Un-balance resistance in motor

Hello Sudh

You do not say whether the motor is new, or if it has been in service.

It is not the DC Resistance in Ohms which is important in AC Induction Motors.

It is the Reactance at the Mains Frequency of each winding which is important.

It may be that you have a fault in the motor, perhaps a short between winding turns.

This is easily checked, by placing an Ammeter in the phase connection of each winding, in turn, with the motor not under load, and taking careful note of the respective currents in Amperes.

If the motor makes a grumbling sound, or the Amperages differ by more than 3%, then almost surely you have a faulty winding, which will require a visit to the rewind shop.

Advise further, with

Kind Regards....

Thanks Guru,

This motor is actually new installation motor. The motor data is as follows:-

7.5KW, 380-415V AC, 50 Hz, 3 ph AC induction motor. Generally in new motor we check the Megger values & resistance value to check the healthiness of new motor. Kindly discuss if AC motors are checked with some more methods.

With the un-balance resistance ,we are little bit doubtful about the winding resistance. We think if it is taken on load it may get overload on other windings and burn the windings.

Regards,

Sudhir

If you have such big difference in the windings, you will most definitely have a different load on each phase. This should cause the 3 phase trip switch to trip shortly after start up or after some time when motor heats up enough.

We had one pump motor doing exactly the same and kept tripping. We used 3 analogue am-meters one on each phase and it showed that the load was a bit higher on one phase only which slowly increased and finally tripped the overload. When resistance check was done, we had a marked difference as well.

My suggestion is to try it but make sure that you use the correct trip switch and use the physically linked type so that if one phase falls over, the other come out with it.

Sudh; you might check the winding or connections between 2 & 5 to see if that is open or with 2 & 5 disconnected check 1 to 8 or 3 to 8 to see if 8 to tie point is open perry

Motor is toast. Send it back to factory. The Ohm readings should be within 1-2% of the other.

The problem would appear to be in L3, it has a higher resistance than either of the other phases. Or both L1 and L2 have the same defect, which I doubt seriously...

Normally a single field's DC resistance must be around 0.615 ohms. L3 seems to have around 1.865 or so for some odd reason.

Can you disconnect the point at which all phases are connected, is it accessible? If yes, you could prove out the above quite easily....

I would send the motor back and request a replacement with in-balance field resistances....

It is strange the resistance reading is about half of the other two phases, are you sure you do not have a mid point tapped phase windings which have had there connections mixed up. If not then your motor has a shorted phase.

Dear mr. Sudhir,

I think this motor is not workable. Insulation resistance of one coil partially damaged. Meger test is only solution.

Thanks,

Tariqul

Where I work, we have underwater motors which have the same characteristics. This is due to an interference reduction circuit (capacitor/diode), and the motors work fine.

How would a capacitor diode give a DC resistance reading? except diode in parallel with capacitor which would not work to well with AC voltage across it.

Where are you at Lieth docks?

Don't know, but that's all of the parts that are visible - may be more to it. Thought it was a bit strange that only 2 pairs were fitted.

I'm at the other end of the city.

Suppression fitted as shown gives the irregular resistance values noted, but has no detrimental effect on the operation of the motor - as long as the values used correspond to the frequencies of operation. This method of suppression is only useful for constant speed use.

The diagram appears to be missing something, can you check and see if it is showing exactly what you intended please?

Bold lines are the 3 phases to the motor connections.

As the suppression pairs are only between 1&2 and 2&3, the resistance measured across 1&3 is different.

Why are L1 and L2 seemingly not connected to anything? where is L3? (I assume a 3 phase motor with at least 3 field coils, please correct me if I am wrong!)

Sorry, I do not call that a diagram!! I usually say a picture is worth a 1000 words, in this case it is worth 1 word = nothing!! I am fairly sure that it does not represent what you wanted it to represent somehow.....

Ah (Amp-hours?) the conufsion!!!!

I've not much time to do a better diagram, but will try to explain....

The three bold lines are what you refer to as L1, L2, and L3 - the three phases of supply. At one end will be the supply, at the other the motor.

L1 & L2 as marked are Inductors - also commonly symbolised by L.

C1 & C2 are capacitors - hopefully no confusion there.

Wired in parallel with Phase1/2 and phase2/3, gives a difference in measured resistance across phase1/3.

The only way that the resistance of a motor winding will change, is if another "resistance" (could be in the form of a coil for example) is connected across it.

As you have neglected to connect the two inductances to anything at all in your diagram at all, only in your your third or fourth post was this finally mentioned, it was VERY difficult to follow in a logical manner.....(I hope that you are not planning ever to become a teacher, if you do, do not give up the day job!!)

May I suggest (assuming you use one of the Windows derivatives) that you acquaint yourself with the small free graphics program called "Paint", you could produce a CORRECT diagram in about 3 minutes.....or draw something, borrow a scanner and scan it into the computer if that is too difficult.

So my thoughts on a ferrite capacitor circuit were in the ball park.

I am still unsure from your drawing how the connections are made?

For some reason I cannot seem to log on under my normal name so this will come up as a guest my log on is accepted but the post page still asks me to log on. Any suggestions anybody.

Cheers Garth.

GEEEeehh! It does sound like something wack out creating some unbalancing conditions. Will be better to start looking for a replacement puppy in the meantime and have ready to swap it to keep downtime to a minimum at the worst case scenario. You know " MURPHY LAW" time it happen always, no joke.

So I agree it will be wise to start checking out so you always will have an spare one ready for action if this happen again. Allset easy does it now. And Lock and Tag Out any energy source around such puppy for safety.

Keep it Cool,

MC

It would appear that the one group of windings is shorting against the other. This will cause an overheat situation and probably draw high amps as well. The insulation between the groups of windings has probably worn thin. This causes a drag on the motor and it has a hard time coming up to speed. You should have someone look at this motor to determine if that is the problem.

Cool stuff here to learn always at CR-4 forum. You have the Technology Pal's. Hehahahee...

Good Techs,

MC

Actually, if you look at this as a sum of 3 field vector resistance info, one field (L3) has a higher DC resistance than the other two......

As the possibilities of two fields having identical problems are far less than a single field, don't you agree, the L3 field has the highest chance of being defective....

We were not told that any one field has a short to another and usually all the field windings come to a common position, also they do not run together in most motors.....

Hopefully the final fix will let us in on what really happens here!!

The motor is brand new and never run by the end user as he is unhappy with the resistance checks.......this should not be forgotten!

I agree, but the difference is not usually that pronounced. Further, if you rotate the leads, in other words, put L1 where L2 is, L2 where L3 is and L3 where L1 was, this sometimes brings the amperage draw on each leg to a better balance. You don't, of course want to reverse the motor rotation, so the shifts must be made such that the legs are always in the same sequence as when you started. If you don't get any better balance, you can do it one more time. After that you're back where you started. I really believe that there is a winding short somewhere. When I gave my fisrt response, I hadn't read far enough to see that this is a new motor. I'd send it back.

It struck me that the only way a possible short would affect the problem is if the motor is connected in Delta format. (We have not been told if the motor is Star or Delta connected as of yet!)

I would also assume (not without danger of course) that anyone checking a motor out with a meter, would automatically check for shorts between windings and let us know if he had found something!!! that was not mentioned!!

In a Star connection.

For some strange reason L3 has a HIGHER resistance than either L1 & L2......that is the problem......

In a new motor, I do feel that is most likely a bad connection between the star (assuming star connection) and L3, I have asked if this point is accessible, but had no reply up to now.

In a Delta connection.

It is possible that the field that connects between L1 and L2 has a lower resistance than the other two fields.

It would be nice to know if the motor is Star or Delta connected too, though I personally have not had any major dealings with delta connected motors, only transformers...

Looking inside the motor connector, you may see something similar to the following:-

Star connected motor.

Delta connected motor.

Power is shown coming in on L1 to L3, so only the links need to be changed over to switch from one to the other, quite a simple operation.

In such a motor, it would be easy to disconnect completely and measure the resistance of each field and check for shorts between fields - before sending the motor back for replacement!!!

If you take the time to sit down and draw it out, you will find (assuming a star connected motor for the moment) that two fields are low (and exactly the same resistance, strange that!!) and ONE field has a higher resistance.....this one field is most likely the problem as two fields going low AND having exactly the same resistance is a reall long shot!!

Read through my last post, it explains it better.....or use a piece of paper and a pencil and work it out for yourself!! Even better!