motor speed controller for kids' electric car

[Michael]

I've got an electric car for the kids that has only an On-Off switch
for speed control. I was hoping to add a better speed controller than
that. I would need to PWM 12V, about 10-15A. I was hoping to use a
555 to generate the pulses to the mosfet gates.

I saw this:
http://zeva.com.au/speedy/

I can appreciate the beefy freewheel diode and mosfets, but would I
really need a FET driver? At what point do I need a FET driver?
Would the output from a 555 suffice?

Thanks,

Michael

 

[Tom Biasi]

"Michael" <mrdarrett@gmail.com> wrote in message
news:15472d02-9e56-4452-ba4f-62c93ae69032@d15g2000prc.googlegroups.com...

I've got an electric car for the kids that has only an On-Off switch
for speed control. I was hoping to add a better speed controller than
that. I would need to PWM 12V, about 10-15A. I was hoping to use a
555 to generate the pulses to the mosfet gates.

I saw this:
http://zeva.com.au/speedy/

I can appreciate the beefy freewheel diode and mosfets, but would I
really need a FET driver? At what point do I need a FET driver?
Would the output from a 555 suffice?

Thanks,

Michael

Before you build look here:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=180387965721

Tom

 

On Mon, 31 Aug 2009 14:49:05 -0400, "Tom Biasi"
<tombiasi@optonline.net> wrote:

"Michael" <mrdarrett@gmail.com> wrote in message
news:15472d02-9e56-4452-ba4f-62c93ae69032@d15g2000prc.googlegroups.com...
I've got an electric car for the kids that has only an On-Off switch
for speed control. I was hoping to add a better speed controller than
that. I would need to PWM 12V, about 10-15A. I was hoping to use a
555 to generate the pulses to the mosfet gates.

I saw this:
http://zeva.com.au/speedy/

I can appreciate the beefy freewheel diode and mosfets, but would I
really need a FET driver? At what point do I need a FET driver?
Would the output from a 555 suffice?

Thanks,

Michael

Before you build look here:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=180387965721

Tom

And you can add a foot pedal throttle here:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=330273410493&ssPageName=STRK:MEWAX:IT

John

 

[DJ Delorie]

In general, a FET driver does a couple of things:

1. More current. FETs have capacitive gates; bigger FETs (or FETs in
parallel) act like bigger capacitors. If you want them to switch
fast (on *or* off), you have to charge them fast - this takes a
HUGE inrush current, which a 555 normally can't provide.

2. More voltage swing. FETs need to be driven to their Vss levels to
shut them off quickly, which for a P-FET might be higher than the
voltage a 555 can put out.

 

[Tom Biasi]

"Michael" <mrdarrett@gmail.com> wrote in message
news:15472d02-9e56-4452-ba4f-62c93ae69032@d15g2000prc.googlegroups.com...

I've got an electric car for the kids that has only an On-Off switch
for speed control. I was hoping to add a better speed controller than
that. I would need to PWM 12V, about 10-15A. I was hoping to use a
555 to generate the pulses to the mosfet gates.

I saw this:
http://zeva.com.au/speedy/

I can appreciate the beefy freewheel diode and mosfets, but would I
really need a FET driver? At what point do I need a FET driver?
Would the output from a 555 suffice?

Thanks,

Michael

I don't know how you calculated your current needs but when the kid goes
uphill or gets stuck the stall current is high.
No need to reverse?

Tom

 

[Tom Biasi]

"Michael" <mrdarrett@gmail.com> wrote in message
news:1119a0a5-2891-433f-80c4-8dea2162fdc0@a39g2000pre.googlegroups.com...
On Aug 31, 12:49 pm, George Herold <ggher...@gmail.com> wrote:

On Aug 31, 2:43 pm, Michael <mrdarr...@gmail.com> wrote:

I've got an electric car for the kids that has only an On-Off switch
for speed control. I was hoping to add a better speed controller than
that. I would need to PWM 12V, about 10-15A. I was hoping to use a
555 to generate the pulses to the mosfet gates.

I saw this:http://zeva.com.au/speedy/

I can appreciate the beefy freewheel diode and mosfets, but would I
really need a FET driver? At what point do I need a FET driver?
Would the output from a 555 suffice?

Thanks,

Michael

Is it one of the small 'power wheels'? (that is the fisher-price name
though there are other manufacturers.) With two small gel cel lead
acid batteries?

If so you can add a simple speed control by putting 'about' a 1/2 ohm
resistor is series with the battery. But I found that after a few
trips the kids want to drive it 'full on' all the time.

George H.

Nope, got it at a flea market. Originally runs on 6V, but that didn't
give a whole lot of power (step on the throttle, and nada). Put in a
higher A-h 6V battery, and works a little better, but still not much
power. Put in a 12V battery from a different project, and whoa, yee-
haw! A bit too much power. Wires got hot, too. So I'm planning on
re-doing the wiring, and adding a PWM control. Maybe a fan, but if I
do that, would that just fan the flames...?

I'm still annoyed that the original 6V battery charger that came with
it gave 8+ volts output across the battery terminals after charging
overnight. I shouldn't have trusted them, but hey... I gave them the
benefit of the doubt. Now I use a dedicated 6/12V adjustable Pb-acid
battery charger.

Right now it's just sitting in the room, no one riding it, because the
6V battery (even the bigger one) doesn't give enough juice. A pity.

Michael

Many years ago, before modern electronics, in a galaxy a lot like our own I
set up a throttle that supplied voltage in 3 volt increments. That's when it
was easy to tap a six volt battery.
The kids liked it, it was like shifting.

Tom


Tom

 

[Tom Biasi]

"Michael" <mrdarrett@gmail.com> wrote in message
news:e3be3c22-f2e7-4d21-877c-2d20bb3d51b0@x5g2000prf.googlegroups.com...
On Aug 31, 1:01 pm, "Tom Biasi" <tombi...@optonline.net> wrote:

~zap~

Many years ago, before modern electronics, in a galaxy a lot like our own
I
set up a throttle that supplied voltage in 3 volt increments. That's when
it
was easy to tap a six volt battery.
The kids liked it, it was like shifting.

Tom

Tom

Hmm... I wonder what will happen if I put the weak 6V and the stronger
6V batteries in series. Maybe that could work... otherwise all I
have are just those two 6V batteries and two more 12V batteries, none
of which are center-tappable.

Thanks,

Michael

Your setup would be like adding series resistance, well not like it, you
would be.
The old motorcycle batteries had the links between the cell exposed as lead
bars. No problem to drill into the bar and add a contact point.
Not so with modern sealed LA batteries.

Tom

 

[Michael]

On Aug 31, 11:49 am, "Tom Biasi" <tombi...@optonline.net> wrote:

"Michael" <mrdarr...@gmail.com> wrote in message

news:15472d02-9e56-4452-ba4f-62c93ae69032@d15g2000prc.googlegroups.com...

I've got an electric car for the kids that has only an On-Off switch
for speed control.  I was hoping to add a better speed controller than
that.  I would need to PWM 12V, about 10-15A.  I was hoping to use a
555 to generate the pulses to the mosfet gates.

I saw this:
http://zeva.com.au/speedy/

I can appreciate the beefy freewheel diode and mosfets, but would I
really need a FET driver?  At what point do I need a FET driver?
Would the output from a 555 suffice?

Thanks,

Michael

Before you build look here:http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=180387965721

Tom

Thanks for the links guys.

Dc Motor Actiyaton, eh?

Michael

 

[David Eather]

DJ Delorie wrote:

In general, a FET driver does a couple of things:

1. More current. FETs have capacitive gates; bigger FETs (or FETs in
parallel) act like bigger capacitors. If you want them to switch
fast (on *or* off), you have to charge them fast - this takes a
HUGE inrush current, which a 555 normally can't provide.

2. More voltage swing. FETs need to be driven to their Vss levels to
shut them off quickly, which for a P-FET might be higher than the
voltage a 555 can put out.

A 555 can source or sink at 200ma. This is more than enough for driving
the gate of a MOSFET at any reasonable frequency for this task. However,
I would use the ebay link if you value the time used on this.

 

[Michael]

On Aug 31, 12:25 pm, DJ Delorie <d...@delorie.com> wrote:

In general, a FET driver does a couple of things:

1. More current.  FETs have capacitive gates; bigger FETs (or FETs in
   parallel) act like bigger capacitors.  If you want them to switch
   fast (on *or* off), you have to charge them fast - this takes a
   HUGE inrush current, which a 555 normally can't provide.

2. More voltage swing.  FETs need to be driven to their Vss levels to
   shut them off quickly, which for a P-FET might be higher than the
   voltage a 555 can put out.

Ah, Thanks!

 

[Michael]

On Aug 31, 12:40 pm, "Tom Biasi" <tombi...@optonline.net> wrote:

"Michael" <mrdarr...@gmail.com> wrote in message

news:15472d02-9e56-4452-ba4f-62c93ae69032@d15g2000prc.googlegroups.com...

I've got an electric car for the kids that has only an On-Off switch
for speed control.  I was hoping to add a better speed controller than
that.  I would need to PWM 12V, about 10-15A.  I was hoping to use a
555 to generate the pulses to the mosfet gates.

I saw this:
http://zeva.com.au/speedy/

I can appreciate the beefy freewheel diode and mosfets, but would I
really need a FET driver?  At what point do I need a FET driver?
Would the output from a 555 suffice?

Thanks,

Michael

I don't know how you calculated your current needs but when the kid goes
uphill or gets stuck the stall current is high.
No need to reverse?

Tom

DMM, steady-state current. No need to reverse. Not anticipating hill
loads. Although I should probably factor in two kids riding at
once...

Thanks,

Michael

 

[George Herold]

On Aug 31, 2:43 pm, Michael <mrdarr...@gmail.com> wrote:

I've got an electric car for the kids that has only an On-Off switch
for speed control.  I was hoping to add a better speed controller than
that.  I would need to PWM 12V, about 10-15A.  I was hoping to use a
555 to generate the pulses to the mosfet gates.

I saw this:http://zeva.com.au/speedy/

I can appreciate the beefy freewheel diode and mosfets, but would I
really need a FET driver?  At what point do I need a FET driver?
Would the output from a 555 suffice?

Thanks,

Michael

Is it one of the small 'power wheels'? (that is the fisher-price name
though there are other manufacturers.) With two small gel cel lead
acid batteries?

If so you can add a simple speed control by putting 'about' a 1/2 ohm
resistor is series with the battery. But I found that after a few
trips the kids want to drive it 'full on' all the time.

George H.

 

[Michael]

On Aug 31, 12:49 pm, George Herold <ggher...@gmail.com> wrote:

On Aug 31, 2:43 pm, Michael <mrdarr...@gmail.com> wrote:

I've got an electric car for the kids that has only an On-Off switch
for speed control.  I was hoping to add a better speed controller than
that.  I would need to PWM 12V, about 10-15A.  I was hoping to use a
555 to generate the pulses to the mosfet gates.

I saw this:http://zeva.com.au/speedy/

I can appreciate the beefy freewheel diode and mosfets, but would I
really need a FET driver?  At what point do I need a FET driver?
Would the output from a 555 suffice?

Thanks,

Michael

Is it one of the small 'power wheels'?  (that is the fisher-price name
though there are other manufacturers.)  With two small gel cel lead
acid batteries?

If so you can add a simple speed control by putting 'about' a 1/2 ohm
resistor is series with the battery. But I found that after a few
trips the kids want to drive it 'full on' all the time.

George H.

Nope, got it at a flea market. Originally runs on 6V, but that didn't
give a whole lot of power (step on the throttle, and nada). Put in a
higher A-h 6V battery, and works a little better, but still not much
power. Put in a 12V battery from a different project, and whoa, yee-
haw! A bit too much power. Wires got hot, too. So I'm planning on
re-doing the wiring, and adding a PWM control. Maybe a fan, but if I
do that, would that just fan the flames...?

I'm still annoyed that the original 6V battery charger that came with
it gave 8+ volts output across the battery terminals after charging
overnight. I shouldn't have trusted them, but hey... I gave them the
benefit of the doubt. Now I use a dedicated 6/12V adjustable Pb-acid
battery charger.

Right now it's just sitting in the room, no one riding it, because the
6V battery (even the bigger one) doesn't give enough juice. A pity.

Michael

 

[Michael]

On Aug 31, 1:01 pm, "Tom Biasi" <tombi...@optonline.net> wrote:

~zap~

Many years ago, before modern electronics, in a galaxy a lot like our own I
set up a throttle that supplied voltage in 3 volt increments. That's when it
was easy to tap a six volt battery.
The kids liked it, it was like shifting.

Tom

Tom

Hmm... I wonder what will happen if I put the weak 6V and the stronger
6V batteries in series. Maybe that could work... otherwise all I
have are just those two 6V batteries and two more 12V batteries, none
of which are center-tappable.

Thanks,

Michael

 

[DJ Delorie]

DJ Delorie wrote:
In general, a FET driver does a couple of things:

David Eather <eather@tpg.com.au> writes:

A 555 can source or sink at 200ma. This is more than enough for
driving the gate of a MOSFET at any reasonable frequency for this
task.

Hence the "in general". The driver the OP referred to could source or
sink 14 amps, which may be needed in really big motor controls, or
controls with really high PWM frequencies.

 

[DJ Delorie]

Michael <mrdarrett@gmail.com> writes:

How is the required gate current calculated?

Assuming my math and assumptions aren't horribly wrong...

I would guess that you'd take the gate capacitance, voltage range, and
switching time, and figure out how much current gives you that time
factor.

Note that the longer a FET is in the "in-between" state, the more
power it wastes, so a faster switching time (for the same PWM
frequency) means more efficiency.

So, let's say you wanted to switch 12v across a 2700pF gate in 10 nS.
I = C * V / T, or 12 * 2700E-12 / 10E-9 = 3.24 amps.

Picking a 20amp fet at random... 2360pF, 5V, 0.2amps means a switching
time of 60nS. If you ran the 555 at 12v, that'd be 140nS, for a
maximum PWM frequency of 3.5 MHz (more likely, much less, if you want
it to have any efficiency).

 

[Jon Kirwan]

On Mon, 31 Aug 2009 15:19:33 -0700 (PDT), Michael
<mrdarrett@gmail.com> wrote:

On Aug 31, 3:07 pm, DJ Delorie <d...@delorie.com> wrote:
DJ Delorie wrote:
In general, a FET driver does a couple of things:
David Eather <eat...@tpg.com.au> writes:
A 555 can source or sink at 200ma. This is more than enough for
driving the gate of a MOSFET at any reasonable frequency for this
task.

Hence the "in general".  The driver the OP referred to could source or
sink 14 amps, which may be needed in really big motor controls, or
controls with really high PWM frequencies.

How is the required gate current calculated?

I'm a hobbyist and haven't any experience with motor drivers, so keep
this in mind. I'm just going from a basic grasp, is all. But the
basic equation for any capacitor is like this:

I = C * dV/dt

A mosfet's gate is roughly just such a capacitor. In the above case,
the "C" used is the capacitance of the mosfet gate, usually referred
to source but not always (I'll get there, shortly.) The gate-source
capacitance is usually found by looking around for "Ciss" on the
mosfet data sheet. There is also something called the equivalent gate
capacitance (Cei, I think) and that is not always found on the
datasheet, directly, but computed from the total gate charge (Qg)
required (sum of a few things like the gate-to-source charge [Qgs]
plus the gate-to-drain miller charge [Qgd] and something a little
extra beyond that for overdrive.) Cei = Qg/Vgs and it is usually a
lot bigger than Ciss. However you decide to get it, you figure out a
C for the above equation. dV is easy. It's just Vgs -- whatever
voltage you plan to use on the gate to turn the mosfet on (assuming
your off state voltage is 0V.) dt is the time over which that voltage
is achieved from the off state voltage. Okay, so that may be a
problem.. figuring out dt. If you know how fast the drive works, you
can plug that in. But that capacitance affects the speed, too. So
that makes for a point of confusion.

Better may be to just realize:

Qg = C * Vgs

You get Vgs for free -- it's whatever you are driving with. Qg you
get by extracting the Qgs and Qgd from the datasheet plus some
estimate of the overdrive part. The effective C falls out. But you
really don't care about it. You are faced with Qg and a designed Vgs
and let the pieces fall where they may. If you know your voltage
source has an eqivalent series resistance to it of 100 ohms and a
Qg/Vgs of about 5nF, for example, then you know that about 63% of the
voltage will be reached after R*C time, or 500ns. Figure three of
these time constants to get to 95%, or 1.50us. Fast enough for some
things. Not fast enough for others.

From the datasheet for that irfp2907z on the web site you posted
earlier:
http://www.irf.com/product-info/datasheets/data/irfp2907z.pdf

There is Figure 6 on page 4. Qgs is the first ramp of that curve.
It's about 46nC. The next flatish part is Qgd. Say that Vds is
12-13V for the car battery, so use the Vds=15V departure... Qgd =
34nC, which is the spread from 46nC to the reading of about 80nC where
the curve rises back up along that Vds=15V line. Now, the overdrive
part is the rest. If your Vgs is 10V, then this is the remainder from
about 80nC to 150nC, or 70nC. But Qg, the total of the three, is just
the sum or else just that final reading of 150nC for the typical gate
charge. Assuming Vgs really is 10V for now, that means 150nC/10V or
15nF for Cei.

You could just use that value with what you feel you know about the
effective series resistance of the driver output and get an idea of
the time it takes to drive the mosfet on, as roughly three times R*C.
Or, if you know what current it can handle well, you can reverse that
and compute Qg/I, or 150nC/I, to get the rough time, too. For
example, someone else posted that the 555 timer's output can handle
200mA. Let's say that's right. Then the required time to turn the
mosfet on is roughly on the order of 750ns... about 1 microsecond, or
so. If that is acceptably fast, then the 555 as granted would seem to
be okay. If you really wanted 25ns instead, though, then the 555
simply wouldn't be fast enough for the job -- you'd need at least 30
times more current or about 6A to get that job done.

Jon

 

[Michael]

On Aug 31, 3:07 pm, DJ Delorie <d...@delorie.com> wrote:

DJ Delorie wrote:
In general, a FET driver does a couple of things:
David Eather <eat...@tpg.com.au> writes:
A 555 can source or sink at 200ma. This is more than enough for
driving the gate of a MOSFET at any reasonable frequency for this
task.

Hence the "in general".  The driver the OP referred to could source or
sink 14 amps, which may be needed in really big motor controls, or
controls with really high PWM frequencies.

How is the required gate current calculated?

Thanks,

Michael

 

[Michael]

On Aug 31, 4:21 pm, Jon Kirwan <j...@infinitefactors.org> wrote:

On Mon, 31 Aug 2009 15:19:33 -0700 (PDT), Michael

mrdarr...@gmail.com> wrote:
On Aug 31, 3:07 pm, DJ Delorie <d...@delorie.com> wrote:
DJ Delorie wrote:
In general, a FET driver does a couple of things:
David Eather <eat...@tpg.com.au> writes:
A 555 can source or sink at 200ma. This is more than enough for
driving the gate of a MOSFET at any reasonable frequency for this
task.

Hence the "in general".  The driver the OP referred to could source or
sink 14 amps, which may be needed in really big motor controls, or
controls with really high PWM frequencies.

How is the required gate current calculated?

I'm a hobbyist and haven't any experience with motor drivers, so keep
this in mind.  I'm just going from a basic grasp, is all.  But the
basic equation for any capacitor is like this:

   I = C * dV/dt

A mosfet's gate is roughly just such a capacitor.  In the above case,
the "C" used is the capacitance of the mosfet gate, usually referred
to source but not always (I'll get there, shortly.)  The gate-source
capacitance is usually found by looking around for "Ciss" on the
mosfet data sheet.  There is also something called the equivalent gate
capacitance (Cei, I think) and that is not always found on the
datasheet, directly, but computed from the total gate charge (Qg)
required (sum of a few things like the gate-to-source charge [Qgs]
plus the gate-to-drain miller charge [Qgd] and something a little
extra beyond that for overdrive.)  Cei = Qg/Vgs and it is usually a
lot bigger than Ciss.  However you decide to get it, you figure out a
C for the above equation.  dV is easy.  It's just Vgs -- whatever
voltage you plan to use on the gate to turn the mosfet on (assuming
your off state voltage is 0V.)  dt is the time over which that voltage
is achieved from the off state voltage.  Okay, so that may be a
problem.. figuring out dt.  If you know how fast the drive works, you
can plug that in.  But that capacitance affects the speed, too.  So
that makes for a point of confusion.

Better may be to just realize:

   Qg = C * Vgs

You get Vgs for free -- it's whatever you are driving with.  Qg you
get by extracting the Qgs and Qgd from the datasheet plus some
estimate of the overdrive part.  The effective C falls out.  But you
really don't care about it.  You are faced with Qg and a designed Vgs
and let the pieces fall where they may.  If you know your voltage
source has an eqivalent series resistance to it of 100 ohms and a
Qg/Vgs of about 5nF, for example, then you know that about 63% of the
voltage will be reached after R*C time, or 500ns.  Figure three of
these time constants to get to 95%, or 1.50us.  Fast enough for some
things.  Not fast enough for others.

From the datasheet for that irfp2907z on the web site you posted
earlier:
 http://www.irf.com/product-info/datasheets/data/irfp2907z.pdf

There is Figure 6 on page 4.  Qgs is the first ramp of that curve.
It's about 46nC.  The next flatish part is Qgd.  Say that Vds is
12-13V for the car battery, so use the Vds=15V departure... Qgd > 34nC, which is the spread from 46nC to the reading of about 80nC where
the curve rises back up along that Vds=15V line.  Now, the overdrive
part is the rest.  If your Vgs is 10V, then this is the remainder from
about 80nC to 150nC, or 70nC.  But Qg, the total of the three, is just
the sum or else just that final reading of 150nC for the typical gate
charge.  Assuming Vgs really is 10V for now, that means 150nC/10V or
15nF for Cei.

You could just use that value with what you feel you know about the
effective series resistance of the driver output and get an idea of
the time it takes to drive the mosfet on, as roughly three times R*C.
Or, if you know what current it can handle well, you can reverse that
and compute Qg/I, or 150nC/I, to get the rough time, too.  For
example, someone else posted that the 555 timer's output can handle
200mA.  Let's say that's right.  Then the required time to turn the
mosfet on is roughly on the order of 750ns... about 1 microsecond, or
so.  If that is acceptably fast, then the 555 as granted would seem to
be okay.  If you really wanted 25ns instead, though, then the 555
simply wouldn't be fast enough for the job -- you'd need at least 30
times more current or about 6A to get that job done.

Jon

WOW thanks. T'will take me a bit of time to digest all that
information. It's much appreciated.

Michael

 

[George Herold]

On Aug 31, 3:57 pm, Michael <mrdarr...@gmail.com> wrote:

On Aug 31, 12:49 pm, George Herold <ggher...@gmail.com> wrote:





On Aug 31, 2:43 pm, Michael <mrdarr...@gmail.com> wrote:

I've got an electric car for the kids that has only an On-Off switch
for speed control.  I was hoping to add a better speed controller than
that.  I would need to PWM 12V, about 10-15A.  I was hoping to use a
555 to generate the pulses to the mosfet gates.

I saw this:http://zeva.com.au/speedy/

I can appreciate the beefy freewheel diode and mosfets, but would I
really need a FET driver?  At what point do I need a FET driver?
Would the output from a 555 suffice?

Thanks,

Michael

Is it one of the small 'power wheels'?  (that is the fisher-price name
though there are other manufacturers.)  With two small gel cel lead
acid batteries?

If so you can add a simple speed control by putting 'about' a 1/2 ohm
resistor is series with the battery. But I found that after a few
trips the kids want to drive it 'full on' all the time.

George H.

Nope, got it at a flea market.  Originally runs on 6V, but that didn't
give a whole lot of power (step on the throttle, and nada).  Put in a
higher A-h 6V battery, and works a little better, but still not much
power.  Put in a 12V battery from a different project, and whoa, yee-
haw!  A bit too much power.  Wires got hot, too.  So I'm planning on
re-doing the wiring, and adding a PWM control.  Maybe a fan, but if I
do that, would that just fan the flames...?  

I'm still annoyed that the original 6V battery charger that came with
it gave 8+ volts output across the battery terminals after charging
overnight.  I shouldn't have trusted them, but hey... I gave them the
benefit of the doubt.  Now I use a dedicated 6/12V adjustable Pb-acid
battery charger.

Right now it's just sitting in the room, no one riding it, because the
6V battery (even the bigger one) doesn't give enough juice.  A pity.

Michael- Hide quoted text -

- Show quoted text -

Hmm, I'm not much of an expert, but I would think that you might end
up frying the motors if you push to much current through them.
They're probably designed to run with the series resistance of the
battery to limit the current. (What's the 6V battery voltage when the
it's putting out 10 or 15 amps?) But if you don't mind wasting the
power a resistor in series with your 12V battery may do the trick.
Unless you are just wanting to learn about PWM circuits and all.

George H.