Quiz: 120% FLA with only 50% of rated slip

[electricpete]

Vertical dual voltage (230vac / 460 vac) motor used in 480vac system.
60hz 4-pole motor.
Nameplate horsepower 20hp
Nameplate full load speed: 1750 rpm
Nameplate full load current on 460vac connection: 25A

Pump BHP vs flow curve is monotonically increasing, but peaks about 12hp. It should not be possible for pump to overload the motor unless there was a rub or something very unusual.

On 6/19/2012, the motor current was measured at 30A (balanced within 1A among phases). That suggests an overload on the order of 30/25 = 120% of 20hp nameplate But based on the BHP curve, the pump should not be capable of loading the motor beyond 12hp (unless there is a rub). AND, the speed is 1775cpm (based on vib spectrum, reasonable frequency resolution), which suggests the load is on the order of (1800-1775)/(1800-1750) = 50%.

On 6/19/12, motor was uncoupled from the pump and uncoupled current measured as 16A and balanced.

I think I know the cause (will see tomorrow). What would you say is the cause and/or what would you check?

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(2B)+(2B)' ?

 

[Skogsgurra]

There are two data that I would check. First, the motor voltage. If high, the current goes up and slip reduces somewhat. Second, if on an islanding generator, I would check frequency. If frequency is off, the calculated slip is wrong. But I do not really think frequency is the cause.
Third? Harmonics? Waiting for answer.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

I would be looking for another ammeter for a reality check, Pete.
I assume that this is in a plant and if there was a voltage or frequency issue you would be asking about a group of motors not just one.
Reminds my of the old days when we did much more DC work. When overhauling DC motors we would clean them up and change the bearings. We would send the armature to the machine shop to have the com' turned and when it returned we would under cut the com'. A helpful machinist would occasionally "clean up" a dirty rotor with one or two cuts. The increased air gap would result in a loss of torque and overheating at rated load.
Anny chance that the air gap has been increased?

Bill
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"Why not the best?"
Jimmy Carter

 

[electricpete]

OK, I'll tell you the "theory" I had when I posted this question. I thought the motor was mis-wired for 230vac connection. I think this would give these exact symptoms. (By the way, another 20hp 1800rpm vertical motor for same application was reported to have 8 amps during a previous no-load run, but I'm not sure if it was truly identical motor... that other motor may have been a different model with same ratings and framesize).

I suggested connections be checked. They were checked overnight by nightshift and the report this morning is that the motor was wired correctly. Hmmm.

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(2B)+(2B)' ?

 

[LionelHutz]

I wouldn't expect the motor to survive for any length of time connected for 230V and operated at 480V. 16A is a higher uncoupled current than I would expect for that motor. However, it is too low for that motor if it was connected for 230V. I'd expect even the open-shaft current to be over the FLA of the motor if it was connected for 230V and running on 480V.

Is this measured directly with a current probe or some other metering system?

Care to share the connections or number of leads for the motor? I could see these readings happen with a 12-lead delta connected motor. There are 2 possible 12-lead motor types, one that is connected Y run and one that is connected delta run. If you wired a delta run motor following the higher voltage wye connection diagram then you'd basically have a 400V motor running on 480V which would be about right to exibit the issues you are seeing.

 

[electricpete]

The motor is 9-lead motor. I inspected term box this morning myself (to double check night shift report). Results are motor was correctly connected for 460vac.
T4 was connected to T7
T5 was connected to T8
T6 was connected to T9

T1, T2, T3 were individually connected to power system leads Black/Red/Orange respectivey.

Motor was installed a few days ago (6/18 I think). At that time the direction of motor rotation was observed and confirmed correct.


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(2B)+(2B)' ?

 

[waross]

New motor? Faulty rotor? Bad ammeter?

Bill
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"Why not the best?"
Jimmy Carter

 

[LionelHutz]

Sounds like either a bad new motor (possibly mis-labeled) or a bad current probe.

 

[electricpete]

"voltage" "frequency"
At our plant, these are somewhat predictable. High voltage is not expected.


"Bad ammeter?"
I sort of doubt repeatable high readings on all three phases.

For various reasons, we elected to recouple the motor so it is available if needed in the short term. We will have a coupled run shortly and I will ask for current readings to be rechecked with a different meter. We will also check the speed again at that time.

"Harmonics?"
Personally my engineer instinct drives me to ask for a current waveform and even better simulataneous measurement of voltage waveform and current. But given this is a small motor, not particularly critical, I am not justified to ask for that and it won't happen unless I get involved in helping set up the instrumentation myself, which I don't have time for.

"New motor?"
Removed from another position and refurbished in 2011.

"Faulty rotor?"
Defect in rotor bars (such as open connection) causes high slip.

But on a related note, I am definitely inclined to suspect some kind of problem with motor. Maybe as you said problem with airgap geometry introduced during refurbishment. Or as Lionel said some mislabeling of the leads..... connecting 4 and 7 together may be putting into the same phase two groups of coils that don't belong in the same phase based on their physical position, which kills the distribution factor and gives lower fundamental flux density, requiring higher current to achieve the same power.

I will definitely recommend to replace the motor (barring some unusual new information during the upcoming coupled run).

Unfortunately, chances are there will not be much interest to take the time to figure out what was wrong with the motor. So this quiz turned out not as interesting as I thought. I'll keep you posted if anything interesting developes.


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(2B)+(2B)' ?

 

[ScusaMe]

EP ..

Vertical Motor...... THRUST BEARING ?????

\ 'scusaMe

 

[ScusaMe]

EP

After motor teardown .. Battery and compass to check polarity of windings
in search of internal mis-connection or lead mis-tagged.

There is a logical answer to mystery.

/'ScusaMe