Imbalance

If the motor is defect free, then Your powering cables for all phases may not have equal lengths or equal conductor diameters or equal number of strands.or combinations of all these three. in other words input impedance for all three phases is not equal. You should check the current drawn by motor instead of resistances.

413 vs 406 ≈ 1.7% imbalance. This does not seem very severe, and might even be due to a loose wire nut or the like.

We did find loose wires under the termination block but this did not cure the imbalance. I am not worried about the level of imbalance but why we only have it when i connect to the motor and only on the yellow phase. The motor still runs no problem it is just a general enquiry to see if someone else has ever had this problem. The core lengths are all the same and the core diameters are all the same. When the cable is not connected i have 1V of a drop between Red/Yellow but when i connect the cable to the motor that reading increases to 7V. It is the same on the other 4 motors the Red/Yellow voltage reading is about 7V lower than the other 2 readings. I just think this is strange and was just trying to understand why this should happen.

the answer is "back to basics"

I'm with you...I wouldn't even notice a small difference like that. It doesn't seem like a "problem" at all.

I wonder what the mfr's specs are? I bet this motor is within tolerance.

are you using flat cables or single core cables to your motors?

Imbalance is a fact of life.

"Imbalance is a fact of life."

Bingo. Although I like to say it as "Imbalance happens."

Nothing on this planet is perfect, there are always small differences in two things, exponentially more in 3 things, such as 3 phases and 3 motor windings. What you have discovered is a phenomenon that motor people often take advantage of in helping to get the most balanced voltages possible. It's called "rolling" the conductors and is used when you find yourself in the exact situation you described, a slight imbalance: move A to B, B to C, C to A. It doesn't change the rotation but it takes advantage of slight variations in conductor resistance, impedance irregularities etc. between the three windings and the three motor leads.

Please measure the harmonics in voltage before and after energising.

you can check the voltage and power between L1,L2,L3 phase when the moto is working

The problem may also be in the source to your factory. If the distribution line to your factory has a PF imbalance the resistance on that phase may be higher IE higher source resistance on that phase due to differential heating. Try taking the measurement again on the weekend or after business hours and see if it is different.

What is your supply transformer kva, and length and gauge of cable runs? With such a small load I am surprised you notice it connected at all. What happens if you connect a balanced three phase resistive load such as an incandescent lamp load?

Good day Bali Boy

I think Rakesh Semwal has the answer. Check the current drawn by each phase, the excitation currents may have a typical H-L-H pattern causing the perceived imbalance. This is typical of transformers as well.

Your source power without load is excellent.

Your voltage drop is due to load activated and doing work.

Your voltage imbalance can lead to current imbalance which is the major issue, but, in this case, the voltage without load is very good from the source, so, the "loading" is coming from the motor and or what it is connected to, or the work it is performing.

As long as your current is on or about the FLA on the nameplate data, its OK. If your over but not over by more than SFA, your imbalance is still OK if imbalance is less than 5%. More than that and you need to perform rotations to find the best balance.

The reasons for loading on just one leg/winding of the motor is multiple. could be dirt, bearing, wear, or aging. monitor for heat build up only if running amps higher than FLA nameplate data.

IF all within parameters, just basic PM, keep circulation area clean. No one gets out alive and all things come to an end. Motors included.

What 'Cable' do you disconnect and still get Voltage readings on your motor terminals? Also are you concerned about the resistance imbalance or the voltage imbalance? Remember a 1% voltage imbalance can cause a 6-7% current imbalance try taking current readings while the motor is under load. Is this a AC Induction motor?

If you interchange the cables on the motor terminals, and still have the same imbalance but related to the same terminals and not the phase colours, AND this happens on all the motors of this same batch, then maybe the imbalance is due to the slight differences in the iron works (Stamped laminated stator and rotor make up. i.e. the magnetic fields. Since it is the same batch, they will exhibit almost the same imbalance due to the fields imbalances. (Lw is more important than the resistance of the windings).

Generally,2% voltage imbalance is allowed without derating of motor Kw capacity. You have mentioned that you have measured motor winding resistances and the figures indicated are also found normal. But which method was adopted for this measurements? You must use Electric motor checker instrument for measurements of % inductances( L-L/ L-N ) and resistances of motor coils . Most probably you may observe that there is a variation in & indutance values and this may be the reason for the voltage imbalance.

Manindra.

I agree with LA LUKE. If you rotate the location of the power leads and the motor terminals keep the same voltage imbalance, the motors are the source of the imbalance, if the power lines keep the imbalance your power supply is the source of variation.

Resistance is only a minor part of the impedance (Z=R+jXl) . The inductive reactance is the main factor that affects the total impedance of each phase. What about currents? (Z=V/I). Note that line to line voltages and phase voltages are not nesessarily the same.

Although motor manufacturers goal is to construct the three phases identical, even shape variations on the windings if not different length or a missing turn, could result on different motor reactances.

Certainlly there are standards that manufactures shall comply with.

I would tend to agree with the following comments:

1.- Less than 2% is nothing to worry about and maybe inherent to the system

2.- The length difference of the cables may also be giving you the discrepancies once the motor is connected.

3.-The power distribution in your plant maybe imbalanced.

4.- Biggest truth in the world "Imbalance exists"

I'm assuming that this motor is being run through a starter. Have you checked line voltage before and after the contacts right in the starter box itself or or you checking at the motor terminal box? Did you remove and check the contacts for signs of arcing or erosion? Additional resistance at the contacts could explain why you're having a slight voltage drop at the motor terminal block or even on the load side of the contacts.

I agree fully with Vargaalex explanation, additionally I recommend you to touch the motor after half an hour running if you can do that then you need not worry and stop thinking about unbalance of voltage.

For the heat? Yeah, that's a good hourly/daily rounds kind of check.