under amping motor

[bkboss]

I have a single phase 230 volt Dayton exaust fan motor. It's rated for 15 amps but only drawing 7. It is overheating and shutting down. Any suggestions to why?

 

[jbartos]

Suggestion: What voltage is it connected to?

 

[bkboss]

There are two lines each measuring 120v.

 

[jbartos]

Suggestion: Please, what do you mean by two lines each measuring 120V? Do you mean that there are two conductors connected to the motor with 120V between them, or do you mean that there are three conductors, two hot ones and one neutral, and there are two 120V voltages, each between each hot and neutral and 230V between hot conductors?

 

[bkboss]

There are two hot conductors and one ground. 230v between hot conductors.

 

[bkboss]

This used to be wired for 3 phase but converted to single. We have been replacing a lot of motors and my repair man doesn't know why.

 

[jbartos]

Please, what used to be wired for 3 phase? The existing single phase motor? Is it original three phase motor converted to a single phase 230V motor?

 

[bkboss]

The original exaust hood had a three phased motor , a single phase replacement was installed. There is a fuse box mounted to the unit with three hot wires coming in from the breaker box. One wire has been eliminated and the other two are connected to wires from the motor. Sorry about the confusion i'm by no means an expert in this area, but I appreciate your help.

 

[busbar]

Any chance the motor is roof-mounted and exhausting air from cooking equipment? In smaller single-phase motors, internal thermal protectors are sometimes furnished. High- ambient temperature could be cycling the motor.

Also, the motor may be fed by a 3ø magnetic starter with an (undersized for the 1ø motor) automatic-reset overload relay.

 

[bkboss]

Exactually, it's located in a Burger King. Seven months ago the motor was replaced because it was pulling too many amps and totally burn up. Our repairman corrected the problem by installing a new motor with an adjustable pully to lower the amps. Now it is getting to hot and shutting down and only pulling 7 amps.

 

[DanDel]

Has this single-phase motor been working ok for seven months and just recently started to get hot and shut down? How does it shut down, from internal thermal protection? Have you tried running the motor unconnected from the load to see what the current draw is?

 

[bkboss]

yes,when it cools down it comes back on and i'll check the current draw without the load, thanks!

 

[pablo02]

What is the location? Does the seven months of run time coincide with the arrival of summer? Is it in the summer sun surrounded by lots of heat? Is a roof top A/C unit blowing hot air on it? Consider what Busbar commented on...

How is this shutting down, is the fuse blowing, it just quits and must cool down to run again, what happens when it quits? Are there motor oveloads in the circuit? If so, were these changed out when re-wired for single phase... may have been OK for 3Ø but marginal for single...

 

[stardelta]

Shorted core laminations can cause the exact fault conditions you mention. Has the motor been rewound in the past or has it been faulty since new?

 

[buzzp]

Since the internal RTD's are causing the trip, it sounds like this is mounted in a hot spot as mentioned above. Perhaps a higher temp motor is in order and perhaps a smaller one if the one you have is pulling 7A with a load.

 

[jbartos]

Suggestion: Try to test the motor with fan at a different location since it has to be uninstalled anyhow, if the motor terminals have correct voltage.

 

[PUMPDESIGNER]

I just gotta love you guys, patient, patient, patient.
bkboss - I hope you do not take this as an insult, not intended as that.

You need a more talented crew for maintenance. Tell headquarters to get someone out there that understands this stuff. I appreciate that you probably already realize this, which is why you are involved, which you probably resent, which ...

Here are the problems that I see:
How do you know that the original motor "burned" up? Did they take the motor apart and examine it? We require that no motor be replaced ever without the technician first committing to a diagnosis as to why the motor failed, and then fixing the problem that burned up the orignal motor if there is a problem elsewhere.

Why would they change a three phase motor to a single phase motor? Big mistake, bad choice, bad decision, sign of ignorance, UNLESS you somehow changed three phase power in the building to single phase.

Busbar is exactly correct about the starter (as he usually is). If the contactor and overload are IEC style, then the new single phase motor must be connected to the old three phase contactor, whith a transfer wire taking the current for one of the phases through the pole not being used any longer, otherwise the overload trip curve will be dramatically shortened and cause problems.

Seeing and speculating what you are dealing with on the technician end, how can you trust anything they do? Is the new motor the same rpm, same horsepower, same torque? Anything they have done is suspect.

On an immediate basis, require that they put the correct motor back in, (3 phase), as specified by the OEM of the equipment. Then talk calmly to the maintenance department if you can and see if they can find a supervisor that is a quaified electrician that also has knowledge about motors.

If the level of competence in the maintenance department is too low, your stores will become what I call "full of fleas". After a few years you will have nothing but jury rigged equipment, faulty connections, poor workmanship, improperly installed equipment, and you will be soooo very miserable, like a dog that cannot walk he has to scratch so much.

PUMPDESIGNER

 

[jbartos]

Suggestion to the previoius posting: A modern electrical maintenace has a laptop with wireless connection to internet where they look for Forums such as eng-tips.com to get going.