As I’m sure you know, with all the storms we have had this summer, there have been a lot of power outages due to power lines getting blown down or knocked down by trees. In most cases, once power is restored, most air conditioner and packaged units start back up with no problems. Occasionally, though, the power company reverses 2 of the phases (power legs) to the building. Now you could have 3-phase blower motors running backwards, or worse, 3-phase scroll compressors running backwards which will cause them to eventually burn up and fail. All it takes to reverse a 3-phase motor or compressor is to switch any 2 of the power legs.
The easiest way to prevent 3-phase motors and scroll compressors from running backwards if the power company reverses any 2 legs of the phase to the building is to add a phase monitor to the system. This is a simple and inexpensive control that can save your customers major repairs or parts from failing due to them running backwards.
The way it works is if all 3 phases are relatively equal and in proper sequence, the normally open contacts will close when 24 volts is applied to the monitor. If the phases are out of sequence, or if one is missing, the contacts will never close. If a phase is lost while the monitor is energized, the contacts will open immediately and will remain open until the error is corrected. Again, to correct the phasing, all you need to do is to reverse any 2 of the INCOMING power legs so you keep the entire unit in phase. Do not just reverse 2 legs at a component or you could still have one or more components running backwards. All units shipped from the factory are phased so if you just reverse the motor, the compressor could still be running backwards and the phase monitor will not allow the unit to run until all 3-phase components are in phase.
Here is a simple wiring diagram for most units:
Now, in the case of units with our Simplicity Board, the wiring is a little different. Since we control all the components in our unit through our Simplicity board, we want to “kill” the power to the board (as opposed to just the contactor as in the diagram above) so we completely shut down the entire unit.
Here is a wiring diagram to make it simple to install the phase monitor on our units with Simplicity boards. I have also attached the spec sheet for the S1- ICM401 monitor which we can factory install in a lot of our equipment and you can field install as an after market add-on.
Phase monitors will save a lot of equipment and, if you have a location that is prone to power failures, storms, imbalanced voltages, etc, then you really should consider talking to your customer about installing a phase monitor on their systems.