Can A Motor Run Backwards by Swapping Run & Start?

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 watch the videos before you get fed up and move on.


In modern residential air conditioning, we see a 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.

That design 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 and 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 three-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 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 an alternating current, so it doesn't travel in only one direction. Still, 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 2000s. 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 the “compressor low” contact. When CH is closed, CL needs to be open; the unit will be in high-speed. When CL is closed, CH needs to be open; the unit will be in low-speed. If you trace it out, you will see that L1 is connected directly to start in low-speed, 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 is of a teardown of this compressor, and another going through the schematic shown above.

—Bryan

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Related Tech Tips

Liquid Line Solenoid Valves: Long Line Applications
Pump down solenoid valves are commonplace for any refrigeration technician. They are energized with the compressor still running, shutting off flow in the liquid line so that the refrigerant is pumped into the condenser and receiver. The compressor will then shut off once a low-pressure switch opens the circuit when the pressure falls below a […]
Read more
Drip Loops and Straws
Many of us are aware that X13 and Fully variable motor failure has peaked over the last few years, and I'm sure there are multiple reasons for that. One of the reasons is fairly simple and can be traced back to two simple installation and service practices that can be easily implemented. Seal all air […]
Read more
Lifting Techniques Part 5 - Really HEAVY Stuff
This is Part 5 of a series by Senior Refrigeration Tech (and prolific writer) Jeremy Smith. Pay attention to this one, folks. I know rigging and safe lifting practices may be boring to some of you, but it could very well save your back or your life. Disclaimer This article is written by a technician […]
Read more
loading

To continue you need to agree to our terms.

The HVAC School site, podcast and daily tech tips
Made possible by Generous support from