180 degree commutation with sensors vs standard 120

General topics and discussions about the VESC and its development.
Hummie
Posts: 107
Joined: 10 May 2016, 04:05

180 degree commutation with sensors vs standard 120

Postby Hummie » 30 Nov 2017, 02:35

I heard with sensors you can commutate at 180 degrees with a 3 phase motor vs 120 and therefore get more torque. do people do this? I thought sensors were only a hair more efficient and at the lowest speeds but if this were the case I'd think you could get greater efficiency at any speed if there was a big load.

devin
Posts: 246
Joined: 08 May 2017, 01:55
Location: San Francisco, California, US

Re: 180 degree commutation with sensors vs standard 120

Postby devin » 30 Nov 2017, 16:11

Doing some reading into “180 degree commutation” it would appear the power would go “in through 2 leads and out through 1 lead” as opposed to the traditional commutation sequence of power “in through 1 lead and out through 1 lead.”

The advantage of this is presumably with a wye motor, all 3 phases are “used” simultaneously instead of just 2.

Problem I see with this is with most motors you can’t tell whether they are delta or wye from the outside, and casually glancing at the circuit diagram of delta vs wye, it appears to my untrained eye if you try to implement a scheme where all 3 phases are utilized with a wye motor, amd then swap out the motor for a delta, now only 2 of the delta phases are utilized instead of all 3... one step forward & one step back?

viewtopic.php?f=6&t=705&start=110

let's imagine in the following diagram showing a wye-wound motor that at a particular time mosfets D, B & C are ON leading to voltage across all 3 wye phases simultaneously:

Image

Now we swap out the wye-wound motor for a delta-wound motor:

Image

^Now, with the delta motor, even though mosfets D, B & C are ON, now there is no voltage across one of the 3 phases (bottom phase).


@Hummie if you want to utilize all 3 phases simultaneously why not just use standard delta rather than wye? Another option I see is “separate excitation” but I believe this would involve 6 leads coming out of the motor instead of 3 and complete hardware re-design of the vesc. What advantage is truly gained via “180 degree commutation” that can’t be obtained with a higher motor amp limit? To me it seems like “180 degree commutation” with a wye motor might be very similar in practice to simply using “120 degree commutation” with delta and a different amp limit?
Last edited by devin on 01 Dec 2017, 02:04, edited 1 time in total.

devin
Posts: 246
Joined: 08 May 2017, 01:55
Location: San Francisco, California, US

Re: 180 degree commutation with sensors vs standard 120

Postby devin » 01 Dec 2017, 00:06

found this:

Image

Image Source: https://www.researchgate.net/profile/Boyang_Hu/publication/238516491/figure/tbl1/AS:393296607236097@1470780626152/Table-1-Switch-sequences-of-120-and-180-degree-commutation.png

the question becomes: does running a wye motor with 180 degree commutation instead of 120 degree commutation improve the electrical to mechanical conversion efficiency at the same rpm, battery amps and electrical wattage?

it would appear to my untrained eye that utilizing 180 degree commutation could possibly have the effect of lowering effective winding resistance and possibly increasing kv compared to 120 commutation, but at this point it's purely my speculation. in theory lowering the effective winding resistance should give more motor amps for the same number of battery amps, in theory giving more torque, but if the kv increases also as a result, then the KT torque per amp might decrease, so even though it's possibly more motor amps for the same battery amps, if the torque per amp is less, the greater motor amps @ same battery amps might result in the same net effective torque.

if (big IF) the "effective kv" is different in each mode then switching between the 2 modes could be another way to "switch gears" so to speak (besides delta/wye changeover switch) for either a higher torque mode or a higher top speed mode.
Last edited by devin on 01 Dec 2017, 02:31, edited 3 times in total.

Hummie
Posts: 107
Joined: 10 May 2016, 04:05

Re: 180 degree commutation with sensors vs standard 120

Postby Hummie » 01 Dec 2017, 02:14

i'm still stuck on the old question of why a wye motor doesn't suck with a high load as it seems to only be using 2/3 the teeth and would be more likely to saturate having less iron. I feel like this question has to be answered before getting any more complex especially since ironically people say wye runs smoother with a heavy slow load. I thought somewhere around here someone surprisingly said that with FOC all phases are electrified. maybe I have it wrong in my memory. Last someone told me there was some crazy transformer black magic going on...that was the latest explanation I heard. It seems such basic basic circuitry but somehow not.

devin
Posts: 246
Joined: 08 May 2017, 01:55
Location: San Francisco, California, US

Re: 180 degree commutation with sensors vs standard 120

Postby devin » 01 Dec 2017, 08:49

i'm going to go out on a limb and make a strange prediction that switching from 120 degree to 180 degree commutation increases the "apparent" kv of delta but not wye.

i base this on the fact that when rewinding motors, shortening the conductor length increases the kv, but increasing the conductor thickness doesn't change kv. in the case of delta, we'd be effectively shortening the conductor length of one of the 2 paths the electricity takes through the motor, but in the case of wye, switching from 120 to 180, the conductor length isn't shortened... it's effectively thickened... and thickening doesn't change kv when rewinding motors.

i wouldn't be surprised if i'm wrong-- but i theorize perhaps 180 vs 120 degree commutation can reveal whether a motor is delta or wye from outside measurements based on whether or not there are changes to the "apparent kv."

Hummie
Posts: 107
Joined: 10 May 2016, 04:05

Re: 180 degree commutation with sensors vs standard 120

Postby Hummie » 02 Dec 2017, 00:32

i'm going to go out on a limb and make a strange prediction that switching from 120 degree to 180 degree commutation increases the "apparent" kv of delta but not wye.
what's apparent vs real. ill assume you mean real would be from generated from the motor and apparent with an applied voltage to the motor. apparent not so appearent

i base this on the fact that when rewinding motors, shortening the conductor length increases the kv, but increasing the conductor thickness doesn't change kv. in the case of delta, we'd be effectively shortening the conductor length of one of the 2 paths the electricity takes through the motor , but in the case of wye, switching from 120 to 180, the conductor length isn't shortened... it's effectively thickened... and thickening doesn't change kv when rewinding motors.Id like to see a visual of the rotor and stator to see what we are talking about in terms of the timing of the fields. I'd have thought we were limited to certain combinations of magnets and teeth by the 120. but there are simulation programs and they're probably pretty right and I could do that. I know you could too. but can you do it on the vesc with sensors and what am I even talking about? ...someone told me with sensors you could do 180 vs 120 as is necessary using the back emf to orient. Does it do 120 or does it do 180 as standard with sensors?


i wouldn't be surprised if i'm wrong-- but i theorize perhaps 180 vs 120 degree commutation can reveal whether a motor is delta or wye from outside measurements based on whether or not there are changes to the "apparent kv."

devin
Posts: 246
Joined: 08 May 2017, 01:55
Location: San Francisco, California, US

Re: 180 degree commutation with sensors vs standard 120

Postby devin » 02 Dec 2017, 04:14

@devin wrote:

i'm going to go out on a limb and make a strange prediction that switching from 120 degree to 180 degree commutation increases the "apparent" kv of delta but not wye.

i base this on the fact that when rewinding motors, shortening the conductor length increases the kv, but increasing the conductor thickness doesn't change kv. in the case of delta, we'd be effectively shortening the conductor length of one of the 2 paths the electricity takes through the motor, but in the case of wye, switching from 120 to 180, the conductor length isn't shortened... it's effectively thickened... and thickening doesn't change kv when rewinding motors.

i wouldn't be surprised if i'm wrong-- but i theorize perhaps 180 vs 120 degree commutation can reveal whether a motor is delta or wye from outside measurements based on whether or not there are changes to the "apparent kv."


@hummie wrote:

what's apparent vs real. ill assume you mean real would be from generated from the motor and apparent with an applied voltage to the motor. apparent not so appearent


@hummie - i mean with delta, if the max rpm per battery volt of the motor changes when switching between 120 degree and 180 degree commutation, this would be an “apparent” change in kv, even though the motor itself was not changed in any way.

devin
Posts: 246
Joined: 08 May 2017, 01:55
Location: San Francisco, California, US

Re: 180 degree commutation with sensors vs standard 120

Postby devin » 02 Dec 2017, 05:09

after a bit more reading, i have a question— are “FOC” and “180 degree commutation” basically just 2 different words for the same thing? i should bow out of the conversation at this point as im only speculating.

rew
Posts: 928
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: 180 degree commutation with sensors vs standard 120

Postby rew » 02 Dec 2017, 19:26

No, From what I read above, the "180 deg commutation" is simply BLDC but with an extra motor coil powered up. (Y or Delta does not matter).

But if we think about what we want to achieve.... We have a permanent magnet in an electromagnetic field. We want to set up that magnetic field that has creates the most torque on that permanent magnet. Well... That's exactly what FOC does: It optimizes the D field vector to be just enough, minimizes the Q component. (the D causes the maximum torque per applied current, the Q field vector does not cause any torque).

This way of commutating would improve performance if the motor amp rating of say 15A would mean: The motor wires blow up at 15.1A. That is not the case. So running two coils at slightly less current instead of just one is going to win you very little.

devin
Posts: 246
Joined: 08 May 2017, 01:55
Location: San Francisco, California, US

Re: 180 degree commutation with sensors vs standard 120

Postby devin » 02 Dec 2017, 22:26

@rew wrote:

No, From what I read above, the "180 deg commutation" is simply BLDC but with an extra motor coil powered up. (Y or Delta does not matter).

But if we think about what we want to achieve.... We have a permanent magnet in an electromagnetic field. We want to set up that magnetic field that has creates the most torque on that permanent magnet. Well... That's exactly what FOC does: It optimizes the D field vector to be just enough, minimizes the Q component. (the D causes the maximum torque per applied current, the Q field vector does not cause any torque).

This way of commutating would improve performance if the motor amp rating of say 15A would mean: The motor wires blow up at 15.1A. That is not the case. So running two coils at slightly less current instead of just one is going to win you very little.


@rew do you suspect running either delta or wye in bldc 180 degree commutation mode vs 120 degree would cause any change in the “apparent kv” max rpm per volt of either delta or wye, considering 180 degree commutation with delta (but not wye) would effectively “shorten” one of the 2 simultaneous pathways the electricity takes through the delta in bldc mode? shortening (halving the length of) this electical path would have the effect of doubling the drift velocity at stall for a given effective voltage in that section of the conductor, and as we have previously discussed... i have observed that doubling the drift velocity at stall in a wye motor either by doubling the battery voltage or halving the winding length doubles the max rpm of the wye motor.

it also seems 2 outputs instead of one would lower the apparent positive-to-negative winding resistance of the motor, in theory leading to more motor amps for the same battery amps and electical wattage at the same rpm.


Return to “General”

Who is online

Users browsing this forum: No registered users and 1 guest