Motor Question

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bkludecke

Senior Member
Location
Big Bear Lake, CA
Occupation
Retired Electrical Contractor
About 6 months ago I was called out to check on a 3hp 3ph pump motor @ a propane plant. The motor had tripped the overloads so I reset and checked the current under load. The motor current was way below the nameplate FLA but one of the phases was about 12% higher than the other two. The motor was replaced by another company today (not electricians) and they told the plant operator it was not getting enough amps.:cool:

I went out and checked the motor connections, OK. I checked voltage, AB=251 AC=249 BC=249 AG=129 BG=128 CG=218. It's a 120/240 3ph delta system. The motor is a wierd size 3 1/2 HP with a nameplate rating of 13.4 FLA at 230volts. I measured the current A=7.6 B=8.4 C=7.6 amps. There was very little load on the motor so I was not suprised except for the middle phase being that much higher. Is this within normal limits or is there a problem somewhere?

The pump seems to be operating OK.
 
a small amount of voltage imbalance will result in a larger amount of current imbalance in 3 phase motors. Usually on order of 10 times.

I calculate voltage imbalance is 0.534%

I calculate current imbalance is 6.77%.

Looks reasonable to me. However, if overloads are blowing that could mean that some other time when you are not there measuring, the imbalance could become worse and overload a winding.

However, if the motor never runs more loaded than when you ran it, you might be able to adjust the trip settings of the overload relay to trip at a higher amount of current without damaging the motor. What are the overloads set to trip at now and are they adjustable?
 
bkludecke said:
... and they told the plant operator it was not getting enough amps.:cool:
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Gotta love those kinds of "experts" eh?

What kind of OL relay? Some solid state OLRs have built-in phase imbalance protection that is overly sensitive but cannot be disabled and give users a headache with nuisance tripping.

If it is a 3 phase motor in an area that is predominantly single phase residential loads, you can end up with severe voltage imbalances from the utility at different times of day because of the unpredictable nature of residential loading. Some people are home, some are not, the utility may not be reacting fast enough to prevent a severe imbalance on the few 3 phase services in the area. We get that a lot around where I live as old farm land is taken over by housing developments.

What is the load in that propane plant? Is it a compressor or transfer pump? If so, and it is a positive displacement pump (such as a gear pump), you may also have a sticking valve or some other sort of malfunctioning mechanical protection device on the propane system that is supposed to shut the motor down long before it overloads. But it isn't working right, so the OL is the last line of defense. If so, you had better find that other issue because if that is the case, adjusting the OL relay beyond the motor FLA rating may just create a disaster.
 
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Thanks for the help guys. Tomorrow I'll get the poco to install their chart recorder for both voltage and current on the service for a week. That should answer the power question. I'll put my recorder on the machine to see what it is doing.
 
bkludecke said:
I went out and checked the motor connections, OK. I checked voltage, AB=251 AC=249 BC=249 AG=129 BG=128 CG=218. It's a 120/240 3ph delta system. The motor is a wierd size 3 1/2 HP with a nameplate rating of 13.4 FLA at 230volts. I measured the current A=7.6 B=8.4 C=7.6 amps. There was very little load on the motor so I was not suprised except for the middle phase being that much higher. Is this within normal limits or is there a problem somewhere?
Click to expand...

Your voltage readings seem to be okay. The nominal voltage on a 240/120 3PH 4W system are supposed to be Vag = 120V, Vbg = 208V, and Vcg = 120V.

An almost 10% current difference (from high to low) should be investigated. Unless you have some type of current limiting impedance in the lines the power system will deliver whatever amps the motor wants to draw, well at least up to the point the protective devices operate.
 
"The motor was replaced by another company today (not electricians) and they told the plant operator it was not getting enough amps."

You can get a bucket of amps from lowes this week their on sale. Simply ad them to your circuit and turn it back on.
 
I would suspect that the motor leads were not spliced properly with this much of a current diference from the nameplate. It almost seems like the motor was wired for high voltage with the windings in series. And I would check this asap being in an explosive atmosphere.
 
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The motor lead connections looked good but I'll double check. The weird part of this is that the old motor had the same symtoms (one leg reading higher than the other two). I even considered an insulation leak to ground, maybe I'll disconnect the motor leads and check for current..

I think HD has a better price on amps this week but they are a different brand and come packaged in bags rather than buckets. I'll double check today & let y'all know:D. Thanks a heap for that suggestion.:smile:
 
Rotate the leads. A to B, B to C, and C to A. Do this until you have the least amount of imbalance. Works sometimes. Don't know why.

Thought it was only for open delta systems but we dropped imbalance by 2% on a 480v Wye doing that today. Imbalance is still high but better.
 
ptonsparky said:
Rotate the leads. A to B, B to C, and C to A. Do this until you have the least amount of imbalance. Works sometimes. Don't know why.

Thought it was only for open delta systems but we dropped imbalance by 2% on a 480v Wye doing that today. Imbalance is still high but better.
Click to expand...

Won't rotating the leads change the rotation direction of the motor?
 
wirenut1980 said:
Won't rotating the leads change the rotation direction of the motor?
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No, you don't swap them, you just "roll" all 3 of them one position. A to B, B to C, C to A. The relative rotation stays the same, but you are connected to different legs.

By the way, if the imbalance follows the roll, the problem is not the motor. If the imbalance stays the same, the problem IS the motor. If it isn't the moor you can repeat the process at the motor starter end; if the problem still follows the roll, it isn't the motor leads either, etc. etc. Not too difficult on a 3HP but lots of work if it's a big motor.
 
wirenut1980 said:
Won't rotating the leads change the rotation direction of the motor?
Click to expand...

As you may notice he did not change to order of the leads. the reason why this will work is that each load phase have different resistance and each line voltage is different. Finding the optimum combination will produce the closest equal current AND the motor will be operated at maximum efficiency on the given system.
 
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