Can a single phase motor run backward when start and run are swapped? The answer is (generally) yes. Is the motor designed to run backward by simply swapping run and start? The answer is (generally) no with a few notable exceptions.
Before we jump in, this article has two purposes. #1 – It helps you understand a compressor design you may find in the field and #2 – It will help new techs with reading and understanding wiring schematics and diagrams.
If it gets too technical for you, jump down to the bottom and just watch the videos before you get fed up and move on.
In modern residential air conditioning, we see this design where the motor can run forward and backward depending on the wiring of start and run in the two-stage compressors made by Bristol shown in the USPTO drawing above which activates the full stroke of both pistons in one direction and only one piston in the other direction. This design allows two distinct capacities from a single compressor with no special unloaders, speed changes or bypass.
This is an extension of an earlier design by Westinghouse shown in the image above. The diagram on this one is pretty vague, but the general idea is a swapping of the phases to the compressor motor R & S to reverse the rotation. Now you may be thinking-
On single phase 240v power the two phases are the same and swapping them makes no difference
You would be totally correct in this assertion other than the purpose of the start (aux) winding is to have a force at play on the motor that is out of phase to an extent to provide the necessary starting torque as well as the improved efficiency and power quality that comes along with the constant phase shift provided by the run capacitor.
In layman’s terms –
We are trying to make single phase motors as close as we can to 3-phase motors and capacitors are our best tool to try and get close
Single phase motors are like a two-handed juggler trying to compete with a three-handed juggler by optimizing our toss and catching angles. I’m running out of metaphors here so I hope you’re getting it…
In order to make a motor that works in either direction the run and start windings need to both be designed to carry the continuous amperage that is usually reserved for run. You may think that the start winding draws higher amperage than start because START sounds like it would take the bulk of the amps during start. Actually, the start winding is generally a smaller, higher resistance winding and its amperage is limited by the connected capacitor. In order for a compressor like the one shown below to work, it needs to have a start winding engineered to function as a run winding and vice versa.
This diagram from Bristol really simplifies how they initially envisioned it, I also like how they give directional arrows so you can follow the circuits in both high and low modes. Obviously, it is alternating current so it doesn’t travel in only one direction but it helps you see how the capacitor is connected to Start in High on top and the Run winding on the bottom.
Here is a diagram from a Carrier 38YDB that used this compressor in the early 2000’s and this diagram shows it in the usual schematic form with the addition of a start capacitor and a potential relay.
Look at the left side, CH is the “compressor high” contact and CL is “compressor low”. When CH is Closed, CL needs to be open and the unit will be in high-speed. When CL is closed CH needs to be open and it will be in low-speed. If you trace it out you will see that in low-speed L1 is connected directly to start and in high-speed, L1 is connected directly to run. From there the opposite side is then only capacitively coupled to L1 through the run and start capacitors. This swap in phase is what causes the motor to run in one direction in high which grabs both pistons and the other in low which only pumps one.
Here are two videos that I did recently. One of a teardown of this compressor and another going through the schematic shown above.