# Re: [EVDL] AC motor blues



## EVDL List (Jul 27, 2007)

*Re: [EVDL] AC motor blues*



> --- minhd <[email protected]> wrote:
> 
> >
> > Hi,
> ...


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## EVDL List (Jul 27, 2007)

*Re: [EVDL] AC motor blues*

> Hi Minh,
>
> That's the problem. No motor source for the little
> guy. Here is one 
> http://www.sevcon.com/PDFs/AC%20Induction%20Motors%20ACM1005EN_rev1.pdf
>
>
> But I doubt they will deal with you. There is no
> profit in it for companies to market these products to
> individuals. The only way I see for EV hobbyists to
> get an AC drive is thru a guy like Victor.

It does give an idea what to look for in the forklift graveyard! Maybe 
someone can direct Minh to one nearby, or an online resource for them 
(looks like a job for Plasma Boy).

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## EVDL List (Jul 27, 2007)

*Re: [EVDL] AC motor blues*


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## EVDL List (Jul 27, 2007)

*Re: [EVDL] AC motor blues*


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## EVDL List (Jul 27, 2007)

*Re: [EVDL] AC motor blues*

Hello Minh,

Here is a source of a AC drive unit and controller that comes in a match set 
is from http://www.azuredynamics.com/products.htm.

See John and Julie AC Toyota at http://www.evalbum.com/1430

They live about 42 miles from me, which they come to Great Falls, MT every 
day with this EV, no matter what road conditions, weather, temperature, deep 
snow etc.

This high voltage EV will go both ways without any additional charging.

Roland


> > Hi,
> >
> > I would like to use the Curtis AC1238 controller for
> > my vehicle. I am
> > having some problems finding a motor supplier who
> > makes a motor for that
> > controller. I am looking for a 72V 18-25kW AC
> > induction motor. I have
> > been waiting for the HiPerformanceGolfcars guys to
> > make a motors for me for
> > four months now. I still get empty promises.
> >
> > Does anyone know where I can buy an AC induction
> > motor which would work well
> > with the Curtis AC1238? I would love to send my
> > business to someone who is
> > reputable. Any discussions on this topic would be
> > much appreciated!
> >
> > Who makes fork lift and airport transport AC motors?
> >
> > Minh 

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## EVDL List (Jul 27, 2007)

*Re: [EVDL] AC motor blues*



> minhd wrote:
> > Hi,
> >
> > I would like to use the Curtis AC1238 controller for my vehicle. I am
> ...


----------



## EVDL List (Jul 27, 2007)

*Re: [EVDL] AC motor blues*



> minhd wrote:
> 
> > I would like to use the Curtis AC1238 controller for my
> > vehicle. I am having some problems finding a motor supplier
> ...


----------



## EVDL List (Jul 27, 2007)

*Re: [EVDL] AC motor blues*

Exactly right.

All of people who think that connecting any 3 phase motor to
a 3 phase inverter with just comparable power levels is the same
as any DC motor to any DC controller, are misunderstanding the task.

We talk a lot that a DC motor is AC motor inside, which is
quite broad generalization and given specific motor type
can be completely wrong statement. A series wound DC motor
almost everyone uses is close to a synchronous AC motor with
external rotor excitation via and slip rings (rotor being
electromagnet). Principle of its operation is quite different
vs. for instance induction motor.

Since for a DC motor mechanical inverter is integrated into
it and timing is rigidly set by commutator glued onto the shaft
(leave moving brush assembly aside for now), DC controller
doesn't need to know much about motor, its shaft position,
etc. So a mere PWM chopper with enough power will do.

AC motor has commutation done electronically and it's not
part of the motor itself. So inverter must know (in real time) much
more about running motor - slip (for induction), shaft position,
and other parameters for optimal vector or field oriented controls.
Some depend on the particular motor construction - stator
inductance and impedance, gapping, amount of poles, thermal reserve, 
presence/absence of
perm magnets in the rotor, and these parameters usually exist
in the inverter's software as particular motor's "model".
Therefore manufacturer of the inverter absolutely much know
everything about the motor their inverter is going to be
connected to to make it run smoothly, with no signal
reflections, etc. MES makes "self-tuning" inverters adapting
basic motor parameters, but it still assumes it is MES motor.
Not to say it won't work with others, but it's like transplanting
Ford engine into a BMW - it can be made to run, but the outcome
is neither Ford nor BMW vehicle. OEMs don't do that and won't
allow others to improvise like that with their products.
IT is not approved, guaranteed or safe use of their electronics; 
attempting to link up incompatible (by design) units is out
of question. It is virtually guaranteed that requests of this
sort will be just ignored as not serious - no manufacturer
expecting to be supported is going to ask this.

I see such questions quite often - "can I connect Siemens
inverter to a GE 3 phase motor I have".


Short answer is of course yes, you can.

Long one is - yes, you can, provided any outcome is acceptable
by you, including destruction of motor, inverter, or both,
not even mentioning associated hazard and safety issues.

No one knows what the model of that
motor look like to be stored in the inverter memory, so it may or may 
not run. Shaft encoder may or may not be used. If used, signal
characteristics will be different and must be adapted.
No one will support such hobby experiments, let alone
guaranteeing any positive outcome. If you buy to tinker
(which by definition means any outcome is expected and no complains),
that is OK to do, but if you buy for the purpose, expecting things
to work, and complain when they don't. You will expect 
seller/manufacturer to warranty repair/replace if something
goes wrong and something of course will. It's unrealistic
expectation.

Complexity of AC systems is hidden and from the user point of view
construction AC drive is the same or simpler than DC one - fewer 
connections to make (because of more integrated
hardware), etc. Like more complex CPU architecture of modern
processors does not make insertion of one into a motherboard
more complicated. However, this does not imply that manufacturer' 
requirements can be ignored, proper use neglected or improvisation
"authorized" just because manufacturer's promise more durable hardware.

Siemens inverters work forever - first Simoverts installed
in 1992 in VW Golfs stil run them today. But I sure can blow
one up in 3 seconds if connect to a GE motor just because
both happen to be 3 phase induction machines. Or, blow up
ACP induction motor because it has pathetic thermal reserve.
So do you seriously think ACP will sell (and support/replace)
you their motor if you will not buy their inverter too?

Victor
'91 ACRX - something different



> Roger Stockton wrote:
> 
> > Last I heard, Curtis would not sell their AC controller to anyone who
> > was not partnered with a motor manufacturer. It is not a matter of
> ...


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## EVDL List (Jul 27, 2007)

*Re: [EVDL] AC motor blues*

The Azure AC24 motor and DMOC445 looks promising. This should make a 1000 lb
car perform well. I will check on pricing and availabity. With the lower
torque produced by the AC24, it will be more difficult for me to implement a
single stage reduction using a 530 chain. I will have to look check out the
belt drives.

The 6.5" motors offered by ThunderStruck and Greenmotorsports are a bit too
small for freeway capable lightweight BEVs. It looks like they are a great
solution for the motorcycles.

http://www.thunderstruck-ev.com/AC_drive_performance.htm

Has any of you seen the work Ethan Arnolds is up to? He is awesome!!

http://rc-autopilot.de/wiki/index.php/AC_upgrade

I hope he is successful with his motor rewind project. I wish I know how to
read German. The link he posted is very interesting - too bad it is in
German. I would love to understand how to modify the windings of the AC
induction motors. Time to go to the library - so much to learn

Minh





> Roland Wiench wrote:
> >
> > Hello Minh,
> >
> ...


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## EVDL List (Jul 27, 2007)

*Re: [EVDL] AC motor blues*

While this may be true with EV inverters and motors, it is certainly not 
with industrial inverters. I work with inverters and AC motors regularly 
and the inverters work just fine with motors made by competitors. Allen 
Bradley would certainly rather that I used an Allen Bradley motor with their 
inverter, but it will work just fine (with full performance) with the Baldor 
motor we generally use. The inverter has various tuning parameters that 
must be optimized for maximum performance, but the default values will 
generally work with acceptable performance. This is usually without any 
form of rotor position feedback. About the only time I worry about rotor 
position feedback is when the application needs high torque at zero speed. 
High torque at low speed is available without feedback, but to get all the 
way to zero needs rotor position.

The disadvantage of these adaptable inverters is the cost. I am working on 
a project now with a 75 HP motor. Acceleration to full speed should be 
about 2 seconds and stopping using regen will be about 3 seconds. 
Unfortunately the inverter with the resistor to dissipate the energy of 
stopping will be close to US$10,000 for just the components and that does 
not include the motor.

I agree with Victor that it is not the same as hooking up a DC motor to a DC 
controller, but I don't think it is quite as difficult as he says either.

David, wishing he could afford this in an EV


----- Original Message ----- 
From: "Metric Mind" <[email protected]>
To: "Electric Vehicle Discussion List" <[email protected]>
Sent: Saturday, March 01, 2008 4:57 PM
Subject: Re: [EVDL] AC motor blues


> Exactly right.
>
> All of people who think that connecting any 3 phase motor to
> a 3 phase inverter with just comparable power levels is the same
> as any DC motor to any DC controller, are misunderstanding the task.

>snip

> AC motor has commutation done electronically and it's not
> part of the motor itself. So inverter must know (in real time) much
> more about running motor - slip (for induction), shaft position,
> and other parameters for optimal vector or field oriented controls.
> Some depend on the particular motor construction - stator
> inductance and impedance, gapping, amount of poles, thermal reserve,
> presence/absence of
> perm magnets in the rotor, and these parameters usually exist
> in the inverter's software as particular motor's "model".
> Therefore manufacturer of the inverter absolutely much know
> everything about the motor their inverter is going to be
> connected to to make it run smoothly, with no signal
> reflections, etc. MES makes "self-tuning" inverters adapting
>
> snip
>
> Victor
> '91 ACRX - something different
>


> > Roger Stockton wrote:
> >
> >> Last I heard, Curtis would not sell their AC controller to anyone who
> >> was not partnered with a motor manufacturer. It is not a matter of
> ...


----------



## EVDL List (Jul 27, 2007)

*Re: [EVDL] AC motor blues*

<<<< While this may be true with EV inverters and motors, it is certainly not
with industrial inverters. I work with inverters and AC motors regularly
and the inverters work just fine with motors made by competitors. Allen
Bradley would certainly rather that I used an Allen Bradley motor with their
inverter, but it will work just fine (with full performance) with the Baldor
motor we generally use. The inverter has various tuning parameters that
must be optimized for maximum performance, but the default values will
generally work with acceptable performance. This is usually without any
form of rotor position feedback. About the only time I worry about rotor
position feedback is when the application needs high torque at zero speed.
High torque at low speed is available without feedback, but to get all the
way to zero needs rotor position. >>>>

Interesting you should mention this - I have seen a clip on YouTube of 
a first or early drive of a electrified truck, and at the end they 
open the hood, and the engine bay is filled with one big AC motor 
controller, like something you'd see running an assembly line! Don't 
remember seeing it mentioned here and I didn't bookmark it, but it was 
unusual enough for me to remember clearly!

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## EVDL List (Jul 27, 2007)

*Re: [EVDL] AC motor blues*



> David wrote:
> > While this may be true with EV inverters and motors, it is certainly not
> > with industrial inverters.
> 
> ...


----------



## EVDL List (Jul 27, 2007)

*Re: [EVDL] AC motor blues*

>But we're not discussing industrial inverters, are we?

Well, they pretty much work the same way; field oriented control and 6 IGBTs
running AC induction motors (often without rotor position sensor). They are
generally forced air cooled instead of liquid, but that really isn't
relevant as far as the electronics and motor are concerned. The main
electrical difference is that industrial inverters generally have lots of
big electrolytic capacitors to handle the required ripple current when
connected to AC mains - with a battery you don't have that problem. Also,
they are generally not sealed well enough for automotive use and probably
not as resistant to vibration.

>If Bradley and Baldor cooperate to make motors quite similar or share
>characteristics so inverter can be programmed for either one, no problem 
>*for these two*.

Here you probably have a point. These industrial motors, whoever you get
them from, are generally similar. The first industrial AC drive were
designed to run ACIMs that were typically run without a drive (simply
connected to AC power).

However, these drives have auto tuning software that measures the motor
characteristics and tunes itself to the motor reasonably well. This
software might be more complex than the FOC loop itself and for automotive
inverter manufacturers it is not probably worth the effort.

>My point is, you can't buy inverter only from Bradley telling them
>you'll shop for the motor later, but no matter what you find you expect 
>them to support you (sort of blindly guarantee it will work).

You can buy an industrial inverter from pretty much anyone and they won't
even ask you about the motor.
You can expect that they will support it unless you use some _really_
oddball ACIM. Then again, most of the guys doing this industrial automation
work are engineers and have at least some knowledge of what they are doing. 
I see that on EVDL some guys have good electrical knowledge while some guys
have very little.

I think that as time goes buy and the market matures, the motors and
inverters will become more interchangeable.

Fran

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## EVDL List (Jul 27, 2007)

*Re: [EVDL] AC motor blues*

I think the main thing you have to pay attention is the waveform of the motor and the controller. Industrial motors are designed to run off the AC mains, and are designed for a sine wave. Therefore, industrial controllers are designed to output sine wave AC to the motor. When designing a controller to run off batteries, it's easier to just chop the DC to produce a "square" wave (actually trapazoidal, as people here have been saying) by turning it on and off rapidly. The waveform is very rich in odd harmonics, particularly the third and fifth harmonics. Sine wave motors don't like these harmonics and will produce less power and more heat. RV and off grid home inverters (the cheaper ones) produce a "modified sine wave" which is actually the same trapazoidal wave talked about here. Many electronics, air conditioners, etc. don't like these and require a true sine wave inverter. Lights don't care. If you are a manufacturer and know you are going to be running off of pure!
DC, you can save money by designing the controller to chop the DC rather than produce sine waves, and then design the motor (by winding it differently) to be happy with the chopped wave. I think the other parameters would be easy to adjust on most industrial controllers (usually referred to as Variable Frequency Drives) so they could be used with most any sine wave motor. My guess is that the situation would be the same for BLDC controllers working with other brands of BLDC motors, provided the motors are built for the same waveform. Since there's no real standard for the trapazoidal waveforms produced, compatibility will be chancier, but I'd certainly give it a try if that's what I had already or had ready access to.

By the way, although I haven't tried it, I believe a BLDC (trapazoidal wave form) motor will be much happier with a sine wave than the sine wave motor will be with a trapazoidal wave. If I was going to try to run a sine wave motor from a trapazoidal inverter, I would try placing a pi circuit (inductor with a couple of large capacitors) to filter out the higher harmonics. If you are a ham or other radio person, this is a called a low pass
filter. Take the highest frequency your controller will produce and
filter out everything above that. However, this will decrease efficiency and will likely be heavy and expensive for the currents you need. If you have access to cheap or free surplus stuff, it may be worth trying for something lightweight like a bicycle or scooter where currents are more reasonable.

Dave


> From: xxx[email protected]
> To: [email protected]
> Date: Sat, 1 Mar 2008 22:11:58 -0600
> Subject: Re: [EVDL] AC motor blues
> 
> While this may be true with EV inverters and motors, it is certainly not 
> with industrial inverters. I work with inverters and AC motors regularly 
> and the inverters work just fine with motors made by competitors. Allen 
> Bradley would certainly rather that I used an Allen Bradley motor with their 
> inverter, but it will work just fine (with full performance) with the Baldor 
> motor we generally use. The inverter has various tuning parameters that 
> must be optimized for maximum performance, but the default values will 
> generally work with acceptable performance. This is usually without any 
> form of rotor position feedback. About the only time I worry about rotor 
> position feedback is when the application needs high torque at zero speed. 
> High torque at low speed is available without feedback, but to get all the 
> way to zero needs rotor position.
> 
> The disadvantage of these adaptable inverters is the cost. I am working on 
> a project now with a 75 HP motor. Acceleration to full speed should be 
> about 2 seconds and stopping using regen will be about 3 seconds. 
> Unfortunately the inverter with the resistor to dissipate the energy of 
> stopping will be close to US$10,000 for just the components and that does 
> not include the motor.
> 
> I agree with Victor that it is not the same as hooking up a DC motor to a DC 
> controller, but I don't think it is quite as difficult as he says either.
> 
> David, wishing he could afford this in an EV
> 
> 
> ----- Original Message ----- 
> From: "Metric Mind" <[email protected]>
> To: "Electric Vehicle Discussion List" <[email protected]>
> Sent: Saturday, March 01, 2008 4:57 PM
> Subject: Re: [EVDL] AC motor blues
> 
> 
> > Exactly right.
> >
> > All of people who think that connecting any 3 phase motor to
> > a 3 phase inverter with just comparable power levels is the same
> > as any DC motor to any DC controller, are misunderstanding the task.
> 
> >snip
> 
> > AC motor has commutation done electronically and it's not
> > part of the motor itself. So inverter must know (in real time) much
> > more about running motor - slip (for induction), shaft position,
> > and other parameters for optimal vector or field oriented controls.
> > Some depend on the particular motor construction - stator
> > inductance and impedance, gapping, amount of poles, thermal reserve,
> > presence/absence of
> > perm magnets in the rotor, and these parameters usually exist
> > in the inverter's software as particular motor's "model".
> > Therefore manufacturer of the inverter absolutely much know
> > everything about the motor their inverter is going to be
> > connected to to make it run smoothly, with no signal
> > reflections, etc. MES makes "self-tuning" inverters adapting
> >
> > snip
> >
> > Victor
> > '91 ACRX - something different
> >


> > > Roger Stockton wrote:
> > >
> > >> Last I heard, Curtis would not sell their AC controller to anyone who
> > >> was not partnered with a motor manufacturer. It is not a matter of
> ...


----------



## EVDL List (Jul 27, 2007)

*Re: [EVDL] AC motor blues*

>By the way, although I haven't tried it, I believe a BLDC (trapazoidal wave
form) motor will be much happier with a sine >wave than the sine wave motor
will be with a trapazoidal wave.

I have tried it (BLDC with sine excitation) and the result was poor. Note
that you can excite a BLDC motor with sinusoidal _current_ and the result
will be OK. You can also excite a PMSM motor with trapezoidal _current_ and
the result will be OK. But most controllers use PWM to control _voltage_
and have a PI loop looking at the current and adjusting the voltage to
maintain the target current. With the motor turning relatively quickly, the
PI loop is not fast enough to adjust PWM to keep the current where it wants
it if the motor's counter EMF shape is not what it expects it to be.

>If I was going to try to run a sine wave motor from a trapazoidal inverter,
I would try placing a pi circuit
>(inductor with a couple of large capacitors) to filter out the higher
harmonics.

This would not help. It is not the high frequency harmonics that would hurt
you. Remember that with a PMSM motor (one that expects sinusoidal
excitation) is still run by PWM from a DC bus (just like BLDC). The only
difference is that the BLDC controller will hold the PWM duty cycle constant
while the PMSM controller will vary the PWM duty cycle to approximate a sine
wave.

For a PMSM drive, the power electronics section is identical to BLDC. The
only difference is that you need enough processing power (usually a DSP) to
do the math which is more complex than for BLDC. However, once you have a
PMSM controller, you can turn it into a BLDC controller with a trivial
software change.

Fran

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## EVDL List (Jul 27, 2007)

*Re: [EVDL] AC motor blues*

> Date: Sun, 2 Mar 2008 11:06:06 -0800
> From: [email protected]
> To: [email protected]
> Subject: Re: [EVDL] AC motor blues
> 
> > 
> I have tried it (BLDC with sine excitation) and the result was poor. Note
> that you can excite a BLDC motor with sinusoidal _current_ and the result
> will be OK. You can also excite a PMSM motor with trapezoidal _current_ and
> the result will be OK.

Depending on the motor and controller combo, apparently not always OK 

> But most controllers use PWM to control _voltage_
> and have a PI loop looking at the current and adjusting the voltage to
> maintain the target current. With the motor turning relatively quickly, the
> PI loop is not fast enough to adjust PWM to keep the current where it wants
> it if the motor's counter EMF shape is not what it expects it to be.
>

Not the kind of pi filter I'm referring to. I'm an old analog guy.....


> >If I was going to try to run a sine wave motor from a trapazoidal inverter,
> I would try placing a pi circuit
> >(inductor with a couple of large capacitors) to filter out the higher
> harmonics.
> 
> This would not help. It is not the high frequency harmonics that would hurt
> you. Remember that with a PMSM motor (one that expects sinusoidal
> excitation) is still run by PWM from a DC bus (just like BLDC). The only
> difference is that the BLDC controller will hold the PWM duty cycle constant
> while the PMSM controller will vary the PWM duty cycle to approximate a sine
> wave.
> 

No, you're confusing a PWM controller for a brushed DC motor with a BLDC controller (or else, I'm reading your comment wrong). Like the AC controller, the BLDC controller adjusts frequency to control motor speed. The PWM brushed DC controller adjusts duty cycle to control speed, by altering the effective voltage. Yes, the AC and BLDC controllers will adjust voltage as well as frequency, but it is the frequency that controls motor speed. By the way, many large AC motors are not Permanent Magnet motors, but use windings on the rotor, and slip rings if it is a syncronous motor. 

If you're referring to the stepping (amplitude adjustment) the AC controller does to better approximate a sine wave, that is effectively reducing the harmonic component of the wave form. You are effective synthesizing a sine wave by modulating a higher frequency pulse waveform. The closer to a pure sine wave your synthesized wave becomes, the lower the harmonic content.

Understanding this requires stepping into the analog world for a bit. To illustrate what I'm talking about, plot a sine wave. Now, add in the 3rd harmonic, then the 5th, then the 7th. The more odd harmonics you add, the more your composite waveform approaches a square or trapezoidal wave. So even though you're generating the trapezoidal wave by turning a switch on and off, the resulting waveform is actually a sine wave at your fundamental frequency along with strong odd harmonics. 

It is these harmonics that cause heating and vibration in a motor not designed to handle them. The closer you can get to a pure sine wave, whether by using a brute force low pass filter, or by digitally adjusting pulse amplitudes to give a closer approximation to a sine wave, the effect is the same: a much lower odd harmonic component to the resulting waveform.

Dave




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## EVDL List (Jul 27, 2007)

*Re: [EVDL] AC motor blues*

>No, you're confusing a PWM controller for a brushed DC motor with a BLDC
controller (or else, I'm >reading your comment wrong). Like the AC
controller, the BLDC controller adjusts frequency to >control motor speed. 
The PWM brushed DC controller adjusts duty cycle to control speed, by
altering >the effective voltage. Yes, the AC and BLDC controllers will
adjust voltage as well as frequency, but >it is the frequency that controls
motor speed. By the way, many large AC motors are not Permanent >Magnet
motors, but use windings on the rotor, and slip rings if it is a syncronous
motor.

You are a bit confused about two things.

First:
For both BLDC and PMSM you do not adjust the "frequency" to get the motor to
go a certain speed. The only controller that does that is for AC induction
machines and it is called a Volts/Hertz controller.

For both AC induction motors and PMSM you generally use FOC (field oriented
control). Here you track the angular position of the rotor and give it the
correct excitation (in the 3 phases) for that particular rotor angle.

For BLDC you essentially do the same thing but in step of 60 degrees (and
the math is somewhat simpler). Don't think of it as the motor staying in
sync with the controller. Think of it as the controller following the rotor
in exactly the same way the commutator follows the rotor.

Second:
There are two frequencies that are in question here - the frequency that
commutation is performed (controlled by the rotor position) and the
frequency of the PWM that the power transistors switch at. A PMSM (or AC
induction) controller does not output a smooth sinewave such as you get from
the grid. It chops DC at a high frequency (typically several kilohertz) to
approximate the sine wave.

And now:
I think I know what you were trying to get at. At first I though you were
simply trying to filter out the high frequency component (typically several
kilohertz) so that the motor doesn't see it. Now I think you were probably
talking about filtering at a much lower frequency. Suppose you have
commutation at 60 hertz at a given motor speed (same as from the grid). Now
you want to use a LC filter to take out the 3,5,7,etc harmonics from the
square wave (ala Fourier) so you are left with basically a sine wave.

Reasons it won't work.

1. The filter would be physically huge - and it would only work for a
limited frequency range (and therefore limited speed range)

2. The filter would introduce a phase lag from the input to the output
(motor side). The phase lag is the last thing you want. Actually, you want
a phase lead because the motor is itself inductive which means that the
current lags the voltage. For BLDC it is common for the controller to lead
the motor as speed increases.

BTW, next time I have the test system together, I can record a little video
of a BLDC motor being driven with sinusoidal excitation and then
trapezoidal. You can easily hear the difference and see the current draw
difference on the power supply.

Fran

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## EVDL List (Jul 27, 2007)

*Re: [EVDL] AC motor blues*

The PWM duty cycle is used to determine the torque. The PWM frequency
is usually a constant. Other than that, you've got it.

However, I think David was talking about the frequency of the signal
seen by the motor, which is probably less than 400Hz.

In EV AC controllers, you don't control the commutation frequency to
control speed; that would just get you out of sync or use way too much
current as you accelerate. Instead, you control the magnitude of the
motor's current, which controls torque, and you make the frequency and
phase of the motor's current follow the mechanical position.

-Morgan LaMoore



> Jeff Shanab <[email protected]> wrote:
> > Uh, I thought the PWM freq was used to determine the torque to create rotation. That when the rotation occurred, a hall sensor (or current sensing in unused leg) then causes the commutation to the next step.
> 
> _______________________________________________
> ...


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## EVDL List (Jul 27, 2007)

*Re: [EVDL] AC motor blues*



> Morgan LaMoore wrote:
> > The PWM duty cycle is used to determine the torque.
> 
> Indirectly. PWM duty essentially determines current and
> ...


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