Electric Motor Thermal Switch

[Whiskey Bent]

I bought a used 5hp motor for my air compressor from a neighbor, he had it on an auger but changed to 3 phase. He said the thermal switch was bad so he had an electrician bypassed it. So I got the motor on my compressor and I hooked the thermal switch up but it will only run for 5 minutes be it pops the switch. I bought a new air compressor pressure switch and it says it has built in thermal protection. So can I bypass the thermal switch on the motor? Picture is the pressure switch I bought.

 

[Steve@Advance]

First thing I would do, check the amp draw of the motor running under load.

That will tell you if the thermal unit is really bad or if the motor is running
within the FLA (full load amps) rating.

On a compressor, when properly set up, (a healthy compressor, running within
rated pressure, the proper pulley ratio, proper motor voltage) the motor will be
loaded right at the FLA rating. Going under the FLA won't hurt, but exceeding the
FLA will overheat and eventually burn out the motor.

Once you know the motor is running at or under the FLA, then you can decide to either replace the overload switch, or get the controller with built in motor protection.

BUT, somehow the controller needs to know the FLA rating of the motor. Either it is adjustable, or more likely a separate overload heater must be purchased to make the switch work. If it requires a heater, it should be listed there on the
page, and you will need the FLA rating from the motor tag to get the correct
overload protection.

 

[Geo-TH,In]

I agree with Steve. check the amps.
If amps are over the rated FLA remove the belt and check amps again.
If amps are below what it was under load, use a smaller pulley.
Motors typically run at either 3550 or 1725 rpms,
You can't take the larger pulley off a 1725 and put it on a 3550.

I have used smaller pulley to decrease FLA.

Don't screw with the thermal switch. It's there for a good reason, to watch the temp of the motor. Measure temp of motor using an IR thermometer.

 

[Butch(OH)]

Thermal switches are provided on smaller
motors because they are often used on
portable equipment. In that capacity they
are plugged into circuits that lack proper
protection or operated via long extension
cords that reduce voltage, both are motor
killers. That being said and as Steve and
George stated it is important to have
correctly rated protection and it matters
not if it is located at the motor or as
part of the starter. Check your operating
amps and the voltage AT THE MOTOR while
under load to see if they fall within the
motor rating, thats tbe only way you can
be certain if you have an overloaded motor
or sone other issue.

 

[buickanddeere]

Why dies it not bother the previous owner your
yourself when a motor protective device trips ? The thermals cut
Our prevents motor failure from overloading .
Either the current is too high , the thermal is damaged or the thermal installed is the incorrect current rating .

 

[Whiskey Bent]

Ok guys thanks. I will check this weekend and report back for more advice!

 

[Whiskey Bent]

Ok guys Im back. Sorry for my ignorance but I cant figure out how to check the FLA, and I dont see a FLA number on my motor. Picture of my multi meter and motor tag. Please help me understand this. Thank you.

 

[Whiskey Bent]

 

[JMOR]

29.5 amp motor and 10 amp meter is a no go.

 

[buickanddeere]

Full Load Amps of 29.5 amp and a 1.0 service factor

 

[Handsome Devil]

I had a small motor that overheated and shut down tried all sorts of tricks. Ended up the motor had been in a filthy environment and needed to be cleaned out. It had oily residue that held all the loose dirt and dust. It couldn't cool itself as designed. Once cleaned out it worked till I sold it 3 years later.

Couple years ago I had to clean a 5 hp 3ph motor on my radial drill press. HD GE monster industrial type motor. That motor had gone completely submerged in two floods before I got it. Then sat under a tarp for 15 years. Actually the whole drill press was submerged. After getting it apart it was pretty simple to clean but in that process you got to see how the motor cooled itself by fan and airways in the castings. Not saying this is the problem but have seen it before. OP says was used on an auger. That often hints to dirty or hostile conditions on a farm if it came from one? Good luck. Regards, John.

This motor from the flood was froze solid, blowing out with air was not going to fix it. Elbow grease, check and pack bearing, meg for short and shazam back in business.

This post was edited by Handsome Devil on 11/17/2021 at 08:17 am.

 

[buickanddeere]

I seen a 200HP , 600V open frame , air cooled electric motor operate while fully submerged in a sump. Never tripped the breaker or the thermal overloads.
We drained the sump, washed out the seaweed, meggered the motor to ensure it was good. Placed the motor back in service . Still operating no problems 8 years later when I transferred to another site .