When a technician finds that a three-phase motor has failed, replacing the motor is only the beginning of the repair. A critical thing to consider is whether or not the three phases of current applied to the motor are not in a stage of imbalance, which could be the underlying cause of the motor failure, and could shorten the life of the new motor. Consulting NEMA (National Electrical Manufacturers Association) Standard MG1-14.35 offers information on this subject. It reads:
Three phase induction motors are designed and manufactured such that all three phases of the winding are carefully balanced with respect to the number of turns, placement of the winding, and winding resistance. When line voltages applied to a poly-phase induction motor are not exactly the same, unbalanced currents will flow in the stator winding, the magnitude depending upon the amount of unbalance. A small amount of voltage unbalance may increase the current an excessive amount. The effect on the motor can be severe and the motor may overheat to the point of burnout.
From a technician’s perspective, this means that our responsibility is to make that that the voltages applied to each phase are evenly balanced as closely as possible, and the most effective way to determine the condition of the electrical supply is use a professional grade digital meter in order to obtain accurate measurements that will provide us with an average.
On the subject of the effects of imbalance, the NEMA standard also states:
The effect of unbalanced voltages on poly-phase induction motors is equivalent to the introduction of a “negative sequence voltage” having a rotation opposite to that occurring with balanced voltages. This negative sequence voltage produces in the air gap a flux rotating against the rotation of the rotor, tending to produce high currents. A small negative sequence voltage may produce in the winding currents considerably in excess of those present under balanced voltage conditions.
What this means to technicians is that when there is excessive current draw caused by a voltage imbalance, the end result is that the motor will operate at a higher-then-normal temperature, which can lead to premature failure. From a mathematical perspective, the percentage increase in temperature rise will be approximately two times the square of the percentage voltage imbalance. What this boils down is that what may seem like a small problem is actually a big problem. For example, a motor operating with a voltage imbalance of more than 2% will experience a temperature rise that is more than 10% above normal, And, yes, a 10% excess, when it comes to motor operating temperature, is a big deal.
To understand the importance of proper phase-to-phase voltage balance, consider an example….A 220-volt, three-phase motor that is operating with the following voltage measurements.
From L1 to L2: 216 Volts From L2 to L3: 223 Volts From L1 to L3: 225 Volts
With accurate measurements accomplished, the next step is to find the average of the three readings. We do this by adding the three readings together, then dividing the total by 3:
216 + 223 + 225 = 664
And….664 / 3 = 221.33
With an average calculated, we’re then going to apply that number individually via subtraction to each phase voltage reading to find out how much each individual phase is out of balance. One important factor to keep in mind about this step in the process is that we need to wind up with a positive number when we subtract. So, we need to ask the question….Is our calculated average voltage higher than any of the individual voltage readings we found?
The answer to that question is yes. One of our voltage readings (L1 to L2) was 216 volts, and our average is 221.33 volts, so our subtraction for each individual phase-to-phase reading will look like this:
221.33 – 216 = 5.33 Volts
223 – 221.33 = 1.67 Volts
225 – 221.33 = 3.67 Volts
Now, for those who easily wrap their heads around mathematics and formulas, the one that applies in our next step is:
Voltage Imbalance = 100 x Maximum Deviation From Average Voltage Divided By Average Voltage
And, for those who make sense of things from a less formulaic perspective….when you look at our calculations above, the largest imbalance is 5.33 volts, so that’s the number we’ll use in our next step in finding the amount excessive temperature we’re experiencing in our motor windings. We accomplish this by dividing 5.33 by our calculated average, multiplied by 100. We’re doing this so we’ll wind up with a percentage:
5.33 / 221.33 x 100 = 2.4%
So what we’ve determined with our basic arithmetic is that the motor in our example is operating with an imbalance beyond 2%. And, plugging that number into our next step is going to tell us what our excessive temperature rise is (in a percentage) in regard to this motor. To do this, we’ll take our calculated percentage of imbalance (2.4%) and square it:
2.4 x 2.4 = 5.76
Which brings us to our last step in calculating the percentage of excessive temperature in our motor windings. We need to plug what we know to be the maximum percentage of imbalance (as we said, 2%) into the actual imbalance percentage we calculated (2.4% squared….which gave us 5.76%), and we find:
2 x 5.76 = 11.52% Temperature Rise
An important thing for us to understand relative to this subject is that what may seem on the surface to be insignificant, is in reality a critical issue. For example, studies have shown that when a voltage imbalance reaches a level of 3%, it reduces the life of the winding insulation by one-fourth. And, if we found an imbalance situation that resulted in an excessive temperature of 50-degrees, the life of the insulation would be cut in half.
And, on this subject, I’ll leave you with a final thought:
Working from the theory that, as technical professionals, we need to invest time in studying so we can have a complete understanding of a given subject or situation, and then use the tools available to us to make things simple….in this case, not going through all the steps of calculation we just reviewed…. here is a link to a calculation tool that allows you to plug in your voltage readings and find out if the motor you’re evaluating is operating normally.
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