How does the Delta Wye Switch Work? Help! 4

[DKirkham]

Dear Gods of Electrical Motor Theory,

I am just a lowly automotive/manufacturing guy most humbly petitioning you, the Gods of Electrical Motor Theory, to grace me with your knowledge on how the delta wye switch works.

Yes, I have searched Google. I have searched this site (you guys understandably gloss over it). I have searched high and low but can not find the answer to my question. I recognize my own electrical stupidity is probably hindering me.

If anyone had a few minutes, I would be most grateful.

I just read this

http://www.mmsonline.com/articles/wyedelta-in-perspective.aspx

My question is, does the delta/wye extend the torque range in a VFD 3 phase motor? How? and most importantly, why?

From the Haas website:

On-The-Fly WYE-DELTA Switching
Many Haas VMCs feature the Haas Dual-Drive
system, which consists of a dual-winding motor –
Wye and Delta – and an electronic switch to change
between the two windings. Selecting the best
winding for low-rpm cutting and the best winding
for high-rpm cutting yields higher torque over a
wider rpm range. The Haas Dual-Drive system also
provides constant horsepower over a much wider
rpm range, and on-the-fly winding change
provides the wide power range necessary for
constant surface speed cuts.

David

http://www.kirkhammotorsports.com

 

[dpc]

If you have motor on a VFD, I don't think this device is going to do much for you.

Normally a delta-wye transition is used only during motor starting. Windings connected wye see less voltage than when they are connected in delta, so the current is reduced AND the torque is reduced. Torque is roughly 1/3.

I've never heard of switching between wye and delta when the motor is running under load.

But with a modern VFD, you will have much better control over the motor over the full range of motor torque than you would ever get with by switching between wye and delta.

It would help if we knew what the application was.

 

[DKirkham]

dcp,

If Haas is using this (see above quote and link) they why are they using it? Or, is this a bogus sales pitch? They say they switch "on-the-fly" to provide constant power over a wider range.

I will try to post a screen shot of the graph they publish.

David

www.kirkhammotorsports.com

 

[dpc]

I'm really not sure what they are trying to accomplish. Constant power over a wider range of what? Speed?

A VFD can vary the voltage and frequency of its output to provide virtually any torque (within the motor's and drive's capabilities) at any speed. Anything that could be accomplished with the wye-delta switching could be accomplished more effectively with a properly sized and rated VFD with the right power supply.

It does seem a bit like a marketing gimmick, but I'm certainly not an expert, especially since I have no idea what you are trying to accomplish with this motor.

In standard industrial motor applications (fans, pumps, etc), I've never heard of switching between wye and delta while the motor is running under load.

 

[DKirkham]

oops!

Sorry, application, my fault. I should have stated that right off.

I am looking for 2 things. One is knowledge on such subjects as the mystery of electrons.

The other is I am fantasizing about retrofitting a large 5 axis mill I have and I am wondering if I can spread the torque curve, as Haas says they can, on the spindle.

In a milling application, you need high torque at low speeds (100-500 for cutting steel) and high rpm's (15,000 or so) to cut aluminum--but you need every speed in between as well. In a lathe, you need to cut at constant surface feet so you can start a cut at a low rpm on the OD and finish in the middle at a much higher rpm--all the time loading the motor.

http://www.kirkhammotorsports.com

 

[dpc]

An induction motor with a VFD should be able to handle these applications, if I understand them. DC motors used to handle things like this, but there are very few applications left where an induction motor with VFD can not do the job these days. Certainly it will work much better than crudely switching between wye and delta connections.

 

[itsmoked]

That technique is so baroque that anyone embarking on it will be sorely disappointed.

Yes it can increase the torque. It is essentially a two position reduced voltage system. Theoretically it does not change the speed as that is set by the frequency.

Your applications would benefit greatly from a vector drive. It would likely benefit nothing from wye/delta.

On a different note: Whatever happened with the Electric Kirkham?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[DKirkham]

Keith,

I am curious as to why Haas does it then? Marketing? (Haas uses a vector drive, so why do they do the switch?).

Electric Kirkham is a side project that is currently in an engineering black hole--the place from which no product escapes! (At least without a massive tirade from the CEO). Actually, I am finishing up another massive project right now that has almost sucked the life out of me.

http://clubcobra.com/forums/showthread.php?t=85278

After you read that thread, you will realize why my thoughts are with an old 60 x 60 x 60 5 axis mill that needs a ton of work to run again. I would love to make some crazy parts on it. I would like to get the best performance out of it that is possible (for a reasonable amount of money).

David

http://www.kirkhammotorsports.com

 

[waross]

I have to think abut this for awhile. How does the torque/speed curve of a delta connected motor compare with the torque/speed curve of a wye connected motor? Is it possible that with the appropriate adjustment of the V/F ratio the delta will produce more torque at low speeds/frequencies?
By the way, delta and wye are two methods of connecting the same windings, not two separate windings.
If there are two separate windings, then anything is possible. We would need to know a lot about each winding before we could make any suggestions.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[DKirkham]

Bill,

I understand the delta connection refers to a "triangular" shaped connection of each of the 3 phases (with a wire between each of them) and the wye refers to each of the ends of the 3 phase windings all connected together at one point. Somehow this is related to series and parallel connections, (between the phases) but that relation is not somehow clear in my mind how this causes a rise/drop in voltage, current, torque...

That is the real question here.

As I did the searches here and on the web, everyone kept talking about the different connections (delta/wye) being used for different input voltages...that is not what Haas is doing as they use a VFD??? Am I right? So, why are they doing it (on the fly switching)? I can't imagine they would simply lie in their advertising.

So, what is it that drives torque in a motor?

Something tells me Haas would not do it if it were not valuable as they could be easily outed by an EE. (But not so easily by a ME, it seems )

David

http://www.kirkhammotorsports.com

 

[waross]

OK I got it!
This is a technique for running VFD driven motors above base speed. If a motor rated at 230 volts is run above base speed, the volts can not increase to maintain the "Volts per Hertz Ratio". At 3600 RPM, an 1800 RPM motor could be supplied with 460 volts with a corresponding increase in power. Some manufacturers use 460 volts and a VFD to gain 200% to double the power out of a motor. (200% voltage and 200% frequency and speed at normal torque).
The wye delta switching technique allows the effective voltage rating of a motor to be lowered to extend the operating range at speeds above base speed.
A motor rated at 10 HP at 1800 RPM would deliver 17.3 HP at 3117 RPM.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

It is most likely what Bill said. You can't get 200% using a Wye-Delta though, the delta speed would be 173% of the wye speed.

When I read it though, I though they may be using a 2-speed 2-winding motor. Basically, have something like a 400rpm winding and a 3600rpm winding.

In either case, you should be able to get reasonable performance from a fixed speed motor with a open loop flux-vector type VFD. If that isn't good enough, a motor with an encoder providing feedback to the VFD would work.

 

[DKirkham]

Lionelhutz,

I would not call a 2-speed 2 winding motor an "on-the-fly" delta/wye switch.

Pretty much all CNC spindle motors these days have encoders on them for the rigid tap and spindle orient commands.

Anyway, what am I missing? (Beside brains).

David

http://www.kirkhammotorsports.com

 

[jraef]

If you used an IEC motor which is dual voltage, 230/400V, then set up the VFD output to max out at 400V, you would have Bill's scenario. In Delta, the motor is running on the 230V pattern and when the frequency goes above base speed, torque is maintained. In Star (Wye), the motor is in 400V mode so full low speed torque is maintained.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[dpc]

I think waross and jraef may have the answer. That could allow operation over a greater speed range on the VFD.

 

[itsmoked]

I still don't see it.

You have two winding schemes so that the same V/Hz and speed occurs with two different supply voltages. Why does switching them allow any change to speed, or for that matter torque? How would this be any different from getting a 240V motor and running it up to some higher voltage without the complexity?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

Ya, I'm with you Keith. I know what they are saying they do but I don't understand why they are bothering either. A good VFD should be able to handle a fixed voltage motor across the whole speed range.

 

[Skogsgurra]

Hello David!

I remember your postings about a year ago on electric cars.

Now, the Wye/Delta switch as such is not very problematic. But I think that the application you seem to think about is a bit extraordinary. Have I got it right when I think that you want to have the motor in Wye for high torque up to a certain speed and then switch to Delta so that speed can be taken up furter to 1,73 times base speed? If so, it still sounds perfectly normal.

You would probably do this so you can get full motor power over a larger speed range without having to oversize the inverter. If the VFD size reduction outweighs the complexity is something one has to think about.

The problem seems to be that you want to do it on the fly without causing any torque shocks in the shaft. That would be impossible if you are using an electro-mechanical switch.

If you use solid state switches instead, it seems to be quite easy. You will need a fast frequency inverter with a specially designed control program that:

1. Disconnects motor from inverter.
2. Measures back EMF and adjust new output voltage to have same frequency, amplitude and phase as existing back EMF.
3. Opens the Wye switches.
4. Closes the Delta switches.
5. Connects motor to inverter.

The whole thing could probably be somewhat simplified. But that seems to be the general idea.

It is necessary to adjust the inverter output so it takes both new voltage level and the 30 degree phase shift in back EMF into account. There will probably be a short (a few milliseconds) paouse in torque, but you can probably live with that. It is much better than having a torque shock like the one you get from the phase and voltage mismatch if you do not do it right.

Does this sound like the thing you are telling us about?

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[DKirkham]

Gentlemen,

I can not imagine someone (Haas) doing something (on the fly switch) they could be called on the carpet for.

Gunnar, I thank you, as always, for your help

David

http://www.kirkhammotorsports.com

 

[DKirkham]

Keith,

You stated the technique is "so baroque that anyone embarking on it will be sorely disappointed." I am reminded of Bach, Handel, and Scarlatti. Their music is far deeper and more complex than can be appreciated with a cursory glance. Hundreds of years after their music was written, it continues to grace our existence. Perhaps there is merit in "baroque" ideas.

David

http://www.kirkhammotorsports.com