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'[EE]: Brushless motors'
2002\08\04@194511 by Sean H. Breheny

face picon face
Hi all,

One of the things I'm working on involves controlling brushless DC motors.
As a first step to understanding them better, I built a three half-H bridge
driver and I am trying to just get one of them to run, driven by my driver
circuit. The motor I'm using is a Maxon EC45 with Hall effect sensors. I
was under the impression that each hall effect sensor output simply told
you what polarity to feed to it's particular winding (ideally, you would be
feeding it with a sine wave but I think that a square wave should be close
enough for a rough test if not for most applications). I.e., all I need to
do with my three half H-bridge circuit is to connect each winding to +
voltage when its hall output is high and to gnd when its hall output is
low. When I do this, the motor runs, but at about 1/4 of the correct RPM
and drawing about 40 times the correct no-load current. I have tried
swapping the sensors around in every combination possible (i.e., use sensor
1 to control winding 2, etc.) and even tried inverting all the hall outputs
and then trying all combinations again. The best I can get is to have it
run at about 1.5 times the correct rpm for my input voltage (!) drawing
about 7 times the correct current.

The motor datasheet does not describe how to do the commutation, and the
info I have found on the web so far seems to vindicate my initial simple
guess.

Any ideas?

Thanks!

Sean

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2002\08\04@203605 by Morgan Olsson

picon face
Never done thnis, just thinking...
One idea might be commutating bipolarily, not on-off.
And plus-off-minus-off-plus etc is for efficiency even better than plus/minus

Like
L1: +++000---000+++000---000
L2: +000---000+++000---000++
L3: 00---000+++000---000+++0

Not seeing it clear though, it is very late here...

/Morgan

Hej Sean H. Breheny. Tack för ditt meddelande 00:42 2002-08-05 enligt nedan:
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2002\08\04@215113 by Sean H. Breheny

face picon face
Hi Morgan,

Thanks for the quick reply. As for bipolarity, I think that this is
completely equivalent to a bipolar drive (just like feeding a brushed motor
with an H bridge and flipping the polarity around). Of course you are
right, it is probably more efficient if you include more points of the sine
wave (you wouldn't really want off, you would want 0 volts, which is
different), but I don't think that a square wave would cause 7 or 40 times
more current.

Sean

At 02:20 AM 8/5/2002 +0100, you wrote:
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2002\08\05@080025 by Morgan Olsson

picon face
Hej Sean H. Breheny. Tack för ditt meddelande 02:47 2002-08-05 enligt nedan:
>Hi Morgan,
>
>Thanks for the quick reply. As for bipolarity, I think that this is
>completely equivalent to a bipolar drive (just like feeding a brushed motor
>with an H bridge and flipping the polarity around). Of course you are
>right, it is probably more efficient if you include more points of the sine
>wave (you wouldn't really want off, you would want 0 volts, which is
>different),

I believe binding to zero only cause power losses.
Not binding it to zero volts will let the magnetic play more freely.  I think freewheeling is better regarding efficiency, heat up, and putput power.
Freewheeling between + and - (and PWM, current sensing, lots of calculations...) is how Variable frequency drives drive 3-phase async motors.

> but I don't think that a square wave would cause 7 or 40 times
>more current.

Right.

Either commutation sequence, levels, or the phasing with sensors to commutation are wrong, or both wrong...


I wrote

>L1: +++000---000+++000---000
>L2: +000---000+++000---000++
>L3: 00---000+++000---000+++0
>
>Not seeing it clear though, it is very late here...

Above is 60° phase shift...
Um, i think it is better to use 120° phase shift ;)
  L1: +++000---000+++000---000
L2: 00---000+++000---000+++0
L3: -000+++000---000+++000--

As a test could you program a pic to output that sequence, and feed the pic with a variable oscillator (for easy stepless adjustment), and the drivers with variable voltage, and see if you can get the motor freerunning fast with low power?  (Not using hall sensors, just see of we can push the rotor correctly)  Then we probably have the correct commutation :)

Next you compare the sensor signals with the transistor drive using a oscilloscope, while running, and revise the PIC code.

/Morgan

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2002\08\05@080233 by Olin Lathrop

face picon face
> One of the things I'm working on involves controlling brushless DC motors.
> As a first step to understanding them better, I built a three half-H
bridge
> driver and I am trying to just get one of them to run, driven by my driver
> circuit. The motor I'm using is a Maxon EC45 with Hall effect sensors. I
> was under the impression that each hall effect sensor output simply told
> you what polarity to feed to it's particular winding (ideally, you would
be
> feeding it with a sine wave but I think that a square wave should be close
> enough for a rough test if not for most applications). I.e., all I need to
> do with my three half H-bridge circuit is to connect each winding to +
> voltage when its hall output is high and to gnd when its hall output is
> low. When I do this, the motor runs, but at about 1/4 of the correct RPM
> and drawing about 40 times the correct no-load current. I have tried
> swapping the sensors around in every combination possible (i.e., use
sensor
> 1 to control winding 2, etc.) and even tried inverting all the hall
outputs
> and then trying all combinations again. The best I can get is to have it
> run at about 1.5 times the correct rpm for my input voltage (!) drawing
> about 7 times the correct current.

The whole subject is too complicated to go into here.  But briefly, here are
some points to keep in mind.

1  -  The motor also works as a generator all the time.  Shorting undriven
windings shorts the generator output, which will act like viscous drag.

2  -  The windings are inductors and can store significant energy.  At the
least you have to make sure this energy doesn't fry something when the
winding is turned off.  A much more efficient scheme is to recycle this
energy.  The best scheme is to transfer the energy in one winding to the
next one in the sequence.  An alternate strategy is to dump the energy back
into the power supply, although this can be trickier than it may seem,
especially while avoiding the viscous drag from #1.

In short, you need to study up on this stuff.  None of it is magic, but
there are lots of issues the novice might overlook, and lots of wrong ways
to do it.


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(978) 742-9014, http://www.embedinc.com

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2002\08\05@110559 by Derek Cowburn

flavicon
face
There have been LOTS of discussions on this subject on the Electric Vehicle
list.  I haven't been there for years now but the topic always intrigued me.

-Derek

> {Original Message removed}

2002\08\05@170315 by Peter L. Peres

picon face
You can replace your control electronics with three opamps, each wired
with gain x 1000 differential input, across the Hall sensor outputs. This
is a tried and working scheme.

To determine the Hall phases etc, you can use the following method:

- supply the hall sensors.
- supply one phase of the motor (I assume it is wye) with dc from a
current limited psu.
The motor will turn and lock on that phase.
- measure Hall outputs and fill in into a table on paper or computer
- feed the next phase. Continue for each phase.

- finally, connect the dvm across one Hall sensor (your choice), and turn
the motor by hand against the energized winding to make sure that the
maximum voltage occurs at the magnetic detent caused by a phase. There are
freak motors that have Hall sensors offset for high speed operation. I
hope that you do not have one of those.

The resulting table will tell you more than you want to know about the
needed controller.

To reduce the current with your simple controller, you need to apply
'magic sine' technology. In this case you should replace the opamp with
two comparators (one for H one for L in each half H bridge) so no current
is driven into a winding when a Hall sensor is not full on (either fwd or
bwd). This may impair self starting capability so you will need to play
with the comparator levels etc. IC brushless controllers do almost
exactly this and more.

Peter

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2002\08\05@170322 by Peter L. Peres

picon face
On Sun, 4 Aug 2002, Sean H. Breheny wrote:

>Hi Morgan,
>
>Thanks for the quick reply. As for bipolarity, I think that this is
>completely equivalent to a bipolar drive (just like feeding a brushed motor
>with an H bridge and flipping the polarity around). Of course you are
>right, it is probably more efficient if you include more points of the sine
>wave (you wouldn't really want off, you would want 0 volts, which is
>different), but I don't think that a square wave would cause 7 or 40 times
>more current.

Unless the original driver uses a chopper and/or a buck regulator to feed
power to the windings, which is highly likely because that's the state of
the art.

Peter

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2002\08\05@171549 by Roman Black

flavicon
face
Hi Sean, first make sure you get any 3-phase wiring
details sorted, like if it soupposed to be star or delta
connected.

Second, brushless motors are current operated devices
(obviously) and you should operate them at the correct
spec current. Like steppers they usually have low-turns
(low ohms) coils, and are meant to be driven with some type
of constant current supply, like a chopper etc. In short,
you need some type of pwm etc on the motor coils to establish
correct running current.

If you don't need too much torque, the motors can usually
be run very efficiently with a tiny pulse on each coil a
correct time after the hall is triggered, ie 2 time delays.
This tiny pulse should give good speed regulation on most
flat type motors (decent flywheel) and use almost no current.
But, it will have lots of torque ripple and with any real
load attached the load will be moved in 3 "jerks" per
rotation.

Most VCR's flatmotors (capstans) have a cheap IC, available
in many styles, that interface direct to the coils, and
the 3 halls, and the chip accepts a train of speed pulses
from a digital input. The chips are sophistacted and drive
the motor well, with the "compensated" sinewave made by
correct PWMing of the coils. These are made to give very
smooth speed and torque with a cheap 3 coil motor. I can get
some chip numbers for you from some of our VCR manuals
if you want to go that route, and chip prices for the more
common chips. :o)

Since you already have the 3-bridge there are some options
for making it work with a PIC, but you haven't said what
this is going to run, which really determines the best way
to drive it. :o)
-Roman



Sean H. Breheny wrote:
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2002\08\05@175730 by Peter L. Peres

picon face
>Second, brushless motors are current operated devices
>(obviously) and you should operate them at the correct
>spec current. Like steppers they usually have low-turns
>(low ohms) coils, and are meant to be driven with some type
>of constant current supply, like a chopper etc. In short,
>you need some type of pwm etc on the motor coils to establish
>correct running current.

This is new to me. Could you indicate a reference for brushless motors
being made for use with constant current supplies ?

Peter

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2002\08\05@190507 by Roman Black

flavicon
face
Peter L. Peres wrote:
>
> >Second, brushless motors are current operated devices
> >(obviously) and you should operate them at the correct
> >spec current. Like steppers they usually have low-turns
> >(low ohms) coils, and are meant to be driven with some type
> >of constant current supply, like a chopper etc. In short,
> >you need some type of pwm etc on the motor coils to establish
> >correct running current.
>
> This is new to me. Could you indicate a reference for brushless motors
> being made for use with constant current supplies ?


Hi Peter, maybe "constant current supply" is a little
too literal, I meant that the motors are very low turns
and attention to CORRECT current needs to be a function
of PWM current limiting etc, ie deliberate, rather than
just connecting a power supply which will cause the
symptoms he saw, of 40x the specified current.

Most of the VCR 3-coil flatmotors I have here have
only a handful of turns on each of the 3 coils, they
are almost short circuit to measure.

So by "constant current supply" I meant a "current-controlled
supply that only allows a fraction of the current that
would flow it it was turned on constantly". ;o)
-Roman

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2002\08\05@203308 by Sean H. Breheny

face picon face
Thanks to all who responded.

I am doing this to learn more about driving brushless motors. This model is
a 30W unit but I may eventually have to do a design for a ~300W motor
(going in an electric powered model helicopter). I may have given people
the wrong impression with my message: I'm not usually the sort of person
who just "tries all combinations" until something works. The only reason
why I did that was because I had such scant info on how the hall outputs
corresponded to the phases that I thought it couldn't hurt to try every
combination as an easy way to start (as long as I didn't let the current
flow long enough to damage anything).

As it turns out, I was given an older copy of the Maxon catalog today and
it has a diagram in it that isn't included in my motor's datasheet, which
shows how their motors are meant to be driven. I must say, Morgan is right,
they apparently do expect you to tristate each phase for part of its cycle
(actually, this would be roughly the same as setting it to half voltage
since the wye neutral point sits around half voltage anyway, and this
happens when the back emf for that phase is near zero).

As for current vs. voltage drive, I haven't heard that either, Roman,
although I suppose it doesn't matter which way you do it as long as you
achieve the same operating point. The maxon motor has about 1.2 ohm from
phase to phase. Of course, the 300W one that I may have to do later has
only a few tens of milliohms.

Tomorrow night or Wednesday I hope to try running it with the sequence from
the catalog and see what happens. I thought it through and I think I can
see why it would help reduce the current greatly (since the way I was doing
it before causes full voltage to be applied even during the point in the
cycle when the back emf is near zero). I'll let you know how it goes.

Thanks!

Sean

At 09:01 AM 8/6/2002 +1000, you wrote:
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2002\08\06@094258 by Morgan Olsson

picon face
Hej Sean H. Breheny. Tack för ditt meddelande 01:29 2002-08-06 enligt nedan:
>Morgan is right, they apparently do expect you to tristate each phase for part of its cycle
>(actually, this would be roughly the same as setting it to half voltage
>since the wye neutral point sits around half voltage anyway, and this
>happens when the back emf for that phase is near zero).

I don´t think it is the same at all.  "Tristateing" Freewheeling it as i say, i mean shut of both transistors and let the inductance shoot current through the freewheeling diodes you should have across the transistors, regaining some current to the power supply / battery / other pahse drivers / other electronic devices.

I think holding it to zero will only turn that energy to heat in the motor, plus occasionally give a negative momentum.

Also: Driving all phases strong, will probably induce lots of corrent in the windings.  The system is "overdefined" if driving more than two pases concurrently.  Ot works if you drive each with sinus 120° apart.

I once put a scopte to a large brushelss fan, and what i saw was:
When the transistor shut off, the voltage wol go to opposite state, making current flow in the other transistors parrallel diode for a short while, then it will fall back to an EMF induced voltage (with some ringing)

Hum... with an A/D and some clever algorithm this could be used to drive the motor without the hall sensors...?

/Morgan

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2002\08\06@100036 by Sean H. Breheny

face picon face
Hi Morgan,

I will have to think about this more when I get a chance, but essentially this is what I am thinking:

You could drive one of these motors with three phase AC and it would work well (as far as I know). So, therefore, our job is essentially to approximate three sinusoidal signals, each 120 deg later in phase than the last, with our three half H-bridges. A sine wave has no freewheeling period, it always presents a rather low (zero, ideally) impedance to each motor winding, but with a variable voltage. However, there comes a point in the cycle (actually twice per cycle per phase) where the voltage of the sine wave matches the back EMF plus inductive voltage of each phase and no current flows in that phase. At this point in the cycle, it would not matter if you disconnected that phase for a few degrees. Since our half H-bridges can only produce high, low, or tristate output, we can approximate a sine wave with just a square wave (high for 180 deg, low for 180 deg) but we can do it a lot better with a three state wave (high for 120 deg, tristate for 60 deg, low for 120 deg, tristate for another 60 deg).

Is this wrong somehow?

By the way, you are exactly right about using the back EMF to provide feedback rather than the hall effect sensors. Many controllers do that.

Sean

At 02:14 PM 8/6/2002 +0100, you wrote:
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2002\08\06@113047 by Morgan Olsson

picon face
Seem OK.
Tweak: For light load high efficiency decrease on-period; for high torque increase on-period.  
Sine driving do not need freewheeling since the voltages are following the EMF

I believe it´s possible to approach sine using PWM, and make sure not driving all three phases at the same time.   Hmmm also depends on Y or D winding...  Hmmm

>By the way, you are exactly right about using the back EMF to provide feedback rather than the hall effect sensors. Many controllers do that.

Must be complicated if using PWM at the same time.  Current sensing each phase too?  Think Variable Frequency drives do so.

/Morgan

>Hej Sean H. Breheny. Tack för ditt meddelande 14:55 2002-08-06 enligt nedan:
>Hi Morgan,
>
>I will have to think about this more when I get a chance, but essentially this is what I am thinking:
>
>You could drive one of these motors with three phase AC and it would work well (as far as I know). So, therefore, our job is essentially to approximate three sinusoidal signals, each 120 deg later in phase than the last, with our three half H-bridges. A sine wave has no freewheeling period, it always presents a rather low (zero, ideally) impedance to each motor winding, but with a variable voltage. However, there comes a point in the cycle (actually twice per cycle per phase) where the voltage of the sine wave matches the back EMF plus inductive voltage of each phase and no current flows in that phase. At this point in the cycle, it would not matter if you disconnected that phase for a few degrees. Since our half H-bridges can only produce high, low, or tristate output, we can approximate a sine wave with just a square wave (high for 180 deg, low for 180 deg) but we can do it a lot better with a three state wave (high for 120 deg, tristate for 60 deg, low for 120 deg, tristate for another 60 deg).
>
>Is this wrong somehow?

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2002\08\06@115402 by Katinka Mills

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face
On Wed, 7 Aug 2002 00:35, you wrote:

> Must be complicated if using PWM at the same time.  Current sensing each
> phase too?  Think Variable Frequency drives do so.
>
> /Morgan


Hi all,

I will have a URL I hope soon to post for a PhD thesis on a motor controller
designed for 3phase brushless DC motor control, IIRC the paper discused a
design for a 120Vdc system and has been used in a number of solar cars. I am
hoping to improve on it using an AVR (ATMega128) as it does high speed PWM.

Will pass URL on when it arrives :o)

Regards,

Kat.



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2002\08\06@121117 by Katinka Mills

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On Tue, 6 Aug 2002 23:57, you wrote:
{Quote hidden}

Here is a search result URL for the info, not sure which link contains the
thesis, but the few things I have picked out like the controller chip from
motorola (MC330033) looks good.

http://www.google.com/u/uqitee?q=motor%20controller

Regards,

Kat.

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2002\08\06@123141 by Katinka Mills

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On Wed, 7 Aug 2002 00:14, you wrote:
{Quote hidden}

Us EE's have nothing better to do than converse at 00:34, here is the URL I
was looking for :

http://celab21.pc.elec.uq.edu.au/~beef/thesis2.PDF


Regards,

Kat.

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2002\08\06@135817 by Peter L. Peres

picon face
On Tue, 6 Aug 2002, Roman Black wrote:

>Hi Peter, maybe "constant current supply" is a little
>too literal, I meant that the motors are very low turns
>and attention to CORRECT current needs to be a function
>of PWM current limiting etc, ie deliberate, rather than
>just connecting a power supply which will cause the
>symptoms he saw, of 40x the specified current.
>
>Most of the VCR 3-coil flatmotors I have here have
>only a handful of turns on each of the 3 coils, they
>are almost short circuit to measure.

Yes but they also have a buck regulator built in or somewhere out of sight
that supplies them with voltage on demand (under servo control).

>So by "constant current supply" I meant a "current-controlled
>supply that only allows a fraction of the current that
>would flow it it was turned on constantly". ;o)

Ok, I just through I'd remind that a brushless PM motor behaves like its
brushed counterpart and the voltage sets the RPM and the current sets the
torque. So current limiting is what you want to prevent shorting the
supply, the control is done in voltage (excepting where torque control is
required).

Peter

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2002\08\06@191444 by Morgan Olsson

picon face
Hej Katinka Mills. Tack för ditt meddelande 17:35 2002-08-06 enligt nedan:

>> Here is a search result URL for the info, not sure which link contains the
>> thesis, but the few things I have picked out like the controller chip from
>> motorola (MC330033) looks good.
>>
>> http://www.google.com/u/uqitee?q=motor%20controller

Interesting thingy :)

One thing i noted, was that is is possible to configure for 2-phase motors.

One customer have need for low rpm high torque low inertia compact driver, so a conventional 200 step/rev stepper and this unit.

Now... problem is mounting the hall sensors that close and inside... impossible!

But... maybe an optical decoder on the shaft... if i can get hands on one with suitable resolution and phase angle.

(I suggested an air motor with planet gear - whis is probably much less expensive than paying me to play with this, but...)

>Us EE's have nothing better to do than converse at 00:34,

I know... 01:10 here...
> here is the URL I
>was looking for :
>
> http://celab21.pc.elec.uq.edu.au/~beef/thesis2.PDF

Later... Thank you :)

/Morgan

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2002\08\07@062655 by Roman Black

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Morgan Olsson wrote:

> >> Here is a search result URL for the info, not sure which link contains the
> >> thesis, but the few things I have picked out like the controller chip from
> >> motorola (MC330033) looks good.
> >>
> >> http://www.google.com/u/uqitee?q=motor%20controller
>
> Interesting thingy :)
>
> One thing i noted, was that is is possible to configure for 2-phase motors.
>
> One customer have need for low rpm high torque low inertia compact driver, so a conventional 200 step/rev stepper and this unit.
>
> Now... problem is mounting the hall sensors that close and inside... impossible!
> But... maybe an optical decoder on the shaft... if i can get hands on one with suitable resolution and phase angle.


A 200 step motor has 50 magnetic poles so a quadrature
encoder with 25, 50 or 100 slots would do it.
The hard part would be adjusting the angle of the encoder
disc relative to pole angle, this is critical for decent
power/efficiency if you are going to use it for simple
commutation. With only 1.8 degrees for a quarter pole,
the encoder would have to be adjusted *finely* within a
short range within that 1.8 degrees. :o)

Surely using a micro to control stepper motor current and
phasing is going to give better control of speed and
probably better torque than commutation anyway??
-Roman

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2002\08\07@124249 by Sean H. Breheny

face picon face
Thanks again to all who responded, and thanks to Kat for the interesting link.

Just as I had suspected (and as Morgan had recommended in the first place)
splitting the commutation among high, low, and open (tristate) did the
trick. My driver circuit now runs the motor almost dead on according to the
specs (12v applied results in 250mA at 4500 rpm, the specs say it should
draw exactly that amount of current and run at 4400 rpm).

What a great resource the piclist is!

Sean

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2002\08\07@171108 by Morgan Olsson

picon face
Sean H. Breheny wrote:
>Thanks again to all who responded, and thanks to Kat for the interesting link.
>
>Just as I had suspected (and as Morgan had recommended in the first place)
>splitting the commutation among high, low, and open (tristate) did the
>trick. My driver circuit now runs the motor almost dead on according to the
>specs (12v applied results in 250mA at 4500 rpm, the specs say it should
>draw exactly that amount of current and run at 4400 rpm).

WOW!
You probably have better drivers than they :)

Have you tested torque?

Simple method: attach a cylinder to the axis of known radius.  Mount he motor so axis is horizontal.
Then wind a wire (for larger motors a leather rope) some turns (seven use to be recommended) non-overlapping.
In one end of the wire hang a known weight so you get a known force.
Secure the other end on something nonmoving.
When cylinder is rotating the right way it wil try to lift the weight, and due to the many turns the force in the other end if the wire is negligible.


          _ secured wire end
          |
[MOTOR]-[///] Cylinder with wire wound around
        |
        |
       ###  Weight

Torque is simply radious times force of weight

Instead of weight you could use a weighing scale arranged some intelligent way.  I used a letter scale once, placed above engine and wire attached to letter plate lifting the wire)



...

Once upon a time i was thinking about converting an car engine generator to motor to drive a small electric vehicle.

Theese are three phase synchronous generators with electromagnet rotor.
-We then have the extra benefit of varying the magnet force: High for high torque; low for high speed (less back-EMF = no need for extreme voltages, better optimizing phase drivers for one voltage in th emiddle, and power supply can be direct battery)
(and of course it works nicely as electric generating brake)


>What a great resource the piclist is!

Agree

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'[EE]: Brushless motors'
2002\09\07@213803 by Sean H. Breheny
face picon face
Hi all,

As a part of my research into small autonomous flying vehicles, I have to
simulate how brushless motors work with PWM. In other words, I need to
write a simulator that can tell me, given the motor parameters and input
voltage, etc., what speed it will run at with a given torque load and PWM
duty cycle. I'm working with the type of hobby brushless motors typically
used with prop-driven fixed wing model aircraft.

I have been reading a book on brushless motors and motor drives and it
suggests that brushless DC motors act almost identically to brushed DC
motors in that they have a linear torque-speed curve and torque is simply
proportional to current, etc. However, it doesn't address (at least not in
very clear terms) what differences might exist in how they respond to PWM
drive. For example, I usually model a brushed motor as a generator in
series with an inductor and a resistor for the purpose of determining how
it will respond to PWM. Is this a good model for brushless motors, too? It
would seem to me that the fact that the commutation uses three phases would
affect the way the inductance changes the current waveforms.

In fact, this book that I'm reading (and all those that I have found so
far) seem to be geared toward industrial motors since it talks about doing
current regulation with the PWM, which I don't think is typically done in
the little controllers for brushless motors in RC planes.

For that matter, I never understood what happens in a brushed motor when
the commutation occurs. There must be quite an inductive voltage spike when
the winding is momentarily disconnected, right?

Why am I so concerned? Because we are using these to make an electric
powered helicopter and as such are trying to eek every ounce of efficiency
out of the system that we can. We need to answer questions like: is it
better to run with a few percent more voltage and few percent lower PWM
duty cycle or the other way around? (This question arises because of the
discharge curve of NiMH batteries not being completely flat, so we want to
know how to make best use of the higher voltage during the early part of
the curve).

Thanks for any help you can give,

Sean

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2002\09\08@184756 by Sam Littlewood

flavicon
face
The speed of a brushless motor is directly controlled by the frequency of
the three phases generated by the driving electronics - PWM will be used to
control the torque. How the contoller relates the two things is up to the
software, eg: most flyers, when using brushless motors in model helis, run
the controller in a constant RPM mode - the controller applies torque via
PWM as necessary to maintain head speed.

If you are looking at squeezing the last few percent, then I think you will
have to get physical - it will be all the little things that get left off
the whiteboard that will be biting. You can use a brushed DC motor as a dyno
(along with a control system to maintain constant current etc.). However, as
you have a single application in mind, my first reaction  would be to rig a
rotor head w/ a balance outside ground effect, and make measurements - I
suspect there will be much richer performance picking in such things as
blade profile/planform, head speed, low disk loading etc.

Brushed motors do put out horrific voltage spikes - and happily smoke the
attached control electronics on a regular basis!

What is you target takeoff weight?

SamL

PS: Also, selection and maintance of you cells will make a huge difference -
discharging them at 20A or more is _slightly_ outside spec :-)


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