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Join Date: Jan 2018
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Current Unbalance in Three-phase Induction Motor Under No Load

06/11/2018 2:53 AM

Good morning friends,

I am having problem with my 3 phase induction motor after it come from rewind.

Motor detail 30 hp, 3465 rpm

When offline test was conducted winding resistance of each winding was found equal... But when the motor was connected to supply voltage and current measured was found highly unbalanced

U=10.7 A.

V=10.5A

W= 3.3 A

Why does this unbalance in motor current was found.i checked no unbalance in voltage was found

Is there any tolerance for no load current balance..does this problem indicate rewinding or it's ok.

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#1

Re: Current unbalance in 3 phase induction motor under no load

06/11/2018 5:34 AM

Cr4 does not provide a "magic crystal ball", so we can see your supply voltage.

Guessing your voltage as 480, because 3465 rpm suggests 60Hz supply, I estimate full load as 34 amp/line.

In that case a "no-load" current of 10 amps/line looks OK, with a range of 10% or more, but 3 amp is wrong.

Since I cannot think of any motor problem which would give 3 amps, I would ask....

  1. Have you got a bad connection in one line?
  2. Have you measured voltage at motor terminals or supply or both?
  3. How you measured the current and where? Clamp-on meters can get magnetic field from cables other than one you put-around - so apparent 3 amp could be zero.
  4. Did you measure the voltage at same time as current?
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#2
In reply to #1

Re: Current unbalance in 3 phase induction motor under no load

06/11/2018 5:50 AM

Sorry for missing out the voltage it's 380v 60 hz..

1. It's a new winding.

2.measured voltage and current at motor end.

3.clamp on meter was used and voltage current were measured at the sane time.

What's worries me is the 3.3 amp reading, though windings resistance are same for u, v and w, no voltage unbalance..

I read somewhere this unbalance in current under no load is normal and when load increases the unbalance in current is reduced.

Winding not good? Or this is happening due to mass unbalance?

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#4
In reply to #2

Re: Current unbalance in 3 phase induction motor under no load

06/11/2018 7:50 AM

PWSlack advice to put a load on is good. Have the rewind people been advised of the currents and your concern? Did they measure the no-load current (was the motor tested at all?), what readings did they get?

Is the motor connected star or D? There is more potential for unbalance if star, because in D the winding voltages are fixed by supply voltage.

If you connect the motor wrong, it can only rotate the wrong way.

Since resistances are balanced, it is unlikely one phase has more turns than another.

So this leaves a magnetic unbalance - a difference in the permeability of the iron or the air gaps between phases.

The permeability of iron varies a lot with the magnetisation, which is due to a remanent permanent magnetisation and the winding ampere turns. Every time the motor stops or starts the currents/voltages in each phase can be different to previous history - an individual phase could be disconnected at zero current (near voltage max) but reconnected at zero voltage, which causes maximum current with saturation peaks (the inrush surge, which decays to normal current in a fraction of a second).

Since you are seeing, I assume, a consistent low current in one phase, those effects do not explain your problem.

The biggest gap is between rotor and stator. It could be the stator is not concentric or co-axial with rotor. If there was a lamination overlap between two phases not bolted tight, it would affect those phases more than the third - but that would make higher than normal magnetising current, you have abnormally low.

Law One of magnetic circuits is "the voltage sets up the flux", so loading the motor has little effect.

Measuring cold winding resistances, then running motor and observing phase currents on-load and resistances [de-energised, stationary!!!!] after warm-up (several hours) could be done. Winding resistance indicates temperature of windings.

But you should not continue load running if any line current exceeds "continuous rated current" - which should be on motor rating plate. Be particularly careful if motor rating plate specifies minutes/hours rather than "continuous" rating.

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#9
In reply to #2

Re: Current unbalance in 3 phase induction motor under no load

06/11/2018 6:02 PM

I did not reply on your "mass imbalance" question.

I guess you mean mechanical balance of rotor or other fault in rotor.

  1. No changes have been made to rotor, so it is not likely to be faulty.
  2. Ball/roller bearings with cages pressed into casing keep the rotor very accurately in position, so variation of air gaps & magnetic/current effect is negligible, even if bearing load is increased by unbalance.
  3. No-load, the slip of rotor is going to be around 60 rpm, 1 rev/second.
  4. Over 1 second approx, the rotor is going to take every possible position relative to stator - so any defect in rotor is going to affect all stator phases the same.
  5. In each phase current, fluctuations over slip cycle might be seen due to rotor fault - if the ammeter was fast enough response.
  6. Moving coil meters [the typical analog multimeter] have a sluggish response, I have measured needle flicker with 2 Hz sine imposed on DC as 20% of peak-peak AC. A little flick in each cycle will be well suppressed.
  7. Digital meters usually make several measurements in a second, number can depend on current measured. An unsteady input causes a variation between measurements, but this is difficult to interpret, the time slot of measurement is rolling/slipping relative to rotor - maximum to minimum most usefull.
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#3

Re: Current unbalance in 3 phase induction motor under no load

06/11/2018 6:03 AM

Put a load onto it, and see what happens then.

If there are any remaining concerns, then take it back to the re-winder and claim under their warranty arrangements.

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#5

Re: Current Unbalance In 3 Phase Induction Motor Under No Load

06/11/2018 8:32 AM

Check that the W winding is connected the same way as the U and V ones. One might expect something strange if it were 180deg out-of-phase.

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#8
In reply to #5

Re: Current Unbalance In 3 Phase Induction Motor Under No Load

06/11/2018 3:31 PM

PWS, I give you G.A. for that - just came back to pose same question myself.

The question for Quaiser OP is ..

Does the motor have three or six terminals or leads?

With six, the installer could have made a polarity error on-site. With three, it is a motor internal problem.

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#10
In reply to #5

Re: Current Unbalance In 3 Phase Induction Motor Under No Load

06/11/2018 10:28 PM

I'm thinking that you might have hit it on the head.

If W were connected backward, it would be bucking the stator field produced by U and V, resulting in less current, which is what we're seeing. The reversal could be external or internal to the motor.

To detect a reversal inside the motor, here's an idea (which I've never tried, but I think it will work).

If you energize only one winding with low voltage AC, there should be an induced voltage on the other two windings. If everything is correct, the voltage on these two windings should be in opposite phase with each other. (See diagram above)

But if W is reversed:

If you energize W, U and V will be out-of-phase. (both U and V are OK)

If you energize U, W will be in-phase with V. (both W and V are not OK)

If you energize V, W will be in-phase with U. (both W and U are not OK)

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#11
In reply to #10

Re: Current Unbalance In 3 Phase Induction Motor Under No Load

06/12/2018 12:20 AM

I'm thinking that you might have hit it on the head.

If W were connected backward, it would be bucking the stator field produced by U and V, resulting in less current, which is what we're seeing. The reversal could be external or internal to the motor.

To detect a reversal inside the motor, here's an idea (which I've never tried, but I think it will work).

CORRECTION: (Sorry, got it backward)

If you energize only one winding with low voltage AC, there should be an induced voltage on the other two windings. If everything is correct, the voltage on these two windings should be in phase with each other (since one leads physically by 120 degrees and the other lags by 120 degrees).

But if W is reversed:

If you energize W, U and V will be in-phase. (both U and V are OK)

If you energize U, W will be out-of-phase with V. (both W and V are not OK)

If you energize V, W will be out-of-phase with U. (both W and U are not OK)

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#13
In reply to #11

Re: Current Unbalance In 3 Phase Induction Motor Under No Load

06/12/2018 5:43 AM

With a reversed winding the current in that winding goes up not down. The other tell-tail is a growling noise from the motor.

Rixter’s test method in #11 is correct and works.

Another way of finding a reversed winding is by trial and error. Reverse one winding and run the motor, if it makes no difference, swap it back and try each winding in turn.

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#6

Re: Current Unbalance In 3 Phase Induction Motor Under No Load

06/11/2018 8:38 AM

There's a pretty easy way to check where the problem is - just swap all supply leads to different motor terminals and recheck the currents, if the low reading is still in the same supply lead then the problem is with the supply, if it has now shifted to another supply lead then the problem is with the motor.

If it's the motor then send it back, if it's the supply then further examination is required in that direction.

Make sure you are not going to have a mishap if the motor runs backwards during this test

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#7

Re: Current Unbalance in Three-phase Induction Motor Under No Load

06/11/2018 10:55 AM

I'm with Spades about isolating the source of the problem, and I'm willing to bet it's the motor rewinding, especially since nothing about the supply or the motor iron should have changed. More than likely there's a pair or more of individual coils that are either out of sequence, with reversed polarity, in the wrong slot, wound in the wrong physical direction, etc., etc.

The DC resistance test will only show that the windings are approximately equal in resistance as a group, but will not pick up an individual coil that is wired backwards, in the wrong sequence, or in the wrong slot.

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#12

Re: Current Unbalance in Three-phase Induction Motor Under No Load

06/12/2018 2:53 AM

Have you checked the 3p input voltage? Last week mine was 128v x 1, 280v x 2.

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#14

Re: Current Unbalance in Three-phase Induction Motor Under No Load

06/12/2018 6:40 AM

In a 2 pole / 30 HP motor it is likely that the 2 groups of coils are connected parallel and that each coil is build up by using 2 or more cupper wires parallel. The reason for this is that the ratio current over cupper-wire-area must be in the range 8-12 A/mm square. If this is not the case the cupper wire will heat up and the wire isolation will be destroyed.

A second reason to use wires parallel is that it is much prettier to work with small diameter wire, they are much easier to handle.

So a 30Hp / 380V motor takes full load +/- 46A (P = I * U * sqrt3 * power factor * efficiency) or (22000W = 46A * 380V * 1.73 * 0.8 * 0.9)

When connected in delta, this gives per phase 46 / 1.73 = 27 A

A 2 pole motor is build up with 2 equal groups of coils. If these groups are connected parallel (which is mostly done in this size of motor), each coils carries 27/2 = 13 A. With a ratio current to cupper-wire-area estimated at 8A/mm2 on needs a cupper-wire-area from 1.66 mm2.

Normal used wire diameter is 30 Hp range motor are 0.90 - 0.95 - 1.00 or 1.05 mm.

Using this sizes of wires, one needs at least 2 wires parallel for each coil. One cupper wire diameter 1.00 mm (= 0.785mm²) used in parallel with one cupper wire diameter 1.05 mm (=0.865mm²) gives 1.65 mm2 cupper wire area. This means that the motor current (46A) is divided over 4 wires (2 x 1.00mm + 2 x 1.05mm)

After mounting the coils in a motor, the coils-ends are de-isolated and connected together to be brought outside the motor.

It is likely that at this connection point, one of the 4 wires is broken.

This is a fault that is difficult to trace.

The resistance of the bad coil will be higher than the 2 good coils, but (point one) the resistance of a coil in a 30Hp 2 pole motor is very low (+/- 0.5 till 0.7 ohm). To measure these small resistances you need at least a 4 wires-measuring-instrument and some experience in connecting the measuring wires to the measuring points. The surface resistance between the measuring points and the measuring leads will try to corrupt your measuring result.

Further (point two) the difference between the good coils and the bad coil is small. Suppose the coils are made up of all the same wire-diameter and the resistance of one wire equals R. In this case the total resistance one measures at the outside of the motor is 0.25 times R (suppose two wires used parallel in each coil and two groups parallel).

If one wire is broken at the connection point, one will measure a resistance 0.333 times R.

This means that the resistance of the bad coil will be 0.33/0.25 = 1.33 times higher than the good coil.

Suppose the coil resistance from a good coil is 0.70 ohm then the broken coil will measure 0.93 ohm.

If you have no professional measuring instrument and some experience, you can’t measure this difference.

When you use a motor with this kind of fault, it will work apparently normal. Magnetically everything is ok, the coils will create a rotating magnetic field and the rotor will turn.

But (at load) one coil will heat up more and burn over time.

The motor current is a vectorial combination from an active current (caused by the actual work load, the ohm resistance of the coils and some other losses in the motor) and the magnetization current. At no-load the magnetization current is much higher than the active current. (This causes the low power factor at no load…)

When working at no-load, the active current will mostly be determined by the ohmic resistance of the coils. If the ohmic resistance is higher in one coil, the current through this coil will be less. This will cause a lower magnetization in this coil which results in a higher permeability of the magnetic core. (normally the magnetic core is used just before saturation and at saturation the B-H curve will flatten, so just before saturation the permeability is higher)

A higher permeability cause a higher inductivity of the coil which leads to a lower magnetisation current.

Both, the higher ohmic resistance and the lower magnetisation decrease the supply current in the faulty coil.

My advice, back to the repairshop with the motor.

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#15
In reply to #14

Re: Current Unbalance in Three-phase Induction Motor Under No Load

06/17/2018 5:03 AM

Thank you , I did the same but no difference.. send it to repair still the same... the air gap between the stator and rotor is not uniform... and when @full load it's making sound like rotor is coming in contact with the stator..

Does air gap creates this imbalance

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#16
In reply to #15

Re: Current Unbalance in Three-phase Induction Motor Under No Load

06/17/2018 11:38 AM

A non-uniform airgap flux could cause an imbalance, but if the rotor and/or bearings are out of alignment then there could be some sort of mechanical problem. Has the rotor been balanced both statically and dynamically, what about any eccentricity on the load that it drives, is the entire drive-train properly aligned and balanced?

One more electrical thing to look at might be a broken/disconnected rotor bar. As a last resort you could bring the motor to a rewind shop that knows what they are doing.

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#17
In reply to #15

Re: Current Unbalance in Three-phase Induction Motor Under No Load

06/17/2018 6:56 PM

I can imagine the stator inside being oval. This could give smaller gap on one phase and bigger gaps on others. But how would stator become oval?

The effects of broken or wrong size wire have been mentioned.

Another possibility is a slot coil connected to wrong phase - it would follow that a second coil must be wrong.

A swop at the phase change point on circumference, would I think, look like a retarded angle on one phase & advanced on next (a circumferential displacement is angle cisplacement).

This might show up with a search coil, rotor out, Stator energised at reduced voltage. A misconnection could be hidden in the end connections.

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#18

Re: Current Unbalance in Three-phase Induction Motor Under No Load

06/21/2018 1:51 AM

Hi there. This is only a suspicion. I have already came across this kind of problem in a rewinding shop where I once worked. Probably the problem in your motor is the copper conductor grouping. Please review again carefully the grouping. It is is a tiresome procedure and requires patience as the motor is three-phase and you can not anymore unwind and remove the copper windings.

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67model (5); minio (1); PWSlack (2); Quaiser (2); RAMConsult (2); Rixter (2); rudy.leurs (1); spades (1); Stuart21 (1); TonyS (1)

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