|Motor users and installers get concerned when they detect unbalanced phase currents on a 3-phase motor. The question is frequently asked: “Is there something wrong with the motor?” The other question is: “How much current unbalance can be tolerated?” This paper will attempt to answer those questions.
In the “Good Old Days” about the only sources of unbalanced phase currents was either a problem in the motor, such as an unbalanced number of turns in the windings, an uneven air gap or unbalanced phase voltages. Winding or air gap problems are definitely motor related. On the other hand unbalanced phase voltages are a power system problem. Unbalanced voltages will generally produce unbalanced currents that are many times greater than the percentage of voltage unbalance. The ratio used is close to 8:1. In other words, a voltage unbalance of 1% could create unbalanced phase currents of as much as 8%.
A very unscientific way of looking at the problem is as follows: Suppose a motor has a nameplate full load current of 10 amps. At full load the amps on each leg of the 3 phases added together would be 10 + 10 + 10 or 30. However, if the load is the same but the phase currents are unbalanced, the total of the 3 legs added together will always be more than the total of the balanced currents. In this case the currents might be 10.5, 11.3 and 12.1 for a total of 33.9. This is a very unscientific way of looking at it, but it is accurate in describing the effect. What this means is that high current on one leg doesn’t mean that the other two legs will be reduced by an equal amount. It can be said that unbalanced currents always result in higher operating temperature, shortened motor life and efficiency reduction.
The next question is “What creates unbalanced currents?” In years past, if the motor was not the problem — the source of unbalanced currents was unbalanced phase voltages. When measuring line to line voltages from phase A to B, B to C, and C to A, detectable differences in the voltages would show up. The voltage differences would account for the unbalanced currents.
In today’s world there are other problems that are frequently not detectable with simple voltage tests. One problem of growing concern, is voltage distortion caused by harmonics in the power system currents. This can happen if there are loads in the general area that draw non-linear (harmonic rich) currents from the power system, they can create voltage distortion in the normal voltage sine-wave that, in turn, can cause unbalanced currents in motors even when phase voltage differences are not detectable with a voltmeter. For example, if you were to detect unbalanced motor currents and took measurements with a digital voltmeter on the three phases, they might be very close to one another. The natural tendency under these conditions, would be to blame the motor for the problem. When this happens it is necessary to go a step further to identify or dismiss the motor as the source of the problem. The test is to rotate all 3 phases. If the power phases are labeled A, B and C and the motor leads connected to them are labeled 1, 2, and 3, motor lead #1 might be reconnected to power supply lead B; motor lead #2 would be reconnected to power supply lead C, motor lead #3 would be reconnected to power supply lead A. Moving all three legs will keep the motor rotating in the same direction. The currents are recorded on each power line leg before and after the connections are changed. If the high current leg stays with the power line phase (for example, B), then the problem is a power supply problem rather than a motor problem. If, however, it moves with the motor leg, then it is a motor problem. This test will pinpoint the problem to be either power supply or motor.
|HOW MUCH UNBALANCE CAN BE TOLERATED?
In general, this depends on the conditions that are found. If the motor is driving the load and the highest amperage of the three legs is below the nameplate Full Load rating, then generally it is safe to operate. If the high leg is above the nameplate rating, but within the normal service factor amps (for a motor with a service factor, normally 1.15) then it is probably still safe to operate the motor. Also, it is not unusual to find currents more unbalanced at no load than they will be under load, so the loaded amps should be used. Finally, in general, if the high leg is not more than 10% above the average of the three legs, determined as shown in the example, it is probably safe to operate the motor.
Motor Nameplate FLA = 10.0
Determine the Average
(10.6 + 9.8 + 10.2)/3 = 10.2 amps
Determine the % Difference
(Highest Phase – Average)/Average x 100
(10.6 – 10.2)/10.2 x 100 = (.4/10.2) x 100 = .039 x 100 = 3.9%
The following table shows some of the sources of unbalanced voltages and currents along with possible remedies.
Unbalanced currents on 3 phase motors are undesirable but a small amount can generally be tolerated. Excessive unbalanced currents can shorten motor life and increase energy consumption.