Up To Speed: When Reversing The Leads Doesn’t Reverse Rotation

04/16/2018

Up To Speed Graphic Thin VersionUp To Speed is a concise, technical blog compiled by industry veteran Mark Reeder. It provides an interesting fact, answers a thought-provoking question, or offers a cliffhanger from an actual site visit by one of our world-class Field Service Engineers.

“3-phase Franklin Electric submersible motors, as with most 3-phase motors, do not care which way they rotate. Clockwise or counterclockwise, it’s the same to them and the motor will deliver the same amount of power in either direction.

However, pumps do care. They are designed to rotate in one direction and one direction only. When a pump spins in the wrong direction, the falloff in performance is dramatic. Of course, simply reversing two of any of the three motor leads on a 3-phase motor will reverse the direction of rotation and correct the pump performance. But, please note:

  1. This doesn’t apply to single-phase motors. 
  2. Keep in mind that we said reversing or interchanging any two leads, not rotating. We rotate the motor leads to check the 3-phase balance. We reverse two of the three leads to change rotation.Rotation

What about using a 3-phase motor with a 3-phase variable frequency drive (VFD)? There’s 3-phase coming into the drive and 3-phase going out to the motor. What happens when we reverse any two leads of the 3-phase input? The answer: absolutely nothing. The reason: inside a VFD, that incoming AC gets converted and stored as DC. From here, the VFD then synthesizes its own variable frequency 3-phase output. So, reversing the leads on the input side will change nothing in terms of motor and pump rotation. To change motor and pump rotation, we must reverse any two motor leads on the output side of the drive.3 Phase Motor

Seems to make sense, but we’ve seen the following scenario more than once in the field:

  1. A pump is not even close to matching its curve.
  2. The drive’s output frequency (and therefore motor RPM) is verified as correct.
  3. To correct rotation, the drive’s input leads are reversed.
  4. There’s no change in performance and the conclusion is drawn that the pump must be defective.

It’s easy to see how this could happen, and unfortunately sometimes the pump gets pulled for no reason. It’s only after the motor leads were interchanged on the output side that everything falls into place, thereby generating that ‘hard lesson learned’ moment.”

Now, you’re up to speed.