Forwarded Message from Ken Martindale

[Jonathan Wallace <jon@whoweb.com>]

Hello Gary,

I have a 30+ year background in Power Electronics. I read the website
information published by you on your Sub Vindicator. I was surprised
that the motor controller is simply a series of relays (power
contactors). I think this means that all the available battery
capacity is not used, which implies that space, weight and cost could
be saved by using the PWM technique briefly discussed by you.

My sub is a wet sub with a single 70 pound thrust Minn Kota trolling
motor (70AT/H). I  will  use 3 each 12 volt AGM lead acid
batteries. The motor cost $169 without their motor
controller. The motor controller was about $200. They also told me
that I could not operate the motor without the controller since for a
locked rotor condition the current through the armature would cause
sufficient MMF to demagnetize the permanent magnet stator. They were
fairly cooperative and referred me to Lake Motors where I talked to a
design engineer and confirmed the problem with demagnetizing the
permanent magnet stator. I then told the engineer that I had a
background  in this area and would do my own motor controller. I
asked about the operating frequency he would recommend. The Lake
Motors engineer was not able to give me any operating frequency since
he felt that was propietary. He did agree that if I used an operating
frequency of 20KHz I would be safe.

I contacted IXYS and got 4 MOSFETs which I used in the breadboard I
built in my garage for about $10 and  found the controller
to be a simple, average current mode control PWM using an existing
PWM IC. This was done primarily to eliminate the potential current
surge. This also gave me motor control by varying the pulse width.
The currents I switch are 56 amperes and the circuit is simple. If
anyone is interested I will provide a copy of the circuit. I do not
consider it to be anything but standard PWM power switching
technology. The real problem in the motor controller is controlling
the parasitic lead inductances to prevent excessive voltage spikes
when the circuit is fabricated.

I guess the point I am making is that I would be glad to help you
come up with a computer controlled PWM motor controller for your
Vindicator if you are interested. It will  improve your battery
utilization and provide uniform discharge for all the batteries.
Since the discharge current would be less for normal cruising you
should get longer cruising times and would prolong your battery life.
We could estimate costs fairly quickly for the components. My time
would be as a volunteer. It should be fun.

I was much impressed with your Sub and can understand the time it
takes to do something like that.  I was expecting my Sub to take
about 1 year but due to time availability it is taking more than 2
years.


Ken Martindale, EE





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