Electrical Unbalance Kills Motors

By Drew D. Troyer, CRE, Contributing Editor

Most industrial electric motors are three–phase AC units. A condition of unbalanced voltage across the three phases spells disaster for motor reliability. Suppose that you measure the voltage at each of the phases and come up with 235 for phase 1, 225 for phase 2, and 238 for phase 3. Adding the voltages for the three phases and dividing by 3 yields an average voltage of 232.66. We use the phase with the greatest deviation from the average to calculate the voltage unbalance. The voltage for phase 2 has the greatest deviation (233.66 – 225 = 7.66). Dividing that deviation by the average voltage and multiplying by 100 yields the percentage voltage unbalance, which for our example is 3.29%. Why is this important?

When there‘s an unbalanced voltage across the three phases, they have to play catch up with increased current. It’s common for the current unbalance to be 6 to 10 times higher than the percentage of voltage unbalance. Increasing current, though, increases motor temperature, and that’s where we get into trouble.

As a general rule, the percentage increase in temperature for a motor is two times the percentage of voltage unbalance squared. In our case, it’s 2 x (3.29)², or approximately, a 21.65% temperature rise. If the motor normally operates at 60 C, a 21.65% increase in temperature translates to an approximate 13 C-deg. rise in temperature. As we all know, heat is a reliability killer.

According to Arrhenius’ equation, a 10-deg. C increase in temperature doubles the rate of chemical reactions, thus cutting equipment life in half. In the case of electric motors, increased heat increases the rate at which insulation degrades. In our example, the increased temperature cuts the motor’s life by about 60%. It’s serious business.

Measuring and calculating voltage unbalance is relatively easy. All you need is a voltmeter and the ability to do some basic arithmetic. As a rule, you should aim to keep voltage unbalance to less than 1% and to not exceed 2%. Refrain from running a motor with a voltage unbalance exceeding 5%. This should make sense as a 5% voltage unbalance equates to a 50% increase in temperature. For a motor that typically operates at 60 C, this would result in a 30 C-deg. rise in temperature, which would slash motor-insulation life by a whopping 87%.

According to the U.S. Department of Energy, Washington (energy.gov), the most common causes for voltage unbalance in plant include:

• faulty operation of power-factor- correction equipment
• unbalance for unstable utility supply
• unbalance transformer bank supplying a three-phase load that’s too large for the bank
• unevenly distributed single-phase loads on the same power system, e.g. lights or welding equipment
• unidentified single-phase ground faults
• an open circuit on the distribution system primary.

Staying on top of such issues and managing voltage balance in your electric motors (see sidebar above) is important. Besides reducing motor life, voltage unbalance decreases motor output and energy efficiencies. EP

Based in Tulsa, OK, industry veteran Drew Troyer is principal with Sigma Reliability Solutions. Email Drew.Troyer@sigma-reliability.com.