Brushless Motors

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Goombi
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Brushless Motors

Post by Goombi » Sat, 28 Nov 2009, 02:29

I am lookig at brushless motors around 10-15 kw.
How good is brushless architecture for EV?
Does anyone have experience or know about these motors?

Thank you   Eugen

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Post by coulomb » Sat, 28 Nov 2009, 15:45

Well, brushless DC are a bit of a mystery at the small car power level. There are plenty of BLDC bikes, and hybrid cars have been using them for years, but until about a year ago, there were almost none available.

They are interesting, because they are a little more efficient than induction motors. Their newness has led to low adoption rates; I couldn't find one with a quick search of EVAlbum that wasn't a bike or a commercial hybrid.

The quality issues aren't helped by experiences like this:
David85's Saturn SL1 conversion which almost caught fire due to a loose connection. There was a separate loose connection issue that caused earlier, much less spectacular problems. However, a local agent (such as yourself, Eugen), would probably go a long way towards easing such concerns (if it breaks or doesn't work, get a replacement quickly).

David85's is a bit of a special case, since he had already bought his 144 VDC lithium pack when he decided to switch from series DC to brushless DC. The Chinese company made a special motor and controller to order, so this one is presumably not typical of those that roll off the production line (designed for a 300+ VDC pack).
How good is brushless architecture for EV?
In my opinion, very good, assuming that the price and quality are comparable with the current series DC offerings. The higher voltage requirement may put some off, though it's much easier to work with than the 600+ volt pack needed for industrial AC. You get full regen, even slightly higher efficiency than other (induction) AC motors, contactor-less reverse, and no brushes to wear out. There is however the question of how long the permanent magnets will last, especially if driven hard.
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Post by Johny » Sat, 28 Nov 2009, 16:12

Good answer coulomb. Due to the usual use of rare earth magnets the price of BLDC motors is usually quite high as well. They also effectively require a 3 phase inverter/controller.

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Post by David85 » Sat, 28 Nov 2009, 18:40

My setup was rated at 20kw constant and 40kw peak. If it works after the repairs, it will still be worth the trouble even at $3600USD (thats for a turn key system). However I can't recommend that company after my experience. Motor is about 130lbs, and the controller about 30, so the weight is average.

What I have noticed is the chinese are focusing more and more on LiFePO4 batteries and brushless motors for export. Its only a matter of time before they become more reliable and affordable. Warranty service will eventially be taken on by local shops to eliminate the risk to end customers. Rare earth motors are not really that expensive anymore despite the name.

If you ever decide to order something like me, don't take anything for granted. Tear down and inspect everything before using. Truth be told there have been some much more spectacular failures of american made controllers too, so what I went through was not completely unusual. I usually do that as a matter of principle with ICE related assemblies (cylinder heads for example), but didn't with the EV powertrain.

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Post by coulomb » Sat, 28 Nov 2009, 23:22

I've just finished a survey of vehicles on the evalbum, filtering by motor type = brushless DC (186 of them). 95% are bikes, a few are oddities with 3 kW motors, two are EVplus (no longer available, Lest We Forget), and a handful are hybrids.

That left just three. However, on closer examination, all three were in fact brushed DC. I've sent an email to all three owners.

So apart from David85's Saturn, the only other brushless conversion I know of is this Suzuki Swift. It's almost complete, the main problem at present is that a few BMS modules are killing their host cells. Oh, and he's having charger problems.

Edit: so we won't know for a while what the motor and controller are like; added link.

Edit2: Oops, there are also the 15 kW Smart Cars.
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Post by Electrocycle » Sat, 28 Nov 2009, 23:51

Don't forget about Tritium's Civic!
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Post by acmotor » Sun, 29 Nov 2009, 01:26

Don't forget the solartaxi, the first EV to drive around the world used an MES-DEA brushless DC around 10kW. Image

BLDC motors are 3 phase PM(synchronous motors). Often offered in a package with a controller and presented to a user as no more complex than a series DC system (actually probably simpler if you consider reversing and regen thrown in !)
BLDC offers no weight advantage for the same kW at the same revs as most other motors but does offer more constant torque than brushed series DC. Typically RPM ranges are higher.

From the industrial emotor world, BLDC offers no efficiency gain over induction or synchronous (PM rotor) AC motors at >25% load levels. The potential efficiency advantage used to be at the low load areas (mind you EVs don't spend much time at low loads ! Image ) I say used to since modern VFDs are rather cleaver at manipulating the magnetising current that used to be a limiting factor to motor efficiency at low load levels. There are still some large AC sync motors made though (<50kW ?)    The fact the induction/PM sychronous are driven by well regulated sine waves puts them at an efficiency advantage in motor sizes >say 5kW.
The smaller BLDC motors can get away with just squarewave switching the DC to the coils (e.g. computer fans motors)

Prius plays with a combination motor ? some sync AC some reluctance AC but not BLDC. But since this is an emotor in a mechanically complex ICE arrangement, it probably doesn't help with real EVs.

Oh, try a search of 'AC motor' on evalbum. That becomes confusing.

But back to BLDC. It is separated from AC sync and AC induction by the use of 3 rotor angle (position) pickups (often hall effect switches) that determine the switching of the 3 phase drive circuitry. Actually this simplifies the driving circuitry slightly as there is no need to sense the rotor speed. This helps with the zero RPM torque (maximises it) although any 3 phase motor with an encoder fitted produces much the same result. I have experience with most combinations.
Most modern VFDs can run induction or sync AC / BLDC motors (mine can)

I would comment that one difference with most BLDC motors vs series DC is the BLDC is not able to over-rate to destruction (with nominal controller) to get that extra torque punch at takeoff. i.e. the max torque is the max torque, you can't just push a few more amps.

BLDC offers no weight advantage for the same kW at the same revs as most other motors but does offer more constant torque than brushed series DC. Typically RPM ranges are higher to get any higher kW.

I've got one of the Mars BLDC and Kelly controller sets but at 5kW nom. 15kW peak it is only in the motor bike territory.

Most of my BLDC experience is with <1kW servo motors. Not in EVs. So you can disregard all this in light of Goombi's original question. Image
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Post by Goombi » Sun, 29 Nov 2009, 02:19

Aperenly brushless are not used with regen It effects the Abs Brakes.

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Post by coulomb » Sun, 29 Nov 2009, 05:41

Goombi wrote: Apparently brushless are not used with regen. It affects the ABS Brakes.

What? That's crazy. By that logic, no modern vehicle would have regen.

Acmotor, why wouldn't a PM synchronous motor be more efficient than the equivalent-in-all-other-respects induction motor? By exactly the power it takes to energise the field?

Are you saying it's impossible to make an otherwise-equivalent PM motor for a given induction motor? What happens if you take out the induction rotor and replace it with permanent magnets, and tell the controller it's now PM? Does the efficiency somehow go down by enough to swamp the efficiency gain of not having to excite the field?
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Post by acmotor » Sun, 29 Nov 2009, 07:06

I'd need to see that in writing re regen and ABS. Prius uses both already ! Myth buster time !

coulomb,
The magnetising current circulates between VFD controller and motor and mostly is not seen by the battery pack/mains supply. That is one of the big advantages of using a VFD on an induction motor. I have viewed this on logged data in red suzi. I've posted this some where... I'll look for it. 'ere 'tis

I'm no expert on the motor equations but am well aware of the results. Industry goes for VFDs on induction motors. (along with the near unity power factor on AC mains at any motor loading)
So, the PM sync offers little advantage with modern VFDs although there are special cases (and prices) where PM sync are used.

There are some down sides to PM motors. They do not coast well as they are both PM generators (driving the stator coils) and cause drag due to the eddie current losses that occur in the stator iron since the magnets can't be turned off (part of cogging).

A VFD/IM can reduce the rotor currents and in fact only uses them as required ( PM sync still has some advantage at low loads as mentioned, but it is not as simple as subtracting the magnetising current to decide on efficiency ). The VFD/IM can also increase the rotor current maybe 3x before the curve of saturation and then some more at reducing efficiency, where the PM motor pulls out long before since rotor magnetism is fixed at a lower value. Just making the PMs stronger backfires as the back EMF and thus stator voltage increases along with the other PM losses as mentioned.

I do think there is little in the efficiency variations though. It would be more manufacturer/model dependent and purpose built for particular load dependent. i.e. there will always be the ultralight 95+% efficient PM and the off the shelf 1/10th the price heavy old 95% efficient IM !

The ironless multipole high frequency PMs are good. Shame you need an 85% efficient gearbox to bring them back to useable revs !

Hey this is Goombi's thread. BLDC is good ! After all, it is 3 phase AC !
Last edited by acmotor on Sat, 28 Nov 2009, 20:42, edited 1 time in total.
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Post by Squiggles » Sun, 29 Nov 2009, 15:42

acmotor wrote: coulomb,
The magnetising current circulates between VFD controller and motor and mostly is not seen by the battery pack/mains supply. That is one of the big advantages of using a VFD on an induction motor.
Free energy?
acmotor wrote: A VFD/IM can reduce the rotor currents and in fact only uses them as required


How does it do that?

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Post by coulomb » Sun, 29 Nov 2009, 16:12

acmotor wrote: Hey this is Goombi's thread.

Oh, we're respecting topic titles now? Neat!   Image And a good idea. Let's see how well we do.

Continued in BLDC vs induction motor efficiency.
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Post by Goombi » Mon, 30 Nov 2009, 16:04

New generation Brushless motor 12 kw 35 Kg Converter/ controller 21kg also available motor up to 50 Kw Anything over 15 Kw has water cooled controller with a 12 volt waterpump as part of setup, There are no charts available What puzzles me is the say 12 Kw motor top 24KW I would have thought about 40 kw That is rather small gap? Speeds are no dofferent to DC Brush motor up to 3600rpm. Regen not recommended.
What this debate is about is the comparison between brushless DC against AC motor The character is similar, is there any advantage between one and the other? Are chinese EV car manuf. switching to brush DC for any reason? Is there some kind of revival for dc motors?

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Post by Johny » Mon, 30 Nov 2009, 16:17

A Brushless DC motor (BLDC) is basically an AC Induction Motor (ACIM) with permanent magnets.
The BLDC motor has much more in common with a 3 phase AC motor than it does with a DC motor.
As such, both BLDC and ACIM require a 3 phase controller. The BLDC controller is slightly different to an ACIM controller but the power stages are the same. These controllers produce variable frequency 3 phase AC that also has variable voltage.

Brushed DC motor controllers are quite simple in comparison requiring only a variable voltage to control the speed of the motor. The variable voltage is usually accomplished with Pulse Width Modulation (PWM) - on-off-on-off... at a fast rate.

Edit: controllerS
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Post by acmotor » Mon, 30 Nov 2009, 18:23

Ah yes, I see the brushless DC (BLDC) is warming the heart of the DC fraternity. Denial that BLDC is actually 3 phase AC ! Image

BTW, the Chinese use almost exclusively BLDC in their massive eBike market. Having been to China 3 times this year, I see no evidence that the EV market will entertain brushed DC.

OK, point here. All BLDC must have regen ability.
Technically since the BLDC motor is a 3 phase alternator if driven (by the vehicle's inertia) then it will generate back to the DC supply. If you don't then you risk overvoltage and damage to the motor insulations and the controller silicon. i.e. alternator will be pushing open circuit volts. (this is not the case with ACIM as there are no PMs).

Often BLDC controllers state that you must not rotate the motor shaft at any speed with the controller powered off as the generated voltage can damage the controller circuitry.

Some BLDC controllers limit the regen current as they may have saved on controller silicon in the reverse path back to battery. Typically this will be the mosfet type controllers that may use separate return diodes. (IGBTs have full rated diodes built in).

edit: I should also note from experience that servo BLDC operated from a mains power supply (i.e. no battery pack as in EV) have issues with regen as there is nowhere to put the power. The user of a BLDC controller in such situation must make provision for dealing with the regen energy. e.g. change ramp times, mechanical brake, shunt the DC supply. You can't just ignore it, it won't go away ! You have an alternator in your hands.

So a BLDC offered with no regen would come with alarm bells sounding IMHO.

Goombi, as I noted earlier, the peak rating for AC systems is often lower than that 'assumed' for brushed DC since the AC system peak is a manufacturer number and attainable without detriment to the motor whereas some (most ?) brushed DC peaks won't be shown on any manufacturer data. But none the less are quite achievable. i.e. you can push 'till it breaks.
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Post by antiscab » Tue, 01 Dec 2009, 11:42

Rod Dilkes has been playing around with a BLDC motor and controller in his proton jumbuck.

IIRC, its around 15kw cont, and i think hes measured 90kw peak (pulling ~350A from a 288v nominal pack).

though i suspect he doesn't plan on revealing where he got it from :)

kelly also makes BLDC controllers for lower voltage conversions (i think their two biggest are 120v 600A or 144v 400A).

David - who manufactured your motor and controller?

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Post by juk » Wed, 02 Dec 2009, 06:14

acmotor wrote: BLDC is actually 3 phase AC !


If it's AC, does that imply that the current changes polarity? If that's what it implies, does that actually happen?

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Post by acmotor » Wed, 02 Dec 2009, 08:36

Yep, AC it is. (at least alternating current, not always sine wave)

Remember the PMs are North / South so require reversal of coil polarity as the PMs pass the coils. Actually the same as with the armature / commutator / brushes arrangement in a brushed DC motor, there is armature coil polarity reversal. With BLDC, the reversal is carried out electronically under instruction from rotor angle pickups.
Sensorless BLDC (controller looks at back EMF) is also popular with RC model motors, but then that becomes a grey area of naming. The line seems to be drawn that if there are rotor angle pickups and PMs are used then it is BLDC. Also if there is block commutation (on/off drive with reversal to the coils) then it is BLDC. If there is PWM and attempt to create sine waves rather than square waves then it is synchronous AC (PM AC).
Note, to get control to zero RPM with smooth and full torque then PWM not block commutation is required so the BLDC starts to look more like PM AC.

The coil drive comes from a 3 phase H bridge where any coil end can be pulled to +ve or -ve DC supply (0V and V+ if you like) thus allowing reversal of coil current. This is the same for BLDC or AC induction or AC PM. Image

edit: spelling ! ..at least the bits I noticed.
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Post by Electrocycle » Wed, 02 Dec 2009, 13:18

I think brushless DC is a bit of a silly term - which should really only apply to motors with integral controllers.

If you buy a motor and it needs 3 wires connected to it (has no internal commutation), it's AC :P
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Post by acmotor » Wed, 02 Dec 2009, 17:37

Matt, if there is a viable 90kW BLDC on the market from China then rest assured nobody will be keeping it a secret ! or sitting on it for 12 months.   Image
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Post by weber » Wed, 02 Dec 2009, 21:43

Electrocycle wrote: I think brushless DC is a bit of a silly term - which should really only apply to motors with integral controllers.

If you buy a motor and it needs 3 wires connected to it (has no internal commutation), it's AC :P


I totally agree. So-called brushless-DC is really Permanent Magnet (PM) AC. Calling it sychronous AC is correct but doesn't give enough info, as there are synchronous AC motors that don't have permanent magnets, such as reluctance motors and DC-excited wound-rotor motors.

I think I prefer induction rotors over PM rotors because
(a) the magnets don't turn off when you want them to, and
(b) the magnets _can_ turn off when you _don't_ want them to. Permanently e.g. by overheating.

[Edit: "And more specifically it is" -> "So-called brushless-DC is really"]
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Post by juk » Thu, 03 Dec 2009, 02:09

Electrocycle wrote: I think brushless DC is a bit of a silly term


Y'see that was the reason for my question, in that the term possibly didn't describe the actuality.

Thanks for the lesson guys.

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Post by coulomb » Fri, 04 Dec 2009, 19:11

Tom's Swift-E is actually being driven now, although his pack is still wounded from a BMS attack:

http://www.diyelectriccar.com/forums/sh ... post154010 (page 18; page 17 has more on actually driving the car).

This is using a Curtis controller, which is only 108 VDC nominal maximum. He's pushing that limit with 36 LiFePO4 cells (108 V @ 3.0 VPC; 115 V @ 3.2 VPC; 118.8 @ 3.3 VPC; 9 12 V nominal batteries at 13.0 V each is 117 V, so the controller is possibly not expecting more than 117 V). It would sure be nice if Curtis made higher voltage AC controllers.

He's quite happy with the acceleration performance of it, but it sounds like the regen is not to his liking as yet.
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Post by coulomb » Fri, 04 Dec 2009, 19:15

Oops! The Suzuki Swift-E that I'm talking about seems to be induction, not brushless. Interesting, but off topic. Sorry!   Image
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