After being through numerous commissioning activities where motor rotation is corrected by reversing two phases at the MCC unit (T) terminals I have come across a specification that states 'motor rotation to be changed at motor end only.'

It seems to me that this spec requires removal of insulation of connections in the motor terminal box and is 'more work' than changing rotation at the MCC.

The originator of the spec is long gone. Does anyone know the rational for this spec? In both cases the motor ends up with two T leads reversed wrt phase.

Bill.

 

The rational is that your phase markings on the wire remain correct and consistent. It can be a real headache when you connect a machine or something that can be damaged by improper phasing. If you always do this, you don't have to run around reversing motors. It's particularly important with movable equipment that every other box isn't wrong.

Regards,
cww speaking from experience.

 

ccw has it correct.

A modification to that spec I have seen (and used) is to switch the downstream leads at a local motor disconnect. This does not disturb the motor junction box connections but accomplishes the phase reversal quickly. The wiring and labeling between the disconnect and the motor starter remains intact. Quite often I see no labels in use for the wires downstream to the motor (I agree, this is not good practice but that is too often reality).

An exception to the above is where 4/C cable (3 phases and ground) is in use all the way to the motor rather than the use of individual (typically black) wires for each. With multicolor cables in use, the color code should match even from the disconnect switch to the motor. If that is the case, switching leads at the disconnect is not a good idea. (Would you want to see a brown phase A connected via a local disconnect to a yellow wire from it to the motor?)

Make the change at or near the motor, you (or someone down the line) will regret not doing that in the future.

Russ K

 

Dear Bill,

Actually these are only some standards. In an MCC, there are more than one motor connected so there is a uniform pattern of cable wiring (coloring of each phase). So it is better not to disturb this layout.

But regarding your problem - changing the two wires in MCC is the easiest way- then go ahead with it. no any clear cut rule for these things

 

Lets say at the time of construction, all motors are (correctly) connected the same way. Now when I bump to check for rotation, I find a motor having rotation that needs to be reversed (I do not normally find all motors rotate CW or CCW for fwd). Now I have the option (ignoring the spec for the moment) of swapping two phases at the MCC unit T terminals, or in the motor j-box. Down the road, say the motor fails. If I had changed the T-leads in the MCC this failed motor j-box will be wired the same as all other motors in the facility. My thinking is a motor replacement is more likely than a branch circuit cable replacement down the road. However, if a branch circuit cable is being replaced, it unusual if the phases have been swapped at the MCC or at the motor. What am I missing? I just don't see any advantage from changing the rotation at the motor.

 

Bill,

Of course it's easier to change the rotation at the MCC but that's just being lazy. If your wires are identified by colour or numbers you should fix it at the motor.

That's my opinion for what it's worth.

Roy

 

Yes, we all have done the bump and switch. The value of having everything consistent is that, unless the motor is mismarked, you can _know_ which direction it is going to turn the first time you start it. Some gadgets like carbon vane air pumps and expensive machines don't like being even bumped in reverse. And if for example you steal a motor from one machine to get another running, you can generally bet it's going to start in the right direction. I've worked in a place without the standard and saw hours or even days lost because something was running backwards. It's not always obvious.

Regards,
cww

 

I'd vote for changing it at the motor if any conductors are color coded or have Wire#. We have all movable equipment and go to great pains to keep all cord conductor colors and twist-lock receptacles wired the same throughout the plant. Would much prefer techs change the wiring at the motor.

 

I know it sounds obvious but it's worth noting that you must ensure that maintenance check the direction after replacement. We were in the middle of a conveyor system test when we realised that one conveyor was running too slow. It was replaced with another and they forgot to check the direction as it was a rush in the middle of the test. The technician had (correctly) connected BN > L1, BK > L2 & GY > L3. We were all waiting to see the bags and wondered what was going on until we went back and found the conveyor running in reverse and had "eaten" a few of the test bags!!!