# Choice of motor. AC, DC - brushed or brushless?



## freddyflatfoot (Oct 6, 2008)

Ok, I have checked out the Wiki, and there seems to be no discussion on the advantages/disadvantages of each type of motor.
I am coming from the e-bike world, where brushless (and now sensorless) DC motors are ruling the day. Brushed motors are seen as being noisier and inefficient.
I currently have a BLDC hub motor, and love the simplicity and ease of use. I may consider converting the controller to sensorless down the track.
However, when it comes to looking at traction motors for EV's, I see that brushed DC motors seem to be the order of the day. I don't think I have come across any reference to a brushless DC traction motor.
I see there are some AC induction motors, and a local EV convertor is using AC motors and offering kits.
My query is, what are the advantages and disadvantages of each type of drive system?
Is it cost? Weight? Complexity?
Which is the way of the future?
Which is more efficient?
Maybe someone can write up a Wiki addressing the differences?
I hope I'm not the only one trying to work through some of these issues!
Thanks.


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## saab96 (Mar 19, 2008)

The Mitsubishi iMiev is using brushless DC. You can't get motors like that for DIY conversions.


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## pquang (Oct 17, 2008)

freddyflatfoot said:


> Ok, I have checked out the Wiki, and there seems to be no discussion on the advantages/disadvantages of each type of motor.
> I am coming from the e-bike world, where brushless (and now sensorless) DC motors are ruling the day. Brushed motors are seen as being noisier and inefficient.
> I currently have a BLDC hub motor, and love the simplicity and ease of use. I may consider converting the controller to sensorless down the track.
> However, when it comes to looking at traction motors for EV's, I see that brushed DC motors seem to be the order of the day. I don't think I have come across any reference to a brushless DC traction motor.
> ...


-You can use the AC motor, but you have to change the DC into AC ; the energy loss for this transfer which is about 25% , it means that If you want to get 10KW you have to supply the power which is higher than 25%.I don't talk yet about the effect of the motor, you loss about 20% for this, and if you want to control the speed you have to use the frequency inverter as so as the power coefficiency is better than the other method .Because of this , I don't choose this way.
If you want to use the DC motor, you must choose the high power motor at least 5KW or higher, the brush method is complexer than the brushless method . Due to the high intensity of the current which will be mainly obstacle.

Thus , You only have one solution for choice of the brushless motor and the PWM controller for the speed control .
I have one solution in this topic as so as you can refer this link:
http://www.diyelectriccar.com/forums/showthread.php/help-motor-selection-20948p2.html
Hope that you can find the DC brushless motor & controller.
Sussess or loss since our choice and our decision.
Thanks


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## major (Apr 4, 2008)

pquang said:


> -You can use the AC motor, but you have to change the DC into AC ; the energy loss for this transfer which is about 25% , it means that If you want to get 10KW you have to supply the power which is higher than 25%.I don't talk yet about the effect of the motor, you loss about 20% for this, and if you want to control the speed you have to use the frequency inverter as so as the power coefficiency is better than the other method .


Hi pquang,

It should be noted that these are your opinions and not fact. I have used AC motors and drives for battery powered vehicles. The "frequency inverter" as you call it takes care of the DC to AC conversion and typically has efficiencies in the 95 percent range.

Glad to see you interested in EVs and motor drives. Keep at it and good luck with your endeavors.

major


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## pquang (Oct 17, 2008)

major said:


> Hi pquang,
> 
> It should be noted that these are your opinions and not fact. I have used AC motors and drives for battery powered vehicles. The "frequency inverter" as you call it takes care of the DC to AC conversion and typically has efficiencies in the 95 percent range.
> 
> ...


As you know the name of "the inverter" which is many uses and kinds

The Inverter is the equipment for changing DC into AC with square wave, or with sine wave .
The frequency Inverter is the equipment which can change the frequency from low frequency to high frequency, it is usefully for increasing the power coefficiency of the motor, and the motor speed can changed.

If you use the motor. , you have two choices as follows:
1/first choice : using the AC motor.
a/- You have to use the Inverter to change DC to AC , but it must be only the sine wave for the AC motor , the efficiency is about 85%, you can not use the square wave, the efficiency of this kind is 90 - 95%.Otherwise, you can use the square wave but you must use the OTT filter ( 2 Capacitors & 1 coil, 2 coils & 1 Capacitor , they are connected by dual T-type ) to rectifier the square wave into the simulated sine wave.
* After you have the sine wave, if you want to change the speed of the motor, you have to use the AC frequency inverter for this purpose, this equipment can reduce the frequency from 0,5Hz or increase to 400Hz so as you can change the speed of the AC motor

b/-Otherwise, you can use only one equipment of frequency inverter using directly the DC batteries , the output is the AC for the AC motor, the efficiency is the same with the above method while transfering the DC to AC and then controlling the speed.
(Pls refer the frequency inverter of VF-S11 made by TOSHIBA, with this type you can use directly the DC batteries into AC 3 phases for your AC motor and it can control the speed of your motor. )

2/ 2nd choice : using the DC motor.
You can use directly the DC into the PWM for controlling the speed of your DC motor, the efficiency is 90-95%.
Thanks


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## saab96 (Mar 19, 2008)

pquang said:


> a/- You have to use the Inverter to change DC to AC , but it must be only the sine wave for the AC motor , the efficiency is about 85%, you can not use the square wave, the efficiency of this kind is 90 - 95%.Otherwise, you can use the square wave but you must use the OTT filter ( 2 Capacitors & 1 coil, 2 coils & 1 Capacitor , they are connected by dual T-type ) to rectifier the square wave into the simulated sine wave.
> --------
> 2/ 2nd choice : using the DC motor.
> You can use directly the DC into the PWM for controlling the speed of your DC motor, the efficiency is 90-95%.
> Thanks


Let's bring this out of theory and into the real world. The only affordable AC system for DIY conversions is the Solectria one (i.e. Azure Dynamics Force Drive). From their PDF:

http://www.azuredynamics.com/products/force-drive/documents/AC24_DMOC445ProductSheet.pdf

Peak Efficiency % [email protected] volts [email protected] volts

So I don't see a huge difference in efficiency between AC and DC systems, especially when you consider the ~10% or so range boost you get with an AC system due to regen.


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## pquang (Oct 17, 2008)

saab96 said:


> Let's bring this out of theory and into the real world. The only affordable AC system for DIY conversions is the Solectria one (i.e. Azure Dynamics Force Drive). From their PDF:
> 
> http://www.azuredynamics.com/products/force-drive/documents/AC24_DMOC445ProductSheet.pdf
> 
> ...


 
Many thank for your guide, but I think that you have been misunderstood my opinion.

I wrote that the inverter can transfer DC into AC , there are two kinds as follows

-If the inverter converts DC to AC with the square wave form , then the lost is about 5-10% . With this wave form, you can not use for the motor, you have to rectifier the square wave into the simulated sine wave by the OTT filter, the lost shall be added about 10% more.

-If the inverter converts the DC to AC with the sine wave form , then the lost is about 15-25%

By any transfer, you could not find which is no lost.

I only talk about the lost while it is the transfer from the DC energy into the AC energy.

Refer your link, this is the DC controller for the AC motor with the PWM control , now, this motor is not truly the AC motor, because you could not find the specifications which were shown the AC voltage, AC current…..in this device specifications. 

It means that the AC motor now is used as the DC motor with the PWM control . The normal think, the AC motor is used with the alternative current with the regular frequency as the 3 phases motor which works at only one sine wave frequency , Otherwise, we can state that this motor is the multipurpose . However, this motor can use directly with DC, it means that it is the DC motor .
For using with DC , the motor must be made by the variable reluctance as I have been designed and shown this design in the topic link:

http://www.diyelectriccar.com/forums/showthread.php/help-motor-selection-20948p2.html

With this method, you can modify the AC motor into the DC motor using directly for PWM controller.

So much words for the thank of your reply, I think that I will receive much more guides from the friend in the world .
Thanks
Pquang


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## major (Apr 4, 2008)

pquang said:


> -If the inverter converts the DC to AC with the sine wave form , then the lost is about 15-25%


Hi pquang,

This statement is incorrect. The typical 3 phase sinewave inverter, DC to AC, efficiency is on the order of 96 to 98 percent. 



> Refer your link, this is the DC controller for the AC motor with the PWM control , now, this motor is not truly the AC motor, because you could not find the specifications which were shown the AC voltage, AC current…..in this device specifications.


Just because they do not list the nameplate AC specifications for the motor does not make it NOT an AC motor. It is a three phase induction motor, AC. It would run from the 3 phase mains if connected. It is sold as a drive package with the DC to AC inverter, so the seller sees no reason to give the AC specs for the motor alone.

The output from the DC to AC inverter is a synthesized AC waveform using PWM. The motor on the output of the inverter acts as a filter and resulting phase current waveforms are pretty close to sinewave.

Regards,

major


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## pquang (Oct 17, 2008)

major said:


> Hi pquang,
> 
> This statement is incorrect. The typical 3 phase sinewave inverter, DC to AC, efficiency is on the order of 96 to 98 percent.
> 
> ...


Please refer this link as follows:
http://www.santak.com/productsinner.php?id=8

The UPS w/ 3 phases , it can convert DC to AC as the true sine wave form.
For the input : power factor is 0.95
For the output : power factor is 0.7

If you use this UPS with the full power , you shall be lost the total loss which is 35% .
I think that any AC motor using the variable reluctance, it can steady run with the PWM as the stepper motor.
For example , the stepper motor is one of this type , but it has [N * four windings] , this motor can run with PWM in DC or with AC sine wave.
Thank so much your reply
Pquang


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## major (Apr 4, 2008)

pquang said:


> The UPS w/ 3 phases , it can convert DC to AC as the true sine wave form.
> For the input : power factor is 0.95
> For the output : power factor is 0.7
> 
> If you use this UPS with the full power , you shall be lost the total loss which is 35% .


Hi pquang,

You confuse power factor and efficiency. Not the same thing.

Look into it.

major


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## frodus (Apr 12, 2008)

Don't confuse Power factor with efficiency. An inefficient motor can have a high power factor.

http://www.brithinee.com/Power_Factor_Efficiency.htm

Power factor is simply a measure of the phase displacement or time lag between the current through the motor and the voltage that’s applied.

Also, don't look at sinde wave UPS inverters, they're notoriously inefficient.

If you use that UPS at full power, you cannot look at the power factor ONLY and determine efficiency. You have to look at power input versus power output as phasors. Power factor is only how the phase angles of the input on the UPS apear to the source. In this case 95% is lagging by 5%, which is typical for most switching power supplies. Some of those power supplies are 97+ percent efficient.


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## frodus (Apr 12, 2008)

major said:


> Hi pquang,
> 
> You confuse power factor and efficiency. Not the same thing.
> 
> ...


damn, beat me to it!


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## major (Apr 4, 2008)

pquang said:


> .
> I think that any AC motor using the variable reluctance, it can steady run with the PWM as the stepper motor.
> For example , the stepper motor is one of this type , but it has [N * four windings] , this motor can run with PWM in DC or with AC sine wave.
> Thank so much your reply
> Pquang


Hi pquang,

I think we're talking apples and oranges here. These are induction motor drives, not variable reluctance or stepper motors. And the PWM is a method used by the inverter to create the sine wave. Look at the link. The arrows below the diagram will advance picture to show what the sine wave looks likes.

Regards,

major

http://www.sea.siemens.com/step/templates/lesson.mason?ac_drives:3:1:2

l


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## frodus (Apr 12, 2008)

Inverters use PFM and PWM to control the created waveform, in this case, sinusoidal. Not sure why he's automatically assuming since its PWM, its DC... PWM can be used to create a sinusoidal output. The inductive windings smooth out the PWM so it looks almost sinusoidal, and thats exactly how inverters work.... PWM, its all digital on-off's. 6 PWM circuits controlled by a microcontroller can drive a 3 phase motor (1 positive and 1 negative PWM circuit for each phase of the motor, times 3 phases).


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## pquang (Oct 17, 2008)

major said:


> Hi pquang,
> 
> You confuse power factor and efficiency. Not the same thing.
> 
> ...


I always talk about the lost.

First way, the lost issues from the elecrical property
I talk about the lost that the power factor participates very much into the effective power.
P=U*I *(Power factor)

Power factor is the COS coefficiency of the angle of [U,I] ; (the voltage and the current .)

If you want get 1KW from the generator , but the power factor is 0.7, it means you must supply 1/0.7=1.43KW , and you lost 30% power .

2nd way, the lost issues from the energy transfer , ie: the electrical energy is changed into the mechanic work, the lost is issued from the energy transfer
I can explain the efficiency of the motor or the equipment as follows
If you want to get the power of the motor which is 1HP or 736W, you must supply higher 736W, for example , the power need 1000W for transfer the electrical energy into the mechanic work , the lost ratio is 
(1000-736)/1000=26.5% and the efficiency is 1-0.265=63.5%.

Please try to ask some experts about my opinion as so as you can find out the result , it is wrong or correct.
Thanks
Pquang


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## pquang (Oct 17, 2008)

frodus said:


> Inverters use PFM and PWM to control the created waveform, in this case, sinusoidal. Not sure why he's automatically assuming since its PWM, its DC... PWM can be used to create a sinusoidal output. The inductive windings smooth out the PWM so it looks almost sinusoidal, and thats exactly how inverters work.... PWM, its all digital on-off's. 6 PWM circuits controlled by a microcontroller can drive a 3 phase motor (1 positive and 1 negative PWM circuit for each phase of the motor, times 3 phases).


You are very correct in your argument, I always talk that the AC motor made by the variable reluctance which can use in DC by PWM .

When you use one AC motor with the highest efficiency, you have to use the frequency inverter for increasing the power coefficiency (about 0.95), this machine is the equipment using the PWM method for simulating the sinusoidal output and the frequency can be changed optionaly and you can use this equipment for changing the speed of the motor instead of the gears.You can reduce the electric bill about 30% for this way, if you use the AC motor at home ( this is one of the application of the PWM comtroller)
I hope that I often hear all of your opinion for rectifing some deviations in my mind.
Thanks
Pquang


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## kugmo (Oct 31, 2008)

either one as long as you get what you want..


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## pquang (Oct 17, 2008)

frodus said:


> Inverters use PFM and PWM to control the created waveform, in this case, sinusoidal. Not sure why he's automatically assuming since its PWM, its DC... PWM can be used to create a sinusoidal output. The inductive windings smooth out the PWM so it looks almost sinusoidal, and thats exactly how inverters work.... PWM, its all digital on-off's. 6 PWM circuits controlled by a microcontroller can drive a 3 phase motor (1 positive and 1 negative PWM circuit for each phase of the motor, times 3 phases).


 

Please refer my design in this link
http://www.diyelectriccar.com/forums/showthread.php/help-motor-selection-20948p2.html

thanks


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## major (Apr 4, 2008)

pquang said:


> Please try to ask some experts about my opinion as so as you can find out the result , it is wrong or correct.
> Thanks
> Pquang


Hi pquang,

You are wrong on a few points here.


> If you want get 1KW from the generator , but the power factor is 0.7, it means you must supply 1/0.7=1.43KW , and you lost 30% power .


No, you need 1.43KVA. 1.43KVA is the APPARENT POWER. Which is 1.00 KW of ACTIVE (or REAL) POWER and 1.02 KVAR of REACTIVE POWER. There is no "lost" power in this example.



> If you want to get the power of the motor which is 1HP or 736W, you must supply higher 736W, for example , the power need 1000W for transfer the electrical energy into the mechanic work , the lost ratio is
> (1000-736)/1000=26.5% and the efficiency is 1-0.265=63.5%.


First, 1 HP = 745.7 W. Second, 1-0.265=0.735 (or 73.5%). Everybody makes typos.

As far as the motor goes, efficiency is defined as Power Out / Power In. Or Pout/Pin. Usually expressed in a percentage. So Efficiency = Pout/Pin * 100%. Loss is Pin - Pout. All power figures used for efficiency and loss calculations are REAL (watts). Not APPARENT (VA) or REACTIVE (VAR).

Regards,

major


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## major (Apr 4, 2008)

Hey pquang,

I am surprised not to see a post from you. I did not mean to insult you or anything. Hopefully I didn't come across that way. Just trying to help.

Regards,

major


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## pquang (Oct 17, 2008)

major said:


> Hey pquang,
> 
> I am surprised not to see a post from you. I did not mean to insult you or anything. Hopefully I didn't come across that way. Just trying to help.
> 
> ...


Hi Major,
Many thanks for your concern, my mother was died on last week. Maybe , next week, I will continue to discuss with you.
Thanks
B.R
Pquang


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## brdriver (May 26, 2009)

dear,
you have to choose the bldc motor drive system base on its transfer effieciency, safety,and smoonth,etc.and its costly is as same AC control system.
you can look at http://www.brdriver.cn/product-101.html,
you will unerstand using its reason.
successfully to you!

best regards!



freddyflatfoot said:


> Ok, I have checked out the Wiki, and there seems to be no discussion on the advantages/disadvantages of each type of motor.
> I am coming from the e-bike world, where brushless (and now sensorless) DC motors are ruling the day. Brushed motors are seen as being noisier and inefficient.
> I currently have a BLDC hub motor, and love the simplicity and ease of use. I may consider converting the controller to sensorless down the track.
> However, when it comes to looking at traction motors for EV's, I see that brushed DC motors seem to be the order of the day. I don't think I have come across any reference to a brushless DC traction motor.
> ...


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## frodus (Apr 12, 2008)

my god.... that website made my head hurt.....

ALL of the text on that page is flashing over and over and over.... very hard to read and very badly formatted and organized.


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## brdriver (May 26, 2009)

thanks for your response,later report to relating responsibility leader.
if this product can help you,we'l be pleasure .
let me wait for your reading again deeply!
regards!



frodus said:


> my god.... that website made my head hurt.....
> 
> ALL of the text on that page is flashing over and over and over.... very hard to read and very badly formatted and organized.


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## frodus (Apr 12, 2008)

You guys have controllers.... what about matched motors?


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## brdriver (May 26, 2009)

dear old brother:
if you choose a kind controller from our list,and need matched motor,i will go on help you to looking for the supply.they are special manufacture BLDC motor.and you would know all world 's motor is easy produce,only almost controller.
so ,don't worry my help of complete.

best regards! contact me:MSN:[email protected]



frodus said:


> You guys have controllers.... what about matched motors?


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## frodus (Apr 12, 2008)

I don't understand anything you just said....


English please.....


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## Anaerin (Feb 4, 2009)

frodus said:


> I don't understand anything you just said....
> 
> 
> English please.....


I have a feeling that English isn't this particular poster's first language, and he may be using a machine translation system to post here.


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## tomofreno (Mar 3, 2009)

This seems to have gone afield from the originators question. I’ll try to bring it back with some observations. 
Some people make the blanket statement that AC induction motors/controllers are more efficient than DC series motors/controllers (a comment in this thread is the only time I have seen the opposite stated). I guess due to the added power loss mechanism of brushes/commutator (controllers for both AC and DC seem to have efficiencies in the mid-90's percent). Based on the little data I’ve seen there doesn’t seem to be a lot of difference, and it depends on the motor/controller. For example, the Azure Dynamics AC24LS motor/DMOC445 controller seems to have efficiency similar to that typically given for series DC motors, and higher torque but lower efficiency than the AC24 made by the same company. AC motors seem to have higher efficiency in the higher voltage wye configuration compared to delta config. The Azure Dynamics website gives efficiency curves for their motors, and if I remember correctly the evsource website gives torque-speed and efficiency curves for some series DC motors. At vehicle speeds below 50 mph the input voltage to a series DC motor is much lower than those typically given in torque-speed curves, and efficiency may be significantly different than that given on such curves. Anyone know what efficiency is at lower input voltages? Also, AC motor efficiency decreases with increasing slip resulting from higher load torque. I don’t know what the slip is for the curves on the Azure Dynamics site. 

Brushes on series DC motors do eventually wear out and need to be replaced, but this seems to be a fairly quick and easy operation. Arcing can occur at the brushes/commutator, but this seems to mainly be an issue in high stress applications like racing (major can address the specifics better than I). AC motors seem to be virtually maintenance-free.

Regenerative braking is often stated as an advantage of AC induction motors. The numbers I’ve seen thrown about are 15 to 30% gain in vehicle range for driving with fairly frequent stop/starting. My routes typically don’t have much stop/starting so I don’t see this as a significant advantage. I do see electric braking (motor slowing down the vehicle due to opposing torque when operated as a generator) as a significant advantage in reducing brake wear and providing quicker stopping. If you coast to a stop with a series DC motor, the motor may still be spinning due to flywheel inertia. If you then engage the clutch, the vehicle may jump or buck. Seems this would be avoided with an AC motor and electric braking, but evidently people easily learn to avoid this so it is not much of an issue.

The Metric Mind website touts the “flat torque-speed curve” (torque independent of motor rpm typically below 3000 to 4000 rpm) of AC induction motors as an advantage due to less required gear shifting. It seems that the available torque of series DC motors is limited by most controllers at lower rpm making the available torque-speed curve effectively flat like that for an AC motor. I don’t mind shifting anyway.

Then there is the oft-cited safety issue of mosfet failure in a series DC motor resulting in full pack voltage applied to the motor. These motors have been used by many people in evs, forklifts, and other industrial vehicles for many years and I’ve not read of many such failures. Most mount a breaker within easy reach of the driver to cut power should such an event occur. However, the risk is there.

BLDC motors of larger power are available. There is a link to Chinese manufacturers on this site – check the forums for the thread on Chinese BLDC motors. This type of motor seems to be the most efficient.

I think most people have an opinion on this for reasons they don’t really know. They then stress the appropriate differences to self-justify their opinion. The more time/money/energy they have invested in one technology the more strongly they self-justify, and the stronger their opinion. So what is a trivial difference to one person becomes a very important difference to another. 

I’d be interested to hear Major weigh in with an opinion based on his greater experience/knowledge.

Tom


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## Falcon4 (Oct 11, 2013)

any good update comments for this thread now in 2013?


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## dougingraham (Jul 26, 2011)

Falcon4 said:


> any good update comments for this thread now in 2013?


Things have not changed all that much. There are a few more affordable AC motors from HPEVS. Curtis is now making a higher current controller (650amps) and higher voltage but lower max current controller (144v 500 amps). The bankruptcy of Azure Dynamics made available at more reasonable prices a bunch of Siemens motors and DMOC 645 controllers available via EVTV but those are not quite ready for people to just plug and go although it is getting close.

The AC problem is not motors. It is controllers at a reasonable price. At the moment Curtis is the only game in town for a reasonable AC controller solution and we need them to bump the voltage some more. 200V at 500 amps would help and I think this is almost just a FET swapout.

These problems will vanish in a few years as the numbers of builds increases. I expect the hot rodders will discover magnetic drive and then we will have exactly the opposite problem, too many choices.


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