Defining and Calculating Voltage Imbalance
Improper voltage is the one of the greatest causes of failed compressors in three-phase units
Improper voltage is the one of the greatest causes of failed compressors in three-phase units. We are allowed a 10% deviation over or under the unit nameplate-rated voltage. Most technicians know to check supply voltage when starting a new unit. Consider a unit rated for 208/230 volts. The allowable limits would be 10% less than 208 volts and 10% greater than 230 volts. This allows us an operating range of 187-253 volts. It’s important that the supply voltage remain within this range at all times, because the compressor needs to see the proper voltage when starting as well as when running.
When considering three-phase supply voltage, another critical factor is voltage imbalance. What is voltage imbalance? Voltage imbalance relates to the measured voltage on each leg with respect to the other two legs. Example No. 1 represents perfectly balanced voltages on a 230 volt / three-phase system.
L1-L2=230 volts L1-L3= 230 volts L2-L3= 230 volts
A three-phase motor running with unbalanced voltages generates extra heat in part of its windings. The extra heat that is generated in the unbalanced area of the winding is two times the square of the imbalance!
This example represents an unbalanced voltage condition.
L1-L2 = 230 volts L1-L3 = 233 volts L2-L3 = 245 volts.
Let’s calculate how much imbalance is represented in this example. First add all the voltages together:
230+233+245 = 708
Now divide by 3 to get the average volts.
708/3 = 236
236 volts represents our average voltage.
Calculate which reading is farthest from the average.
The reading between L2-L3 is 9 volts away.
245-236 = 9 Divide 9 by 236.
9/236 = 0.038
Now multiply by 100
0.038x100 = 3.8%.
This unit is operating with a voltage imbalance of 3.8%.
What happens in the compressor winding that is operating at 3.8% imbalance? Remember the formula is two times the square of the imbalance or 2x(3.8x3.8)=28.88. That part of the winding is running more than 28% hotter than normal. The maximum allowable imbalance is 2%. A compressor operating at 2% imbalance would be 8% hotter. As you can see, 3.8% imbalance generates substantially more heat than the allowable 2%.
I always recommend to use a voltage monitor that shuts off the compressor when 2% voltage imbalance is experienced. Saving one compressor will cover the cost of a voltage monitor and then some.
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