# [EVDL] permanent magnet AC synchronous motors from China



## EVDL List (Jul 27, 2007)

>I believe permanent magnet AC aka synchronous AC is the same as 
>brushless DC. but not entirely sure

Very similar but not quite the same. Brushless DC generally has trapezoidal
counter EMF while PMSM (permanent magnet synchronous motor) has sinusoidal
counter EMF. Also, for good starting torque you will need some kind of
rotor position sensor. PMSM will run on a BLDC controller (and vice versa),
but not well.

Fran

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## EVDL List (Jul 27, 2007)

fsabolich skrev:
>> I believe permanent magnet AC aka synchronous AC is the same as 
>> brushless DC. but not entirely sure
>> 
>
> Very similar but not quite the same. Brushless DC generally has trapezoidal
> counter EMF while PMSM (permanent magnet synchronous motor) has sinusoidal
> counter EMF. Also, for good starting torque you will need some kind of
> rotor position sensor. PMSM will run on a BLDC controller (and vice versa),
> but not well.
> 

what about the design of the motors makes that difference?

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## EVDL List (Jul 27, 2007)

On Thu, Feb 28, 2008 at 6:38 AM, Dan Frederiksen


> <[email protected]> wrote:
> > what about the design of the motors makes that difference?
> 
> It's the way the coils are wound: the winding density of the coils vs.
> ...


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## EVDL List (Jul 27, 2007)

> Morgan LaMoore wrote:
> > [email protected]x.xxx> wrote:
> >
> >> what about the design of the motors makes that difference?
> ...


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## EVDL List (Jul 27, 2007)

The optimal waveform for an AC motor's coils depends on both how the 
windings are positioned, and how the iron is shaped. The easiest way to 
see the optimal waveform for a given motor is to spin it as a generator, 
and look at the waveform it generates.

Let's say you have a 2-pole motor. As you go in a circle around the air 
gap between rotor and stator, the magnetic field produced by the rotor 
magnets is all North poles for half of the circle, and all South poles 
for the other half. If we "unwrap" these poles and lay them 
horizontally, you get something like

NNN...SSS... (one complete revolution)

As they rotate, these magnetic poles pass each wire in the stator once 
per revolution. Passing the North pole raises the voltage; passing the 
South pole lowers the voltage. Passing the gaps leaves the voltage constant.

If the strength of the magnetic field is the same across the pole faces, 
and the width of the gaps between poles is the same as the with of the 
poles themselves, then you get this kind of waveform.
___
/ \ /
/ \ /
/ \___/

This is called a trapezoidal waveform.

To make it more square, you make the poles very narrow and the gaps 
between the very wide. This waveform is easy for the electronics to 
produce, but hard on the motor because all the magnetic flux is 
concentrated into these narrow pole faces. Like trying to have very high 
current in small wires, it increases losses.

N.....S.....
_____
| | |
| | |
| |_____|

If you want a sinewave, you need to taper the poles so the field is 
stronger in the center and gradually tapers off at the edges. This is 
good for the motor, but harder to handle with the electronics (lowers 
controller efficiency).

nNn...sSs...
___
/ \ /
| | |
/ \___/

You can also shape the waveform by not winding the coils in one thick 
bundle, but instead by slotting the pole pieces and spreading the wires 
out between them.

-- 
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in -- Leonard Cohen
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net

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## EVDL List (Jul 27, 2007)

>If you want a sinewave, you need to taper the poles so the field is 
>stronger in the center and gradually tapers off at the edges. This is 
>good for the motor, but harder to handle with the electronics (lowers 
>controller efficiency).

I understand that this is more complex than BLDC (6 step commutation), but I
fail to see how it lowers controller efficiency. If you accurately track
the rotor position and adjust the PWM accordingly, how is efficiency lower
than with BLDC?

Fran


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## EVDL List (Jul 27, 2007)

> fsabolich wrote:
> >> If you want a sinewave, you need to taper the poles so the field is
> >> stronger in the center and gradually tapers off at the edges. This is
> >> good for the motor, but harder to handle with the electronics (lowers
> ...


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## EVDL List (Jul 27, 2007)

>If the controller only needs to make square waves, the transistors only 
>switch on/off once per cycle. At 60 Hz, they switch on/off 60 times a 
>second. Switching losses are thus very low.

Well yes, but this is a corner case that is valid for full speed only. If
running at let's say 65% speed it makes no difference if you keep PWM
constant at 65% or vary PWM to fit a sinewave.

>If the controller needs to create sine waves, the transistors have to 
>switch on/off dozens or even hundreds of times per cycle. To synthesize 
>a 60 Hz sinewave can require switching on/off 6000 times per second. 
>Switching losses get larger, so controller efficiency falls.

With actual measured data from my first controller prototype - switching
losses at 12kHz modulation were about 7W at 70V using 100V 220A MOSFETs (ST
Micro STE250NS10). So even if you are relatively conservative with the
controller (rate it 70V 100A for 7kW), those kind of losses compared for 7kW
are a drop in the bucket.

Of course this is with MOSFETs with are very good high speed switches and
have low switching losses provided that you get the gate drive circuit
right. IGBTs are a different beast because of tail current.

But anyways, since with BLDC you can go to 100% duty PWM (thereby avoiding
high frequency switching) only at full speed, I consider it a non issue.

Fran

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