Change in Phase Sequence

Changing a phase on any 3 phase motor causes a change in direction of rotation. compressors will usually run in only one direction. Copland Scroll compressors will self distruct after a brief time running in the wrong direction.

You are right with the scrool but piston compressor can often run both directions. It is not always a problem.

Top drawing is 3 phase and the bottom is single phase.

Single phase power is generated for two legs 180° apart. Poly(3) phase has 3 legs 120° apart. The frequency of the sine waves(in both cases) are dependant on the rpm of the generator and will not change without a SCR, nor the time or distance between phases.

So call the brown phase A or 1. Then orange B or 2. And then yellow C or 3.

In an induction motor the poles will attract and repel through a cycle of the sine wave, and they will cause the rotor to turn. If the motor is checked for rotation, without anything coupled to it, and it turns backwards---No big deal--Swap any two legs and it will reverse directions.

Touch brown 1st- orange 2nd- yellow 3rd---Clockwise, right. Change any two of them, brown 1st- yellow 2nd- orange third--Counterclockwise.

The generator spinning causes this and it can be a thousand miles away(if the generator slows or speeds up, you motor will do the same).

Single phase induction motors will not start turning on their own unless the have a starting winding or capacitor.

Hi

The phase sequence is determined (at the powerhouse) by the rotation of the generator. Likewise it determines the direction (clockwise, counterclockwise) that your compressore motor will try to run to "imitate" the generator. Thus if you change the sequence (reverse any two leads) it will want to run backwards, which it should not for either mechanical or electrical reasons. Since we normally don´t really know the sequence, sometimes we test motor rotation momentarily to see if is right, then switch 2 leads if it is not. I am no expert, but likely there are testers that will tell you the same thing.

Jesse

Yes, I agree.

There are testers and I have wired hundereds of three phase motors and have only heard of them. As a normal practice, if there is the possibility of damaging equipment the motor drives--A quick rotation check is always a good idea before coupling it up. Sometimes it is easier to make up the motor and then change the rotation at the motor starter if it turns the wrong direction. A motor that is disconnected should get phase tape on the line and motor leads, for reconnection(still checking rotation after).

Kilgore, The guys who wear out the phase rotation meters are the guys re-connecting after repairing problems on the distribution grid. Not too handy to do a quick rotation check by touching a couple wires for a second.

You are absolutely right.It is beeter to check phase seq.before coupling to ensure correct direction of rotation particularly on uni directional equipments coupled to motor.