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

22K views 30 replies 11 participants last post by  dougingraham 
#1 ·
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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#3 · (Edited)
-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
 
#4 ·
-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
 
#11 ·
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.
 
#14 ·
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).
 
#16 · (Edited)
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
 
#26 · (Edited)
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:szbangrong@hotmail.com
 
#29 ·
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
 
#31 ·
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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