Motors Running FLA's

[stripedbass]

I have 2 motors at 2 different job sites running Liquid Ring Vacuum Pumps from the same company. One motor is 10 HP and the other is 25 HP. Both motors are single voltage 3 ph. 208 VAC although both have been rewound because the orginal ones roasted. Both motors are running at their max FLA. The wiring to the both motors appears adequate. Other than being undersized, is there anything else that could cause the motors to run at full load? I was wondering about the power phase coming in from the street. Is all power at the street 3 ph. What should I be looking for?

thanks,

stripedbass

 

[jbartos]

Suggestions:
1. Check the shaft load HP. The shaft load may have increased.
2. The motor terminal voltage may have decreased. It is supposed to be rated voltage indicated on the nameplate.
3. Check the voltage harmonic content (Total Harmonic Distortion THD of the power supply voltage is supposed to be less than 5%).
4. Check the voltage phase imbalance. It should not be more than 3%.
5. The licensed electrician/service, specialized in motor service, may diagnose it accurately.

 

[stripedbass]

The voltage phase to phase is within 2.2% so that should be O.K. The motor/load match is in question but the manufacturer says the amps are too high as opposed to their design testing. The voltage at the motor is O.K. Please explain phase imbalance and THD. These motors are wired directly to the power company lines at the street. I'll need to look at that. It's a long run but it was approved by the local electrical inspector.

 

[jbartos]

1. THD<5% may be found/measured by a harmonic analyzer. Nowadays with a lot of nonlinear load, the harmonic content is appropriate to check since the true RMS of current increases because of added current harmonics since the total Irms = (I1**2 + I2**2 + I3**2 + ....)**0.5
where
** stands for an exponent
I1 is fundamental
I2 is second harmonic (usually negligible)
I3 is third harmonic
etc.
The added current harmonics due to voltage harmonics in the power source increase the motor full load total rms current.
2. Voltage V or current I imbalance (=unbalance synonymously) is calculated as:
(|Vab| + |Vbc| + |Vca|)/3=Vav
for a,b,c phase subscripts, then
Vunb(%)=[(Vphasetophasemax-Vphasetophasemin)/Vav]x100% < 3%
Similarly for phase to neutral voltage that must have voltage unbalance (=imbalance) < 3%

 

[don01]

Stripedbass,
a few suggestions: measure the supply voltage again
1) at the supply to your plant
2) at the contactor to your motor
3) at the motor (if safe)
have got voltage between all 3 pahses??? i presume so
take a tong tester and measure the motor current again. It should be approximately the same across each phase. (as per jbartos).
consider what % of the name plate data this is. >100%??
If possible decouple the motor and check the no load current it should be close to the manufacters spec. Give the rewind man a ring or check the makers website for no load current.
My gut instinct is that you will find a substantial leak or carry over in your vaccumm system. Check the pressure at the pump / capacity tank then go down the line(s) looking for a sharp change this may indicate where your load is
All that failing get the motor rewinder out and the vaccuum pump man out at the same time

Plse keep us informed
Don

 

[Wirenut]

It sounds like to me that your load is too high for your motors. Simply increase the horsepower rating of the motors. Make sure your wiring, motor starters, etc can handle the additional horsepower.

 

[stripedbass]

I talked to the manufacturer, they said that 10 H.P. is the motor that was selected and that it ran about 32 amps during their test. I can't figure out why they would do that unless it has something to do with torque. Anyway, they're going to look into it further. I'll let you know

stripedbass

 

[Wirenut]

Pumps are funny. Especially vacuum pumps. If the air exhaust is not throttled correctly, your load and therefore your current can rise above FLA.

 

[stripedbass]

It turns out that the service factor is really 1.28 not the stamped 1.15. The motor name plate is stamped 28 amps. The manufacturer says that the motor is good for 36 amps. We also found today that the LRP pump oil heat exchanger cooling fan is blowing air the wrong way. The cooler and fan(radiator)is located outboard of the motor. The fan is pulling air from the motor side through the cooler. IOW's the motor housing is not getting any cooling at all. Air should be blowing over the motor. (The motor is totally enclosed; Class 1, Div 1). Question: When air flow is restored over the motor housing (cooling the motor some)should we see a small drop in amperage?

thanks,

stripedbass

 

[jbartos]

Some small amperage drop will be experienced, depending on a size of the blower. If the motor windigs are damaged, e.g. by interturn shorts, the motor will draw higher current and become warmer.

 

[don01]

strippedbass,
I have read your last post 2 or 3 times now & maybe there is a little terminology difference here. Are you saying the load on the motor is 128% of name plate rating? AND the pump set manufacturer says this is ok?
Down here under the rest of the world if the name plate says x then the overload is not set to x+ something. I cannot find one reference that promotes this.
Lets face it the maker is saying his /her motor can draw x amps on continous duty with out the iron & copper losses causing a rate of rise for internal temp that will lead of a breakdown of insulation.
Talking to some of my peers here one comment was &quot;is that a manufacturing plant or a casino&quot;
Now I've sounded off Someone will show me how to eat humble pie again

Don

 

[Wirenut]

Don,

Right on!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Wirenut of Virginia

 

[stripedbass]

Here's the story: The motor is running 32 amps. The nameplate is stamped 28 amps. The service factor is stamped 1.15. However...the pump manufacturer issued a certification stating the service factor is 1.28. The motor is a Reliance that was originally wound for 230 VAC but was re-wound for approx. 212 VAC to match PSEG's power supply. The voltage on the namplate is stamped 212 The pump manufacturer says that, according to their pre-release tests, the motor runs at 32 amps and that is acceptable to their engineering department since the service factor is 1.28. This is a new job I'm working and all these problems were pre-existing. We started the pump/motor today with the new fan. How long do you think it will run?

 

[jbartos]

I have seen a maintenance man to replace a burned 5HP induction motor at a pump by 10HP induction motor. It ran o.k. but the pump is endangered.

 

[Wirenut]

If you burn up a 5 hp motor and then you burn up your pump, you need a higher rated motor and pump.

The solution is that simple. Wirenut of Virginia

 

[rhatcher]

It is not uncommon for OEM manufacturers of pumps, compressors, and blowers (variable torque loads) to supply motors with their equipment which must run at the service factor to provide the necessary power to the load. Personally, I disagree with the practice, but there is (somewhat?) of a justification for doing so. I am willing to go into that if anyone is curious.

In your case, the original rating of 32A at 230V is the power requirement of the pump. The motor rating of 28A at that voltage for a 10hp motor is correct by NEMA standards. The pump manufacturer's assertion that the motor can operate at 32A continuously in this application is correct if the motor has a service factor of 1.15 and the ambient temperature is 25C or less (ie 28*1.15=32). Beyond this your situation seems sketchy.

In your case, the namplate data for the new rating (212V) is somewhat ambiguous. Superficially it makes sense that with a reduction in voltage from 230 to 212 that the service factor will (should, could?) change from 1.15 to 1.28 to allow the maximum allowable current to increase proportional (from 32-36) to the reduced voltage. However, the idea that a proportional increase in current will offset a decrease in voltage with the resulting power being the same is a fallacy because of the relationship between volt/hertz and torque (power) in an induction motor.

It would appear from your post that the only nameplate values that were changed when your motor was re-rated were the voltage and service factor. If in fact you have motor whose nameplate states 10hp at: Xrpm, 212V, 32A, and 1.32SF and the motor current when supplying a 10hp load is 36A then I would question that rating based on NEMA and EASA standards for motor ratings and re-ratings respectively.

With respect to the heat exchanger fan direction, it makes sense that with a totally enclosed motor (specifically totally enclosed, not TEFC) on a &quot;package&quot; motor/load system that the heat exchanger fan would blow away fom the motor since a totally enclosed motor rating is based strictly on ambient temperature. This is because the only cooling provided (or necessary) for this frame type is internal to the motor. If the motor is exposed to greater than ambient temperatures (ie air blowing across it after passing though a heat exchanger) then it would have to be derated.

In sum, I would be very suspicious of the situation you are describing based on the information provided. In the best case you have a motor that was actually rewound to provide NEMA rated hp (and the implied service life) but has an incorrect nameplate. I doubt whether this is the case. In the worst case you have a motor that is not properly rated (or nameplated) and will last perhaps as long as the warranty for the equipment with which it was supplied. As a test of where you stand, take a temperature reading of the stator of the motor in question and compare it to the temperature rating of the insulation class.

 

[rhatcher]

If I may elaborate on a point from the previous post:
&quot;However, the idea that a proportional increase in current will offset a decrease in voltage with the resulting power being the same is a fallacy because of the relationship between volt/hertz and torque (power) in an induction motor.&quot;

This statement is incomplete. An induction motor &quot;ideally&quot; will produce the same torque (and power) at rated speed with reduced voltage and increased current. However, in a practical sense this will not be the case since increased current will cause increased temperature and, more importantly, saturation in the motor core will produce a nonlinear voltage/current relationship. As well, a motor operated below nameplate voltage will not produce NEMA specified torques and currents and if nameplated for such duty will require all of the nameplate values to change, not just the voltage and SF.

 

[don01]

Stripedbass,
yeh, I still don't feel right about this motor set. Ihave said &quot;in this forum&quot; that I try to size motors close to the projected load. But this &quot;just don't feel right&quot;
Put it on the pump people- give a warranty or guarantee. I am assuming you are having problems with trips etc. Maybe check it out with Reliance. My experience with thier controls people is they are very helpful.

Please keep us informed & good luck
Don

 

[stripedbass]

I'm happy to say that the company is reimbursing us for the rewind and some expenses. The motor is still running flat out (so to speak).

Thanks to everyone who contributed. If anything exciting happens, I'll let you know.

stripedbass