# Checking Rotation When a VFD is Involved



## Peewee0413 (Oct 18, 2012)

Nope. Best bet is use a rotation tester or slow the motor down and creep up the speed until its starts to move.. Disconnect the motor and test.. Many options but line side swapping is not one. 

Sent from my VS995 using Tapatalk


----------



## MDShunk (Jan 7, 2007)

Doesn't matter how the input phases are arranged. The output, when in forward, will be A, B, C rotation. Check the output briefly with your phase rotation meter before you connect the motor for real.


----------



## mitch65 (Mar 26, 2015)

If you set a VFD to run CW then swap the leads in the motor peckerhead, does the VFD know the motor changed directions? can you bump them at lowest speed with VFD?


----------



## MDShunk (Jan 7, 2007)

mitch65 said:


> If you set a VFD to run CW then swap the leads in the motor peckerhead, does the VFD know the motor changed directions? can you bump them at lowest speed with VFD?


No. The VFD only knows "run forward" and "run reverse". What you call forward and reverse is up to you and how you wire it to the motor.


----------



## Peewee0413 (Oct 18, 2012)

mitch65 said:


> If you set a VFD to run CW then swap the leads in the motor peckerhead, does the VFD know the motor changed directions? can you bump them at lowest speed with VFD?


If you swap rotation in the pecker head it will reverse rotation, regardless of the forward or reverse on the drive. If the drive is able to be keypad control and has a pot then put it in keypad control and turn the speed down. Start the drive and increase the speed. All depends on the parameters for min freq and the equipment. You didn't give enough info for a straight forward answer. 

Sent from my VS995 using Tapatalk


----------



## MDShunk (Jan 7, 2007)

A "bump at lowest speed" would be unlikely to produce observable results. It would probably just charge the capacitor that is the wire and motor windings. Putting it in run forward and bringing the speed up a little at a time, until the load just starts to budge, is a decent way if the driven load can tolerate going backwards just a hair.


----------



## just the cowboy (Sep 4, 2013)

If motors are new check specs on motor and VFD. Most will state when output is connected A/U to T1 B/V to T2 C/W to T3 __ direction is achived when looking at motor shaft.


----------



## Wirenuting (Sep 12, 2010)

If your drive has a bypass feature check it before attaching the motor output leads also. 
Then do as others above stated. 


Ya, I’ve seen the factory make mistakes on the bypass contactor.


----------



## JRaef (Mar 23, 2009)

Wirenuting said:


> If your drive has a bypass feature check it before attaching the motor output leads also.
> Then do as others above stated.
> 
> 
> Ya, I’ve seen the factory make mistakes on the bypass contactor.


You have just described the solution to the #1 most common mistake when a bypass is used with a VFD.

So just to reiterate and clarify:


There is ZERO relationship to the incoming power phase rotation and the outgoing power phase rotation across a VFD.
The input to a VFD is simply the "raw material" from which the VFD creates DC to feed its transistors, then the transistors create the pseudo-AC output from a VFD that is, for all intents, *a new source of power going to the motor*.
The output phase rotation from a VFD is always ABC, but can be changed at the flip of a bit to CBA, because all it needs to do is change the pattern of firing the transistors.
You do not know what direction a 3 phase motor will rotate until you try it (or use an expensive phase rotation checker, which are still only about 95% accurate). That's why you do a "bump check" on the motor every time. Even the motor mfr does not know when the motor is manufactured, but SOMETIMES a good mfr will TEST it at the factory before it ships and mark it on the motor for you. That is NOT always true however and I have always made it my policy to check it even if it does say it. I have found two that were wrong over the past 30+ years.
You cannot even trust the utility! Here in California for example, PG&E, the largest utility, uses CAB rotation, not ABC. Screws with people who don't know that all the time...
If you have a VFD that has a Bypass arrangement, ALWAYS do the bump test in bypass FIRST, then swap leads on the output of the VFD to make it match the motor rotation when in bypass (which is of course what Wirenutting said above).


----------



## glen1971 (Oct 10, 2012)

JRaef said:


> You have just described the solution to the #1 most common mistake when a bypass is used with a VFD.
> 
> So just to reiterate and clarify:
> 
> ...


Excellent post! We always bump test uncoupled.. Sometimes the guy using the rotation meter doesn't know which way it is supposed to go...

JRaef - shouldn't #5 be CBA, since CAB is ABC?


----------



## JRaef (Mar 23, 2009)

glen1971 said:


> ...
> 
> JRaef - shouldn't #5 be CBA, since CAB is ABC?


LOL, yes. Spell check "fixed" it for me...


----------

