Vibrating Wires?

[Johnmcca]

Roger, I'm with Larry on this one. The only effect swaping lead will have is to reverse the rotation of the motor. Draw a delta or wye transformer diagram and label the phases and then swap them around they are never more than 120 degrees apart, just the rotation direction changes. The motor itself doesn't care which way it spins, the equipment it drives is another matter.

 

[LarryFine]

Right. If you look at a reversed 3-phase system as 240 degrees instead of 120, it's still the same thing, only with the rotation direction reversed.

 

[ramsy]

Roger, I'm with Larry on this one. The only effect swaping lead will have is to reverse the rotation of the motor.

Servey says, motors are damaged by an (A,C,B) phase rotation. This story below described (A,C,B) motor centers causing severe enough heat to stress the AC and employees.

"Normal electrical power phase rotation is A, B, C. Fifth (5th) harmonic current has a phase rotation of A, C, B. This reverse rotation will cause a counter torque on motors resulting in overheated windings, and eventually insulation breakdown." Public Works Digest:"Problems at Fort Tank"

Last month Rick Hart encountered these 5th order harmonics on his motors, and I shared this reference.

"The negative sequenced harmonics are the 5th, 11th, 17th, etc. They present a force that opposes the motor rotation and tries to make the motor rotate in the opposite direction. The force of these harmonics acting upon each other creates heat which leads to premature failure." Controlled Power Company

Motors loads are joined by some UPS's that fail to operate entirely on miswired phase rotation.

(If) "Phase rotation is C, B, A. This is incorrect and must be corrected before continuing with startup. Call Active Power Customer Service." CleanSource UPS (SMS)

 

[kingpb]

What about the case when the utility is using ACB as their rotation, does this change the harmonics you describe? Just because ABC is "normal" at Fort Tank, does not mean it is "normal" everywhere else.

The fact that the 5th, 11th, and 17th, etc. cause internal reverse sequence current, is inherent to a standard induction motor design, without a drive installed, whether the system is an ABC, OR ACB, is inconsequential.

Your reference to problems is based on 6-pulse drives operating on non-inverter duty rated motors. Yes, 6-pulse drive create problems, especially on long runs of cable between the motor and the drive. But that's another topic.

 

[ramsy]

If you look at a reversed 3-phase system as 240 degrees instead of 120, it's still the same thing, only with the rotation direction reversed.

Yes, I saw this.

#1) I traced (A,C,B) with my finger on a circle with 3 points, it went perfectly backwards.

Then I tried to see what this load rotation and opposite svc. phase rotation did together.

#2) I traced both phase rotations on the same diagram with my fingers, circling opposite directions, and saw the xfmr firing (B,C) at the same time, before realizing the svc. won't do that.

#3) When I finally realized one finger would have to trace a mirror image, since C&B coils trade angles to run opposite phase rotations, I was seeing DC or VFD's --240 degrees apart-- and forgot about #1)

#4) By the time I realized no matter which order a tripods legs hit the ground they alway circle 120 degrees apart, I was seeing 5th harmonics on (A,C,B) phase rotations, and forgot about VFD's.

#5) So, do I get it yet?

Overheating with 5th order harmonics is a product of DC simulated 6-pulse phase rotations using VFD's, and mixing up 3-phase rotation will always hit AC motors 120 degrees apart, changing the direction of the motor only?

Or are 5th order harmonics also inherent to a standard induction motor design with a negative phase rotation?

 

[peteo]

Roger, your quote made my day. Past that you've lost me. A VFD or a utility 'sees' a three phase motor at the leads.
Kingpb, as far as I know you're correct. "To reverse the direction of a three phase motor reverse any two leads." Any side effects from reversing or swapping various leads may have on harmonics or other equipment, is something I'm just not familiar with.

msoe03, Do I think the B phase could be mechanically 'exciting' the other wires to vibrate? Yes and no. This is just a thought. I'm not advocating a position, just bringing up an observation which agrees with theory. When something vibrates at its natural pitch,
1) It takes very little to get it started (galloping gertie for example. That stupid rattling thing in the dash of my car for another.)
2) It is almost fruitless to try to find the source of the excitation. Could be anything. Could be a freeway down the road for all I know.
3) Even a very simple system is difficult difficult to model. Overall, you've got THAT wire with its stiffness, attached to a lug at one end with the panel's stiffness and at the other? It would be folly to even attempt a guess at frequency response.