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  • When commissioning an electric motor and current unbalance is detected, the question frequently asked is: "What's wrong with my motor? Another question that might be raised is: How much current unbalance is the motor able to operate with? To identify the cause of the current unbalance it is very important to have the motor inspected at the application. The motor should only be removed from the application when we are 100% sure the problem is with the motor. Current unbalance is caused, in most cases by unbalanced phase voltage. However it can also be caused by an unbalanced number of turns in the windings and an uneven air gap. Each one of these causes, will be discussed below:

Unbalanced phase voltages:

  • Voltage unbalance in the electrical system can lead to a number of problems related to utility and customer operations. From the utility perspective, unbalanced voltages may be caused by load unbalance or possibly by fuses being blown on distribution capacitors. For the customer, voltage unbalance can cause adverse effects on three-phase motor loads, resulting in the necessity to de-rate motors under unbalanced conditions. When measuring the line to line voltage from phase R to S, S to T and T to R, detectable differences in the voltages would show up. Unbalanced voltage of 1% will generally produce unbalanced currents up to 5%.
  • The amount of unbalanced voltage is calculated as follows:
    Max. voltage deviation
    from average
    voltage % voltage unbalance = 100 x --------------------------------------- .
    Average voltage

Example:

  • In case R, S and T are 400, 418 and 409 volts respectively, the maximum deviation from the average is 9 and the percent unbalance is:
  • 100 ∗9/409=2.2%
  • According to ABNT 7094 / 96 standard (Attachment B), an electric motor will be able to supply its rated output when the voltage unbalance is not greater than 1%. In situations where it is greater than 1%, a derating factor must be applied to the motor rated output. Derating factor as follows:
    Voltage Unbalance kW derating factor
    1% 1
    2% 0.951
    3% 0.9
    4% 0.84
    5% 0.76