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'[EE] 7.2 V motor at higher voltage'
2006\07\06@035051 by Wouter van Ooijen

face picon face
I have a bunch of nice 7.2V geared DC motors. Could I run these motors
at let's say 24V if I keep the PWM on-percentage sufficiently low, I
would guess (7.2/24)^2 = 0.09 at 24V would corespond to 1.0 at 7.2V? Or
doesn't it work that way with motors?

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


2006\07\06@044325 by Luis.Moreira

picon face
Hi Wouter,
I am just looking at it from a inductor point of view, so I may be
wrong, but I think you may have problems with the current on the
windings. Your di/dt will increase as you have more voltage v = L di/dt,
so the problem may be that your windings will not be able to handle the
current. You also may have problems with control system you use.
I may be wrong...
Best regards
                       Luis



{Original Message removed}

2006\07\06@051358 by Spehro Pefhany

picon face
At 09:52 AM 7/6/2006 +0200, you wrote:
>I have a bunch of nice 7.2V geared DC motors. Could I run these motors
>at let's say 24V if I keep the PWM on-percentage sufficiently low, I
>would guess (7.2/24)^2 = 0.09 at 24V would corespond to 1.0 at 7.2V? Or
>doesn't it work that way with motors?
>
>Wouter van Ooijen

If you operate at high enough PWM frequency that the current through
the motor inductance has little ripple, then the duty cycle for full speed
would be 7.2/24, not the lower duty cycle to give a 7.2V RMS you'd expect
with a resistive load.

The more ripple, the higher the I^2R losses, and the more current through
your switch (and thus higher losses there too). Of course at high PWM
frequencies you get more switching losses, so there is a trade-off
depending on the motor characteristics.

You may wish to measure and limit the peak current through the motor to
datasheet maximum (or failing a good data sheet, not much above
7.2V/armature resistance) to prevent possible damage such as
demagnetization.  Or test one or two to possible destruction to make sure
it's not a problem if your micro locks up for a few seconds. In that case,
a simple polyfuse could protect against burning the motor out.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
spam_OUTspeffTakeThisOuTspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->>Test equipment, parts OLED displys http://search.ebay.com/_W0QQsassZspeff


2006\07\06@061231 by Vasile Surducan

face picon face
On 7/6/06, Wouter van Ooijen <.....wouterKILLspamspam@spam@voti.nl> wrote:
> I have a bunch of nice 7.2V geared DC motors. Could I run these motors
> at let's say 24V if I keep the PWM on-percentage sufficiently low, I
> would guess (7.2/24)^2 = 0.09 at 24V would corespond to 1.0 at 7.2V? Or
> doesn't it work that way with motors?


I don't think. The current pulse at 24V will eat the brushes quite soon.
Not talking about the power you've got at the motor shaft with very
low dutycycle.

Vasile

2006\07\06@072330 by olin piclist

face picon face
Wouter van Ooijen wrote:
> I have a bunch of nice 7.2V geared DC motors. Could I run these motors
> at let's say 24V if I keep the PWM on-percentage sufficiently low,

Yes.  The limiting factor for the voltage rating of a motor is the power it
can dissipate at whatever the rated load is.  Motors are slow mechanical
devices and temperature also responds slowly, so these devices are well
suited to PWM drive of a few 100Hz and up.  The limiting factor on the PWM
input voltage is the insulation breakdown, which is very unlikely a problem
even well beyond 24V.

You can also run the motor above 7.2V average depending on conditions.  What
load is the 7.2V rating for?  Is there some minimum speed associated with
that spec, or is 7.2V OK indefinitely at stall?  A motor voltage spec really
doesn't describe the limitations very well, because motors aren't devices
inherently limited to a particular voltage.  The real limitation is the
maximum internal motor temperature.  Unfortunately that doesn't convert
easily into dumbed down electrical specs for the masses.

Motor dissipation is electrical power in minus mechanical power out.  This
does not directly translate into a voltage.  What you want to know in your
case is what the maximum dissipation power is.  You can sometimes infer this
from the other specs.  The easiest is if they give you maximum sustainable
stall voltage or current.  Stall means there is no mechanical output power,
so everything goes to heat.  One meaurement will tell you what the input
stall power is, and therefore the sustainable dissipation.  Sometimes they
will characterize voltage and current at a rated power output.  This can be
useful, but is often below the maximum sustainable motor dissipation.
Things can get tricky.

Once you do come up with a power figure, you can use the PIC to take a good
enough guess at the motor internal temperature if you know how fast the
motor is spinning and the no load speed at a rated voltage.  I did this once
in a 12V gearhead motor used in a servo mechanism driven from a 16F876.
Every PWM pulse, I added an increment to the temperature value.  This
increment was the 12V PWM input voltage, minus the assumed back EMF from the
speed, times the PWM duty cycle.  Then I also subtracted a small fraction of
the temperature from itself.  This fraction was adjusted so that the 1/2
decay time was about 45 seconds, which we decided modeled the motor cooling
well enough.  The maximum allowed temperature was then adjusted to that
value where the cooling would just ballance the power input at the long term
sustainable level.  If a PWM pulse would result in exceeding the temperature
limit, the duty cycle was reduced so that it would just hit the limit.

While this wasn't a perfect model for motor temperature, it did a pretty
decent job all around.  It allowed us to get high torque for short periods
when needed, but still protect the motor in case this was called for too
often.  All in all the system worked very well.


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2006\07\06@100542 by Gerhard Fiedler

picon face
Vasile Surducan wrote:

> On 7/6/06, Wouter van Ooijen <wouterspamKILLspamvoti.nl> wrote:
>> I have a bunch of nice 7.2V geared DC motors. Could I run these motors
>> at let's say 24V if I keep the PWM on-percentage sufficiently low, I
>> would guess (7.2/24)^2 = 0.09 at 24V would corespond to 1.0 at 7.2V? Or
>> doesn't it work that way with motors?
>
> I don't think. The current pulse at 24V will eat the brushes quite soon.

Since the motor is basically inductive, there isn't really a "current
pulse", is there? At least not one that you couldn't reduce by a higher PWM
frequency. Given a high enough PWM frequency, the current should have
little ripple and it should be possible to keep it in the same range as if
the motor was driven with 7V.

On the electrical side, it's the current that determines the operating
point of the motor. (E.g. if you have a certain torque curve on the
mechanical load side, it's the current that determines the motor speed.)
You can set up a current control loop, and you can put the motor on any
point in its current/torque/speed/power diagram almost independently of the
driving voltage. (As long as the PWM frequency is high enough to guarantee
a low-ripple current.) See this
http://www.casaferreira.com.br/pdf/CEP453042.pdf for a typical diagram for
a DC motor. These diagrams are all very similar; just a few corner points
change.

> Not talking about the power you've got at the motor shaft with very
> low dutycycle.

Of course... but Wouter said that already. The motor's max. power would be
way below 100% duty cycle, but that's not really a problem usually. (With
the 10bit PWM of a typical PIC you get 1% resolution even if you only use
10% of the full range. Good enough for most motor applications.)

Gerhard

2006\07\06@112804 by Bob Axtell

face picon face
Wouter van Ooijen wrote:
> I have a bunch of nice 7.2V geared DC motors. Could I run these motors
> at let's say 24V if I keep the PWM on-percentage sufficiently low, I
> would guess (7.2/24)^2 = 0.09 at 24V would corespond to 1.0 at 7.2V? Or
> doesn't it work that way with motors?
>
> Wouter van Ooijen
>
> -- -------------------------------------------
> Van Ooijen Technische Informatica: http://www.voti.nl
> consultancy, development, PICmicro products
> docent Hogeschool van Utrecht: http://www.voti.nl/hvu
>  
>
>  
Yes, you can. The torque will be very high, but it won't be damaged as
long as the overall duty
cycle is small. Be sure to enclose the wires to the motor in a shield,
as they will radiate a lot of
RF.

--Bob

2006\07\06@141440 by Martin Klingensmith

flavicon
face
Low voltage PMDC motors also have low inductance. I would expect to find
high ripple current. This may or may not be a problem depending on the
rest of your system. I would measure the ripple at whatever PWM
frequency you use and increase that frequency if the ripple is too much.
--
Martin K


Wouter van Ooijen wrote:

{Quote hidden}

2006\07\06@142133 by Vasile Surducan

face picon face
On 7/6/06, Gerhard Fiedler <.....listsKILLspamspam.....connectionbrazil.com> wrote:
> Vasile Surducan wrote:
>
> > On 7/6/06, Wouter van Ooijen <EraseMEwouterspam_OUTspamTakeThisOuTvoti.nl> wrote:
> >> I have a bunch of nice 7.2V geared DC motors. Could I run these motors
> >> at let's say 24V if I keep the PWM on-percentage sufficiently low, I
> >> would guess (7.2/24)^2 = 0.09 at 24V would corespond to 1.0 at 7.2V? Or
> >> doesn't it work that way with motors?
> >
> > I don't think. The current pulse at 24V will eat the brushes quite soon.
>
> Since the motor is basically inductive, there isn't really a "current
> pulse", is there? At least not one that you couldn't reduce by a higher PWM
> frequency. Given a high enough PWM frequency, the current should have
> little ripple and it should be possible to keep it in the same range as if
> the motor was driven with 7V.

I think you have right. But if the frequency is too high, there will
not be a real PWM switching and the H bridge will become hot. 24-7=17V
Unfortunately I don't know the current flow through the motor.

greetings,
Vasile

2006\07\06@143225 by Spehro Pefhany

picon face
At 02:14 PM 7/6/2006 -0400, you wrote:
>Low voltage PMDC motors also have low inductance. I would expect to find
>high ripple current. This may or may not be a problem depending on the
>rest of your system. I would measure the ripple at whatever PWM
>frequency you use and increase that frequency if the ripple is too much.
>--
>Martin K

Or, if the motor is of a type that has very low inductance, just add a
series inductor to keep the PWM frequency reasonable. Needless to say(?)
switching losses will tend to be higher to begin with, all other things
being equal, just because of the low duty cycle, and thus low on time
in relation to (fixed) switching times.

Best regards,

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




2006\07\06@145619 by Martin Klingensmith

flavicon
face
That's a good idea. Usually people opt for fewer components and things
like inductors especially. If he's PWMing with a MOSFET (which is a
smart thing to do) - a few KHz is possible directly from a PIC pin, and
if you add a few cheap transistors you can switch at whatever frequency
you desire, up to the max PWM freq. of the PIC. I guess if he knows that
though, I'm sure.
--
Martin K

Spehro Pefhany wrote:

{Quote hidden}

2006\07\06@152907 by Vasile Surducan

face picon face
On 7/6/06, Vasile Surducan <KILLspampiclist9KILLspamspamgmail.com> wrote:
{Quote hidden}

 If it's indeed 1A, I have doubts will work, maybe someone could tell
me why I'm wrong with math ecuations.

thx,
Vasile

>
> greetings,
> Vasile
>

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