motor troubleshooting question.

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Fulthrotl

Fulthrotl

~Autocorrect is My Worst Enema.~
Location
HB, CA (19 Hrs. 22 Min. from Winged Horses)
Occupation
E
air compressor, 1ph/208v motor.

running current is 23.8 working under load. 5.3kw. 7.5 hp TEFC

starting is another thing......
stutters badly for five or six seconds, then lights up
and runs smoothly.

i'm guessing the centrifical switch has either stuck
open, or closed. if stuck open, there is no starting
cap.

if stuck closed, it burnt out the starting cap.

i usually just remove and take to a motor shop.
is there a way i can easily test the centrifical switch?

or, does it sound like something else entirely?

it was working fine, until they moved it about 7'
further down the wall. same circuit, same feeders,
same everything. now, it's like trying to kick start
a harley with one plug wire disconnected.

motor's been in service a year and a half.
 
Barbqranch said:
Can you hear it unloading the air pressure on the head when it turns off?
Click to expand...

One you get above 5HP, it is not uncommon to have unloaders that hold the suction valve open as opposed to dumping the head pressure. Either way, if the compressor is not properly unloaded, it could be the problem. Does it start ok when the entire system is at zero pressure? If so, unloading is likely your problem.
 
If open capacitor circuit, whether centrifugal switch is stuck or if capacitor is burned out, it probably won't have enough torque to start at all.

I agree with others to make sure pressure unloader works, also maybe make sure if there is multiple capacitors (series or parallel) that all of them are good.

Some chance there is a fault in aux winding or even fault in a start capacitor but still works enough to get it started, once up to speed the aux winding is out of the circuit and it runs fine.

Poor contact in that centrifugal switch is also a possibility.
 
I'm with the others.

I'd bleed the whole system down to nothing, and clip some test leads on the bottom of the starter to monitor the voltage while you fire it off. Then go from there...
 
Cow said:
I'm with the others.

I'd bleed the whole system down to nothing, and clip some test leads on the bottom of the starter to monitor the voltage while you fire it off. Then go from there...
Click to expand...

yeps. i'll log voltage and current at the contactor.... i was looking
at the current when starting, and instead of a spike and drop off,
it kept fluttering between 50 and 110 or so amps, until it hit synchronous
speed.
 
The centrifugal switch you can only check by stripping the motor end bell.

This is what I found inside a S Ph motor

Centrifugal-switch_zpsavxmdyxa.jpg
 
drcampbell said:
Is the motor actually designed/rated/labeled for 208 volts? A 230-volt motor on a 208-volt circuit might be hard to start.
Click to expand...

Very good point. I spent a good part of my career in the compressor business and don't recall ever seeing a 208 V single phase motor. The company I work for never offered such a beast.
 
drcampbell said:
Is the motor actually designed/rated/labeled for 208 volts? A 230-volt motor on a 208-volt circuit might be hard to start.
Click to expand...
Unless the 208 volts runs at the low end of what is acceptable, it probably works fine. Here 208 volt systems almost always start out unloaded near 215 volts. 207 volts is -10% of motor rating of 230.
 
Fulthrotl said:
yeps. i'll log voltage and current at the contactor.... i was looking
at the current when starting, and instead of a spike and drop off,
it kept fluttering between 50 and 110 or so amps, until it hit synchronous
speed.
Click to expand...

If there is a contactor, make sure it is not "chattering" when current is high - like you would have during starting. If so - you may have too much voltage drop during starting and it is dropping the contactor out, then it pulls back in when current stops because voltage went back up, repeat cycle. If it is lucky enough to accelerate the motor this goes away once current reaches a certain level.

If this always worked before, something has changed.
 
Tony S said:
The centrifugal switch you can only check by stripping the motor end bell.

This is what I found inside a S Ph motor

Centrifugal-switch_zpsavxmdyxa.jpg
Click to expand...

That looks nasty. Years ago I worked for a compay that manufactued some of their own printed circuit boards and all that fiberglass dust would get inside of the motors.

A friend of mine worked for a chicken processing plant and would tell horror stories about what they would find in motors.
 
growler said:
That looks nasty. Years ago I worked for a compay that manufactued some of their own printed circuit boards and all that fiberglass dust would get inside of the motors.

A friend of mine worked for a chicken processing plant and would tell horror stories about what they would find in motors.
Click to expand...
I thought it looked relatively clean compared to some I have seen:)
 
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