Odd kinda-sorta Wye/Delta starter

TwoBlocked

TwoBlocked

Senior Member
Location
Bradford County, PA
Occupation
Industrial Electrician
Got a trouble call to an LNG outfit about the wiring on a cryogenic pump motor. I had seen this same setup years before at the same place. The problem was that they pulled the assembly to replace the pump, but swapped a motor lead for the ground wire when re-installing. No damage done.

Wondering if anyone knows why a motor starting circuit would be wired this way. I had seen the printed schematic before, and it is intentional, not some mistake made in the field. See sketch.

20251022_034306.jpg
 
Is it possible that there are additional contactors that change where L1, L2, and L3 are connected during the switch from start to run?

In the two configurations that you drew, the phase angle supplied to coils that are in the same 'set' changes. For example, in the 'start' configuration the phase angles from terminals 3 to 6 and from terminals 6 to 12 are the same, but in the 'run' configuration they are different. But the phase angle change is small and combined with a large voltage change that seems pretty ugly.

Take a look at: https://www.electricaltechnology.org/2023/09/dahlander-motor-connection.html

In this setup the phase angle change is intentionally _huge_, flipping half of the coils 180 degrees and changing the motor pole count (and synchronous speed).

I've never seen the arrangement in your schematic and am having a hard time wrapping my head around it. I would expect some strange circulating current in that delta as drawn.
 
winnie said:
Is it possible that there are additional contactors that change where L1, L2, and L3 are connected during the switch from start to run?

In the two configurations that you drew, the phase angle supplied to coils that are in the same 'set' changes. For example, in the 'start' configuration the phase angles from terminals 3 to 6 and from terminals 6 to 12 are the same, but in the 'run' configuration they are different. But the phase angle change is small and combined with a large voltage change that seems pretty ugly.

Take a look at: https://www.electricaltechnology.org/2023/09/dahlander-motor-connection.html

In this setup the phase angle change is intentionally _huge_, flipping half of the coils 180 degrees and changing the motor pole count (and synchronous speed).

I've never seen the arrangement in your schematic and am having a hard time wrapping my head around it. I would expect some strange circulating current in that delta as drawn.
Click to expand...
This is a rare example, it is necessary to carefully analyze the original design purpose of this circuit!
 
When I saw this set up 4 years ago at the same place, they had a D-size drawing of the schematic. I remember doing a search and found just one reference to what I remember being called "outside wye, inside delta." It was in a white paper talking about how there are advantages to it for very large motors, but didn't explain just why. And it wasn't about a low voltage starter, like the standard Wye/Delta. The motor would be permanently wired that way.

Understanding magnetic circuits is sometimes confusing to me. When I look at the Wye portion, I see four coils between each two power lines. For a standard Delta, I see 2 coils between each two power lines. Now for this outside Wye, inside Delta, I see 3 coils between each two power lines. A smoother transition from reduced voltage to full voltage? I'm probably wrong.

I should have jotted down the manufacturer and contacted them. Maybe next week I'll reach out to the outfit to get that.
 
So the motor certainly has fat more than 4 or 6 coils; think 36 or 48 or 72.

It is quite common to have 9 or 12 coils per pole, for example.

Coils are grouped together in 'pole-phase groups', and then these groups are placed electrically in series.

Each 'coil' in the schematic represents many coils in the actual motor winding.

Is it possible that this is some sort of wound rotor motor, and the wye/delta change is only happening to the rotor?
 
Joethemechanic said:
Did you get a good look at the contactor? I'm trying to picture it
Click to expand...
The way I saw it was two main contactors in parallel, each with it's own overload; one for start, one for run. Then a regular looking pair of wye/delta contactors with one having a set of terminals jumpered together to make the wye. Musta been either a timing relay or a current relay to switch the wye delta. There was also a phase rotation relay in case somehow the incoming power got reversed, which could ruin the pump. They are always VERY concerned about that when changing out a pump, which requires wiring and unwiring the motor.

The motor leads came into a terminal strip with the nine leads. The 1st, 4th, and 7 had continuity. Same with the 2nd, 5th, and 8th; and the 3rd, 6th, and 9th.

There were three guys that worked like half a day trying to figure out why the one overload was tripping with one of the legs drawing 100A. They were down under the LNG "queen" trailer messing with the wiring in the motor box while I did the trouble shooting. They were at their wit's end because the boss said he swapped wire for wire and they weren't thinking outside the box. So, they called for help. (The one guy was so relieved he gave me $20! First time I ever got a tip for a service call.)

So, the 3rd, 6th, and 9th lead showed a dead short. The 3rd motor lead from the control cabinet was green and was taken to the ground in the motor box while the actual ground wire from the control cabinet was black, and was connected to motor lead 3 in the motor box. We corrected that and everything ran.

The leads coming out of the motor were numbered 1, 2, 3, 4, 5, 6, 10, 11, 12. So 7, 8, and 9 must be bugged to 4, 5, and 6, respectively, internal to the motor. The one guy was having a fit because he was used to bugging 4 to7, 5 to 8, and 6 to 9, but couldn't because 7, 8, and 9 weren't available.

The cabinet details and the troubleshooting I explained don't have much to do with the what I am wondering, but I am sure you fine folks would want to hear the grisly details. :) Much of what I did was explain the operation to them and get them calmed down. The big question they had was "How could that happen when the boss was certain he went wire by wire?" My answer was , "Because he's a f'n human." :D :D :D
 
winnie said:
So the motor certainly has fat more than 4 or 6 coils; think 36 or 48 or 72.

It is quite common to have 9 or 12 coils per pole, for example.

Coils are grouped together in 'pole-phase groups', and then these groups are placed electrically in series.

Each 'coil' in the schematic represents many coils in the actual motor winding.

Is it possible that this is some sort of wound rotor motor, and the wye/delta change is only happening to the rotor?
Click to expand...
I understand what you are saying. The drawing I saw years ago showed a regular induction motor.
 
Ok, some more thoughts on this:

In a normal stator winding you have some large number of coils N, say 36. These coils are divided up into N/3 (12 in this case) for each phase. Each coil must sit in two slots; in a normal 'lap' winding each coil takes up half a slot so you have 36 coils with 2 half coils sitting in each of the 36 slots.

In a typical 12 lead motor, these 12 coils would be further divided up into 2 sets (N/6, in the specific case 6 per set) so that you can have 2 independent sets per phase. The two coil sets that make up each phase must be symmetrically matched, so that you could place them in series or parallel and not get excessive circulating currents.

I was imagining connecting only one of the 2 subsets in delta while the other subset remained in star, and having these 2 symmetric sets connected differently would likely cause problems (uneven pole spacing, circulating currents, etc).

But you can wind a motor with two completely separate windings. Keeping the 36 slot example, you would have 72 coils, with _4_ half coils sitting in each slot. Of course the coils themselves have to be slimmer (fewer turns or lighter gauge wire or some such) in order to fit. One set of 36 coils is the inner winding, the other set of 36 coils is the outer winding.

These two winding could be built with different numbers of turns, or otherwise adjusted to work properly with different supply voltage; they are not necessarily designed to be series or parallel. So having one winding in star while the other is in delta wouldn't cause symmetry problems in the motor. I could see benefits where the delta winding acts to smooth out some harmonics in the magnetic field, or were only having part of the winding switch from star to delta prevents transients during the starting process.

I'd love to get more information on the setup if you go back to ask.

Jonathan
 
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