As part of my ongoing series of electrical diagnostics for air conditioning systems, in this posting we will discuss voltage imbalance in 3-phase systems.
Most technicians know low voltage causes problems with motors, compressors, and other components in a HVAC system. Since low voltage can result in increased amperage draw and higher motor temperatures, I felt it would be good to write a post on problems caused by voltage imbalance in 3-phase systems.
While low voltage is always an issue, voltage imbalance is a more subtle problem. It is a condition when the voltage to the major 3-phase components is significantly different between the 3 phases. When voltage imbalance is too different, you can have excessive component temperatures and even burnouts.
Many compressors and motors are replaced without determining the cause of the failure. Often the cause is the result of voltage imbalance in the 3-phase power.
So what is voltage imbalance? Three phase power consists of 3 “hot” wires each having full line voltage with respect to the other two. These three voltages should be nearly, if not exactly, equal in voltage to each other. If the voltages are too far out of balance, components (like motors and compressors) will start to over heat. Motors and compressors operating with too high a voltage imbalance may continue to run but at elevated motor winding temperatures. This, in turn, will reduce the life of that component. It is generally accepted that the maximum allowable voltage imbalance is two percent.
Voltage imbalance can raise the motor winding temperature significantly. The winding temperature increases by 2 times the square of the percent of imbalance as shown in the following table:
As the table shows, a small voltage imbalance will cause the motor windings to operate at highly elevated temperatures over the normal operating temperatures thus reducing the motor life.
So, knowing that voltage imbalance can cause motors to over heat and eventually fail, how do we check to see if we have a voltage imbalance in our system.
Voltage imbalance can occur anywhere in the electrical circuit to the component. It can occur only at the motor, below the motor at the contactor or starter, at the disconnect, or at the service coming into the space. For example, if the imbalance exists at the load side of the contactor or starter but is not present at the line side of the contactor or starter, the source of the imbalance is the contactor or starter. A voltage drop across the contacts indicates poor contact connections. This is remedied by replacing the contactor or starter and saving the motor or compressor.
To actually check for a voltage imbalance, voltage measurements need to be taken across legs A,B, & C at several location to determine where the voltage imbalance exists. This actual voltage needs to be done with a good digital volt meter to get accurate readings ( see illustration below)
Once these reading are obtained, you can calculate the voltage imbalance by using the following formula:
% voltage imbalance = Maximum deviation from the average ÷ average × 100
Let’s do an example
If your voltage readings were 221volts from A to B, 224 volts from A to C and 215 volts from B to C, the first thing we do is add these 3 reading together and divide by 3 to get our average voltage for this point. 221 + 224 + 215= 660 ÷ 3 = 220 volts. The maximum deviation from the average is the measured voltage value furthest from the average. The difference between 221 and 220 is 1 volt. The difference between 224 and 220 is 4 volts. The difference between 215 and 220 is 5 volts. So the maximum deviation in this example is 5 volts. Now we use the equation:
% voltage imbalance = 5 V ÷ 220 V = .0227 × 100 = 2.27%
The voltage imbalance here means the motor will operate 10.31% hotter than normal and the motor’s lifespan will be reduced accordingly by referring back to the table above.
So what could cause this voltage imbalance? As I said earlier in this post, it could be ANYWHERE in the circuit. It could be poor wiring connections. Such connections could be at the main power panel, the disconnect, the contactor (either line or load side) at the motor or any other point on the 3-phase system. Poor contacts are a point of electrical resistance and cause voltage drops. Mixed wire sizes, especially on longer runs, drop voltage. When 3-phase wires are not all the same size (wire gauge) the smaller wire(s) will drop more voltage creating the imbalance. And lastly, it could be the motor or compressor itself. A partial winding to winding short can cause a voltage imbalance. If you remember, when ohming out any 3-phase motor, all winding should read the same.
Keep this test in mind when troubleshooting problems and component failures in 3-phase units. Many motors and compressors have failed unnecessarily from this preventable condition. Checking voltage imbalance should be a regular part of service on 3-phase units. It should especially be performed when you do have a motor or compressor that has failed so the new one does not have the same issue down the road!
York Central Tech Talk 
The thing that must be considered when checking for voltage imbalance is the meter you are using. In the above post is is mentioned to use a digital meter. This is not a good practice for this type of test. An analog meter should be used because overwhelming majority of digital meters average. In my working life I have found very few technicians that really understand what a digital meter is telling them, sure it reads voltage but at what part of the sine wave? If you have an averaging volt meter most technicians do not even know how to turn that feature off in their meter. This can and will lead to a very different outcome when doing this type of test. If you have a phase imbalance and you are going use capacitance to correct it you will be totally off the ranch if you use a averaging digital voltmeter. I would ALWAYS recommend an analog meter such as the old Simpson or one like it that most have in their truck. I realize its like carting around a brick, but its accurate and for doing calculations and capacitance calculations in particular their is no substitute. This type of meter should always be in your service truck anyway because of its megging capabilities.The technician that is prepared and knows how to use his arsenal of tools will be the successful technician and the customer gains the benefit. Be that technician today.
thank you for that information , i really appreciate it.
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Thanks for the knowledge.
I am having problem with voltage unbalance on project I am installing a/c units,i have voltage monitors installed in condensing units ,voltage imbalance when calculated is 2.5 %, trying to convince power company of problem but they are saying there voltages are within there limits line a-b240 volts,line b-c 231 v,line a-c 232 volts,can you give some assistance
go back to your manufacturer and request their voltage requirements for the specific unit. Then take that to the power company.
Great explanation for voltage checks. Thank you. When checking amp draw for a three phase compressor do you check each leg, add for total then divide by three or do you take each legs value for what it is.
Amperage is separate from VOLTAGE imbalance. On a 3-phase compressor, the amperage should be taken at each leg value for what it is and should all be about the same.
You will be interested in my letter to the December issue of the E&T magazine, from the Institution of Engineering Technology. Millions of £ could be wasted on the proposed HS2 rail link for the reason that you give.