 20min or so. Set meter to MAX and it hit 28.3 right before cut out at 175psi.

No tripping today????

What size heaters are used? It should have tripped by 28.3a nevermind 30-32a.

 

[manwithtools]

mm08822 is correct, the heaters should trip closer to the 28.3 amps if they are properly sized. I asked about voltage drop earlier - curious if after significant run time you are seeing a drop in supply voltage - could be due to poor connection somewhere along the line.

 

[wyliesdiesels]

Geez 32a and or 28a on a 21a motor? Ouch!

 

[bochnak]

I adjusted the pressure switch to cut out at 145psi. Current right before hits 27.4A.

 

[bochnak]

This! Anything over 24.15A is overloading the motor. The solution is to slow down the RPM. Is the motor sheave adjustable? If so, just turn it out (make the diameter smaller by moving the sides apart) a half turn, re-adjust the belt tension and check the current with your clamp meter. Repeat until it is under the max load. Forget the whole RPM BS.

No. I bought a new pulley for the new pump install.

 

[bochnak]

Ok, wasn't sure it this was just converted and in initial start-up.

Check the caps and make sure cent switch is opening. Could be dirt/rust preventing free movement of switch.

You said trips after 10-15 mins. Is it running 100% of the time or cycling?

Can you plot a current vs. pressure curve?

Pull the belt and measure the no load current?

I run it in continuous mode most of the time (intake unloaders).

15A when the intake valves are open.

I adjusted cutout to 145psi and hitting 27.4A.

No tripping today and have been blasting on/off all day.

 

[bochnak]

I'd like to know this answer as well. What is the voltage measured at the motor starter while it's running? Do you have voltage loss due to long wire runs?

I'll look into this. Thanks.

 

[SGKent]

lower the pressure to 125 PSI. That is still adequate to blast and bead, just about anything. At 145 psi the particles are blowing apart into dust when they hit and cratering / spreading the surface metal. We used to use glass beads at 90 PSI for a poor man's shot peen. You can actually see the surface metal flow.

 

[ClappedOutBport]

lower the pressure to 125 PSI. That is still adequate to blast and bead, just about anything. At 145 psi the particles are blowing apart into dust when they hit and cratering / spreading the surface metal. We used to use glass beads at 90 PSI for a poor man's shot peen. You can actually see the surface metal flow.

I can't say for certain, but if he's a sane man he surely has a regulator. So the addition pressure would be for headroom.

 

[wyliesdiesels]

I run it in continuous mode most of the time (intake unloaders).

15A when the intake valves are open.

I adjusted cutout to 145psi and hitting 27.4A.

No tripping today and have been blasting on/off all day.

27.4a is still way too high for that motor. Youre shortening its life.

what are the ratings of the heaters in the starter?

Also, breakers for motor circuits are not sized to trip on overload because of inrush current. Breakers for motor circuits are ONLY for short circuit and ground fault protection.

The overloads in the starter or integral to the motor are for protecting the motor and circuit wiring.

 

[SGKent]

I can't say for certain, but if he's a sane man he surely has a regulator. So the addition pressure would be for headroom.

hence there is no need for headroom if the motor is way over current. If the voltage is holding steady then another alternative is to modify the pulley sizes again so it works less hard. Seemed dropping the pressure would be easier.

 

[bochnak]

I'd like to know this answer as well. What is the voltage measured at the motor starter while it's running? Do you have voltage loss due to long wire runs?

I was only able to measure voltage at my disconnect switch during running, and it was 240-245V. I need to measure at starter etc..

 

[bochnak]

NEC FLC table lists 5HP as 28a

So 30-32 is high.

If the nameplate lists 22a and its running 30-32a, then youre overloading the motor.

The heaters are doing their job to protect the motor from overheating.

Nameplate pic attached

What current should I be pulling? 24A?

 

[strutaeng]

What's the story prior to this issue? Is this a compressor you just bought? Someone changed something on it prior to being sold?

Or has it always been running like this and suddenly you start having a problem?

I've got a 7.5 to 10 HP compressor pump that I tried to run off a 5HP motor at the minimum RPM (1725 RPM motor and 550 RPM pump.) It ran briefly and eventually the heaters on the magnetic starter would trip. Heaters were sized correctly for 5HP.

I had to switch motors and found a 10HP. Used the same pulley ratio, and hits around 30A right before it switches off at 125 psi. It's a single stage, but still applies if it is 2 stage.

What's the spec. on the pump?

 

[walta]

Face the fact anytime you are drawing over 21 amps the motor is overloaded your motor has special service factor rating of 25 amps for a short time period.

You have two choices one keep overloading this motor and replace it with a 7.5 HP motor when its insulation gets too hot and fails letting the smoke out of the motor.

Option 2 is to reduce the load on this motor by lowering the cut out pressure and or changing the pulley and slowing down the pump.

Walta

 

[bochnak]

What's the story prior to this issue? Is this a compressor you just bought? Someone changed something on it prior to being sold?

Or has it always been running like this and suddenly you start having a problem?



What's the spec. on the pump?

I recently replaced the pump. Old pump was only putting out 9-10cfm, so the load/current was less and did not have the problem.

Pump is a eaton, or saylor 705 clone:
https://eatoncompressor.com/product/5-hp-air-compressor-pump-19-cfm/

Eaton uses a 7.8" pulley (so am I). Saylor docs state to run pump at 850 rpm with 5hp motor.

 

[bochnak]

Face the fact anytime you are drawing over 21 amps the motor is overloaded your motor has special service factor rating of 25 amps for a short time period.

You have two choices one keep overloading this motor and replace it with a 7.5 HP motor when its insulation gets too hot and fails letting the smoke out of the motor.

Option 2 is to reduce the load on this motor by lowering the cut out pressure and or changing the pulley and slowing down the pump.

Walta

I'm with you. Current pulley is 7.75. What pulley size would you guess I need. Hate to keep buying pulleys until I hit 21A. Maybe 80% reduction?

 

[bochnak]

I'm also wondering if a BRAND new motor would resolve this? I mean this pump is used on 5hp units.

Man, motor prices have skyrocketed in the past year. At least with Baldor.

 

[Aceman]

You have three choices the way I see it:

Return the pump and get one with a lower cfm rating.

Slow the pump down to drop your amperage. (you'll have to verify the oiling system is okay with the slower speed)

Replace the 5HP with a 7.5HP motor. (requires larger wire, breaker, and possibly a call to the utility co. first to make sure they're okay with starting that motor)

 

[strutaeng]

Are the Eaton guys blowing smoke? How is a 5HP pump making 19 CFM @ 100 psi and 17 CFM @ 175?

None of the 5HP compressors put more than 15-16CFM.
https://www.aircompressorsdirect.com/tools/5-horsepower-2-stage-air-compressors.html

I seem to recall the Eaton guys were not known to be the most honest of the crowd. I bet there's something fishy about the pump specs.

You do have a 1725/1750 RPM motor right?

The amperage draw is more on par with a bigger motor requirement. You need to plot a pressure vs. amp graph to see if it is even possible to get by lowering the pressure. I would be calling Eaton before spending more money. It is possible you have a defective pump.

 

[sberry]

I am not sure what Eaton says but the general rule for real 5 hp 2 stage is aprox 4 cfm per hp. Little lighter motors probably pull a little less.

 

[bochnak]

Are the Eaton guys blowing smoke? How is a 5HP pump making 19 CFM @ 100 psi and 17 CFM @ 175?

None of the 5HP compressors put more than 15-16CFM.
https://www.aircompressorsdirect.com/tools/5-horsepower-2-stage-air-compressors.html

I seem to recall the Eaton guys were not known to be the most honest of the crowd. I bet there's something fishy about the pump specs.

You do have a 1725/1750 RPM motor right?

The amperage draw is more on par with a bigger motor requirement. You need to plot a pressure vs. amp graph to see if it is even possible to get by lowering the pressure. I would be calling Eaton before spending more money. It is possible you have a defective pump.

Same specs on the SB705. I'm getting those specs, I have timed the fill and calculated the CFM.

"Cheaper" pumps at 5hp do get 15-16 CFM. Most of these are reed. I wanted discs.

My motor is 1725.

Eaton replied back and they use a 7.8" pulley (so 800ish pump rpm) and their motors pull 24A. I don't know what motor they are using.

I will try to get amperage on lower PSI. I vapor blast at 60-70psi so I can modify unloader to unload at 80psi or something much lower.

 

[bochnak]

You have three choices the way I see it:

Return the pump and get one with a lower cfm rating.

Slow the pump down to drop your amperage. (you'll have to verify the oiling system is okay with the slower speed)

Replace the 5HP with a 7.5HP motor. (requires larger wire, breaker, and possibly a call to the utility co. first to make sure they're okay with starting that motor)

At this point I will get amperage readout at lower PSI. Or slow down pump speed.

Otherwise I will sell this unit and buy a 7.5hp unit. I'm done messing with this 89' unit.