You've just had a motor repaired or purchased a replacement, and you're about to connect it. What critical task must be performed for the correct operation of the motor's connected load? That's right: Determining the motor's correct rotation.
We all know that the direction of rotation of a 3-phase motor can be changed by reversing two of its stator leads. This interchanging, if you will, reverses the direction of the rotating magnetic field within the motor.
If we know that the connected load will not be affected by the motor's reverse rotation, we can temporarily power the motor and observe its direction of rotation. If it's the wrong direction, we can simply interchange any two line leads.
But what can we do if the connected load would be damaged with the motor's reverse rotation? We have to determine the correct rotation before the motor is connected to its load. We can temporarily power the motor while it's uncoupled from its load and observe its direction of rotation. And, after making the lead interchange, if required, the motor then can be coupled to its load. There's another option, one that's less time consuming and more efficient.
Using a phase rotation meter
A phase rotation meter, through the use of its six terminal leads, compares the phase rotation of two different 3-phase connections. Three leads, labeled "A," "B," and "C," are connected to the test unit's side labeled "MOTOR." Three other leads are labeled the same but are connected to the test unit's other side, which is labeled "LINE." The meter also has a zero-center voltmeter, with one side labeled "INCORRECT" and the other labeled "CORRECT."
First, you "zero" the meter per the manufacturer's instructions. Then you position the meter's selector switch to "MOTOR" and connect the three MOTOR leads to the motor's leads. Finally, you hand-turn the motor's shaft in the desired direction while watching the voltmeter, which immediately will swing to the CORRECT or INCORRECT direction. A special note: Although the needle will swing in the opposite direction after the shaft stops turning, you should use the voltmeter's first indication as to the status of rotation direction.
If you're lucky and the first connection configuration is correct, you label the motor's leads "A," "B," and "C" to coincide with the connected leads from the phase rotation meter.
Suppose you're unlucky and the voltmeter reads INCORRECT. Then you should swap any two of the MOTOR leads and hand-turn the motor shaft again. Now, the voltmeter should read CORRECT, and you should label the motor's leads "A," "B," and "C" to coincide with the connected leads from the phase rotation meter. You're not done yet, though, with the motor installation.
Determining rotation of power source line
Now, you have to verify the rotation of the line supplying power to the motor. Your phase rotation meter can be used here, also.
After deenergizing the motor's power feeder and applying required lockout/tagout devices, you position the unit's selector switch to "LINE" and connect the three LINE leads to the feeder. Then, energize the feeder and watch the voltmeter.
If you're lucky again and the voltmeter reads CORRECT, you label the power feeder leads "A," "B," and "C," after the feeder is deenergized and with lockout/tagout devices again installed, to coincide with the connected LINE leads from the phase rotation meter.
If you're unlucky again, you deenergize the motor's power feeder and apply required lockout/tagout devices, and swap any two LINE leads. With the feeder reenergized, the voltmeter will read CORRECT. Now, you can label the feeder conductors "A," "B," and "C" to coincide with the connected LINE leads from the phase rotation meter.
All that's left to do is to match the labeled motor leads with the labeled power feeder conductors and make the required connections. The motor will run with the right rotation.
Another phase rotation meter application
Just as we verified rotation of a motor and its power source, we can do the same for two separate power sources. Suppose you've constructed a temporary service with overcurrent protective devices while changing out an existing switchboard or panelboard. Obviously, the temporary power's rotation must be the same as that of the existing service in order for any connected motors to run in the right direction.
After making all lockout/tagout requirements, you first connect the LINE leads of the phase rotation meter to the load side of the existing service switch, which is in the open position. Then, with the device in the closed position, you check the phase rotation meter's voltmeter. If it reads CORRECT, put the service disconnect device in the open position and label the load side conductors to coincide with the connected LINE leads from the phase rotation meter. If the voltmeter reads INCORRECT, swap two LINE leads, and make the appropriate labeling on the load side conductors.
Next, before connecting the temporary service to the existing load, you connect the phase rotation meter's LINE leads to the line side of the temporary service. With the temporary service disconnect in the open position, close the existing service disconnect and view the voltmeter. If it reads CORRECT, label the temporary service line leads to coincide with the connected LINE leads from the phase rotation meter. If the voltmeter reads INCORRECT, swap any two LINE leads and make the appropriate labeling.
