# Overvolting DC motor



## mecanum (Jan 8, 2015)

Hi,

I have a 36v Li-ion battery pack & 12v Minn kota trolling motor.
I bought a PWM controller (that can sustain up to 48v / 3000W). 
The motor has 4 forward speed / off / 2 reverse speed. 
If set on the fourth speed, the current is directly going to the motor. Using the PWM controller, I can control the power output.
Following the logic that if I can control the power output, then everything should be fine - (right?). 
My second concern is the efficiency. I suppose it should be higher since the power dissipated is proportional to I^2. Am I forgetting something here? 

Typically, I am running at around 80-100w which is fast enough for me. 
The battery pack I am using is limited to a 2.5A draw at 36V (90W max). 
Given this, shouldn't I be fine to run on higher voltage?


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## madderscience (Jun 28, 2008)

The PWM controller effectively steps down the voltage. So as long as the motor type on the minn kota is compatible with the controller (DC permanent magnet brushed most likely) then it should work. Hopefully the controller is configurable, If it is you might be able to limit the output voltage to a preprogrammed maximum.

Failing that, I assume the throttle is a 5Kohm pot; that is usually the case. You can run an expirement starting from zero throttle and figure out where (under load) on the throttle you get to 12V across the motor. That will be different in the water vs. on land so make sure and do the test wet. Assuming everything is proportional a guess would be to limit your throttle to the first 1/3 of travel, which you can do with a simple resistor network, like perhaps a 2K pot in series with a 3k fixed resistor.

good luck.


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## mecanum (Jan 8, 2015)

madderscience said:


> The PWM controller effectively steps down the voltage. So as long as the motor type on the minn kota is compatible with the controller (DC permanent magnet brushed most likely) then it should work. Hopefully the controller is configurable, If it is you might be able to limit the output voltage to a preprogrammed maximum.
> 
> Failing that, I assume the throttle is a 5Kohm pot; that is usually the case. You can run an expirement starting from zero throttle and figure out where (under load) on the throttle you get to 12V across the motor. That will be different in the water vs. on land so make sure and do the test wet. Assuming everything is proportional a guess would be to limit your throttle to the first 1/3 of travel, which you can do with a simple resistor network, like perhaps a 2K pot in series with a 3k fixed resistor.
> 
> good luck.


Hi, thanks for you answer.

This is the PWM controller I have:
http://www.ebay.ca/itm/DC-10-50V-60...734?pt=LH_DefaultDomain_0&hash=item2ee09b77be

I can adjust power output with the knob. I have a watt meter that calculate the power thus, I can easily set the load at the desired value. 

I don't understand the part of your post about the throttle (actually the second paragraph). I would really appreciate some elaboration (I am a newbie). 

Thanks again.


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## jwiger (Oct 18, 2014)

mecanum said:


> ...I don't understand the part of your post about the throttle (actually the second paragraph). I would really appreciate some elaboration (I am a newbie).


Pots, or potentiometers are basically variable resistors. Think a volume knob on an old radio. The PWM controller sends a set voltage to the pot (usually 5 volts) and depending on how much comes back through the pot it throttles the big motor. 

If one third throttle on your motor controller gives you the right voltage at the motor, you could add another resistor, in-line to the pot. Which would allow you to use the full sweep of the throttle with-out going over the power limit of your motor. 

The knob you mentioned it probably that pot. Unless you are talking about an extra knob on the PWM motor controller.


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## madderscience (Jun 28, 2008)

Looking at the link to the motor controller you sent, yes, jwiger is right. The knob is the pot, or variable resistor, or throttle control.

A typical potentiometer has 3 wires going into it: the two ends of the fixed resistance, and a wiper (the variable piece) which moves back and forth across the fixed resistance.

I see the controller you have has TWO sets of 3 wires going into the speed control knob. this means that there are two variable resistors in there, probably for safety reasons if I had to guess. You would need to make the same series resistance modification to each. Another way which might be easier if you aren't electrically inclined is just design a control panel which mechanically limits the travel of the knob to your desired range.

Good luck.


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## mecanum (Jan 8, 2015)

Thanks guys for your help!


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