Motors Running FLA's
Motors Running FLA's
(OP)
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
thanks,
stripedbass





RE: Motors Running FLA's
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.
RE: Motors Running FLA's
RE: Motors Running FLA's
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%
RE: Motors Running FLA's
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
RE: Motors Running FLA's
RE: Motors Running FLA's
stripedbass
RE: Motors Running FLA's
RE: Motors Running FLA's
thanks,
stripedbass
RE: Motors Running FLA's
RE: Motors Running FLA's
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 "is that a manufacturing plant or a casino"
Now I've sounded off Someone will show me how to eat humble pie again
Don
RE: Motors Running FLA's
Right on!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Wirenut of Virginia
RE: Motors Running FLA's
RE: Motors Running FLA's
RE: Motors Running FLA's
The solution is that simple.
Wirenut of Virginia
RE: Motors Running FLA's
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 "package" 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.
RE: Motors Running FLA's
"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."
This statement is incomplete. An induction motor "ideally" 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.
RE: Motors Running FLA's
yeh, I still don't feel right about this motor set. Ihave said "in this forum" that I try to size motors close to the projected load. But this "just don't feel right"
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
RE: Motors Running FLA's
Thanks to everyone who contributed. If anything exciting happens, I'll let you know.
stripedbass