# pondering brushless DC



## dtbaker (Jan 5, 2008)

I am in rough planning stage for my next EV, and considering brushless DC so that I can consider grabbing some of that lovely regen power when braking... but I don't know much about the pros/cons/costs of brushed versus brushless DC, or how it would stack up against AC for performance and cost.

There has to be a good reason the high performance e cars are going with AC; but what is it? why not brushless DC and skip the need to invert?

Why are the brushless DC motors not common for EVs so that we can grab some regen?


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## major (Apr 4, 2008)

dtbaker said:


> There has to be a good reason the high performance e cars are going with AC; but what is it? why not brushless DC and skip the need to invert?


Hi dt,

You do need to invert with BrushLess DC (BLDC). BLDC should be considered an AC motor. It may not be sine wave AC, but never-the-less goes positive to negative, back and forth, like up to 300 or more times a second.

And some of the high end, or production EVs use or will use BLDC, or PMAC motors instead of ACIM (induction motors). The use of permanent magnets has advantages, and some disadvantages. 

"Brushless DC" has always been a misleading phrase when it comes to motors.

Regards,

major


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## Bentzon (Sep 5, 2009)

Isn't pretty much any new electric car coming to the marked powered by either a asynchronous or AC synchronous motor?

Doesn't AC synchronous motors provide better low speed control than DC powered synchronous motors?


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## dtbaker (Jan 5, 2008)

ok.... I guess I need a primer. Can you back up to a high-level comparison of the motor types at a level to help an average DIY make a choice? I don't really care what is inside the case, I just need to know if there are other things I would need OUTSIDE the case that might make the wiring super complex or drive the cost way up.

What I really want to know is... for 'similar' performance what are my options and what ARE the pros/cons?

lets say for a typical 'small' car EV, with a 120v battery pack, what are my motor options and aproximate cost.... ? perhaps comparing to a typical 8" DC as the baseline

8" DC at 120v - continuous hp=22, 5 min=37hp, peak=83hp
peak torque at
peak hp at
cost about $1400, controller about $1000, 
15* 8v FLA batteries for 120v about $2000, 
giving 12kWhr using 100ah effective available (at 1 hr rate for EV use)
... or 40x100ah Li for $4400 plus maybe an extra $1000 for BMS
no regen (reasonably)

BLDC - ...... ? typically require higher v, which would mean Li battery pack ?

AC - ?


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## Bentzon (Sep 5, 2009)

Probably AC induction(asynchronous) since its much cheaper to manufacture than synchronous motors but theres just not many affordable motors around build for EV purposes.


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## major (Apr 4, 2008)

dtbaker said:


> Can you back up to a high-level comparison of the motor types at a level to help an average DIY make a choice?


Hi dt,

In the EV, you need a controller regardless of the type of motor. For the DC series motor, it is a simple single quadrant chopper, or buck converter. For AC motors (including BLDC, PMSM and ACIM), you need a poly phase inverter, usually 3 phase. This most often gives you 4 quadrant control without any extra parts, meaning forward/ reverse/ motoring/ regeneration.

The AC controllers require more power silicon and end up costing more. There are also fewer choices for suitable AC motors. So, the AC/DC debate continues. Volumes have been posted on this forum and others. I have no easy answers. I like the AC systems better. But have built far more DC systems. Usually boils down to the budget and component availability, doesn't it?

Regards,

major


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## dtbaker (Jan 5, 2008)

I am trying to quantify, at least roughly, what the ballpark of 'more expensive' means in a typical build, and what the rough pros/cons are for the un-informed like me.  

Is it hundreds, or thousands to get that extra 20% from regen? (all else being 'equal')
Is there significant difference in torque curves for DC, BLDC, AC?
Has the limiting factor been high voltage requirements for BLDC, AC,which can now be addressed with Li packs easier than typical Lead?


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## major (Apr 4, 2008)

Hi again dt,

Here's a couple of my opinions 



dtbaker said:


> Is it hundreds, or thousands to get that extra 20% from regen? (all else being 'equal')


Thousands. And more likely 5% from regen in terms of range, depending on all sorts of factors. Maybe a commercial heavy vehicle doing 200 stops a day could realize 20%. But if you're counting on numbers like that for daily driving, think again.



> Is there significant difference in torque curves for DC, BLDC, AC?


Can be a real big difference, depending on the particulars. And with those particulars, you may need to gear it differently.



> Has the limiting factor been high voltage requirements for BLDC, AC,which can now be addressed with Li packs easier than typical Lead?


Yeah, I think the newer batteries help one get higher voltage packs and high voltage has been the realm of AC. But there are low voltage AC systems hitting the market in recent years.

Remember, just my opinions. Not trying to start the AC/DC war again.

major


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## Bentzon (Sep 5, 2009)

I don't know what size car you are planning but if your only after regen, and not zero maintenance and wide rpm range the AC motors offer, wouldn't dual or triple brushed PMDC motors like the AGNI or LEMCO motors be a easier and cheaper way to go?
Its been done on a ford fiesta with dual AGNI motors and the manual transmission.


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## dtbaker (Jan 5, 2008)

major said:


> Thousands. And more likely 5% from regen in terms of range,
> 
> Torque
> ------
> Can be a real big difference, depending on the particulars. And with those particulars, you may need to gear it differently.


...not trying to start a war,just trying to quantify the pros/cons to dollars and specifics.  

It is sounding like the range extension to be gained by BLDC or AC would be more expensive than just getting bigger AH batteries if measured in dollars per range mile for a given target range.

Can you just say a LITTLE more about the torque characteristics? Seems like DC has highest torque from 0rpm and tails off... what does BLDC and AC typically do?

...given all of this, what would you (or the upcoming all-e from chevy etc) choose DC vs BLDC vs AC, and why ?


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## dtbaker (Jan 5, 2008)

Bentzon said:


> I don't know what size car you are planning but if your only after regen, and not zero maintenance and wide rpm range the AC motors offer, wouldn't dual or triple brushed PMDC motors like the AGNI or LEMCO motors be a easier and cheaper way to go?
> Its been done on a ford fiesta with dual AGNI motors and the manual transmission.


I don't know anything about other types of motors... what are the torque characteristics and cost for similar peak and continuous HP ? Can you help fill in a comparision against the 'typical' DC motor example?


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## major (Apr 4, 2008)

dtbaker said:


> It is sounding like the range extension to be gained by BLDC or AC would be more expensive than just getting bigger AH batteries if measured in dollars per range mile for a given target range.


I've said this very thing many times, especially to those adding mass as well as dollars to a series DC system to get regen 

With an AC system, you get regen for free. And I really like it. It's great for drivability. But as for paying a lot extra just for regen, like you say, spend the money in other places and you'll go further.



> Can you just say a LITTLE more about the torque characteristics? Seems like DC has highest torque from 0rpm and tails off... what does BLDC and AC typically do?


 
See: http://www.diyelectriccar.com/forums/showthread.php/pure-horse-power-dc-vs-ac-14110p13.html 

Post #124 has a nice plot showing AC and DC. But notice the rest of the thread. You trying to start that again 

And then this: http://www.diyelectriccar.com/forums/showthread.php/torque-irrelevant-relevanti-36904.html 

Was about motor torque with trying to not get into the AC/DC war. 



> ...given all of this, what would you (or the upcoming all-e from chevy etc) choose DC vs BLDC vs AC, and why?


Production EVs and HEVs or PHEVs from the major companies will have AC motors and drives, IM or PM. No doubt in my mind. Many reasons. (edit) Biggest reason; they can afford it.

Regards,

major


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## dtbaker (Jan 5, 2008)

hhhmmm, those discussions seemed to have deteriorated pretty fast.  I have no real 'favorite' and am just after the most cost effective delivery of good power from a stop for typical in-town driving balanced with reasonable range. If the extra cost of 'built-in' regen is higher than buying bigger batteries, then the re-gen factor goes out the window. Unless there is some OTHER big factor for BLDC or AC for in town?

maybe just answer this:

given motors with similar peak HP, which (DC,BLDC,AC) will have more torque available from 0 rpm? This to me is very important for in-town drivability. The regen is less so because my range requirement is within limitations of FLA even without regen.


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## 1clue (Jul 21, 2008)

dtbaker,

You're going to have to sit down and start reading.

There are large differences between types, and there are adherents to each strategy who make the debate look like a religious war.

Personally, I'm looking to do AC. Most people tend to do DC, because it's cheaper to get something that runs, and in some cases the benefits of AC don't seem to outweigh the extra cost.

The problem is, both sides are right. It all depends on how much money you think is a lot, how much resources you have, how much various things matter to you.

If you just think AC gives you regen, then screw it and go DC. The thing is, it gives a whole lot more depending on what type of AC you're talking about, and what type of controller you use.

Now, as far as AC/DC/BLDC, here's how I look at it: It's really down to internal commutation or external commutation. The commutator on a DC motor is what the brushes ride on. ALL motors are AC when it gets to the coils inside the motor.

A DC motor has a set of brushes and a commutator to continually change which wires have current, and in what direction. 

A pure AC motor, like an AC induction motor (ACIM), has no commutator. It gets AC into the coils, and the design of the motor causes the motor to spin. The frequency of the AC power causes it to spin at a certain speed and no faster.

Some motors which have brushes work on either AC or DC. These motors do not have permanent magnets.

A brushless DC (BLDC) motor is essentially identical to a permanent magnet AC motor (PMAC) in any mechanical sense. I personally think it was pure marketing that came up with the BLDC term. However, BLDC implies an external controller, and that implies synthetic frequency control, which generally means that the nature of the iron core is different than would be on a plain-jane PMAC motor. It would be made so it could handle higher frequency power, to get the RPM up there.

The thing is, that's not tried and true either. There are lots of motors of every designation that are able to handle higher RPM and designed to handle the higher frequency AC power.

A BLDC motor could run off 3-phase AC power if the frequency and voltage are within reason. Nobody would really do that, though, you would want a controller. The controller could be viewed as an external commutator.

The WIKI has lots more on this, and they will probably explain it a lot better than I could.


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## major (Apr 4, 2008)

dtbaker said:


> maybe just answer this:
> 
> given motors with similar peak HP, which (DC,BLDC,AC) will have more torque available from 0 rpm? This to me is very important for in-town drivability. The regen is less so because my range requirement is within limitations of FLA even without regen.


Hi dt,

You can't beat the series wound DC motor with a monster controller (high current limit) for stall torque in a given package size. 

But having said that, you can play games with gear ratios and do a lot of shifting so the other types of motors (with similar peak power) could get enough wheel torque. But you'd come up short in a drag race. I think I showed that in the relevant torque thread.

Regards,

major


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## dtbaker (Jan 5, 2008)

I got a reply from M&C regarding their BLDC motors. I asked them to quote me a price, and send specs for a motor and controller with continuous rating of 20kW, and peak over 70kW.

DL-35 motor, req 320vDC, rated speed 3500rpm, max torque=210N.m. weighs 114kg, air cooled for $1470 plus shipping (from china) rated power 35kW, max 75kW

controller CNV-120, capacity 120 kVA at 320v, weighs 25kg, 450mmx320x160, air cooled for $2941 plus shipping.

so..... while the continuous power is great, the max is not as high as plain DC of similar size, and the motor itself looks to be significantly bigger and heavier. The controller is WAY more expensive as well as bigger and heavier than typical DC setup, but I don't know how big a similar regen capable controller from Kelly or others would be. Another issue is that running at 320v pretty much requires the battery pack to be Li which drive the initial price up further.


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## yarross (Jan 7, 2009)

1clue said:


> ALL motors are AC when it gets to the coils inside the motor.


You are right - almost. There's a DC motor that is a true DC one - homopolar (unipolar) motor. Unfortunately, it's completely not suitable for traction applications (it's a very low voltage, very high current system).



1clue said:


> A brushless DC (BLDC) motor is essentially identical to a permanent magnet AC motor (PMAC) in any mechanical sense.


At first glance. BLDCs are intended to be fed with rectangular voltage waveforms, while PMSMs with sinusoidal. A good BLDC is designed with special attention to pole shapes (a claw-pole design or any other one that provides triangular flux as a function of shaft angle) to avoid torque ripple.


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## order99 (Sep 8, 2008)

I think i'm missing something-nothing new there, right? 

I'm currently playing with some 12V radiator motors(blame Amberwolf and his Crazybike blogs!). I haven't time to open one up and watch the parts fly all over the place yet-but I can feel a light cogging when turning the motor, generate current by same and mildly attract metals through the casing, so it's a PM motor. I've been testing with a single 12V battery and no controller (one's on the way) and the motors run in either direction with ease. Good torque too!

Given that I have a PM and am using no inverter-should I find brushes when I open one up? It seems counter-intuitive that such a long-running DC motor would have brushes, and even less-likely that i've got an AC motor meant to run on DC current without stall-start issues...I won't have time to pry one open until the weekend, so any theories in the meantime?

Not to veer even further off topic, BUT-I just found a Pick-a-part place within 25 miles of my house...$2 to get in, hope you can find what you want(no inventory control) but really nice prices. I can grab radiator motors for roughly $10 apiece-adapt some burned-out Powertools to the 12V motors,one Deep-cycle battery and a cheap Solar panel off Ebay and i've got a self-contained workshop!

But the motors off the 1980's-90's Volvos are the best-twice the size of the standard ones and so much torque that 2A at 12V sent the Volvo motor flying loose from its moorings and across the room into a couch! I've never been so happy about two sprained fingers in my life...

I think I have a Bicycle motor in my hands here.  My first EV, at last? My hands have begun to itch...

Sorry about the topic drift, but I had to tell somebody!


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