# AC motor, Power = amps AC, or amps DC into motor control?



## Sheepdog 44 (Dec 27, 2013)

I was looking at the specs of the ME1115 PMAC motor and i came across this part about amps.

- Current: 125 Amps AC continuous (180 Amps DC into the motor control)
- Peak current: 420 Amps AC for 1 minute (600 Amps DC into the motor control) 

http://electricmotorsport.com/me1115-brushless-motor-24-96v-5000rpm-12-kw-cont-30-kw-pk.html

To find out the continuous power output of the motor at 96 volts, do i multiply by 125 amps AC for 12kw, or 180 amps DC for 17.28kw?

I don't understand why there are two different amp ratings?


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## major (Apr 4, 2008)

Sheepdog 44 said:


> I was looking at the specs of the ME1115 PMAC motor and i came across this part about amps.
> 
> - Current: 125 Amps AC continuous (180 Amps DC into the motor control)
> - Peak current: 420 Amps AC for 1 minute (600 Amps DC into the motor control)
> ...


Yep, that is confusing. But the specification claims 12 kW continuous rating. And 12,000 = 96 * 125  So what is that? DC Volts times AC phase current? Doesn't seem right 

The 3 phase motor controller is like a combination of six buck converters. A buck converter is essentially a DC motor controller. So, the phase voltage is always lower than the DC voltage. And from my experience, the AC phase current is always higher than the DC current. But when you look at the governing equations (at least for sine wave AC), the DC can be higher than AC current for certain power factor. But I have never seen this case, but my experience is mostly with ACIM. At and above base frequency, I see the RMS phase current very close to DC level.

Personally, I'd like to see 125 Amps AC with 180 Amps DC as that spec infers. I don't think it is reasonable. I expect about 125 Amps DC at that point.


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## gunnarhs (Apr 24, 2012)

major said:


> Yep, that is confusing. But the specification claims 12 kW continuous rating. And 12,000 = 96 * 125  So what is that? DC Volts times AC phase current? Doesn't seem right
> 
> The 3 phase motor controller is like a combination of six buck converters. A buck converter is essentially a DC motor controller. So, the phase voltage is always lower than the DC voltage. And from my experience, the AC phase current is always higher than the DC current. But when you look at the governing equations (at least for sine wave AC), the AC can be higher than DC current for certain power factor. But I have never seen this case, but my experience is mostly with ACIM. At and above base frequency, I see the RMS phase current very close to DC level.
> 
> Personally, I'd like to see 125 Amps AC with 180 Amps DC as that spec infers. I don't think it is reasonable. I expect about 125 Amps DC at that point.


You are right about this.
The explanation for the RMS value for a AC-Voltage (sinus for example) is the effective voltage to produce the same Power (Current over a load) as the equivalent DC (without losses theoretically).
For a sinus at 50 Hz for example 230V AC RMS gives us a Voltage top of 320V. 
In a DC->AC-conversion the DC-system must match at least the top of the AC-Wave, meaning we need a 320V battery to make 240V Sinus RMS.
As power is equal in an Ideal DC-> AC (in the ideal case) P (dc) = P (ac)
and according to definition current should be the same over the load we have:
P(dc) = (320/rmsfactor)* I (load) = P(ac) = 240*I (load), 
having rmsfactor = 1,4 for our sinus 50 Hz case.

We can also (very seldom seen) define the Power source as a current source instead of voltage. In that case we would define a current which would produce a steady voltage over a load. in that case the rnsfactor would apply to the current. I would assume this is the above case having the rmsfactor = 180/125 which gives 1,44 (surprise, surprise  )

Either way the (continuous?) power would be 
P = (96 V/1,44) * 180A = 12 kW
P = 96 V * (180A/1,44) = 96*125A = 12 kW

When monitoring such a system in action in an EV the practical result would look like this (measuring DC voltage and DC amperage battery and rms AC at motor side)
Assuming a 96V PWM Sinewave-controller of 12 kW , assuming near continuous power ( 12 kW) during all cases, assuming full battery, assuming rms-faktor to go from 1,60-> 1,44 -> 1,2 according to motor-frequency:
(remember ideal: Power battery side = Power motor side
1) Vehicle slowly accelerating at low speed: 
Voltage battery side is 96V, "DC voltage" motor side is maybe 24V. Current battery side is then maybe 200A current motor side is 800A , P (battery) = (96V/1,60)*200A = P(Motor) = 24V(dc)/(1,6)*800A = 15V(rms)*800A = 12 kW
2) Vehicle at medium speed (little dragforce, motor around base speed): 
Voltage battery side is 96V, "DC voltage" motor side 48V DC. Current battery side is then maybe 100A current motor side is 200A , P (battery) = (96V/1,44)*100A = P(Motor) = (48V/1,44) *200A = 6,7 kW
3) Vehicle at maximum speed (more dragforce, motor over base speed): 
Voltage battery side is 96V,"DC voltage" motor side 80V . Current battery side is then maybe 125A current motor side is 150A , P (battery) = (96V/1,2)*125A = P(Motor) = (80V/1,2)*150A = 10 kW

What the point was. You usually see a higher current motor side than battery side during operation. And higher battery voltage than motor voltage. 
When reaching max speed motor Voltage becomes near battery voltage and motor current near battery current but never nearer than the rms-factor allows.
So the motor specs in the description are bullshit (should rather rate max power, same goes for controller specs which often are rated in max amps rather than max power).
The interesting thing is the continuous power (60 minutes), and the maximal power (1-5 minutes). Both voltages AND currents must be considered.


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## major (Apr 4, 2008)

gunnarhs said:


> You are right about this.


Thanks, but I did correct an error in my post.....too many AC.DC.AC things 

I think we agree, AC phase current is higher than battery current


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## liveforphysics (Jan 16, 2014)

major;376221
I think we agree said:


> It starts higher for certain, but it ends up lower than battery current by quite a bit, even before field weakening (timing advance), at which point its not uncommon to see Ibatt's reach 1.5-2x Iphase.


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## gunnarhs (Apr 24, 2012)

major said:


> Thanks, but I did correct an error in my post.....too many AC.DC.AC things
> 
> I think we agree, AC phase current is higher than battery current


He,he, did not see your typo until I compared the text  
Yes, that is the point, Motor current higher than battery current when looking at values during tests.
Therefore strange to see a motor / controller spec with it the other way round.


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## gunnarhs (Apr 24, 2012)

liveforphysics said:


> It starts higher for certain, but it ends up lower than battery current by quite a bit, even before field weakening (timing advance), at which point its not uncommon to see Ibatt's reach 1.5-2x Iphase.


Hmm, only when looking at one phase then of the three.
When calculating a balanced 3 phase current system in motor 
I(totalmotorPhase) = 1,7 *I(oneMotorPhase).

Most suppliers I know of controllers and motors give you the specs of I(totalmotorPhase).
(the others I know have problems selling their product )


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## major (Apr 4, 2008)

liveforphysics said:


> It starts higher for certain, but it ends up lower than battery current by quite a bit, even before field weakening (timing advance), at which point its not uncommon to see Ibatt's reach 1.5-2x Iphase.





major said:


> I think we agree, AC phase current is higher than battery current


The above statement was in context with a previous post shown below. And I have heard tell of reports of the DC current higher than phase current for ACIM, and believe it possible for certain conditions, but I've never seen it happen. I haven't done much with non-sinusoidal poly phase machines, so don't care to comment about that.



major said:


> But when you look at the governing equations (at least for sine wave AC), the DC can be higher than AC current for certain power factor. But I have never seen this case, but my experience is mostly with ACIM. At and above base frequency, I see the RMS phase current very close to DC level.


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## major (Apr 4, 2008)

major said:


> ....when you look at the governing equations (at least for sine wave AC), the DC can be higher than AC current for certain power factor.



P = √3 * U * Irms * cosΘ

That is power in a 3-phase sinusoidal system. P in Watts, U is line to line RMS voltage, Irms is RMS phase current and cosΘ is power factor.

Of course the battery power is Vb * Ib.

So, Vb * Ib = √3 * U * Irms * cosΘ 

Or, Ib = √3 * (U/Vb) * Irms * cosΘ

With an inverter synthesized sinewave, the peak voltage cannot be greater than Vb, and the maximum RMS value of U = Vb/√2. Substitute that into the equation:

Ib = √3 * ((Vb/√2)/Vb) * Irms * cosΘ

Or, Ib = (√3/√2) * Irms * cosΘ = 1.24 * Irms * cosΘ

So it would appear that battery current can exceed RMS phase current at full voltage to the motor and when the power factor is better than 0.81. Of course this assumes no inverter losses for simplicity sake.

I do have a difficult time envisioning how this happens in our typical 3 phase bridge circuit


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## major (Apr 4, 2008)

liveforphysics said:


> It starts higher ...........


BTW, welcome to this forum. I didn't notice it was your first post here until just now. I'm aware of your participation on other forums and kind of assumed you'd been here before. We met at the Infineon TTXGP a few years ago. Glad to see you contribute over here


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## gunnarhs (Apr 24, 2012)

major said:


> P = √3 * U * Irms * cosΘ
> 
> ...
> Or, Ib = (√3/√2) * Irms * cosΘ = 1.24 * Irms * cosΘ
> ...


Ok, this is actually an interesting deduction 
(assuming we ignore the sinepart which is the "blindloss" and very low in this "high load "condition)
I have never looked at it from only one phase (usually I measure at least 2 Phases motor-side):
The formula Ib = 1.24 * Irms * cosΘ 
-> rewritten ->
cosΘ = Ib/ (Irms*1.24) when U(motor) near U(battery) at "maxspeed" (speed more than base and just before(?) entering field weakening)


But it could give a good indication of the speed limit one can drive with still keeping good efficiency (without applying fancy FOC-calculations)

I have to think this through, this seems so simple ,


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## liveforphysics (Jan 16, 2014)

major said:


> BTW, welcome to this forum. I didn't notice it was your first post here until just now. I'm aware of your participation on other forums and kind of assumed you'd been here before. We met at the Infineon TTXGP a few years ago. Glad to see you contribute over here



Thank you Major. I've met so many awesome DIY EV building folks it's tough to remember everyone, but if you're local to me feel free to meet up and I can show you on an instrumented dyno exactly the relationships between phase current and battery current look like in practice on a modern BLDC controller with timing advance (field weakening). 

Essentially Ibatt at stalled rotor condition is just the copper/Si losses to hold the phase current setting and nothing more. So, starting from a stop with a non-moving rotor is the lowest I-batt for full phase current. As it gets rotation and builds some BEMF, I-batt rises linearly with RPM while I-phase stays maxed out. 

Once you get to a point that Vbatt is limiting your ability to maintain max RMS I-phase, you can be certain your Ibatt has exceeded Iphase at that point (but they are typically close to each other). Then you add commutation timing advance (sevcon's settings call it "field weakening"), then you see I-batt increase further as RPM's increase (as sharply as you want really) from the timing advance.


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## Tesseract (Sep 27, 2008)

major said:


> ...
> With an inverter synthesized sinewave, the peak voltage cannot be greater than Vb, and the maximum RMS value of U = Vb/√2. Substitute that into the equation:
> 
> Ib = √3 * ((Vb/√2)/Vb) * Irms * cosΘ
> ...


You've corrected me on something similar before, so I guess turnabout is fair play, as the old saying goes... 

So you are correct that the usual carrier-based PWM scheme used in many inverters (including the Curtis 123x series) can only produce a maximum RMS AC voltage of 0.707x the DC input voltage. However, if space vector modulation, or other more exotic modulation schemes such as "neutral point clamping" are used then another 15% of the DC voltage can be put to use, getting the phase voltage up to ~81%.

Additionally, the magnetizing current required to set up the field in the induction motor is not necessary in the PMAC motor, and since the magnetizing current basically sloshes back and forth between the stator inductance and the inverter capacitance, the only current required from the battery to sustain it is that needed to overcome switching and conduction losses. Hence why the DC link current in an inverter driving an induction motor will rarely (if ever) exceed the phase current, even if NPC or SVM switching schemes are used.

However in a PMAC (aka - BLDC) motor there is no need for magnetizing current up to base speed so all of the phase current goes towards producing torque, and the power factor - at maximum torque load - should be close to unity. So the DC input current could very well approach a multiple of ~1.22x the current in any one phase.

Note to gunnarhs - this has nothing to do with FOC; FOC is a means of calculating the required timing and magnitude of the currents to deliver to each phase while the maximum magnitude of the voltage (and therefore the current) that can be delivered to each phase is determined by the switching scheme, and some schemes, like SVM or NPC, are able to extract more effective AC voltage from a given DC link voltage.


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## liveforphysics (Jan 16, 2014)

Tesseract said:


> So the DC input current could very well approach a multiple of ~1.22x the current in any one phase.



Yep. This is how it works for PMAC. I see low 500'sA I batt values on controllers peaking I phase of 440A. Or my bicycle, which draws a peak of ~750A Ibatt (on 28s) to supply 660A I phase RMS as I reach power peak. The same 660A I-phase costs me a mere 15A Ibatt to sustain with the rotor locked.


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## major (Apr 4, 2008)

liveforphysics said:


> Tesseract said:
> 
> 
> > So the DC input current could very well approach a multiple of ~1.22x the current in any one phase.
> ...


Thanks for the data points. Those are about 114% of phase current for the battery current, within the 122% from Tesser. And fitting my equation for PF better than .81. But still a lot less than the 1.44 ratio in post #1 or 1.5-2 in post #5. 

Like I said, most of my experience is with ACIM and wound field DC. With those you see a decreasing peak power as you further weaken the field. I suspect the same holds for the PM motors.


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## Tesseract (Sep 27, 2008)

major said:


> Thanks for the data points. Those are about 114% of phase current for the battery current, within the 122% from Tesser. And fitting my equation for PF better than .81. But still a lot less than the 1.44 ratio in post #1 or 1.5-2 in post #5....


I should clarify that the battery (input) current to an inverter can reach 1.22x the current in any one phase leg if the inverter uses carrier-based PWM. If SVM, NPC or the various harmonic injection schemes that litter IEEE Xplore these days are employed then the DC input current could go even higher.

That is to say, to "sum" the RMS currents from all 3 phases together into a single effective RMS current you multiply by √3, but to convert RMS voltage to DC you divide by √2 with carrier based PWM, then divide* that number by as much as 1.15 for SVM, NPC, etc... so DC input current could go as high as 1.41x the RMS AC current in any one phase.

Not that I have personally seen this happen, but it is theoretically possible.


* - EDIT: wrote multiply instead of divide


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## major (Apr 4, 2008)

Tesseract said:


> for SVM, NPC, etc... so DC input current could go as high as 1.41x the RMS AC current in any one phase.......


So, back to the OP.


> Continuous current of 125 amps AC (180 Amps DC into the motor control).


 Do you think that is a valid advertisement for the ME1115 PMAC motor?


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## Tesseract (Sep 27, 2008)

major said:


> So, back to the OP. Do you think that is a valid advertisement for the ME1115 PMAC motor?


Nope - that is very much false advertising as the 180A input current spec pertains to the inverter, not the motor.

Also, 180A [DC] is 1.44x 125A [AC RMS] per phase output, and that exceeds what I understand to be possible with state of the art modulation schemes in the inverter. So that spec doesn't pass the smell test regardless.


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## major (Apr 4, 2008)

Tesseract said:


> major said:
> 
> 
> > So, back to the OP. Do you think that is a valid advertisement for the ME1115 PMAC motor?
> ...


O.K. That was the question at hand. I hope we have helped Mr. Sheepdog 44. I do think the discussion on the topic was good and should be useful to others. Thanks.


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## liveforphysics (Jan 16, 2014)

I don't know that there is in fact some hard limit multiplier if the controller has no limitations on the timing advance it enables. 

There is a point of rapidly diminishing returns on motor output with advancing timing radically, but I think you can choose to draw tremendous I-batts if you choose to keep switching on that field before the BEMF has risen it in from the magnet moving passed. I know you can get >30% over packV *ke RPM and still have big torque if you manipulate the timing right, that requires some mega I-batt to power.


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## Tesseract (Sep 27, 2008)

liveforphysics said:


> I don't know that there is in fact some hard limit multiplier if the controller has no limitations on the timing advance it enables. ...


WhatchootalkinboutWillis?

Perhaps I am missing something here, but it sounds like you are saying that by advancing the timing of the stator waveform with respect to the rotor waveform you can get battery current to increase substantially without a corresponding increase in phase current. That just seems to be another way of saying you can get the efficiency of the inverter to go way down?! Owing to your long and storied history on EndlessSphere, I suspect this is not what you mean...


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## gunnarhs (Apr 24, 2012)

Tesseract said:


> Note to gunnarhs - this has nothing to do with FOC; FOC is a means of calculating the required timing and magnitude of the currents to deliver to each phase while the maximum magnitude of the voltage (and therefore the current) that can be delivered to each phase is determined by the switching scheme, and some schemes, like SVM or NPC, are able to extract more effective AC voltage from a given DC link voltage.


I know, I am sorry I went this way in the last point, I should not have mentioned FOC in this context as this was more about maximal use of DC-Bus Voltage in case of using an adequate modulation.
I was more thinking about a discussion with one member here which measured Current DC-side instead of AC, and deducted torque/speed/efficiency from it.
(I myself prefer to measure three AC current Phases and calculate the current situation from the FOC-model.)
So lets erase my last point there it was obsolete in this discussion
But lets stay to the fact that in every situation I have measured (Induction, DC and Sepex) the (total current battery side) <= (total current motor side).

Interesting to hear it is not the case with some PM-motors...


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## liveforphysics (Jan 16, 2014)

Tesseract said:


> WhatchootalkinboutWillis?
> 
> Perhaps I am missing something here, but it sounds like you are saying that by advancing the timing of the stator waveform with respect to the rotor waveform you can get battery current to increase substantially without a corresponding increase in phase current. *That just seems to be another way of saying you can get the efficiency of the inverter to go way down?*! Owing to your long and storied history on EndlessSphere, I suspect this is not what you mean...



It is exactly as you described. Swapping in efficiency for RPM and an extended high RPM torque range. You can swap in a LOT of efficiency to get a LOT more high RPM torque range than would otherwise be possible. The efficiency hit of the timing advance starts out very small, but can become quite substantial if you start trying to over-extend the functional range of a motor. 

This is done as a pretty standard part of PMAC control in modern EV's due to the shape of the power curve being so much wider. Some applications don't need it, some applications just use a tiny bit of timing advance, and some use a BUNCH of timing advance like my bicycle. No timing advance on my bicycle and it's torque curve has nosed over by 70mph (depending on the gearing I'm running of course), with lots of timing advance setup I can make it continue to pull at nearly full torque to 85-90mph, which makes me another ~>10hp+ peak power at the cost of another hundred amps I-batt and greater motor heating. 

Well worth the trade IMHO. If you re-geared it to have torque at 85-90mph with no timing advance, you would have less torque everywhere all the time. With timing advance I can still achieve the high speed performance I want, while keeping gearing that makes the bike terrifying through the whole speed range. The time it's an efficiency penalty is rarely more than a few seconds at the end of straights on racetracks, so net impact on motor heat load or battery consumption is fairly minimal in exchange for the cool ability stay geared short and still have brutal top-end pulls. 

Timing advance is a good thing. Yes it costs a little efficiency, but I don't believe there were many areas of my entire power range efficiency plot that even dipped as low as most brushed type EV motors can even peak, so the efficiency hit isn't as extreme as you many be thinking.


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## major (Apr 4, 2008)

liveforphysics said:


> Tesseract said:
> 
> 
> > liveforphysics said:
> ...


You have me confused. What would be the loss mechanism in the inverter to account for the decreased efficiency?


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## Tesseract (Sep 27, 2008)

liveforphysics said:


> It is exactly as you described. Swapping in efficiency for RPM and an extended high RPM torque range. You can swap in a LOT of efficiency to get a LOT more high RPM torque range than would otherwise be possible. The efficiency hit of the timing advance starts out very small, but can become quite substantial if you start trying to over-extend the functional range of a motor.


Err... hmmm... Field-weakening of a PM motor does exact a penalty in efficiency - because some of the phase current has to be used to counteract the PM field from the PMs - but I wouldn't think this would amount to more than a 1-2 percentage point increase in loss.

Just like for a series DC motor, when you field-weaken a PMAC motor you are really trading torque for RPM. With any decent inverter the total 3ph. power into the motor will be within 2-3% of the DC input power. Keeping in mind that the current waveform will undergo some phase displacement and distortion relative to the (integral of the) voltage waveform in the field-weakened region, as a bit of negative Id needs to be synthesized.


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## liveforphysics (Jan 16, 2014)

Tesseract said:


> Err... hmmm... Field-weakening of a PM motor does exact a penalty in efficiency - because some of the phase current has to be used to counteract the PM field from the PMs - but I wouldn't think this would amount to more than a 1-2 percentage point increase in loss.
> 
> Just like for a series DC motor, when you field-weaken a PMAC motor you are really trading torque for RPM. With any decent inverter the total 3ph. power into the motor will be within 2-3% of the DC input power. Keeping in mind that the current waveform will undergo some phase displacement and distortion relative to the (integral of the) voltage waveform in the field-weakened region, as a bit of negative Id needs to be synthesized.



You can have the timing advance cause as much or as little efficiency penalty as you choose depending on the extent you push things. In my bicycle, we push advance hard enough to double inefficiency at it's most extreme, but since it's entirely a non-penalty for the RPM range I'm operating in >95% of the time, it's not much of an increased heat source or net system efficiency impact.

Neutral timing tends to optimize torque vs phase current as well as efficiency, but it has a more restrictive range of high RPM high power output than a system that can advance it's timing. This enables a system with good timing advance to gear itself more optimally for good acceleration, yet still be capable of reaching high speeds, and of course accomplish it without adding a transmission that would likely be adding more drive-train losses than the entire rest of the system combined.

IMHO, the optimization of EV drive is the solution which involves the least stages of power transfer and loss. The concept of using legacy ICE drivetrain parts like 90deg power transfer rear-ends in oil baths and mechanical differentials and clutches and transmissions and things just screams fail to me. Every power transfer means loss. Every power transfer means new failure modes. Every power transfer means additional NHV (from mild to extreme). Every power transfer stage also takes up weight and space. 

What do power transfer stages give back in return for all the harsh penalties? They function as a band-aid to let low specific-torque topology motor designs work to power EVs. I think the path looking forward is simply a pair of motors designed to deliver the intended wheel torque over the desired RPM range and mounted either inboard with a direct drive-shaft to the wheel (Yasa motors are amazingly good for this) or directly in the wheel itself. 

As industry moves more towards that inherently thermodynamic loss minimized drive-train topology, it will become even more critical moving forward to have as wide of useful RPM/torque range as possible with a given control scheme. I think this means we will see higher motor phase counts soon become more common, likely will see the end of making delta-wye connections, running 6 motor phase leads to the controller will make the lives of high power BLDC/BLAC controller power stages so much easier. It's so much easier to switch a bunch of 500A stages than make multi kA stages share current effectively enough to both survive and be small and compact and cheap.


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