Re: posted email 27 mar,1998 17:43

[Subscuba@aol.com]

Shayne,

The problem I was referring to occurs with the starting surge of current that
happens when the motor has a sudden application of full running voltage such
as, in your case, 12 volts. What happens is that your motor armature cannot
come up to speed and generate a back EMF or voltage that would limit the
current. When the armature, that's the part that rotates and drives the shaft
and propeller, is stationary, the only thing that limits the current is the
resistance of the armature and brushes. The rate of rise of current with a
stationary armature is limited by the motor inductance. So if you have a
locked or jammed propeller the current can reach levels of 50 to well over 100
amperes. This current flows in the armature windings and generates a strong
magnetic field. This strong magnetic field can demagnetize your permanent
magnet stator. Once the magnets are demagnetized the motor doesn't drive your
Sub at Warp speed anymore. Impulse power is also low.

The above is somewhat of a generalization since in some permanent magnet
motors the magnetics can resist the armature fields for the locked case. The
motor I am using is Minnkota's 70 pound thrust and is rated for 24 volts. I
talked to the people at Lake Motors about this problem and for my motor
current limiting is necessary. It may be possible that your lower voltage
motor can stand a locked rotor condition. I would ask Minnkota. Bear in mind
they sell motor controllers for their motors. As far as I know their
controllers are OK, but for mine was too expensive so I designed and built my
own.

What is meant by active current limiting is a circuit in series with your
motor that senses the armature current and usually by PWM techniques limits
the maximum current to some safe level. This type of active current limit also
will control the motor speed by applying a variable average voltage. I would
again suggest calling Minnkota and find out if you really need a controller.
If you don't, then don't use one. It will keep your system simpler. A simple
contactor with an ON/OFF switch is hard to beat.

I hope this answers your question. It is difficult to tell how much power
electronics experience this forum has so I tend to assume the worst. If you
are in engineering see if you can get a copy of Electrocraft's book on
permanent magnet motors. It will answer all your questions.

Good luck with your Sub, I wish you the best. I have finished mine but have
been unable to get enough free time to launch it. It looks real pretty in my
garage but I would like to see how pretty it is in the water.

Ken Martindale