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'[EE]: Driving DC motors from higher voltage?'
2003\06\13@192957 by Chris Emerson

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face
Hello,

I want to occasionally control a ~1.5V DC motor from a PIC circuit
running from 3V.  The speed isn't critical.  Power consumption, both
while the motor is running and when not, is important (it's battery
powered, and this will be where the bulk of the energy goes), and space
could be important later.  It's a one-off hobby project, so I'm not
worried about saving every last penny in components.

It seems like overkill including a full SMPS.  Would it work to feed a
pulsed 3V to the motor through a transistor at a suitable (possibly
empirically determined) duty?  I'm a bit worried about (a) ruining the
motor, and (b) wasting lots of current in the protection diode during
the off periods.

I'm intending to use a 12F629 or 675 which only has one spare pin,
although it's possible I could free up another for a comparator/ADC.

I'd be grateful for any ideas!

Cheers,

Chris

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2003\06\13@200352 by Herbert Graf

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> I want to occasionally control a ~1.5V DC motor from a PIC circuit
> running from 3V.  The speed isn't critical.  Power consumption, both
> while the motor is running and when not, is important (it's battery
> powered, and this will be where the bulk of the energy goes), and space
> could be important later.  It's a one-off hobby project, so I'm not
> worried about saving every last penny in components.
>
> It seems like overkill including a full SMPS.  Would it work to feed a
> pulsed 3V to the motor through a transistor at a suitable (possibly
> empirically determined) duty?  I'm a bit worried about (a) ruining the
> motor, and (b) wasting lots of current in the protection diode during
> the off periods.

       Well, it all depends one what you want to do. You could simply drive the
motor with a 50% duty cycle waveform and probably get an average pretty
close to 1.5V. The one great thing about motors is that they have a very
high inductance, which means they filter incoming pulsed waveforms quite
well. The frequency of operation isn't too critical, I'd say anywhere from
400Hz to maybe 2000Hz would PROBABLY be OK. Going lower might result in some
vibration, going higher will get into efficiency issues. If audible noise is
a concern of yours you may have to add more filtering elements, a cap will
likely be enough, something more elaborate may be required in some
situations though. How "beefy" a motor are we talking about? TTYL

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2003\06\14@052627 by Chris Emerson

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Thanks for the quick response!

On Fri, Jun 13, 2003 at 08:02:30PM -0400, Herbert Graf wrote:
> > I want to occasionally control a ~1.5V DC motor from a PIC circuit
> > running from 3V.  The speed isn't critical.  Power consumption, both
> > while the motor is running and when not, is important (it's battery
> > powered, and this will be where the bulk of the energy goes), and space
> > could be important later.  It's a one-off hobby project, so I'm not
> > worried about saving every last penny in components.
[snip]
>
>     Well, it all depends one what you want to do. You could simply drive the
> motor with a 50% duty cycle waveform and probably get an average pretty
> close to 1.5V. The one great thing about motors is that they have a very
> high inductance, which means they filter incoming pulsed waveforms quite
> well. The frequency of operation isn't too critical, I'd say anywhere from
> 400Hz to maybe 2000Hz would PROBABLY be OK.

Ok, I'll have to experiment with different frequencies and duties.  An
excuse for another project - I keep thinking that a PWM generator would
be useful.  :-)

> Going lower might result in some vibration, going higher will get into
> efficiency issues.

What are the efficiency issues?

> If audible noise is a concern of yours you may have to add more
> filtering elements, a cap will likely be enough, something more
> elaborate may be required in some situations though. How "beefy" a
> motor are we talking about? TTYL

It's a small pager motor, taking about 10mA when connected straight to a
single AA battery, so not very beefy.

Thanks,

Chris

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2003\06\14@071240 by Herbert Graf

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> >     Well, it all depends one what you want to do. You could
> simply drive the
> > motor with a 50% duty cycle waveform and probably get an average pretty
> > close to 1.5V. The one great thing about motors is that they have a very
> > high inductance, which means they filter incoming pulsed waveforms quite
> > well. The frequency of operation isn't too critical, I'd say
> anywhere from
> > 400Hz to maybe 2000Hz would PROBABLY be OK.
>
> Ok, I'll have to experiment with different frequencies and duties.  An
> excuse for another project - I keep thinking that a PWM generator would
> be useful.  :-)
>
> > Going lower might result in some vibration, going higher will get into
> > efficiency issues.
>
> What are the efficiency issues?

       With any switching scheme a concern is the amount of time a switching
device is in the "process" of switching versus the whole cycle time. When a
device is "switching", power is being wasted (through a variety of
mechanisms that depends on the device used and therefore I won't get in to),
so you want to limit the ratio between the time "in switching" and the total
time, per cycle. What is a "good" number depends on you, obviously there are
diminishing returns on this sort of thing, but by keeping the switching rate
lower you minimize this loss. The frequencies I quoted are usually, with
today's devices, perfectly fine.

> > If audible noise is a concern of yours you may have to add more
> > filtering elements, a cap will likely be enough, something more
> > elaborate may be required in some situations though. How "beefy" a
> > motor are we talking about? TTYL
>
> It's a small pager motor, taking about 10mA when connected straight to a
> single AA battery, so not very beefy.

       Ahh, kinda figured that's what you're up to, in that case the switching
noise wouldn't be a problem (most likely), experiment with the frequency and
see "how low you can go"! :) TTYL

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2003\06\14@082900 by Spehro Pefhany

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At 10:16 AM 6/14/2003 +0100, you wrote:


>It's a small pager motor, taking about 10mA when connected straight to a
>single AA battery, so not very beefy.

I suggest a bipolar half-bridge drive at that voltage and current, with
perhaps 500uA of base current on each transistor. You don't need a
flyback diode; the transistor will give you much higher efficiency, and
since it conducts roughly half the time, the drop is important in that
respect.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speffspamKILLspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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2003\06\14@083525 by Olin Lathrop

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> It's a small pager motor, taking about 10mA when connected straight to a
> single AA battery, so not very beefy.

In that case you can drive it directly from the RA4 open drain output with
50% duty cycle software PWM.  Just don't forget the diode from RA4 to Vdd.
Schottky might be appropriate here.


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2003\06\14@112952 by Sean H. Breheny

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It isn't very relevant to running a little tiny 10mA motor but IMHO, it is
a mistake to assume that 400 to 2000Hz is a good typical PWM frequency.

(WARNING: here comes a long explanation :-)

There are two regimes or modes that PWM can operate in: low frequency and
high frequency.

Which one you are operating in depends on whether your PWM period is
significantly higher or lower than the motor's inductive-resistive (RL)
time constant, which is L/R (so it is determined by whether your frequency
is much larger or smaller than R/L). If your switching element and wiring
has significant resistance, you need to include this in the R here.

If the low frequency region, you usually connect the motor to the power
supply during the "ON" time and leave it open circuited (with a flyback
diode) during the "OFF" period. The current waveform in the motor then
looks like a constant current during the ON time and the current very
quickly decays to zero at the beginning of the OFF period and remains zero
for the majority of the OFF time.

This results in torque pulses being applied to the motor (since torque is
proportional to current). You rely on the mechanical time constant of the
system to smooth out these torque pulses into steady motion.

In this region of operation, your main concern is that you do not want so
low a PWM frequency that you cause mechanical vibration and you don't want
so high a frequency that it becomes very audible and annoying.

The average motor current equals the average battery current and the
applied torque (at a fixed motor RPM) is a linear function of the PWM duty
percent.

The motor output power is average current times back EMF (motor open
circuit voltage as it spins and acts like a generator). System input
power is average current times battery voltage. Roughly, the motor speed
and therefore EMF will go up linearly with duty percent, so you can have a
very inefficient situation in this PWM region. Consider a situation where
the motor EMF is only 1 volt and your supply voltage is 10 volts (PWM duty
of about 10%). This means that your system will only be 10% efficient. The
extra power is lost in I^2*R losses in the motor winding resistance, which
is NOT Iavg^2*R but is actually the average(I^2)*R, which is much higher
since your current consists of very large pulses of short duration.

The other mode of PWM operation is high speed mode, where your PWM period
is so short that the RL time constant tends to keep the motor current
constant during each PWM cycle.  Here, you connect the motor to the supply
during the ON time but SHORT it during the OFF time.

This rubs a lot of people the wrong way because they realize that normally,
shorting a motor causes breaking (the back EMF acts to put current through
in the direction that causes torque opposite the direction of rotation).
However, what they don't realize is that when you suddenly short a motor,
you actually get positive torque for a very short period of time (due to
the stored energy in the inductance being used very efficiently to drive
the motor) followed by breaking. If you are using a high enough PWM
frequency, then the breaking never happens (unless you are at a lower PWM
duty than you should be for the current speed and load torque).

In the high frequency mode, the motor and PWM switch are being used like a
little buck SMPS (switching power supply) to efficiently convert from the
high supply voltage down to the lower back EMF of the motor. This happens
because the inductor forces a constant current, and so has a high voltage
across it when the supply is connected to the motor. This high voltage
bucks the supply and prevents the current from increasing. Since voltage
times current equals power, this represents power going into the inductor
during the on time, Then, during the off time, the inductor acts like a
little power supply most of that power back to the motor.

The net result is that there is very little current ripple in the motor so
that things are efficient (Iavg^2 is about equal to average(I^2)) and also
the applied torque is very uniform.

For typical motors, we will be taking about somewhere around 10kHz as a
typical high frequency PWM, although plenty of motors need 30 or even 60kHz
drive to get well into this region (you usually want less than 30% current
ripple for high efficiency). This is especially true of very high
performance motors for electric flight, for example, which have low inductance.

Another plus of this method is that you get the option of breaking as a
free byproduct. If you set the PWM duty to zero, you get full breaking and
anything lower than the equilibrium point that sustains your speed will be
breaking to some extent. This is different than the low-speed method where
you need a separate switching element to do breaking (or at least a
separate control signal). Setting the PWM duty to a lower value than the
equilibrium in low speed operation just causes the motor to slow down due
to the attached load, which might be very light and cause a slow speed
change. This can be a problem for feedback control systems because your
ability to apply torque is highly asymmetrical (you can apply high positive
torque but almost no negative torque).

The down-side of the high frequency method is that you need to design your
switching circuit very well so that it can switch efficiently at these high
frequencies. As Herbert said, you need to minimize the time spent in
transition. You also still need a flyback diode because you need to prevent
voltage spikes during each transition (voltage spikes are due to the
inductor trying to enforce constant current through a circuit with high
resistance, such as when both switching elements are open).

Average supply current is now duty percent times motor winding current. In
both PWM methods, if your battery or supply has a significant internal
resistance, you often want to bypass it with large capacitors (that can
handle the ripple current) so that your supply voltage doesn't go up and
down with each current pulse. Even though the high frequency method causes
constant current in the motor, it still draws pulses from the supply.

Another minor downside is that the equations become slightly more
complicated. Current is now a nonlinear function of duty percent and you
can get situations where peak motor torque is achieved at a PWM duty
percent below 100% and actually drops off above that. In extreme cases
(which only occur when your supply resistance is very high), you might
actually need to take this into account to achieve maximum torque during
acceleration. In other words, a higher PWM duty will always result in a
higher final speed, but just pegging it at 100% and then dropping it down
to the equilibrium PWM duty for your new speed, as you would assume would
give the fastest response, might actually be slower than, say, going to
80%, sliding up to 90% during the acceleration, and then bringing it down
to the new duty.

When you bypass your power supply with capacitors such that it sees a
constant current, then you get:

Imotor = d*Vb/(d^2*Rw+Rm)
Ibatt=d*Imotor

where Imotor is motor winding current, Vb is battery (or supply) open
circuit voltage, Rw is the resistance of the battery (or supply) and all
the wiring connecting it to the circuit. Rm is the resistance of the motor
plus switching element plus any wire between them.

Note that this becomes the simple linear formula when Rw is much smaller
than Rm (as might be the case with NiMH or NiCd batteries).

I just went through all of this with a high performance brushless motor
controller design and I managed to get somewhere around 90% efficiency from
battery to load even at 50% duty. It sure took a while to figure it all
out, though! It is also amazing how few people seem to know this, although
I did find one site that explained it well.

Sean






At 07:11 AM 6/14/2003 -0400, you wrote:
> > >     Well, it all depends one what you want to do. You could
> > simply drive the
> > > motor with a 50% duty cycle waveform and probably get an average pretty
> > > close to 1.5V. The one great thing about motors is that they have a very
> > > high inductance, which means they filter incoming pulsed waveforms quite
> > > well. The frequency of operation isn't too critical, I'd say
> > anywhere from
> > > 400Hz to maybe 2000Hz would PROBABLY be OK.
> >

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2003\06\14@114808 by Sean H. Breheny

face picon face
Quick correction to my last post. The formula should have been:

Imotor = (d*Vb-Vemf)/(d^2*Rw+Rm)
Ibatt=d*Imotor

where Imotor is motor winding current, Vb is battery (or supply) open
circuit voltage, Rw is the resistance of the battery (or supply) and all
the wiring connecting it to the circuit. Rm is the resistance of the motor
plus switching element plus any wire between them.

Vemf is the motor back EMF. This is what I left out.

Sean

At 07:11 AM 6/14/2003 -0400, you wrote:
{Quote hidden}

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2003\06\14@115425 by Bob Axtell

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Extremely well written.

All I was going to say was that the PWM method of driving a low voltage
motor at high voltages allows significantly better torque than normal
voltages. Sometimes that can be a real plus in the application, such as
when driving a motor who is designed to vibrate with an eccentic cam; they
are easier to startup.

--Bob

At 11:29 AM 6/14/2003 -0400, you wrote:
{Quote hidden}

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2003\06\14@221238 by Herbert Graf

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> It isn't very relevant to running a little tiny 10mA motor but IMHO, it is
> a mistake to assume that 400 to 2000Hz is a good typical PWM frequency.

       Agreed, which is why that recommendation was for THAT PARTICULAR CASE.

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2003\06\14@222730 by Sean Breheny

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Hi Herbert,

For one thing, I thought you were saying that this frequency range was
good as a general recommendation. If you were not, then I misunderstood
you and I'm sorry. I know that I've seen previous discussion on the list
about motor PWM center around this frequency range.

Also, since I've been doing a lot of work with motors lately, it was on
my mind and I wanted an opportunity to get on my soapbox :-)

Sean


On Sat, 14 Jun 2003, Herbert Graf wrote:

> > It isn't very relevant to running a little tiny 10mA motor but IMHO, it is
> > a mistake to assume that 400 to 2000Hz is a good typical PWM frequency.
>
>         Agreed, which is why that recommendation was for THAT PARTICULAR CASE.
>
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2003\06\15@163902 by hard Prosser

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A very nice explanation of driving DC motors with PWM.

My question is - in the high frequency case (snipped below) - what
direction is the current flowing through the short circuit? As I read it,
if the current is a result of motor inductance, rather than back emf,  a
diode should be able to handle it without the danger of entering the
breaking region. (I.e. it becomes a simple buck regulator using the motor
inductance).

Any current resulting from back emf would be in the opposide direction and
therefore not pass through the diode.

A separate breaking transistor could then be used if this mode is required
- but special switching arrangements would not be needed provided drive &
break were not applied together.

Richard P





It isn't very relevant to running a little tiny 10mA motor but IMHO, it is
a mistake to assume that 400 to 2000Hz is a good typical PWM frequency.

(WARNING: here comes a long explanation :-)

snip....

The other mode of PWM operation is high speed mode, where your PWM period
is so short that the RL time constant tends to keep the motor current
constant during each PWM cycle.  Here, you connect the motor to the supply
during the ON time but SHORT it during the OFF time.

This rubs a lot of people the wrong way because they realize that normally,
shorting a motor causes breaking (the back EMF acts to put current through
in the direction that causes torque opposite the direction of rotation).
However, what they don't realize is that when you suddenly short a motor,
you actually get positive torque for a very short period of time (due to
the stored energy in the inductance being used very efficiently to drive
the motor) followed by breaking. If you are using a high enough PWM
frequency, then the breaking never happens (unless you are at a lower PWM
duty than you should be for the current speed and load torque).

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2003\06\15@190214 by Mike Singer

picon face
Chris Emerson wrote:
> I want to occasionally control a ~1.5V DC motor from
> a PIC circuit running from 3V.
...
> It's a small pager motor, taking about 10mA...


Why not just "DAC" of few resistors as current source?

Efficiency wouldn't be much worse compared to the switching
coils variant at these conditions, if would at all.

Anyway, if the motor datasheet has nothing about motor
PWM-ing: do not PWM it.

Mike.

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2003\06\15@215123 by Olin Lathrop

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> Anyway, if the motor datasheet has nothing about motor
> PWM-ing: do not PWM it.

This is silly.  Few motor data sheets specifically mention PWM, but high
speed switching is a valid way to drive a motor with good efficiency.  It
works because of the inherent physics of the motor, not because the motor is
"rated" for it.


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2003\06\16@002147 by Sean H. Breheny

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Hi Richard,

First of all, I want to mention that, as Lee Jones pointed out offlist, I
used the wrong word. It should be braking not breaking :-)

Yes, as you say, the current is in the forward direction and so a diode
would work. However, the diode voltage drop is likely to be higher than the
drop due to the on resistance of a good FET, so there would be slightly
higher inefficiency and possibly much greater heat-sinking needs. For
example, in my design for the four-rotor helicopter, I currently do not
need any heat sink on the FETs even though they run at around 23 amps at
hover. If I used the diodes for full off-time conduction, I'd have enough
heat generation to need a sink.

Also, there is no danger of entering the braking region improperly if you
merely have a high enough PWM frequency. In fact, as I pointed out, it
makes for a more linear drive system to have braking occur below the
equilibrium PWM duty.

If there were some reason why you never wanted braking to happen, then,
yes, you could use the scheme you suggest.

Sean

At 08:41 AM 6/16/2003 +1200, you wrote:
{Quote hidden}

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2003\06\16@002152 by Mike Singer

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Olin Lathrop wrote:
> > Anyway, if the motor datasheet has nothing about motor
> > PWM-ing: do not PWM it.
>
> This is silly.  Few motor data sheets specifically mention
> PWM, but high speed switching is a valid way to drive a
> motor with good efficiency.
> It works because of the inherent physics of the motor, not
> because the motor is "rated" for it.


This differences a professional and a hobbyist (even golden)
approaches. Who are you: a Nobel Prize Winner to judge about
"inherent physics of the motor"?

As a professional you are supposed to work within datasheet
specifications. Any uncertainty must be treated as "no" thing.

There are a lot of uncertainties when applying 100% pulsed
voltage to inductive coils of a motor instead of slowly
varying voltage. (Yes, I know the current would be ALMOST
constant :-)

Mike.

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2003\06\16@004941 by hard Prosser

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Hi Sean,
Yes thanks,
The other reason for using an active switch in this position I guess is if
the drive is bidirectional anyway -e.g.. a full H bridge.

Richard P



Hi Richard,

First of all, I want to mention that, as Lee Jones pointed out offlist, I
used the wrong word. It should be braking not breaking :-)

Yes, as you say, the current is in the forward direction and so a diode
would work. However, the diode voltage drop is likely to be higher than the
drop due to the on resistance of a good FET, so there would be slightly
higher inefficiency and possibly much greater heat-sinking needs. For
example, in my design for the four-rotor helicopter, I currently do not
need any heat sink on the FETs even though they run at around 23 amps at
hover. If I used the diodes for full off-time conduction, I'd have enough
heat generation to need a sink.

Also, there is no danger of entering the braking region improperly if you


snip.....

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2003\06\16@023523 by Jinx

face picon face
> There are a lot of uncertainties when applying 100% pulsed
> voltage to inductive coils of a motor instead of slowly
> varying voltage. (Yes, I know the current would be ALMOST
> constant :-)
>
> Mike.

What kind of uncertainties ? There would be millions of DC
motors run on PWM, and AFAICT it's always the driver that
suffers damage (probably because they've not been designed
properly and aren't tough enough). Is this what you mean or
is there some degeneration in the motor itself ? From a
practical point of view PWM seems to be a legitimate, reliable,
and dare I say "harmless", way of driving DC motors

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2003\06\16@061348 by Mike Singer

picon face
Jinx wrote:
> > There are a lot of uncertainties when applying 100% pulsed
> > voltage to inductive coils of a motor instead of slowly
> > varying voltage. (Yes, I know the current would be ALMOST
> > constant :-)
> What kind of uncertainties ? There would be millions of DC
> motors run on PWM, and AFAICT it's always the driver that
> suffers damage (probably because they've not been designed
> properly and aren't tough enough). Is this what you mean or
> is there some degeneration in the motor itself ? From a
> practical point of view PWM seems to be a legitimate, reliable,
> and dare I say "harmless", way of driving DC motors

"Practical point" and number of "millions of DC motors
run on PWM" may not get a justification, when motor coil
isolation crashed under high frequency PWM voltage, for
example. Well, maybe it crashed for different reason, but
it's hard to prove, and all the guilt may be laid down
on a developer who set a motor out of spec. Lawyers
sometimes are not as honest as one may imagine :-)

Mike.

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2003\06\16@065109 by Jinx

face picon face
> "Practical point" and number of "millions of DC motors
> run on PWM" may not get a justification, when motor coil
> isolation crashed under high frequency PWM voltage, for
> example. Well, maybe it crashed for different reason, but
> it's hard to prove, and all the guilt may be laid down
> on a developer who set a motor out of spec.

OK, I'll accept that. If it could be proven that a particular
method of driving the motor caused premature failures
then obviously it's the wrong type of drive. If however you
have to do a post mortem looking for "probable" cause
of death in isolated cases then I think you'd be drawing a
long bow to make a generalisation about PWM being
harmful

> Lawyers sometimes are not as honest as one may imagine :-)

You mean they could more honest ?

> Mike.

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2003\06\16@075116 by Olin Lathrop

face picon face
> ... when motor coil
> isolation crashed under high frequency PWM voltage, for
> example.

Huh?  Any isolation in a motor is going to based on a dielectrics and/or
air gaps.  These things don't just "crash" for no reason.  About the only
way electrically you can punch thru an insulator is to apply too high a
voltage.  The highest voltage comes from the inductive spikes when the
current is suddenly shut off.  This happens whether PWM is used or not.
In fact, PWM circuits will include flyback diodes and specific means of
limiting the spike to protect the circuitry and manage the energy in the
coil.  The are therefore less likely to compromise isolation.

A plain old mechanical switch in series with the motor is the worst thing
you can do in regards to voltage spikes.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

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2003\06\16@075543 by Alan B. Pearce

face picon face
Bob wrote about Sean's treatise

>Extremely well written.
>
>All I was going to say was that the PWM method of driving a low voltage
>motor at high voltages allows significantly better torque than normal
>voltages. Sometimes that can be a real plus in the application, such as
>when driving a motor who is designed to vibrate with an eccentic cam; they
>are easier to startup.

I agree with Bob about it being well written.

Just a few points to add, which are not necessarily obvious, and catch
people out.

There is a family of motors which have a "coreless" or "bell shaped"
armature. These have the armature wound as a self supporting structure which
then goes over a large non rotating magnet piece, with another magnet around
the outside. There is no rotating metal in the armature except for the
shaft. These motors are made by several companies, one of the earliest I
know of is Faulhaber, a Swiss company. The design allows very low voltage
operation (I have seen one run off a single solar cell many years ago, an
easily done demonstration now, but not then), and also very low rotating
mass.

However because of the way the winding is self supporting, it is mandatory
to use high frequency PWM with these. 20kHz or more is necessary. Using a
low frequency PWM will cause enough vibration in the armature to make the
windings shake apart with the torque shocks which occur at the PWM rate.

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2003\06\16@085713 by Spehro Pefhany

picon face
At 06:32 PM 6/16/2003 +1200, you wrote:


>What kind of uncertainties ? There would be millions of DC
>motors run on PWM, and AFAICT it's always the driver that
>suffers damage (probably because they've not been designed
>properly and aren't tough enough).

In the case of mains-voltage AC motors there are so-called
"inverter rated" motors that have higher voltage insulation than
regular motors. Seems the extra peak voltage and high frequency
spikes could cause failures of some motor insulation. However, I can't
see any reason to have such concerns at a voltage of a few volts, in
fact the spike appearing when the motor is switched off with a mechanical
switch will far exceed anything the contemplated circuits would
do to it. Also it's really difficult to make wire insulation that bad.

>Is this what you mean or
>is there some degeneration in the motor itself ? From a
>practical point of view PWM seems to be a legitimate, reliable,
>and dare I say "harmless", way of driving DC motors

I would have absolutely no concern provided the peak armature
current is not significantly higher than the stall current at full
rated voltage. Even 2:1 current would be achieved by plugging the motor,
so most motors should not be damaged. Applying something like 10 times
the rated current for a brief time could have undesirable effects such
as demagnetizing the PM. However at 50% duty cycle the coil current
doesn't get higher than normal if the frequency is high enough, and
there's no reason to worry, IME.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
RemoveMEspeffspam_OUTspamKILLspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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2003\06\16@090811 by Spehro Pefhany

picon face
At 12:55 PM 6/16/2003 +0100, you wrote:


>However because of the way the winding is self supporting, it is mandatory
>to use high frequency PWM with these. 20kHz or more is necessary. Using a
>low frequency PWM will cause enough vibration in the armature to make the
>windings shake apart with the torque shocks which occur at the PWM rate.

If the high frequency PWM is a problem, an external series inductor is
always a possibility.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
RemoveMEspeffTakeThisOuTspamspaminterlog.com             Info for manufacturers: http://www.trexon.com
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2003\06\16@093742 by Bob Ammerman

picon face
While I am sure that running a 1.5V motor off 3V PWM'd will cause no
problem, it is worth noting that there is a difference between the
occasional voltage spike caused my a mechanical switch, and a rapidly
repeated spike caused by PWMing.

Bob Ammerman
RAm Systems

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2003\06\16@144213 by Peter L. Peres

picon face
>> Anyway, if the motor datasheet has nothing about motor
>> PWM-ing: do not PWM it.
>
> This is silly.  Few motor data sheets specifically mention PWM, but high
> speed switching is a valid way to drive a motor with good efficiency.
> It works because of the inherent physics of the motor, not because the
> motor is "rated" for it.

I am with Mike here. It is dead easy to cook a motor with pwm, especially
in a servo loop. You need to have SOA limiting and derate power with rpm
(the motor does not ventilate itself when it runs slowly). Also with some
motors the PWM can excite resonance in delicate parts that will fail
early. Typically in very low inertia rotor motors (which happen to be
very expensive). There are several parts in a small motor that can follow
the PWM up into tens of kilohertz range. Like coil self resonant
frequency, magnetostrictive resonance in the core or ferrite magnets and
other things.

Peter

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2003\06\16@145442 by Mike Singer

picon face
Olin Lathrop wrote:
> > ... when motor coil isolation crashed under high
> > frequency PWM voltage, for example.
>
> Huh?  Any isolation in a motor is going to based on a
> dielectrics and/or air gaps.  These things don't just
> "crash" for no reason.  About the only way electrically
> you can punch thru an insulator is to apply too high a
> voltage.  The highest voltage comes from the inductive
> spikes when the current is suddenly shut off.  This
> happens whether PWM is used or not. In fact, PWM
> circuits will include flyback diodes and specific means
> of limiting the spike to protect the circuitry and
> manage the energy in the coil.  The are therefore
> less likely to compromise isolation.
>
> A plain old mechanical switch in series with the motor
> is the worst thing you can do in regards to voltage spikes.

  Isolation was taken no more than for example. The
point was that a motor must be considered as black
box with properties described in datasheet. I.e.
a motor was tested and guaranteed to meet some
requirements under some conditions. If it wasn't
tested with PWM, then, I think, it shouldn't be used
with it in a professional design.
  Yes it's a personal matter how to interpret
datasheet text. (Another extreme: If motor wasn't
tested when painted black, then it should never be
painted black)
  But I'm afraid that PWM-ing is a different thing
than just painting. So to feel myself on a safe side
I'd rather not use PWM with motors not directly
specified for PWM-ing.
  Regarding your cited above passage about isolation,
I found it just childish. Imaging tiny motor with
very thin wires. Are them supposed to "tinkle" under
PWM when got loose a bit, aren't them? If them can
stand billions tinkles, then is "a plain old
mechanical switch" the worst thing for motor?
But continuous PWM-ing could crash isolation in a
finite amount of time, I think.

Mike.

P.S. Was written before reading Peter Peres post.

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2003\06\16@151519 by David VanHorn

flavicon
face
At 10:54 PM 6/16/2003 +0400, Mike Singer wrote:

>Olin Lathrop wrote:
>> > ... when motor coil isolation crashed under high
>> > frequency PWM voltage, for example.
>>
>> Huh?  Any isolation in a motor is going to based on a
>> dielectrics and/or air gaps.  These things don't just
>> "crash" for no reason.  About the only way electrically
>> you can punch thru an insulator is to apply too high a
>> voltage.

Ever hear of resonance? I mean electrical resonance.

Each turn of that winding has inductance and capacitance.
You can hit a self resonant point, and at that point, the voltage between two physically adjacent turns could be very high.

There's also mechanical effects, if the windings aren't epoxied down, where wires can rub their insulation off.

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2003\06\17@025532 by Mike Singer

picon face
Bob Ammerman wrote:
> While I am sure that running a 1.5V motor off 3V PWM'd
> will cause no problem...

He-he Bob, as a professional you were expected to write:

"I take full responsibility to state that running
_ANY_ 1.5V motor off 3V PWM'd will cause no problem."

What does it mean "I am sure": "I believe", "I hope",
"I suspect", "I had a dream"...


Mike :-)

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2003\06\17@133346 by Peter L. Peres

picon face
> What kind of uncertainties ? There would be millions of DC
> motors run on PWM, and AFAICT it's always the driver that
> suffers damage (probably because they've not been designed
> properly and aren't tough enough). Is this what you mean or
> is there some degeneration in the motor itself ? From a
> practical point of view PWM seems to be a legitimate, reliable,
> and dare I say "harmless", way of driving DC motors

Just off of my head a couple uncertainities:

You drive the coils of a 12V motor from 24V with 50% duty cycle, the
kickback spikes at the motor (not at the switcher) reach 50Vpk and it
works perfectly in the lab but 50 of 200 units break down within a month
when used in an industrial environment (hot, vibration, oil mist that
weakens the dielectric strength of the coil insulation).

You drive the coils of a honeycomb wound (coreless, very low inertia) 9V
servo with varying duty cycle from 9V in a servo. You do not pay attention
to SOA. You use the motor at 50% rated power for holding torque (with no
cooling and no motion). One of two things can happen: the coils get hot
enough that they melt the glue that holds them in place.  Poof motor gone.
You set up vibration in some mode in the axle and it widens the bushing to
untolerable tolerances within 2 weeks. The motor becomes noisy for no
reason.

You have a perfectly tested and operating servo, using pwm at 21kHz. It is
mounted into the final product prototype, everything works, at the last
minute the case is modified. With the new case the case 'sings' at a
subharmonic of 21kHz (like 7kHz). You spend weeks to deaden it using foam
pads and rubber gaskets, and make several interesting trips to wherever it
is they are putting the product together.

Magnetostrictive vibration in the core, yoke, and axle is in the few khz
to tens of khz range for most small 'servo' motors.

A simple example of evil sneaky vibration is steppers: Who does not know a
stepper-using device (like a printer) that is perfectly quiet except when
put on *that* table just *so*. In which case it sounds like a small marble
cutter at low rpm.

If you want to use PWM with a magnet or a motor you have to check for
these things. A scope may be enough but be aware that the resonant modes
will be very narrow and may appear as a *dip* in the spike voltages on the
motor proper (since the energy goes elsewhere than where you think) ... so
don't pat yourself on the back when the spikes disappear at some
frequency, and do *not* use that frequency if you can help it.

Peter

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2003\06\18@131939 by Peter L. Peres

picon face
> What does it mean "I am sure": "I believe", "I hope",
> "I suspect", "I had a dream"...

Approximately what they mean by stamping 'Preliminary' or 'Draft' or by
putting a little star on a datahseet data row and explaining that the
parameter is 'not fully tested' and that in general ymmv.

Professionals who make strong assertions are either professionals with
strong legal backing, or marketing types with strong legal backing posing
as professionals, or people with nothing to lose (not even face). At least
in the 'civilized' world, where a lawyer's word is heavier than common
sense. imho.

Peter

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2003\06\18@145250 by Mike Singer

picon face
Peter L. Peres wrote:
> Professionals who make strong assertions are either professionals
> with strong legal backing, or marketing types with strong legal
> backing posing as professionals, or people with nothing to lose
> (not even face). At least in the 'civilized' world, where a
> lawyer's word is heavier than common sense. imho.

In 'uncivilized' world few grams of lead is heavier than lawyer's
word. And I'm not sure it's better.

Mike.

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2003\06\20@060806 by Peter L. Peres

picon face
Mike Singer wrote:
> In 'uncivilized' world few grams of lead is heavier than lawyer's
> word. And I'm not sure it's better.

Well, I don't know about better, but at least it is *cheaper* at least on
the short run. Or so I gather from examining legal fees from some smaller
lawsuits I have been reading up on. I understand that year-long
contry-wide wars have been fought and won/lost for smaller sums in places
like Africa.

Peter

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