Three-Phase Power and Voltage Imbalances
A little electrical knowledge can go a long way in troubleshooting HVAC systems
Keep in mind when reading any article about electrical theory or application that it only scratches the surface of the topic. You can dedicate years of your life to understanding electrical theory and design the way many engineers do and still know just enough to be dangerous. In HVAC, we rarely need to have a deep understanding of electrical design. But there are a few cases where a little understanding can go a long way in identifying issues before they cause trouble, and that is the intent of this short article.
What is Three-Phase Power?
Power is generated at the utility in three phases that are 120 degrees out of phase with one another at 60 Hz. This simply means that 60 times per second, each individual leg of power makes one peak and valley (a full circle), and all three of the phases together split the cycle into thirds (trisect).
What Do HVAC Techs Need to Know About Three-Phase Power?
Three-phase motors don’t require a run capacitor because the 120-degree phase difference is ideal for efficiently spinning a motor so a “start” winding and phase shifting capacitor isn’t needed.
The biggest concern for the techs and installers with three-phase power is getting the phasing correct so that motors run in the appropriate direction. While this doesn’t matter for reciprocating compressors, it is important for condenser fans and blowers, and it is absolutely critical for scroll and screw compressors. Changing the direction of rotation is just as simple as swapping any two phases.
When installing replacement parts and equipment, remember that if you keep the phases connected in the same way, you will generally be in good shape. It is still a good practice to use a phase-rotation indicator to confirm proper rotation. In most cases, clockwise phase rotation is what you are looking for, but I’m sure there are exceptions to that. Alternatively, you can disconnect the compressor that could be damaged by improper phasing and start up the blower to see if it runs the correct direction before bringing the compressor online. One caveat is that when motors use a variable-frequency drive, the phase rotation will automatically correct, making them an unreliable test in those cases.
Electricians are responsible for balancing the amperage of single-phase loads (120-V single-leg and 208-V two-leg loads are typical on a wye three-phase system) so the neutral doesn’t carry high amperage on the 120-V loads and so the one leg of power doesn’t carry significantly more or less load than the other two. As the amperage load on a particular phase goes up, there is more opportunity for voltage drop depending on the size of the load, the size of the transformer feeding the space, as well as wire size and connection quality. This can become a challenge when there is a mix of single-phase outlets, 208-V appliances, and three-phase equipment.
Let’s say someone connects a bunch of space heaters on phase A, a few smaller HVAC systems between phases A and B, and almost nothing on phase C. If you have a large rooftop unit that uses three phases, phase C will tend to have less load and, therefore, higher voltage, while the loads on phases A and B will fluctuate based on when the smaller systems and space heaters go on and off.
This can cause overheating of conductors and damage. It can also cause voltage imbalance, which is a real cause for concern for HVAC technicians.
Voltage imbalance is a motor killer. It causes poor motor performance and increased winding heat, which leads to premature failure. In the case of HVAC blowers and compressors, this additional heat ends up in either the refrigerant or the air. It must then be removed, which further decreases efficiency.
To test for three-phase imbalance, always check from phase to phase, not from phase to ground. You simply check the voltage from each of the three phases to one another and find the average (add all three and divide by three). Then, compare the reading that is farthest from the average and find the percent of deviation.
The U.S. Department of Energy recommends that the voltage imbalance be no more than 1 percent while other industry sources say up to 4 percent is acceptable. In general, you will want to make sure the imbalance is below 4 percent and work to rectify anything over 1 percent.
What Can I Do About It?
You want to first look for the obvious. Melted wires, loose terminals and lugs, undersized wire, pitted contacts, poor disconnect fuse contact, etc. Obviously, if you aren’t licensed or allowed to open a panel, you won’t always be able to fully rectify the issue yourself, but you can go a long way toward the diagnosis.
When checking voltage, it is generally best to do it with the system running as close to the motor you are checking as possible. This is the actual voltage the motor is “seeing” and is what matters most to the operation of the motor. You can then test back toward the distribution point; if you see a big increase in voltage as you test back toward the source, you know you found a voltage drop and a cause or contributor to the issue.
From there, the issues of amperage load imbalance in the panel, service size, and utility issues must be considered once all the basics are covered. Most of all, if the imbalance is severe (over 4 percent), you don’t want to leave your motors running, or you risk damage and expensive repairs.
Publication date: 3/18/2019