compressor motors

[Marktal]

We had a customer change over his original 10 hp, 4 p 3 phase tefc motor, for another. It was runnig a compressor in a refridgeration cool room. The motor provided only started once under load, and took full load amps. It was unable to start under load again. Even with the belts loosened quite much. Once taken the load off, it started quite well.
Are compressor motors desinged differently either with windings, or rotor? Do they start and run under larger loads?
It was assumed that the motor provided, given that it was the same speed, HP, FLA rating, frame size, tefc, that it would have been able to perform the same operation.
Is this assumption incorrect?

 

[waross]

The Nema Design letter indicates the starting torque of a motor. If you have replaced a design "C" with a design "B", or replaced a design "B" with a design "A" you may have starting problems. Check the nameplates of both motors for the Nema Design Letter.
yours

 

[itsmoked]

Could you have lost a phase or have some other electrical problem? Especially since you had to change out the previous unit.

Your compressor can also be failing or you could have a clogged filter or muffler, or liquid line problems.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[bklauba]

It makes wonder if the unloader valve is stuck closed (and whether that was the death of the old motor?)

BK

 

[itsmoked]

Oh yeah, that is very likely too! Good idea bklauba.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[bklauba]

It could also be the check valve 'tween the head and the tank: Same symptoms if it were stuck closed.

BK

 

[waross]

Hey fellas, this is a refrigeration compressor. They don't have a lot of that stuff that air compressors have.
The OP said that it started and ran once with (I understand) normal current. A refrigeration compressor usually starts under full head pressure. I've seen unloaders on old old ammonia compressors, but they were used for capacity control, not starting. They were bigger also.
It could be an internal problem with the compressor, depending on the elevation of the receiver, it could be oil slugging.
Bearing problems with a belt drive refrigeration compressor may show up as leaking seals before they get so bad as to stall the motor.
Marktal, is the machine in fairly good shape? Does it still have the normal controls, ie. high head pressure shut down, low suction pressure shut down, oil pressure shut down?
One thing to check is the motor connections. If it was wired for 480V and connected to 240V or 208V, it may be able to start and run. It wouldn't have much torque but it might be able to keep running. It would be running slow. Once there was some head pressure it would not be able to re-start.
yours

 

[jraef]

Aside from waross' first observation about the motor torque-speed curve design criteria, the only other possibility is a mechanical issue. Obviously it was not a hermetically sealed refer compressor or you would not have been able to swap the motor so easily. So what kind is it?

One major thing to consider. The hermetic units use the refrigerant to lubricate. If this is a different type, the lube may be a separately powered pump or a mechanical take-off from the compressor drive shaft. In either case, you may have lost the lube. That may have also contributed to the demise of the old motor, and the new one was able to restart it once, because it had more torque than the old motor as it was failing, but now the shaft has locked up tight.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[itsmoked]

waross; I beg to differ GREATLY with you. Not only do a lot of compressors have head unloaders a lot have multiple stage head unloaders!!!!

Any AND all Carrier and Trane open type (belt drive) compressors start unloaded then load. (e.g. have head unloaders)

They often need them to modify system capacity and for ease of starting.

If this function is faulty then you could easily prevent a motor from starting.

But alas as the OP mentioned he even loosened the belts and had a start failure which makes me think electrical.

Keith Cress
Flamin Systems, Inc.- <

http://www.flaminsystems.com>

 

[waross]

Hi itsmoked
I stand corrected. I see a few belt drive compressors but mostly semi hermetics. I saw a lot of the old ammonia machines with manual capacity control. They would start without the unloaders. We were comisioning an old, used Vilter. It started fine but we couldn't get the capacity control working. It turned out that even though it had the control solenoid on the outside, all the internal parts of the unloader had been removed.
Back to the problem at hand.
I understood that with the belts loose it wouldn't start but with the belts off it would start.

Even with the belts loosened quite much. Once taken the load off, it started quite well.

That is what made me think possible wrong voltage connection.
respectfully