12V Motor Circuit Design - Bass Fishing Trolling Motor

[BassFisher]

Hey everyone,

I am a Mechanical Engineer by trade so this whole electronics world usually doesn't go over too well with me. I have been scratching my head as to how exactly I can control the propeller direction and speed of my trolling motor.

The model is a Minn Kota 55lb Thrust Endura trolling motor that runs at 12V with around a 55W load at full throttle, which means around 4.6A of current if I am not mistaken.

It broke recently and I have taken it upon myself to come up with a better solution to regulate the speed so it is infinitely adjustable from 0% to 100% and not five different set speeds.

I was thinking of using a DPDT switch wired so that it will reverse the polarity, F-Off-R, but I am having trouble finding the right device to regulate the voltage so I can slow it down. I was thinking of using a potentiometer but I cannot find one rated high enough, thought about using a house dimmer switch but I guess that won't work, a rheostat but I can't find one that will work correctly, and finally an infinite switch such as one used in an electric stove range.

I have no idea what device to use to lower the voltage and how to wire it, so I am hoping some members here can help me out!

And as a side question:

At 12V, 55W, 4.6A does that mean a resistor with a 3ohm value will drop the voltage enough to stop the motor? It should be able to drop it just shy of 14V I think.

Thanks for any and all help!

 

[carbonzit]

I'm going to try to help, but I'm not the expert you need. There are some here, though.

Basically, using a resistor of any kind to control your motor's speed is a Bad Idea, for at least two reasons:

1. Wastes power (as heat)
2. Reduces motor torque

You need some other way of controlling the speed. Without going into the gory details (others can and will do that), one way is PWM (pulse-width modulation), where the motor always operates at the same voltage (but how could that be? you say), but with a square wave with a variable duty cycle (= proportion of on/off time). This includes a square-wave generator (oscillator), a PWM modulator and a pulse-width control, along with an output stage that can handle the motor current. Not too hard to cook up for a medium-sized motor like yours.

 

[BassFisher]

Ah wow I had no idea.

So something like this then:

12V 30A DC Motor Speed Control PWM HHO RC Controller | Virtual Village - US

Will probably be the easiest and cheapest method to solve this problem? That is unless I feel like ordering a PCB and wiring up my own custom circuit...which with my luck will go up in flame haha.

 

[cr0sh]

To get your PWM signal, the best and easiest way would be to use a 555 running as an adjustable astable multivibrator. Use that to drive a MOSFET (which may need a heatsink), the output of the mosfet could then go into a DPDT with center-off, wired in an h-bridge fashion. Alternatively, you could sub a small h-bridge driver (sized for the motor) for the MOSFET, with the 555 output switched via a simple SPDT switch (with center-off) to the enable pins on the h-bridge, and the h-bridge drives the motor.

 

[crutschow]

Generally understood but not mentioned by cr0sh, is that you need a free-wheeling diode across the motor (cathode to positive) to allow the motor current to keep flowing each time the transistor turns off in the PWM cycle (for efficiency and to avoid zapping the transistor).

Here's an example of a PWM circuit.

 

[Reloadron]

Many years ago I used this circuit for some small DC motor applications and as I recall it worked quite well. It is a pretty simple design for a 12 volt motor speed control using PWM. I have no clue how well it will work with a trolling motor it should. For reversing direction with a trolling motor I would just think about a DPDT switch. The circuit I inked to also has the PCB artwork. As the author points out, using a larger transistor for Q1 (the MOSFET) will allow driving the load up to about 10 amps. I would fuse the input side and load side just for safety. Anyway it is something to consider.

Ron

 

[KeepItSimpleStupid]

I've used this kit for a few things and I was pleased. K8004: VELLEMAN INC.: Education & Hobby A DPDT switch for reversing would work fine, just make sure the motor is stopped before doing so.

An "H Bridge", the electronic way to reverse, may waste too much power.

 

[Dragon Tamer]

My suggestion is... keep_it_simple_stupid (wink-wink). If you are using a swithc to control your motors speed and direction, first off you will need a DPTT switch not a DPDT (that would be F/R) Second, the best way to control your motors speed is some sort of PWM (this could even be used to control the motors direction). But if you are looking for an even simpler solution, try a rheostat. It's like a potmeter but at much lower resistances and much hgher rated currets. You should be able to find one that can handle the 55W your motor is running at (go with at leas a 60W to handle the motors startup currnet). Connect it to the positive terminal of the battery and the whiper should go to the ceter pole of your switch. (P.S. Crutshow I enjoyed the link you posted, that guys accent made my day )

 

[BassFisher]

Thanks for all the replies!

Also, I found an ESC laying around...

It is a Novak Super Rooster ESC which can handle 320A at 12V in the Forward direction and 160A in the Reverse direction.

My question is, will this work with my application? And if so how the heck do I wire this to work correctly?

My past experiences with ESCs are solely with radio control applications and I have no idea how to use it in this application.

Thanks!

 

[KeepItSimpleStupid]

Dragon. A DPDT or ON-OFF-ON would sort of work, but a 3 or 4PDT switch that I have used for hoists would work better, That gives you a few contacts for control, so OFF is really off. I'd have to do alot of research to see if switches such as these Switch, Drum Reversing - Manual Motor Switches and Starters - Starters and Contactors - Electrical : Grainger Industrial Supply could work.

I can't figure it out: **broken link removed** I can't even find a wiring diagram. I don't know how people can sell this stuff. I think people are getting dumber.

 

[BassFisher]

Google Super Rooster, second link down and it is a .pdf of the manual. Novak makes a great product that is really reliable and functions very well...these ESCs actually rose in price when they were discontinued because they are so sought after. The forums will not let me post a link yet...

I think I may be able to use a servo driver to control the ESC...now I just need to find the right one!

 

[Dragon Tamer]

Your starting to get more complicated in your design, you need to try to keep this as simple as possible. The more you add, the more likely it is that something will go wrong. Stick with the PWM idea, it is a little more on the complicated side but you can control speed and dirction fairly easily using this method.

 

[ronv]

If you don't want to build something maybe something like this with the DPDT switch to reverse direction would be a good buy.
**broken link removed**

 

[adamey]

I think you've got your current ratings mixed up. A typical 55lb thrust motor running on 12V would consume closer to 50A not 5A. It might only draw 5A running on a bench with no load, but in the water the current is going to be much higher.

That's going to make things a little more complex. You won't be able to use an RC speed control as the way they rate them is a little hokey - when they say 100A they mean 100A peak. You're not going to be able to run 20, 30 or more amps continuously from an RC ESC.

I think what you want is to find a used controller from a golf cart (bigger than you'd need) or maybe a wheelchair (probably much closer to your needs).

The other problem is how your existing motor is wired. Usually the motors with fixed speeds have "coils" which are bascially wire-wound resistors to limit motor output (very inefficient). These coils are often inside the motor housing for heat sinking ability. You'd need to bypass these coils and wire your circuit directly to the motor.

 

[4pyros]

I think what you want is to find a used controller from a golf cart (bigger than you'd need) or maybe a wheelchair (probably much closer to your needs).

Use a "Electric Scooter Controller". Andy