Fractional motors 3 Phase unbalance

[2b2]

We received an italian machine with 7 VFDs (40HP)each one equipped with a small 3 phase motor to operate the cooling fan (spec 460 VAC, .29A).After 5 hours of operation the overloads began tripping. Checked amperage (Di-log clamp meter) T1:.41A, T2:.27A, T3:.19A, Supply Voltage:L1-L2 490.2, L2-L3 491.4, L3-L1: 490.1.
What could it be the source of unbalance?

 

[busbar]

If the readings are accurate, {i.e., made at the motor terminals} then it's likely the motor is defective. Check for unbalanced winding resistance to confirm the condition.

 

[SteveKW]

This site will help you pinpoint your problem.

http://www.franklinelect.com/Manual/AIM_29.htm

 

[busbar]

The Franklin Electric voltage-balance link is interesting, but I believe that unfortunately it only applies to single-bank, single-motor installations, such as would be case of an isolated water pump and associated utility-transformer bank. In an installation of multiple-motor [and potentially other three-and single-phase loads,] the voltage imbalance changes with differing combinations of running motors, so that the lead-rolling exercise becomes futile.

 

[SteveKW]

So what do you want him to do... throw the whole machine away? He has to start somewhere. Doing that test on each individual motor will balance his phases better and also help if he does it on the larger motors to. You add the SUM of all the imbalances to get your total imbalance. Lower all the imbalances and the SUM drops. With that test you can identify if its the motor or the source. I would be interested in what test you think he should do to find the source of the imbalance.

 

[SteveKW2]

This is the key point in the test I posted...

"If, on the three possible hookups, the leg farthest from the average stays on the same power lead, most of the unbalance is coming from the power source. However, if the reading farthest from average moves with the same motor lead, the primary source of unbalance is on the "motor side" of the starter."

If you do the test right at the motor and it follows the motor lead then the fault is the motor.

Doing that test beats having to remove the motor and take it to a shop and have them tell you "its not the motor"!

 

[electricpete]

As busbar points out, it would appear that 2b2 has already ruled out voltage imbalance... which seems to rule out supply problems. So that natural question is: why would several new motors all go south (apparent turn shorts) at the same shortly after installation.

Average current does not appear above FLA. Perhaps you can double check if there remains a motor with balanced current to see if that is above FLA.

Is the motor being operated within nameplate parameters.... ambient temperature, continuous or intermittent rating, proper wiring (delta or wye), correct frequency?

Voltage is a little high but still below 110% (although maybe it is varying over time?)

It seems remotely possible that your drives might be putting voltage spikes onto your 480volt system?

You might want to troubleshoot a little by checking winding resistance, insulation resistance, and if you have access to test equipment... a surge comparison test.

I'm just guessing here. If you can contact motor manufacturere I'm sure he'll help you work through application considerations. He'll also probably be able to tell you what tests were done on the motors before they left the factory. If you can't narrow down the cause I would recommend having the motors torn down for evaluation.

 

[2b2]

I rechecked the readings connecting the amp meter in series (used a Fluke 176) with the following results:

FLUKE CLAMP(DI-LOG) VOLTAJE RESISTANCE
.299 .28 L1-L2 496.1 T1-T2 231.1
.270 .23 L1-L3 496.7 T1-T3 246.3
.311 .327 L2-L3 495.4 T2-T3 238

I will try next rotating the phases. Thanks a lot

 

[electricpete]

You have a 6% range of resistances (3% deviation from average). That sounds like a shorted turn to me.

 

[electricpete]

I have seen limits of 1% and 2% winding resistance imbalance for large motors. I'm not sure if they apply for fractional horsepower motors.

 

[electricpete]

No standard limit for max acceptable winding resistance is identified in NEMA MG-1 or in IEEE112. As a result there are differing opinions

Note that there are 2 different ways to report it:
“range”=(Max-Min)/average
“imbalance”=(max deviation from avg)/avg.
Typically Range~2x imbalance.

For your data of 231, 238, 246, we compute average = 238
range = (246-231)/238~ 6%
imbalance = (246-238) ~ 3%

Here are some references:
EPRI 108773V1 (Large Electric Motor Preventive Maintenance Guidelines) recommends 5% range (~2.5% imbalance)
IEEE 1290 (Nuclear Motor Operated Valves) 5% range (~2.5% imbalance)
EPRI 1000897 (Large Motor Rewind Spec) recommends a maximum 1% winding resistance "imbalance" (approx 2% range.)
EPRI 111195 (Nuclear MOV’s) – 2% range (~1% imbalance).
EPRI 107524 (Large Nuclear Motors) recommends 1% range (~0.5% imbalance)
EPRI 111196 (large motor rewind spec) – 1/2% range (~1/4% imbalance!).

I'll grant you that some of these specs apply to large motors. The MOV specs apply to small motors. Your data with 6% range and 3% imbalance does not meet any of these criteria.

 

[busbar]

SteveKW, I suggested that with the line voltages acceptably balanced, lead rolling will give 2b2 little new information. Unbalanced currents with reasonably balanced voltages have already pointed to the motor. Clearly one phase current on the subject motor is higher than intended, properly tripping the motor-overload relay.

It sounds like other locally connected loads don’t exhibit this problem. The “…throw the whole machine away?” comment was not warranted.

 

[electricpete]

I'd like to throw in one more comment and 1 more question

comment - my comments above imply the motor may be bad with possible shorted turns... but I should add that you must also consider the accuracy of the measurement. Is it repeatable? If not then contact resistance might be affecting it... use 4-wire resistance measurement.

question - is it just one fan that tripped it's overloads or several?

 

[TheDOG]

If the motors are controlled by a VFD, the input voltage will have no affect on motor current. If you have a phase imbalance, it is a fault of the motor or the installation. Was the voltage reading taken at the motor or the input of the VFD? What is the motor nameplate voltage?
The winding resistance imbalance is the reason the motor currents are imbalanced.

 

[SteveKW]

Busbar, let's just say I woke up on the wrong side of the bed when I posted "just throw it away" (before having my coffee). Most likely the motors are also 50Hz. It will be interesting to see what the currents are once he rotates the phases around.

 

Sticking with my KISS theory of troubleshooting. I have measured 100's of 3 phase motors, many in the 1,2, and 3 HP range. The amp differences you describe are seen frequently. In a recent discussion with one of the Certified Energy Manager trainers at AEE, he indicated that this was almost always a result of an overall facility phase imbalance. If this is the case, you could go the whole 9 yards, and balance the faciity -- but hard to know if this is worthwhile compared to just buying bigger motors and overloads...this will not increase energy consumption as the load (the fan) determine the energy consumption. Have you asked the manufacturer what they think? Certainly consider harmonics from nearby large VFD's, these can result in "unexpected results" particularly large currents on nuetrals. Again, the larger motors and overloads could overcome this issue also, be cheap and quick to implement. You may want to consult manufacturer to ensure they don't take a "warranty voided" stance at a later date. I am mechanical, and certainly not as smart as most the people posting here, but would a 6% resistance difference account for a 50%+ amperage difference? I don't see this as being the problem.

 

[dpc]

If these fan motors are connected at the input to the VFD, you may want to check the voltage waveform or harmonic distortion. The waveform could be pretty ugly right at the drive input.

Have you checked the sizing of the overloads? I would agree with others that these very small motors can have considerable variation in current per phase, especially running lightly loaded.

Is this happening in every VFD or only one?

dpc