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Old February 5th, 2018, 04:52 PM   #1
lesmar96
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reverse three phase to reverse rotation

Hey, this might be a simple question, but I don't think I ever ran into it before.

We all know that on 3ph, any two leads can be swapped to switch the rotation of the motor. Does this not work through a vfd? I suppose with the DC bridge, etc. the output phasing is not necessarily sequential with the input phasing.

At any rate, today I had wired up a panel with a couple vfds and all the motors went the wrong way. Normally, i switch the leads in the terminal box, but since all the motors were going the wrong way, I thought I would just try to switch two leads at the incoming power disconnect. Well that didn't make any difference. Swapped two leads on the line side of the drive and they still went backwards.

Is that normal?
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Old February 5th, 2018, 05:03 PM   #2
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Yes, it's normal. The VFD input section (converter) turns that 3 phase into DC, charges up a capacitor bank like a battery, and then the output section turns it back into 3 phase with no real regard for the input section.. This is great since as long as you keep the wires between the drive and the motor straight, you don't have to worry about something happening to change the phase rotation upstream of the VFD. This is not so great if all your motors turn backwards because it means you have to go move a whole shedload of wires to make them right.
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Old February 5th, 2018, 05:22 PM   #3
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You can swap any two phases either at the Input, or Output of the VFD.

This can cause havoc if all the cables in the panel are identified "R, Y, B", people like consistence in the panel, it makes for easier maintenance.

So it might be better to swap the phasing at the motor terminal box, and update the field schematics accordingly.
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Old February 5th, 2018, 05:27 PM   #4
OkiePC
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Quote:
Originally Posted by daba View Post
You can swap any two phases either at the Input, or Output of the VFD.
Uhmm, no. Sorry to disagree, but swapping the input phases will have no effect on a VFD. It will work for a soft starter or for a mechanical motor starter... but not a VFD. You can only change rotation at the output side of the VFD.

Now one other way you can reverse them is through drive programming. That might be easier, but not necessarily the right thing to do.
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Old February 5th, 2018, 05:31 PM   #5
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Great answer, OkiePC. You are exactly correct. As for daba, unfortunately, you are exactly wrong for the reasons OkiePC cites.
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Old February 5th, 2018, 05:32 PM   #6
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Quote:
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Great answer, OkiePC. You are exactly correct. As for daba, unfortunately, you are exactly wrong for the reasons OkiePC cites.
Daba is sharp as a tack, so I BET he meant input to the motor...
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Old February 5th, 2018, 05:47 PM   #7
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Quote:
Originally Posted by OkiePC View Post
Daba is sharp as a tack, so I BET he meant input to the motor...
Yes, I said "Input" to the VFD by mistake, been a long day and didn't think it through.

Perhaps my sharpness is reduced by age, lol

The other considerations I think are valid, though.
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Last edited by daba; February 5th, 2018 at 05:49 PM.
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Old February 5th, 2018, 06:02 PM   #8
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Does the VFD in question have a Run FWD and a Run REV input? Or a direction parameter?

Telling the VFD to run a reversed motor in reverse would put the motion forward.
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Old February 5th, 2018, 06:13 PM   #9
lesmar96
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Quote:
Originally Posted by Aabeck View Post
Does the VFD in question have a Run FWD and a Run REV input? Or a direction parameter?

Telling the VFD to run a reversed motor in reverse would put the motion forward.
Yes. the vfd can run in either fwd or rev by terminal command, but I have been taught and also consider it not proper to run a vfd in Rev just to make the motor go the right way because that could cause confusion to some future person.
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Old February 5th, 2018, 06:20 PM   #10
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Quote:
Originally Posted by lesmar96 View Post
Yes. the vfd can run in either fwd or rev by terminal command, but I have been taught and also consider it not proper to run a vfd in Rev just to make the motor go the right way because that could cause confusion to some future person.
I don't think it matters. There is absolutely no concept of FWD/REV, versus Clockwise/Anticlockwise, etc.

Some manufacturers go with FWD or Clockwise when looking at the shaft, but there's a few that define FWD or Clockwise when looking from behind the motor.

I'm prepared to be shot down on this, but I don't think there's a "standard", especially when shopping "overseas".
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Old February 5th, 2018, 06:41 PM   #11
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Quote:
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I don't think it matters. There is absolutely no concept of FWD/REV, versus Clockwise/Anticlockwise, etc.

Some manufacturers go with FWD or Clockwise when looking at the shaft, but there's a few that define FWD or Clockwise when looking from behind the motor.

I'm prepared to be shot down on this, but I don't think there's a "standard", especially when shopping "overseas".
I too stand to be corrected, but I always had the idea that FWD is the way something needs to go. For example, these were on conveyors that always needed to run the same direction and never go the other way. So I would have said that is fwd. Or an air compressor or blowers that needs to go a certain rotation, I would call in forward. But If equipment runs either or at any given time by system instruction, then there really is not fwd or rev per say. That was always my perception.......
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Old February 5th, 2018, 06:43 PM   #12
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Personally, I prefer to change as few drive parameters as possible from their default values. I'll connect the wires so as to make the motor turn in the direction I want it to go at the default direction parameter setting.
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Old February 5th, 2018, 08:08 PM   #13
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Personally, I prefer to change as few drive parameters as possible from their default values. I'll connect the wires so as to make the motor turn in the direction I want it to go at the default direction parameter setting.
Logically speaking this makes the most sense in my opinion. When setting up several conveyors we used the default settings of the VFD, than swapped the wires on the output side of the VFD, if necessary. These conveyors were only designed to go in one direction...this was a company standard, and part of our buy-off procedure.

Last edited by Sydney; February 5th, 2018 at 08:41 PM.
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Old February 5th, 2018, 08:41 PM   #14
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IEC 60034-8 and BS 4999 part 108 specify that the rotation direction is clockwise when viewed from the "Drive-end".

NEMA MG-1 specifies in Section 2.24 that the motor should rotate counter-clockwise when viewed from the non-drive end.

So they're the same, just from inverse definitions.

Go ahead, ask me when International Standards Day is.
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Old February 5th, 2018, 09:24 PM   #15
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If it were me, I would change the rotation at the local disconnect for the motor. This is a pretty standard practice, and should not confuse anyone. Plus it is a lot easier than rewiring the motor.

Bubba.
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