Re: [PSUBS-MAILIST] Re: Big motor for PSUBS...ATTN: K. MARTINDALE...

[Subscuba@aol.com]

Steve,

Elaboration might be a dangerous thing, but here goes with a little.

The trolling motors I am familiar with have permanent magnets as the stator ( 
the part that doesn't move). The armature or rotating part has brushes and 
coils. The motor is called a constant speed motor since when the armature is 
turning the back EMF just about matches the input voltage. So if the applied 
voltage is 12 volts you get one speed and 24 volts increases the speed. So by 
basically applying a varying voltage you get a varying speed, sort of, 
loading on the motor also changes the speed.

The controllers supplied by the vendors are simple Pulse Width Modulated 
switches operating at about 20 KHz and you're right some of them aren't too 
efficient and waste power. They do limit the motor inrush surge current.

Now comes the down side of not using a controller. If the armature is not 
moving as in starting up, the only thing that limits the input current is the 
armature inductance and resistance. The inductance limits the rate of rise of 
the current to a value of the applied voltage divided by the armature and 
brushes resistance. The current limit set by these resistances is very high 
and might damage the brush materials and more important the current passing 
the armature coils produces a magnetic field which opposes the permanent 
magnet stator. This field is high enough to demagnetize the permanent 
magnets. If this happens then the motor does not work although they can be 
sent in for repair of the brushes and re-magnetization of the stator magnets.

I also thought I didn't need a controller but as above I do need one. That's 
when I found out the controller cost more than the motor. The controller I 
designed and built is, I believe much more efficient than the commercial 
units and for me certainly cheaper. You can buy other controllers other than 
what Motorguide or Minn Kota sell. For the record the controllers are not 
true speed controllers since they do not control the speed just the applied 
voltage, they still work great for this application.

When I talked to Lake Motors (sorry I lost the contact there) they said that 
maybe you could apply 12 volts directly but certainly not 24 volts.

Hope this helps. 

Good luck,

Ken Martindale



In a message dated 9/18/00 11:35:12 AM Eastern Daylight Time, 
s_lindblom@conknet.com writes:

<< Can you elaborate on this - is it just a concern with the higher voltage
 motors, or all of them?
 
 I have a little 30 lb thrust motor I plan to use in a project, and was
 intending to run it flat out without the controller, as I gather a number
 of other people are doing. The controller it came with is fairly bulky, and
 uses the aluminum base of the top unit of the troller motor setup as a heat
 sink. I could just include all the circuitry, but I'm trying to set it up
 as a sealed module, and getting that much heat sink area would be
 difficult. The other thing about the stock controller is that it gets quite
 hot even at full throttle operation which suggests to me that the
 controller is wasting electricity that would better be used driving the
 motor.
 
 
 Oh, any contact phone number at Lake motors?
  >>