PWM Controller

[Subscuba <Subscuba@aol.com>]

Hello Shayne,

On the subject of your need for a motor controller. First of all I would check
with Minnkota to see if the motors you bought require a current limit to avoid
demagnetizing the permanent magnets in the stator. If this is not necessary
then you might be able to avoid the motor controller all together. Be aware
that a locked propellor may cause excessive heating in the motor. This keeps
your system simple which is a very good thing. If the current limit is not
required then you can control the motors with a simple contactor or solenoid
such as a Stancor  70-902 and a diode. The diode greatly reduces the arcing in
the contactor. Make sure the contactor is continuous duty rated. You can also
buy these at a West Marine or E&B Marine. Don't get a starting contactor since
they are normally rated for intermittant duty. The contactors look alike, read
the label for continuous duty. You can control the speed by simply turning the
motor off and on quickly. This assumes you don't require cruising at anything
but full on. Reversing the motor will require a reversing relay.

However, if you still want to do your own motor controller, I would suggest
that you give it a lot of thought. I suspect you will need one. You can buy
Minnkota's controller and that would reduce your labor in doing your own
controller and may be less expensive. I will be glad to help you with the
design of your controller but I need to know what you want the controller to
do, such as reversing with a bridge or use of a reversing relay. Do you want
simple Pulse Width Modulation (PWM) without motor speed feedback or something
more complex? I would also need more information on the motor.

If you want to pursue this further, I will be glad to send you a copy of my
motor controller schematic which your father can easily follow. Bear in mind
my design operates off 36 volts. Yours might be simpler. That would give you a
start on what a simple controller looks like. It may also be possible that the
motors you bought have multiple armature windings for speed control which
makes your problems simpler. Does your motors have multiple leads or just
black and red leads?

You also asked where information can be had concerning design information for
motor controllers. I would suggest that you start by contacting any local
University Library and looking up texts on the subject. There is a lot of
detailed information published which may be more than you need. Another place
to start is the Unitrode Applications manual. Their information would also
give you a starting place.

One thing about constructing your own motor controller is that the physical
construction of the circuits is critical, especially the lead lengths between
the power switch and the batteries or power source. Lead inductances of 1
microhenry can easily cause excessive voltage spikes across the power switches
destroying them. 

My background is power electronics so I didn't need to do much research for my
motor controller. I also, coincidentally, picked the same switching frequency
as Minnkota's design which they consider proprietary. I would like to see
their design, I think their efficiency is lower than my design. I'm good at
patting myself on the back except my elbow gets sore along with my foot for
the times I shoot it. My foot is sore most often. 

Have fun and good luck,

Ken Martindale