Winding Question: does STAR make more torque than DELTA, even with adjusted phase current?

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madin88   1 GW

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Winding Question: does STAR make more torque than DELTA, even with adjusted phase current?

Post by madin88 » Dec 30 2019 1:05pm

Hey guys,

as the thread title says the question is if a given motor wound in STAR can do more torque as same motor wound in DELTA.
Of course if we adjust the phase current by factor 1,73 and all other things left aside, like harmonics, circulating currents etc.
Just raw torque from zero (0) RPM with adjusted phase current.

I would like to discuss about that because i have a motor with changable winding connection, and i noticed that it can do way more torque in STAR @ 200A vs DELTA @ >400A, which i cannot explain by myself since at around 350A (200A x 1,73) they should be on par, and at 400A the DELTA-wound motor should do more torque, but it doesn't :?:

One reason could be that if we apply current between two phases, then in STAR two of the three windings see the full current and the other winding zero, while in DELTA one winding sees 2/3 of the current and the other two 1/3.

Any ideas?

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Re: Winding Question: does STAR make more torque than DELTA, even with adjusted phase current?

Post by district9prawn » Dec 30 2019 8:31pm

How are you measuring phase current?
Neu 8057 6kW left hand drive hardtail with 18 fet Vesc: viewtopic.php?f=6&t=96754

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Re: Winding Question: does STAR make more torque than DELTA, even with adjusted phase current?

Post by madin88 » Jan 03 2020 7:12am

district9prawn wrote:
Dec 30 2019 8:31pm
How are you measuring phase current?
I did not measure it by myself. It was Nucular controller which does show actual phase amps.

The exact settings have been:
With winding in STAR (45kV), 72V 150/200A it had more torque compared to:
DELTA (75-80kV), 72V 150/420A

The motor does come wound in DELTA from factory, but the No-load consumption was within a few Watts identical at given RPM, so there should be no issues with hall offset or other things which could occur when changing the winding connection. It works quite well with both, only peak torque seems to worse with DELTA.

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Re: Winding Question: does STAR make more torque than DELTA, even with adjusted phase current?

Post by larsb » Jan 03 2020 9:07am

How do you measure the torque? Butt measurement, defined slope riding or acceleration times or..

The 30 degrees shift between delta and wye will have some effect as sensors are not perfectly aligned for one winding configuration.

There should be exactly the same torque output with adjusted current and the kV should show the difference.

If wye is 45 and delta is 78 it matches, if it’s higher than that in delta then delta winding is faster/weaker than it should be.

Perhaps the inductance gets too low for controller in delta? Vasili doesn’t recommend the controller for RC motors.
Last edited by larsb on Jan 04 2020 12:17pm, edited 1 time in total.
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Re: Winding Question: does STAR make more torque than DELTA, even with adjusted phase current?

Post by madin88 » Jan 03 2020 1:38pm

larsb wrote:
Jan 03 2020 9:07am
How do you measure the torque? Butt measurement, defined slope riding or acceleration times or..
At 200A in STAR the bike did lift the front wheel, while at 420A in DELTA it does not (all other settings equal).
There should be exactly the same torque output with adjusted current and the kV should show the difference.
What about saturation of the stator could it be that it happens earlier in DELTA vs STAR?
In STAR, 2/3 of the stator (two windings) see the full current, in my case 200A, and the left one zero (0A). Whereas in DELTA, one winding see 420A x 0,666 = 280A and the other two 420 x 0,333 = 140A. I am right with that?

What if the 280A are pushing the associated part of the stator already so deep in saturation that torque output noticeable drops off?
If wye is 45 and delta is 78 it matches, if it’s higher than that in delta then delta winding is faster/weaker than it should be.
The 30 degrees shift between delta and wye will have some effect as sensors are not perfectly aligned for one winding configuration.
Yes it matches exactly, and no-load consumption + overall system efficiency at given RPM or speed on the bike is great in both configurations. Aside from peak torque output there are no noticable differences.
Perhaps the inductance gets too low for controller in delta? Vasili doesn’t recommend the controller for RC motors.
I was thinking about that too. The motor has around 30-50µH in delta, and above 100µH in star. thats a big difference, however, the display shows something like 430-450A phase if i stall the bike against a wall and i believe that we can trust in what the controller measures (i was told that it measures phase current directly and calculates battery current from that). But yes, the current ripple (percent of current which doesn't contribute to torque) will be higher in DELTA.

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Re: Winding Question: does STAR make more torque than DELTA, even with adjusted phase current?

Post by larsb » Jan 04 2020 10:33am

I think you're correct. the 80100 motor of similar size saturates above 200A in delta and steel has to saturate more on the high current coils. never thought about this or hear about it, it surprises me.

This would always put a wye motor as the better choice if you could rev it high enough with the winding and voltage available. Somehow i doubt this. :?:
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Re: Winding Question: does STAR make more torque than DELTA, even with adjusted phase current?

Post by peters » Jan 06 2020 3:39am

I think it's not a saturation issue because a saturated winding produces more magnetic field and so should produce more torque than the same winding not in saturation regardless of the winding config (wye-delta), if the hall sensor vs. phase current position is adjusted (I mean the 30 degrees difference between wye and delta).

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Re: Winding Question: does STAR make more torque than DELTA, even with adjusted phase current?

Post by larsb » Jan 06 2020 3:51am

peters wrote:
Jan 06 2020 3:39am
I think it's not a saturation issue because a saturated winding produces more magnetic field and so should produce more torque than the same winding not in saturation regardless of the winding config (wye-delta), if the hall sensor vs. phase current position is adjusted (I mean the 30 degrees difference between wye and delta).
No, it doesn’t as the torque per amps drop off on saturation. The calculated current to drive the delta motor is based on that neither of the cores saturate as that stops the (semi) linear relationship between current and torque.
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Re: Winding Question: does STAR make more torque than DELTA, even with adjusted phase current?

Post by peters » Jan 06 2020 6:01am

larsb wrote:
Jan 06 2020 3:51am
No, it doesn’t as the torque per amps drop off on saturation. The calculated current to drive the delta motor is based on that neither of the cores saturate as that stops the (semi) linear relationship between current and torque.
I agree that torque per amps drops off, but the torque itself is the highest in saturation because the B-field (and flux and force also) is at its maximum. Just the excess current in saturation does not produce additional torque.

But if the windings remain below saturation, the torque should be the same at the adjusted phase current (x1,73 in delta). In fact in delta the phase current is not divided in 1/3 - 2/3 manner between the windings, because all the 3 phase currents are controlled as per the 3-phase sine waveforms. If everything is balanced, the winding currents are the same in delta and wye at the adjusted phase currents, that't why they produce the same torque, so none of the windings should saturate. There can be differences/imbalances in the windings that can produce some circulating current in delta, but I think that should not make so much torque difference.

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Re: Winding Question: does STAR make more torque than DELTA, even with adjusted phase current?

Post by larsb » Jan 18 2020 9:26am

Sounds reasonable. But then what’s the likely cause?
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Re: Winding Question: does STAR make more torque than DELTA, even with adjusted phase current?

Post by John in CR » Jan 18 2020 10:40am

If all settings equal other than phase current limit, then current is ramping up at the same rate, so the higher current takes a longer period to reach there.

Also, some motors are wound in a manner that they work properly in only one type of termination. I remember when some guys were exploring Star/Delta switching and not all motors worked properly in both.
Last edited by John in CR on Jan 20 2020 1:43pm, edited 1 time in total.

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Re: Winding Question: does STAR make more torque than DELTA, even with adjusted phase current?

Post by madin88 » Jan 19 2020 8:57am

peters wrote:
Jan 06 2020 6:01am
But if the windings remain below saturation, the torque should be the same at the adjusted phase current (x1,73 in delta). In fact in delta the phase current is not divided in 1/3 - 2/3 manner between the windings, because all the 3 phase currents are controlled as per the 3-phase sine waveforms. If everything is balanced, the winding currents are the same in delta and wye at the adjusted phase currents, that't why they produce the same torque, so none of the windings should saturate. There can be differences/imbalances in the windings that can produce some circulating current in delta, but I think that should not make so much torque difference.
Yes i think you are right with that, and Dr. google says the same about 3-phase sine waveform.

But if we talk about trapezoidal 6-step commutation, then one phase is always open and so for DELTA the current follows the 1/3 - 2/3A manner per winding, whereas in STAR one winding sees 0% and the other two 100% of the phase current.

below pic shows a STAR connection where phase A + B are energized and phase C open.
Four of the six coils, or 2/3 of the stator is energized with the full phase current.

Image

Image

Image

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Re: Winding Question: does STAR make more torque than DELTA, even with adjusted phase current?

Post by larsb » Jan 19 2020 10:51am

Interesting. Didn’t think of this difference before.

I have some papers showing max efficiency is when controller and BEMF waveform match - this makes me wonder if there are high-end three-phase square wave controllers, otherwise they’d lose some efficiency right there due to the imbalance.
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Re: Winding Question: does STAR make more torque than DELTA, even with adjusted phase current?

Post by madin88 » Jan 19 2020 12:51pm

I did some more research and have the meaning that even with 3-phase sinewave, there is also quasi one phase open for a very short time (doing nothing), and it is interesting that this happens exactly at the moment when the other two phases see the highest current and probably also producing the most torque?
I made a sketch of how i understand the sinewave current, take a look at point 4-6 (the green lines).

With trapez commutation the time where one phase is doing nothing is much longer.
sine+trapez.jpg
sine+trapez.jpg (191.59 KiB) Viewed 980 times
Last edited by madin88 on Jan 19 2020 5:43pm, edited 3 times in total.

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Re: Winding Question: does STAR make more torque than DELTA, even with adjusted phase current?

Post by madin88 » Jan 19 2020 1:50pm

Lets take a closer look at number 4 and compare the currents which flow in STAR and DELTA. Phase 1 and 2 are engergized with the maximum current while phase 3 is open (at 0V as it is crossing the x-axis). Forget the BEMF thing as our motor is at stall or spinning at low RPM.

in STAR:

winding1 = 200A
winding2 = 200A
winding3 = 0A (because phase 3 is open)

DELTA:

winding1 = (200x1,73) x 0,66 = 228A
winding2 = (200x1,73) x 0,33 = 114A
winding3 = same 114A

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Re: Winding Question: does STAR make more torque than DELTA, even with adjusted phase current?

Post by madin88 » Jan 19 2020 5:53pm

larsb wrote:
Jan 19 2020 10:51am
I have some papers showing max efficiency is when controller and BEMF waveform match - this makes me wonder if there are high-end three-phase square wave controllers, otherwise they’d lose some efficiency right there due to the imbalance.
Wasn't it Lebowski who said this?
Yes, the waveform of the output current and BEMF should match, and in an optimal case it should be sine-waveform.
I do not think that trapez can be "high end" since most of the so called motors do have a BEMF which is somewhere between trapez and sine, so it is not this and that.
Better make nails with head and have a motor with a quasi sine BEMF and sine controller.

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Re: Winding Question: does STAR make more torque than DELTA, even with adjusted phase current?

Post by larsb » Jan 20 2020 12:52am

True, in real life there probably won’t be a motor controller combination like this to be found.
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Re: Winding Question: does STAR make more torque than DELTA, even with adjusted phase current?

Post by Lebowski » Feb 03 2020 3:49am

To correct, I did not say motor BEMF and controller waveforms should match, I tried this for a while but came back on this as there is no point.

Power comes from the motors backemf voltage and current being in phase.

Lets say the motor has fundamental sine wave and a 3rd harmonic (making it mildly trapezoidal). Lets say you match this with a controller with equal output voltage shape. The typical controller will match the phase of the fundamental motor current to that of the fundamental bemf voltage. Then automatically the 3rd order components are not in phase, as the motors LR delay is different for the 3rd order than the fundamental. Making the fundamentel current to be in phase automatically means the 3rd is out of phase.

Now you can build a controller which matches both fundamental and 3rd harmonics phases, but then the controller output voltage will look different from the motors backemf voltage...

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Re: Winding Question: does STAR make more torque than DELTA, even with adjusted phase current?

Post by madin88 » Mar 04 2020 5:12pm

Thanks Lebowski for chiming in and sorry for late response here.
Lebowski wrote:
Feb 03 2020 3:49am
Lets say the motor has fundamental sine wave and a 3rd harmonic (making it mildly trapezoidal). Lets say you match this with a controller with equal output voltage shape. The typical controller will match the phase of the fundamental motor current to that of the fundamental bemf voltage. Then automatically the 3rd order components are not in phase, as the motors LR delay is different for the 3rd order than the fundamental. Making the fundamentel current to be in phase automatically means the 3rd is out of phase.

Now you can build a controller which matches both fundamental and 3rd harmonics phases, but then the controller output voltage will look different from the motors backemf voltage...
There is one good article which describes the problem you were talking about:
http://build-its-inprogress.blogspot.co ... onics.html

What i do not understand yet is how someone can figure out if a given motor does have more or less of those unwanted harmonics.
Lets say i hook up the osci between two phases, spin the motor with a power drill and watch BEMF waveform.
When it looks like a sine wave, does it mean that it doesn't have harmonics?, and the more it deviates from the sine the more harmonics (or the higher the amplitude of them)?

FFT was mentioned which should be used to find it out, but i never worked with that. Measuring phase to phase voltage and hall signals was already challenging enough :)

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