Unfortunately, It isn’t possible to check phase rotation with a standard multimeter. Phase rotation must be checked with a specialized tester like the fluke 9040 or Amprobe PRM-6. These testers allow technicians to ensure equipment like pumps and compressors won’t be damaged by incorrect rotation. If you do not have access to a phase rotation tester then a motor bump test can be performed instead.
How to Use a Motor Rotation Tester
A rotation tester can be connected to three phase motor and supply leads in order to determine the how a motor will rotate once power is applied.
- Disconnect power to the motor and lockout/tag-out as required.
- Plug the three test leads into the rotation tester.
- Attached the alligator clips to the corresponding motor leads. For example, connect the L1 alligator clip to the T1 wire.
- Rotate the motor shaft clockwise. The tester will display either clockwise or counterclockwise rotation.
- If the tester shows counterclockwise rotation while turning the shaft clockwise, swap and two of the leads and repeat the test until the tester indicates the correct rotation direction.
- Label motor leads A, B, C.
Now you’ll need to check the rotation of the supply voltage.
- Disconnect power to the motor supply and lockout/tag-out as required.
- Connect the alligator clips to the three-phase supply.
- Re-energize the circuit and see what direction the rotation tester indicates. If the indicated rotation is different then the desired rotation, lockout again and swap any two leads then repeat the test.
- Label the wires as A, B, C.
- Lockout again and connect corresponding feeder wires to the motor leads with the same label.
If performed correctly this test will ensure that desired rotation direction is achieved the first time. It can take a little more time to use this method but if incorrect rotation could damage equipment there is no better way to go than using a phase rotation tester.
How to Bump a Motor
Bump testing a motor is a perfectly fine way to test rotation as long the connected equipment moving backward won’t cause damage. If possible, the motor can also be bumped without the load coupled to the output shaft.
Depending on how motor connections are made it can be time-consuming to swap leads if rotation is incorrect. If connections are made with split bolts and rubber splicing tape it’s a good idea to purchase a few rubber boots that can cover the split bolts temporarily while the bump test is done.
Always ensure that the wiring is safe and will not short out while performing a bump test.
Some workers will close motor starters with an insulated screwdriver to perform a bump test. This is an unsafe practice and should not be performed. Because an arc flash could occur it’s safer to use an alternative means to bump test a motor such as:
- Jogging from the keypad if the motor is controlled by a soft start or VFD
- Using jog/test buttons
- Activating controls at the operator’s console
- Making edits in the PLC logic to allow for a bump test
When you are ready to bump test have someone watching the motor to verify the rotation direction. Give the motor a quick bump and confirm the rotation direction. Lockout and swap any two leads if rotation is incorrect.
Why Test Rotation?
Isn’t it’s easy enough to just swap the T leads in the morning if rotation wrong?
I was trained to verify everything I do. I truly believe testing your work will set you apart from other tradespeople. The last thing I want is for an operator to start their machine in the morning and have a chain run backward, creating a mess for them, and making me look bad!
And while rotation can’t be tested with a multimeter there are several other options available to us.