AC Motor Supply Wye Delta 1

[7JLAman4]

I have a simple question/inquiry. I'm determining the stator supply configuration for a 6.9kV 3-Phase 12-pole squirrel cage induction motor. What are, if any, the pro and cons to supplying the stator with a WYE connection that does not utilize the neutral vice a Delta connected stator?

 

[RRaghunath]

The post is confusing.
A delta connected stator for 6.6kV rated motor doesn't seem standard.
If the motor is being supplied directly by a transformer (hope my understanding is right), it doesn't matter whether the transformer winding is delta or star connected (not withstanding whether the motor/stator is star connected or delta connected).
If transformer winding is delta connected, there is no earth reference and earth fault protection based on residual voltage measurement will be necessary.
In case the transformer winding is star connected, the star point can be earthed through NER and current based earth fault protection supplied from neutral CT can be used.

 

[7JLAman4]

raghun-
You are correct. This is not a typical motor or motor installation. This will eventually be for offshore marine applications. I'm questioning the motor stator wiring configuration. What are the pros/cons from: a delta wired stator; or wye wired stator where the neutral is either not connected or shorted internally to the motor.
Do I see less eddy currents in a wye connection where the neutral is not connected, or...

This is my question and I'm not sure how else to phrase it. The power supply may be a dedicated generator with a WYE or Delta wired output. Typically on offshore installations, the generator outputs are delta wired.

 

[davidbeach]

Three wires into (or out of) a black box and there are no tests available to determine if the connection in the box is delta or wye. For a given voltage the motor will need to be connected either wye or delta depending on the motor design. Makes no difference. For that voltage though, I'd give serious thought to wye connected and bring all six leads out so you can do differential protection on the windings.

 

[7JLAman4]

Well that's kind of what I'm looking into...the design of the motor while attempting to determine any likes/dislikes about the input connection.

 

[jraef]

I would not use Y-Delta starting on a 6.9kV motor anyway, so I don't see a point in even doing that. But that is just a personal preference. I have seen too many problems with transition spikes on Y-Delta starting for my liking.

 

[edison123]

jlamann

I am assuming that you are designing a new winding, not just reconnecting a wye wound motor to delta (the latter one will kill the winding).

Having said that, I have seen (and rewound) delta connected 6.6 KV stator windings for 3.5 MW/4 MW boiler feed pump motors. Though many MV motors are star connected (thereby reducing the voltage stress across phase to ground), if the MW is high (meaning more current requiring heavier copper cross-sections, which are difficult to handle), a wye connection is discarded in favor of delta connection.

* Women are like the police. They can have all the evidence in the world and yet they still want a confession - Chris Rock *

 

[RRaghunath]

edison123,
Thanks, I can see there is a case for delta connection in MV motors especially in cases where there is limitation on available power supply voltages (like marine applications such as the present case).

jlamann,
What is the motor rating. From protection point of view, I don't see any issue even if the motor is delta connected especially if the CTs are built into each winding of the motor for sensitive Differential protection.

 

[motorspert]

I think the motor vendors would prefer three wire star connected. When connected in delta there will be third harmonic currents circulating which need to be considered in the design. If star connected, these currents cancel out and can be ignored. These affects are negligible on smaller motors but have a slight affect on larger motors.

The comments made above regarding stress and diff protection are important as well.

 

[LionelHutz]

You posted about having the Y connection shorted internally to the motor. That doesn't make any sense. You would not ground the Y connection of the motor.

It really does not matter which connection the motor uses.

The bigger factor than this connection is how the motor is built internally to minimize insulation stresses. But unless you are actually building the motor you really have little control over this.

 

[7JLAman4]

raghun-
The motor rating is approx what edison123 mentioned.

motorspert-
Those are the type of comments I'm looking for. Thankyou. Are there any potential weight savings by designing/building this motor with either a WYE or Delta stator. If incorporating a Delta stator containing 3rd-order harmonics, the space/weight of a harmonic filter may need to be looked at to remain within power quality limits.

 

[motorspert]

jlamann
Firstly, the harmoncs are internal, they will not be seen outside the motor and you will not need any filters - apolgies if I have mislead you.

The effect will be second order in terms of temperature
rise and motor efficiency, the motor designer will incorporate this into the design without you realising.

 

[edison123]

I do not see much cost difference between wye or delta connections in a motor.

As for 3rd harmonics, there are ways to eliminate them by proper winding design. So, performance will remain the same.

 

[RRaghunath]

jilamann,
To your question about weight of the motor, I guess it would be lesser for a delta winding motor when compared to star connected.
This is because of the lesser current that the delta connected windings see and hence lesser heat. This should apply even after making provision for additional heat due to circulating 3rd harmonics in delta.
I guess the weight is a factor in off-shore application as is the limitation on available voltage levels.

 

[edison123]

raghun

Star or Delta, total amount of copper is same.

Star = Less turns, more copper cross-section

Delta = More turns, less copper cs

 

[martinrelayer]

edison123, could you please explain a little bit about eliminating 3rd harmonics with delta connection, or a reference book?

many thanks!

 

[roydm]

"a delta wired stator; or wye wired stator where the neutral is either not connected or shorted internally to the motor"
Surely you are not proposing to ground the supply through the motor.
Q Will the supply be Star or some form of grounded Delta?
Roy

 

[edison123]

martinrelayer

3rd harmonics (and its multiples) can be eliminated by using winding spread of 120 deg. The 3rd harmonic vector closes on itself resulting in zero value.

See if you can get a copy of Performance and Design of AC Machines by M G Say.

 

[rbulsara]

I believe, in old days star wound MV motors used to cost less or was easier to wind because of insualtion (thinkness). It was easier, cheaper to insulate for phase to neutral voltage than phase to phase voltages.

But advances in insulating material has elimilated that factor. So if there is no cost, weight or size benefit, I do not think there is any tehcnical advantage of star over delta windings or vice versa.

 

[ScottyUK]

That technique certainly applies to transformers but is usually reserved for the high voltage designs used on the transmission system. With the rising costs of electrical steels the extra cost of a graded insulation system may become economical at the higher distribution voltages.


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