PWM for a high amp motor?

[Glenn Ashmore]

I have a 1500 watt 12VDC motor on the anchor windlass that I want to give
all the power it needs. I also want to slow it down as the anchor comes
onboard. I have built a little microcontroller program that counts the
revolutions and calculates how much anchor rode is out. When the count gets
less than 10' I want to start slowing the motor. The windlass has positive
forward and reverse inputs and a single common so I assume the speed control
should be on the low side. I figure adding a PWM routine to the PIC program
would be the way to do it.

The motor will draw about 120 amps under full load (which will be rare) and
about 30 amps retrieving the last few feet of anchor anchor rode when the
PWM will be required. I have a pile of IRFZ48V MOSFETS that are good to 72
amps at 12 mOhms on resistance and wonder if they would be of use stacking 4
of them in parallel. It also appears that some sort of diodes would be
required accross the motor terminals to protect the MOSFETs from voltage
spikes the current stops.

The questions are, am I on the right track and if so, what do I use to drive
the MOSFETs?

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

 

[Rich Grise]

On Mon, 14 Feb 2005 23:48:02 -0500, Glenn Ashmore wrote:

I have a 1500 watt 12VDC motor on the anchor windlass that I want to give
all the power it needs. I also want to slow it down as the anchor comes
onboard. I have built a little microcontroller program that counts the
revolutions and calculates how much anchor rode is out. When the count gets
less than 10' I want to start slowing the motor. The windlass has positive
forward and reverse inputs and a single common so I assume the speed control
should be on the low side. I figure adding a PWM routine to the PIC program
would be the way to do it.

The motor will draw about 120 amps under full load (which will be rare) and
about 30 amps retrieving the last few feet of anchor anchor rode when the
PWM will be required. I have a pile of IRFZ48V MOSFETS that are good to 72
amps at 12 mOhms on resistance and wonder if they would be of use stacking 4
of them in parallel. It also appears that some sort of diodes would be
required accross the motor terminals to protect the MOSFETs from voltage
spikes the current stops.

The questions are, am I on the right track and if so, what do I use to drive
the MOSFETs?

Yes, it sounds like you're exactly on track, although this is kind of an
advanced project to do from scratch, depending on how much experience you
have with building stuff.

Since you have 12V, all you need to drive the gates is something that can
provide current to charge/discharge the gate capacitance as fast as
possible. From what I've read, MOSFETs parallel really nice since Ron has
a postiive tempco, or something like that. I haven't been keeping up with
the latest chips and stuff, but I'm sure somebody's making something. Try,
for example:
http://www.google.com/search?q=mosfet+gate+drivers&btnG=Google+Search

Good Luck!
Rich