Three-Phase Motor Current Unbalance

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Three-Phase Motor Current Unbalance By Rodrigo Duarte Page

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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 ABNT 7094 / 96 96 standard standard (Attachment (Attachment B), an electric motor will be able able to supply 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: Volt Volta age unba unbala lanc nce e 1% 2% 3% 4% 5%

kW dera derati ting ng facto actor r 1 0.951 0.9 0.84 0.76

For more information contact WEG Electric Motors - 0800 FOR WEG

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Unbalanced number of turns in the windings and uneven air gap. Nowadays with the quality control systems implemented in the motor manufacturing process by most manufacturers, it is mostly unlikely that the above could be the cause of unbalanced current. However an easy test can be done to check if the motor is causing current unbalance and that is: Step 1: Measure the full load current of the motor and write the readings down. The readings must be taken at the motor terminal block, as shown on picture 1(I R1, IS2, and I T3):

Step 2: Change the phases as shown on picture 2. Observe that the phases have been changed, however the motor will be turning in the same direction as previously. It is very important to change the phases inside the motor terminal box. After that, measure again the full load current of the motor and write the readings down. (I R2, IS3, and IT1):


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Compare the full load current readings before and after the phase change and: - If IR2 = IR1, IS3 = IS2 and IT1 = IT3

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The cause of unbalance is in the electric system.

- If IR2 = IS2, IS3 = IT3 and IT1 = IR1

þ

The cause of unbalance is in the motor.

The balancing of the number of turns in each phase of the motor, can be checked by measuring the resistance of them with an ohmmeter. The resistance unbalance between each phase should not be higher than 3%.


Three-Phase Motor Current Unbalance

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