brushed pwm controller motor "surging"

C

causalitist

Guest
i set up a basic lm324 15kz uni-directional pwm motor controller.
this goes to a tc4422 mosfet driver and then to 2 irfp4668 mosfets that
are in parallel. there is one 10 Ohm gate resistor that both fets share ..
selected at random since i have no idea why one would use a powerful driver
and then reduce its power.(NOT the question at hand) and a gate diode
bypassing the gate resistor to allow fast turn off.

i'm using a second voltage source for the motor/mosfets so i can use more
voltage. when both the lm324 and secondary motor/fet loop get the same
voltage(+-12v) it works great.

when i give the mosfet/motor loop like 45v and the lm324 12v, as i turn
the pwm pot it seems to jump from 0 to 10% instantly (not so at lower
voltage)
but most importantly, the motor surges between the pwm corresponding to
the current pot setting and 100% pwm at a very specific pot setting around
50%.
this is not the case when i use 24v for the motor loop and 12v for the pwm
generating part.

i dont know why, but im guessing i need a bigger gate resistor.
i will try that because i think it solved the problem for me in the past
with a tiny motor, but i dont know if its the right solution. heck, i may
just need a 100nf cap somewhere(there are none on the power side of it
besides the cap required by the driver.. v+ to gnd to help with
instantaneous power to gate)

thanks alot!

end goal is 150vdc, 55mph bicycle. everything but the controller is
finished.
 
I played with PWM several years ago and made this page:
http://www.boldinventions.com/gadget_pwm.html


My knowledge of analog stuff is not great, but I had a lot of fun
with Tina making up circuits and getting graphs of the result. I'm
not sure what your circuit is like, but it sounds like something Tina
could do easily. The fun part is changing the circuit and then
instantly seeing a graph of the results.

I think they (designsoftware.com) have a free demo version which might
be enough for you. Their website is a mess, though.

As I said, this was years ago, I wonder if there are better and
possibly free things out there now.

-Kevin


On Dec 31, 4:45 am, "causalitist" <nate53...@hotmail.com> wrote:
i set up a basic lm324 15kz uni-directional pwm motor controller.  
this goes to a tc4422 mosfet driver and then to 2 irfp4668 mosfets that
are in parallel. there is one 10 Ohm gate resistor that both fets share ...
selected at random since i have no idea why one would use a powerful driver
 and then reduce its power.(NOT the question at hand)  and a gate diode
bypassing the gate resistor to allow fast turn off.

i'm using a second voltage source for the motor/mosfets so i can  use more
voltage. when both the lm324 and secondary motor/fet loop get the same
voltage(+-12v) it works great.

when i give the mosfet/motor loop like 45v and the lm324 12v, as i turn
the pwm pot it seems to jump from 0 to 10% instantly (not so at lower
voltage)
but most importantly, the motor surges between the pwm corresponding to
the current pot setting and 100% pwm at a very specific pot setting around
50%.
this is not the case when i use 24v for the motor loop and 12v for the pwm
generating part.

i dont know why, but im guessing i need a bigger gate resistor.
i will try that because i think it solved the problem for me in the past
with a tiny motor, but i dont know if its the right solution. heck, i may
just need a 100nf cap somewhere(there are none on the power side of it
besides the cap required by the driver.. v+ to gnd to help with
instantaneous power to gate)

thanks alot!  

end goal is 150vdc, 55mph bicycle.  everything but the controller is
finished.
Click to expand...
 
On Wed, 31 Dec 2008 03:45:34 -0600, causalitist wrote:
when i give the mosfet/motor loop like 45v and the lm324 12v, as i turn
the pwm pot it seems to jump from 0 to 10% instantly (not so at lower
voltage)
but most importantly, the motor surges between the pwm corresponding to
the current pot setting and 100% pwm at a very specific pot setting around
50%.
this is not the case when i use 24v for the motor loop and 12v for the pwm
generating part.
Click to expand...
Sounds like you've got a spurious oscillation in your control loop.

Schematic? (most of us don't get the alt groups any more, so try to find
a website where you can post it.)

Good Luck!
Rich
 
causalitist wrote:

i set up a basic lm324 15kz uni-directional pwm motor controller.
this goes to a tc4422 mosfet driver and then to 2 irfp4668 mosfets that
are in parallel. there is one 10 Ohm gate resistor that both fets share ..
selected at random since i have no idea why one would use a powerful driver
and then reduce its power.(NOT the question at hand) and a gate diode
bypassing the gate resistor to allow fast turn off.

i'm using a second voltage source for the motor/mosfets so i can use more
voltage. when both the lm324 and secondary motor/fet loop get the same
voltage(+-12v) it works great.

when i give the mosfet/motor loop like 45v and the lm324 12v, as i turn
the pwm pot it seems to jump from 0 to 10% instantly (not so at lower
voltage)
but most importantly, the motor surges between the pwm corresponding to
the current pot setting and 100% pwm at a very specific pot setting around
50%.
this is not the case when i use 24v for the motor loop and 12v for the pwm
generating part.

i dont know why, but im guessing i need a bigger gate resistor.
i will try that because i think it solved the problem for me in the past
with a tiny motor, but i dont know if its the right solution. heck, i may
just need a 100nf cap somewhere(there are none on the power side of it
besides the cap required by the driver.. v+ to gnd to help with
instantaneous power to gate)

thanks alot!

end goal is 150vdc, 55mph bicycle. everything but the controller is
finished.

have you looked at the gate signal via a scope?
Click to expand...

I know that you say at 24 volts it works how ever,
the extra voltage could be showing you an error that
you may have due to the extra voltage thus allowing
the motor to accelerate better.

It's my guess that the gate's signal is truncated
a bit and the LM324 may be current limiting ..

If you don't have a scope to check this, lower the
carrier freq by a lot!.

http://webpages.charter.net/jamie_5"
 
i put a 220 OHM gate resistor on the mosfets and it works great. gave the
lm324 its little 12v source. ran 60v 20amps through the mosfets and the
motor responded very well (linearly and smoothly) to the speed adjustment.

thing is, i have no idea why! hahaha

it seems that when mosfets and pwm generation use separate sources with
common ground, as the voltage through the mosfets is increased, more
resistance at the gate is needed.
 
On 2009-01-01, causalitist <nate53143@hotmail.com> wrote:
i put a 220 OHM gate resistor on the mosfets and it works great. gave the
lm324 its little 12v source. ran 60v 20amps through the mosfets and the
motor responded very well (linearly and smoothly) to the speed adjustment.

thing is, i have no idea why! hahaha
Click to expand...
LM324 doesn't like capacitors (ex power mosfet gates) connected to its
outputs.

it seems that when mosfets and pwm generation use separate sources with
common ground, as the voltage through the mosfets is increased, more
resistance at the gate is needed.
Click to expand...
yeah, or use a mosfet driver.
 
On Jan 2, 4:02 am, Jasen Betts <ja...@xnet.co.nz> wrote:
On 2009-01-01, causalitist <nate53...@hotmail.com> wrote:

i put a 220 OHM gate resistor on the mosfets and it works great. gave the
lm324 its little 12v source. ran 60v 20amps through the mosfets and the
motor responded very well (linearly and smoothly) to the speed adjustment.

thing is, i have no idea why! hahaha

LM324 doesn't like capacitors (ex power mosfet gates) connected to its
outputs.

it seems that when mosfets and pwm generation use separate sources with
common ground, as the voltage through the mosfets is increased, more
resistance at the gate is needed.

yeah, or use a mosfet driver.
Click to expand...
sorry, im using a tc4422 driver, so i got 9amps peak to drive fet.
im having alot of trouble calculating an estimate for the gate
resistor, that 220ohm was a random guess.
i found a paper stating Rg=2*sqrrt(Ls/Ciss) - (Rdrv+Rg,i)

for my irfp4668
Rg,i = 1ohm (internal gate R)
Ciss = 10,720 pF (input capac)
Rdrv= im assuming this is driver resistence.. which is ~3ohms
Ls= SHOULD be on the mosfet datasheet, but its not! so im stuck.

is there an easier way to estimate gate resistor?

also, as far as a anti parallel diode across Rg to enhance turn off, i
realize the diode is only working when Ig>[(V,diode fwd)/Rg]
so i should look for a diode with the lowest "forward voltage" on its
datasheet ? or is it "forward recovery voltage" ?
 
ok. i just used [5*(mosfet rise+fall times)]/mosfet input capacitence.
i will finalize exact value with experimentation.

i hear about "distance"=bad from mosfet driver to mosfet... i realize what
this is dependant on everything, and why its bad.. but do you guys think 4"
of 12 gauge solid copper wire from tc4422 9amp driver to the mosfet is too
much distance?
 
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