Encoder channel reversing

I will try to keep this simple. I have a machine that is running an emerson vfd running in closed loop vector mode. The direction say to to wire the the encoder as follows:

A to pin 1, -A to pin 2, B to pin 3, -B to pin 4

The unit is wired as follows:

B to pin 1,. -B to pin 2, A to pin 3, -A to pin 4

This reverses both channels. What do this do to the motor and or vfd?

Thanks
Jim Cav

jimcav said:

I will try to keep this simple. I have a machine that is running an emerson vfd running in closed loop vector mode. The direction say to to wire the the encoder as follows:

A to pin 1, -A to pin 2, B to pin 3, -B to pin 4

The unit is wired as follows:

B to pin 1,. -B to pin 2, A to pin 3, -A to pin 4

This reverses both channels. What do this do to the motor and or vfd?

Thanks
Jim Cav

Click to expand...

Nothing. It's an incremental encoder with pulse ouput (ONs and OFFs), it will count the pulses just the same.

quadrature

what if it is a quadrature encoder?

Switching the encoder phasing is done to match the motor phasing. If the don’t match the motor will not function correctly in CLV.

Sometimes this is done because the engineer doesn’t understand why, other times it is done to change rotation direction. This could be required if the motor is running in torque mode, as some won’t run torque mode in reverse.

Best case I would expect the drive to throw a fault and shut down. The fault code would indicate a discrepancy between the commanded direction and direction indicated by the feedback. Worst case the motor accelerates rapidly to an overspeed condition.

The motor really does't care but if before the counts increased while turning clockwise they will now decrease. However the output polarity must be set to match the how the counts increase or decrease or now you will have positive feedback instead of negative feedback.

jimcav said:

what if it is a quadrature encoder?

Click to expand...

Nothing or intentional. Which drive do you have? What is the motor used for? What issues are you having?

It probably means that on initial startup there was a discrepancy between the drive output phasing and the encoder feedback phasing. The commissioning personnel decided it was easier to switch the encoder phasing than it would be to change the motor phasing. They should have updated the schematics to agree with the change but that doesn't always happen.

This some times happens depending on which end of the shaft the encoder is mounted on if it isn't mounted on the motor. We have a convention where a positive output should make the encoder counts/positions increase. If not then there are two options.
1. Reverse the polarity of the output
2. Change the wiring.
Sometimes when the encoder is mounted on the motor the encoder counts may decrease when you want the position units to increase. This will required a negative scale factor so decreasing counts will result in increasing positions and positive velocities. In our case we close the loop around the positions after they are scaled by this may be different with different manufacturers.
I am pretty sure Rockwell does it the same way we do at least for the M02AE.

Lots of VFD’S on the market have an invert parameter for the encoder inputs. If the drive expects to see positive counts, but the feedback is negative, you can simply set the invert bit high, and vice versa.

Im not sure if Emerson has this feature or not.

Kick-It-Up said:

Switching the encoder phasing is done to match the motor phasing. If the don’t match the motor will not function correctly in CLV.

Click to expand...

This is the correct answer.

When starting up a drive, first you get the motor going the right direction for forward and/or reverse via motor phasing. Then the encoder phasing is setup to match it.

If the encoder was wrong, and the drive was set for encoder feedback, then you would trip on an encoder direction fault.