code to change the motor amp limit

General topics and discussions about the VESC and its development.
devin
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Re: code to change the motor amp limit

Postby devin » 11 May 2017, 22:11

@benjamin... question: if motors are delta wound not wye, does one still need to double the resistance figure of the VESC detection to calculate lead to lead resistance?

rew
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Re: code to change the motor amp limit

Postby rew » 12 May 2017, 07:50

Benjamin seems to assume that most motors are Y wound. As far as I know, most motors (that we use) are delta-wound.

All this does not matter, if I allow you to take (only) external measurements of two motors, one Y wound, one delta-wound you cannot tell which is which.

So even when the motor is delta-wound, having a mental model of a Y-wound motor does not harm anything in any way.

Suppose I have a Y-wound motor with 1 ohm winding resistance. I measure the resistance from terminal to terminal and I measure two windings in series: 2 ohms. Now suppose there is a delta-wound motor with 3 ohm winding resistance. again from terminal to terminal I now there is one winding in parallel with the other two. 3 ohms parallel to 3+3, do the math, and again you get 2 ohms.

These two will behave 100% identical for measurements from the outside.

devin
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Re: code to change the motor amp limit

Postby devin » 12 May 2017, 12:50


devin
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Re: code to change the motor amp limit

Postby devin » 13 May 2017, 01:47

i think my original guess was wrong about being able to distinguish delta and wye by comparing 1 to 1 and 1 to 2 lead resistance measurements because all 3 windings are energized at the same time with delta... and only 2 at a time with wye...

so recalculating...

if a lead-to-virtual-ground-point segment of a wye motor is 2 ohms

r=2ohms

1 to 1 wye = 4ohm
1 to 2 wye = 3ohm
3/4 = 0.75

1 to 1 delta = 1.333...ohm
1 to 2 delta = 1ohm
1 / 1.333... = 0.75

if a lead-to-virtual-ground-point segment of a wye motor is 3 ohms

r=3

1 to 1 wye = 6ohm
1 to 2 wye = 4.5ohm
4.5 / 6 = 0.75

1 to 1 delta = 2ohm
1 to 2 delta = 1.5ohm
1.5 / 2 = 0.75

in simple terms i was wrong, rew was right, you cant distinguish wye/delta from outside measurements, even by comparing the ratios between 1 to 1 and 1 to 2 lead resistance measurements.

interestingly it appears wye termination has 3 times greater resistance than delta, rather than 2 times greater as i'd believed previously...

rew
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Re: code to change the motor amp limit

Postby rew » 13 May 2017, 09:15

Best way to learn is to start shouting "your're wrong!!", and finding out for yourself that instead you were wrong. :-) Glad to have contributed to some understanding. :-) (just for reference: This realization of Y<->delta came to me in exactly the same way).

devin
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Re: code to change the motor amp limit

Postby devin » 13 May 2017, 14:00

interesting...

with delta the use of 2 "underpowered" coils which cancel each other out effectively for motive purposes + 3rd coil which does the "work" results in more total "BEMF per RPM" and therefore higher kv and no load speed (delta) (delta is 1/3rd electrical resistance of wye) (at the cost of electrical to mechanical conversion efficiency compared to wye)

than

2 fully powered coils which have lower total BEMF per RPM than delta, with no work canceling and therefore lower kv & lower no load rpm (wye is 3x the electrical resistance of delta)

rew
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Re: code to change the motor amp limit

Postby rew » 13 May 2017, 18:55

"cancel eachother" is not true. Each of the coils produces a magnetic vector. Put phase A at 0/360 degrees, B at 120 and C at 240, and you add the vectors they produce together, you'll get the same results. So for example, in Y you power A-to-B, you get positive current in A and negative current in B. So B becomes 120+180 = 300 degrees. So the vectors add to 330 degrees. Next phase, there will stil be A-positive, but C-negative instead of B. So we get A-C = 0 / 60 , or added to 30 degrees.

The same goes in Delta configuration, but the resulting vector will be a bit rotated. So we power A-B connection, but lets call that coil A. Coils B and C are powered at half the voltage, and if you add them up you get a magnetic vector. Next power the A-C connection, and you'll see the resulting vector rotate by 60 degrees.

devin
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Re: code to change the motor amp limit

Postby devin » 13 May 2017, 19:04

so in simple terms are you saying there is no "wasted electrical to mechanical efficiency" with delta compared to wye considering there are 3 energized windings at a time with delta and 2 energized windings at a time with wye?

in simple terms are you saying all 3 windings are actively contributing to mechanical motion at all times with delta, and no electrical to mechanical conversion efficiency is lost compared to wye in the delta arrangement?

rew
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Re: code to change the motor amp limit

Postby rew » 13 May 2017, 20:36

Correct!

In principle, BLDC motors are a descendant of three phase motor. There you have three coils positions 120 degrees from each other, and three excitation voltages also 120 degrees shifted from each other. Add the three magnetic vectors from each of the coils together and the result is a ROTATING magnetic field. Now each of the coils is excited with a voltage between zero and one of the three phases or between two phases for each coil doesn't matter. In both cases, the excitation voltages are precisely those 120 degree shifted voltages.

In FOC Mode, the controller WILL try to enforce those sinusoidal 120-degree-shifted voltages on each of the three phases. No "unpowered leg" on the Y.

Motors specifically built for BLDC mode, or trapezoidal excitation, have some fancy stuff going on between the magnets and the coils so that not driving one of the coils is quite OK.

devin
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Re: code to change the motor amp limit

Postby devin » 13 May 2017, 21:21

if 100 turns = delta = 100kv

X turns = wye = 100kv

what is X?

if 100 turns = delta = 100kv

100 turns = wye = X kv

what is X?


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