current unbalance in same motor

If it's not in the motor you're looking at, it's somewhere else.

That's all I can say.

Please check up the In coming Supply Voltage in each Phase and see that your Power supply is balanced.There may be single Phase load in your system

Unbalanced voltage results in unbalanced current.Some time incoming supply itself will have this problem .

I would say the previous comments are on the right track incoming lines to motor.

1. Starter contacts

2 voltage to starter over time each phase

3 voltage at motor over time each phase

If these values stay constant there is a remote possibility (remote) winding insulation instability.(but all three phases?)

Is the incoming feed suppy on with a variable frequency drive somewhere down the line if so multiple harmonic disturbances could manifest itself as a changing phase voltage differential you are experiencing . 16%??

Also look at DC resistance of motor windings. Sounds a little like you may have a phase which may have over-heated and starting to short some windings. Phase voltages will affect phase currents also as others have said (backwards, in one case).

If 3 phase motor:

- Check coil resistance. It should be balanced for 3 coils.

- Check insulation resistance (disconnecting star / delta links) seperatly for 3 coils with respect to earth. It shouls be infinity or >20 MOHMS equal for all coils.

- Check all switch gears (contact gaps (if required filler gauge) connected series to motor. There should be no gap in power contactor contacts or in other switch gears.

- Check for any loose connections in terminal connections or thimble crimpings. There should be no loose connections.

Note: If all above said things are ok only problem is in supply voltage. That means supply voltage is imbalanced.

Regards,

Vinod.P

sir,

What should i do,If a total MCC gets imbalance current in one phase. We have checked out the load current at PCC it was normal.but one thing i found out is the system is unbalanced ie:In=80A. Is there any permissible limit for this Neutral current.Please do reply me.

Dear friend

A good idea would be to calculate the sequence components of the currents for each of the three different values.

Perhaps you reach to the conclusion that the unbalance will be dependant in which phases it occurs.

I think it is a path you may follow.

Regards

Tadagids--

I would expect that equipment problems, such as contact resistance or winding faults would be unlikely to be in different phases at different times. The earlier posts are all correct that an unbalanced voltage to the motor will cause an unbalanced current. In the USA, the standard motor designs have predictable characteristics, which are in graphs in the NEMA (National Electrical Manufacturers' Association) handbooks. I don't have one in front of me right now, but a voltage imbalance usually causes a much larger current imbalance. The real problem is that an imbalanced current between the phases causes circulating magnetic eddy currents in the steel laminations, which leads to overheating of the windings and motor failure. I had a large blower motor in an industrial setting, which had about a 4 or 5% voltage imbalance, but a 15 to 20% current imbalance. Although every phase current had been within the nameplate rating for the motor, it burned out.

Check your voltage, phase to phase as well as phase to ground (if it is on a 4-wire wye system). Write each one down. Find the average of the three voltages, and then calculate the % each one is away from the average. If your plant receives its power from someone else, talk with them and explain your problem. They may be causing problems for many people.

In my example above, when I called the electrical utility (late in the afternoon), they called more and more electrical linemen out until they finally found a bad splice in their high-voltage distribution system about 2 km away from our plant.

-JMM

That's right. You need to understand that the current imbalance percentage is logarithmically proportional to the voltage imbalance. A 1% voltage imbalance can easilly show up as a 3% current imbalance, but that 4% V creating a 20% I is to be expected as well.

So if your supply voltage balancing changes during the work day, you can expect a current imbalance to be seen in your motor. This is especially prevalent in summer months where you might have a lot of 1 phase Air Conditioners coming on and off randomly during the peak hours. There is no way for the utility to predict and compensate for the load swings, so the voltage gets unbalanced.

This is my dilemma. You gave these three statements:

what is exact reason for current unbalancing in same motor

all these are in sam,e motor

Mostly in Mill motor.

The first and second statement says that it happens only to one motor. The third statement seems to say that it also happens to other motors but, most of the time, it happens in the Mill motor. Which is it? Does it happen only to one motor? Or does it happen to other motors as well? How you troubleshoot will depend on your answer.

If it happens only to one motor, then I'd check the motor itself, the cables, the contactor, breaker, terminal blocks, or anything else that is associated only with that motor. A problem with your utility provider will not result in only one motor acting this way.

If it happens with other motors, then you have a bigger problem. You'll have to search in the feeders, maybe the substation or, as someone said, you'll have to check with your electrical utility.

I just wanted to point out that accurate statements will go a long way in diagnosing the problem.

Friend,

It`s true such incident happens in our 8 Nos. Mill Motor which are located in different location & supply source is different load center.

I feel the load pofile to be checked. In mill , loads are not uniform, which may result in current unbalance.

On distribution boards fed from 3-phase supplies, it is not unknown for a site electrician to add new single phase loads starting from the top left. As the red phase is usually at the top of a board, that phase tends to see the greatest current and consequently the lowest voltage. If a 3-phase motor is connected elsewhere, then these single phase loads will produce a difference in currents and voltages at the motor.

If the motor is under its full-load-current and its correctly-set overload is still holding in, then normally there isn't a problem. If there is a persistent problem, then one needs to look again at motor sizing, spreading the single phase loads across the other phases instead, and techniques to reduce loads like low-energy lighting and power factor correction. It's all part of the bread-and-butter of the site Electrical Engineer.

Good Moaning in return, Crabtree--

I disagree with your saying "If the motor is under its full-load-current and its correctly-set overload is still holding in, then normally there isn't a problem."

If the current is sufficiently unbalanced, such as by a voltage imbalance of 3% or more, the increased magnetic fields in the steel laminations can easily overheat the windings, even when the currents are all below name plate values. This will cause insulation failure just as easily as overloading will. Most motor starters are supplied with Class-20 overload relays, which will respond only seldom to this sort of a problem. Motor starters with IEC type Class-10 overload relays frequently will respond to the current imbalance, but this is not always assured. Starters with closely sized fuses for running overcurrent protection will do no better than Class-20 overload relays.

However, there have been many (usually older) motors which were rated very conservatively on their nameplate, so imbalances such as have been described were fairly easily handled.

As I read "between the lines" of the original post and the later additions to it, I suspect that this application is in an area with a poorly or inadequately engineered power distribution system, and possibly also has few (or no) code compliance requirements.

--JMM