Controlling an electric trolling motor for boats
tim hirzel
As a big nerd, I am trying to setup autopilot/remote control for my
canoe.
I need some advice. I have a small 12 V dc trolling motor from a
company, Minnkota. Its a 30lb thrust model for small boats, and draws
up to around 25 amps max. Right now, is has 5 forward speeds and 3
reverse speeds. Upon opening it up, I see there are 4 wires leaving
the speed switch assembly going down the motor. There is the black
negative wire, a large red wire, a medium yellow wire, and a smallish
white wire. My assumption is this motor is set up so you can just
apply power to one or more of the three red, yellow, white wires to
adjust the speed. Is it right that each wire connects to a chunk of
the windings on the motor? With no potentiometers etc, is this an
efficient way to control a motor's speed?
My goal is to control the motor speed electronically. Here, I have
two options. One simple option is that I could use some relays and
mimic the current mechanical switch's behavior, switching power to
each of the wires, and adding the ability to reverse polarity for
reverse speeds. Is it possible instead to use a fancier speed
controller and treat all three of those wires as one? The advantage
of this second option would be continuous speed control instead of
discrete steps. On the other hand, I am not sure though whether this
would reduce efficiency or otherwise present problems. Any other
thoughts on controlling this motors speed with 5V signals?
Any thoughts on this control issue, or information about how this type
of multiwire speed control works would be greatly appreciated.
Here is a link to a pdf with a "schematic." It doesn't provide much,
but maybe the picture can help illuminate the situation.
http://www.mikesreelrepair.com/schematics/schematic.php?url=Minn%20Kota/2007/transom/endura/Endura%2030.pdf
Many Thanks,
Tim Hirzel
Toma Dordea
When you say 5 forward speeds and 3 reverse speeds what do you mean ?
What happens if you select one of the positions ? What does the switch
do ?
Eventually you can determine what is connected to what by measuring the
resistance with an Ohmmeter between different terminals for different
switch positions (after separation of the switch from the motor)
DC motors' speed can usually be changed by either changing the applied
voltage to the rotor (field - usually over the assembly
collector-brushes) winding or the voltage applied to the excitation
winding (stator).
Rotation direction (CW or CCW) can be changed by swapping the (+) and
(-) at one of the windings (but not at both, since in this situation
the rotation direction will not change).
The 2 windings can be connected either in series or in parallel.
Rotor wiring has usually larger wire size connected to it. Excitation
has usually a smaller wire to it (if connected in parallel). If there is
a series connection between the 2 windings, the wire size shall be the
same (in this case there are usually only 2 conductor ends ( the end of
field winding and beginning of excitation are connected together).
Toma P. Dordea, PE
ALFATECH CAMBRIDGE GROUP
97 East Brokaw Road, Suite 300
San Jose, CA 95112
Tel.408.487.1200 Fax.408.436.1511
Dir.408.487.1258 Cel.408.726.2897
mailto: toma....@atcginc.net; t.do...@ieee.org
http://www.atcginc.net
tim hirzel
I took your advice and I disconnected the switch from the motor to
test. As far as I could measure with my multimeter, there are no
resistors in the switch mechanism. I did find a pattern of how the
switch's 9 rotational switch positions (3 reverse, one off, and 5
forward) correspond to the powering the wires going to the battery.
Here is a chart I made:
key to the chart:
R: connected to the Red Battery Wire
B: Connected to the Black Battery Wire
/: not connected to either battery wire
(in the special case of speed 2,
the red and yellow motor wires are connected)
Wires going to Motor
switch
position Red Yllw Wht Blck Current (when motor is in the water)
-----------------------------------------------------------------
-3 B B B R NA
-2 / B B R NA
-1 / / B R NA
0 / / / / 0 A
1 / / R B 7 A
2 (---------) R B 9 A (in this case the Red and Yellow wires are
connected just to each other, no battery, I don't understand how this
makes the motor turn faster)
3 / R R B 12 A
4 / R / B 14 A
5 R R R B 24 A
------------------------------------------------------------------
>From this, it appears that powering the additional wires adds speed
and current draw. When unloaded, I noticed that the motor draws
around 2 amps all the time, and the current seems to be associated
more with time (it appears to decrease over time) than with switch
position. I hope this illuminates the scenario somewhat. Another
clue with this motor is that it is about $100, so whatever is going on
is going to be simple and inexpensive. I am wondering if I can
connect the white, red, and yellow wires together, treat them as one,
and control the motor with a brushed motor controller?
cheers,
Tim
> mailto: toma.dor...@atcginc.net; t.dor...@ieee.orghttp://www.atcginc.net
> but maybe the picture can help illuminate the situation.http://www.mikesreelrepair.com/schematics/schematic.php?url=Minn%20Kota/
tmem...@gmail.com
It is difficult to see exactly how the motor is working from this, 2,.
3 and 4 have me thinking at present, however, I would say that you
should be able to connect as 5, and vary the voltage to vary the
output torque.
The problem is the voltage variation at the current that you could be
drawing. Any series element such as a linear regulator, or a resistor
is going to dissipate power and if you are operating off a battery,
that means a shorter battery life.
You could use some form of switching regulator to reduce the losses.
I am not sure what access you have to this type of technology, but it
is worth a try.
Best regards,
Mark.
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