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Thread: Motor running unbalanced amps

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    Question Motor running unbalanced amps

    Good day.

    I have a 200kw motor running through a soft starter at the moment. Previously it ran with a VSD, that burnt due to this matter.

    We have redone a water treatment plant in 2012, and noticed that the voltage was good testing it with a multi meter. As soon as you start a motor it runs at almost 45% amps difference. If you run it through the standby generator, then the amps are fine.

    Its also time dependant, and weather. Between peak hours its worse and if its winter even more.

    Supply voltages: 405V, 411V and 412V.
    Amps on the 200kw motor: 340A, 210A and 315A.

    Generator voltages: 400V, 400V and 400V
    Amps on the 200kw motor: 330A, 325A and 332A

    This has been a big problem, as engineers of the electical supply company and engineers of Johannesburg and PE are batheled with this issue.

    The supply company says they only supply voltage and not amps, duh I know that, but why did our voltage regulator poped as wel as 3 VSDs in 2 weeks time and then the motors started burning.

    Any person than can give advise on this matter please.

    I assume its poor load ballancing of towns on the gridline as we have our own 22kV line supplying the water treatment plant.

    Ivan

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    Is the motor connected in star, 3-wire delta or 6-wire delta?
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    Hi

    I went out to investigate a similar scenario in a plant after a motor had been rewound.

    I tested on other motors within the plant and found the same variance of amperage all on the same phase.The blue phase was drawing 30% less current then the rest of the phases even though the voltage between phases was within 2% .

    The rest was above my pay grade but it basically came down to power factor correction equipment on the MV side that was creating the shift in phase and created uneven current.
    They tested and rectified the power factor correction equipment and it sorted the problem out.

    I would assume that if you put a scope across the phases that you may find they are not at 120 degrees to each other .

    Another quick test to see if it is incoming supply or motor problem is to swop phases around and see if the fault follows the phase or does it follow the motor coil.Have a feeling it will follow the phase.

    Please keep us informed

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    I suspect that the input side of the HV transformer has a damaged white phase. Usually happens when there is a lightning strike on the HV side, and damages the HV side surge arrester, and then damages the transformer winding.

    If you feel the incoming white phase cable (on the plastic of course ) to the incoming breaker, when the motor is running, you will probably feel that that cable has a vibration in it, and if you feel the Red and Blue phase it won't be there.
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    Quote Originally Posted by Ivanjvv View Post
    Good day.

    I have a 200kw motor running through a soft starter at the moment. Previously it ran with a VSD, that burnt due to this matter.

    We have redone a water treatment plant in 2012, and noticed that the voltage was good testing it with a multi meter. As soon as you start a motor it runs at almost 45% amps difference. If you run it through the standby generator, then the amps are fine.

    Its also time dependant, and weather. Between peak hours its worse and if its winter even more.

    Supply voltages: 405V, 411V and 412V.
    Amps on the 200kw motor: 340A, 210A and 315A.

    Generator voltages: 400V, 400V and 400V
    Amps on the 200kw motor: 330A, 325A and 332A

    This has been a big problem, as engineers of the electical supply company and engineers of Johannesburg and PE are batheled with this issue.

    The supply company says they only supply voltage and not amps, duh I know that, but why did our voltage regulator poped as wel as 3 VSDs in 2 weeks time and then the motors started burning.

    Any person than can give advise on this matter please.

    I assume its poor load ballancing of towns on the gridline as we have our own 22kV line supplying the water treatment plant.

    Ivan
    hook up a power quality analyser...like a fluke 435 (what i use ) it tells no lies....it is the only way to figue out what is causing the problem...the rest is just guessing.

    Fluke have even better equipment now for motor testing...contact Gavin van rooy at comtest 011-6088520...find out if they have anyone down in your area with the right equipment to test.

    a power quality analyser can produce a report which can be used to fight with the supply authority ...or at least show them if they are the problem.

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    Good day.

    The motor use to run on a star/delta then was changed to a VSD. It is currently running on soft starter on the one side and a contactor for the return side. Still has the 6x cables connected.

    We have a smaller pump station on that same line and also just the white phase pulls more Amps. I did a test with a no loaded 30kw motor with the meeting I had on site with the engineers, power supplier ect. They argued that the motor had off balanced windings, this after U tested it infront of them. So.... i guessed the only way to prove that its not the motor was to change the phases. I did this 4 different times and all still kept pulling more amps on the incoming white phase and not the motors white phase. This test was done DOL straight from a breaker i added on the LV side of the transformer.

    We did do an inspection from the transformer upto the point where we get the suplly from. It is our own incomming line as they first thought that is where is is due to the town being on the red and blue phases. No one of the 16 people that attended the meeting saw anything on the overhead lines.

    After the new line was installed, thats when the entire plant was redone. We lost the voltage regulator and 3 VSDs in 2 weeks time. I did hook up an supply anilizer and it did have big fluctuations in peak hours but non peak times were still 45% amps differance.

    Recently the supply was swopped, as they tried to balance the loads in town. Thus, just causing the problem to swing over to another phase, cauzing phase rotation issues. They did swing it back 2 days later.

    Kind regards
    Ivan

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    This is the amps for the phases on a good day and not in peak hours. Keep in mind that there are 2 motors running. A 160kw and 200kw, as well as the fact if the motors are not running the plant amps is 13A, 63A and 0A. That is for the dwelling houses and plant lights ect

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    When you hooked up the power anilizer did you record the phase angles ?

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    If the three-phase voltage is not balanced, there will be a reverse current and an inverse magnetic field in the motor, which will produce a large reverse torque, resulting in the unbalanced distribution of the three-phase current of the motor and the increase of the winding current in a certain phase. When the unbalance of three phase voltage reaches 5, the phase current of the motor can exceed 20% of the normal value. The unbalance of three-phase voltage mainly lies in the following aspects:

    (1) abnormal phase occurs in the three-phase winding of transformer, which conveys asymmetric power supply voltage.
    (2) long transmission line, uneven traverse cross section and different impedance voltage drop result in unbalanced voltage of each phase.
    (3) the power and lighting are mixed together, and there are many single-phase loads, such as: household appliances, electric furnaces, welders, etc., which are too concentrated in one or two phases, resulting in uneven distribution of power load in each phase and unbalanced supply voltage and current.

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