How Do You Change Rotation Direction in Three Phase Motors

A question came in from one of our viewers about HOW to change the direction of a three-phase motor. In the latest episode of Electrician U, Dustin answers this question in addition to explaining to us WHY we are able to do this in the first place!!

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With a single-phase motor, we are relying on 180 degree opposing stators to move the center shaft. In fact, with single phase motors, we are actually applying power to the Capacitor which then relays power so said stator. The reason for that is that the central shaft can become stuck in a center state, thus not moving when power gets applied. But by utilizing a capacitor, that capacitor causes a shift when it starts up (sort of acting like a kick starter on a dirt bike motor) that moves the rotor a bit and allows the opposing coils to actually do their job. A three-phase motor operates a bit different. In lieu of having the 2 poles 180 degrees apart, since we are using 3 phases, those poles are 120 degrees apart. Instead of one pole strongly pushing and the other strongly pulling the rotor, on a 3-phase motor, one of the poles is strongly pushing while the other two are each sharing the pull function. As the pushing pole winds down, the other 2 poles are picking up speed/strength and the rotor’s momentum can be kept going until it hits the next pushing pole thus making the motor much more efficient and really negating the need for a capacitor.
So, onto the reason of why and how we can make a three-phase motor spin the opposite direction. The answer lies in the phases we just spoke of above. As the rotor is spinning, the pole that has been strongly pushing, is transitioning to pulling less, while one of the others that has been strongly pulling is transitioning to strongly pushing. If we swap two of the three phases, this causes power to be applied in a slightly different sequence thus causing the motor to spin the other way!
We can utilize this ability to shift in a positive way. If we hook up a three-phase motor only to find it is spinning backwards to the way we want it to do, we can simply swap two of the three phases and have that motor spin the way we want it to! And it can be any of the three phases, not just two in particular. We can swap A & B, or A & C, or any combination of. There are even meters available (called phase rotation meters) that we can connect to a three-phase circuit to see which way it is going. These meters are useful when we are replacing a piece of existing gear feeding equipment! Simply check the rotation BEFORE removing the gear, hook up the new gear in the same phasing as the old (A, B, C= Black, Red, Blue for instance), and then recheck BEFORE you energize any equipment on the new gear. If it is not spinning the same way, just swap two of the legs feeding the gear and you are in business!! To change a motors rotation, you needn’t necessarily do it IN the gear that is feeding the motor, you could do it anywhere between the gear and the motor. Keep in mind, that if you change it on the line side of said gear, you will be changing ALL the load side feeders feeding OUT of that gear, and that could change rotation on OTHER pieces as well! To combat that, just change two of the three wires on the load side of the specific equipment’s breaker only!
We hope this has been an informative look into the way a three-phase motor operates and how you can change the rotation of a three-phase motor! Is there an electrically related topic you would like to see discussed here on Electrician U? Leave a comment in the comment section below and let us know what you would like to see. Please continue to follow Electrician U and Dustin’s adventures as we are constantly adding new content to help our followers be the best electricians they can be!