PWM for the trolling motor?

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kofkorn
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PWM for the trolling motor?

Post by kofkorn » 15 Apr 2015, 14:38

Hi Luvxu,

No reverse. I simply spin the motor around. The newer Powerdrive motors don't have a reverse either. As to the connections, I don't have a diagram made, but it's not terribly complicated. These controllers have power and ground from battery, power and ground to motor, and then three wires for the control (5V+, gnd, Signal)

The only challenging thing on this particular motor was breaking out the wiring AFTER the power was split off to the steering motor. If I had simply placed the controller on the main power leads, the PWM would have been applied to the steering as well. That would have significantly slowed the steering function down depending on the motor speed. So I got into the wiring and spliced into the power lead that went to the propulsion motor only. It was a tight fit, but worked perfectly afterward.

I also added a PWM controller to a cable steer motor. That was much more straight forward. I simply added the controller into the boat wiring under the bow and didn't do a thing to the motor. Then all I had to do was leave the original motor switch on "5" and use the PWM to control the speed instead.

Each motor and controller would be a little different.

Good luck!

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bAcKpAiN
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PWM for the trolling motor?

Post by bAcKpAiN » 15 Apr 2015, 15:14

I may be over simplifying it but couldn't you just put in a variable ohm resistor in the circuit where the current speed setting switch is so you can change the resistance there in a linear way rather than the speed settings? I did something similar when the speed setting switch broke on my old Evinrude and I couldn't find a replacement. I forget the specs, but I was able to find a variable ohm potentiometer in the right range at radio shack and it worked great until it got wet. I then got one on eBay that was waterproof. They are used for the speed settings in wheelchairs/golf carts etc...

Here is one similar to what I got for mine, can't find the exact one. Worked great on a 24V 65lb thrust motor.
https://www.ebay.com/itm/1-X-TOCOS-Throt ... 3f49bb4e15

Again if this isn't what you are looking for I apologize!

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nlester
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PWM for the trolling motor?

Post by nlester » 15 Apr 2015, 22:06

bAcKpAiN wrote:I may be over simplifying it but couldn't you just put in a variable ohm resistor in the circuit where the current speed setting switch is so you can change the resistance there in a linear way rather than the speed settings? I did something similar when the speed setting switch broke on my old Evinrude and I couldn't find a replacement. I forget the specs, but I was able to find a variable ohm potentiometer in the right range at radio shack and it worked great until it got wet. I then got one on eBay that was waterproof. They are used for the speed settings in wheelchairs/golf carts etc...

Here is one similar to what I got for mine, can't find the exact one. Worked great on a 24V 65lb thrust motor.
https://www.ebay.com/itm/1-X-TOCOS-Throt ... 3f49bb4e15

Again if this isn't what you are looking for I apologize!
This is how I understand Speed Coil vs Digital (PWD).

What you are describing is the way a traditional variable speed coil trolling motor works. The speed coil is a variable resister and it burns off part of the voltage as heat so it varies the voltage going to the motor. You get the same amount of time from the battery on the water at the highest speed as you do at the slowest seed.

A PWD switches the motor on/off at high rate of speed so very little power is wasted in heat. Digitally switching the power on/off controls the speed by sending maximum voltage in varying pulses (off/on periods). If you are running at the slowest speed with a PWD, you can get 4 to 5 times the amount of time on the water with the same battery. You are converting your motor to a digital motor similar to Minn Kotas or Motor Guides digital motor.

The more you run at a slower speed, the greater your time on the water with a digital speed controller.
At full speed there a should be little or no advantage.

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kofkorn
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PWM for the trolling motor?

Post by kofkorn » 16 Apr 2015, 09:13

bAcKpAiN wrote:I may be over simplifying it but couldn't you just put in a variable ohm resistor in the circuit where the current speed setting switch is so you can change the resistance there in a linear way rather than the speed settings?
nlester wrote: The speed coil is a variable resister and it burns off part of the voltage as heat so it varies the voltage going to the motor....

...At full speed there a should be little or no advantage.
nlester, you are partially correct. The speed coil is actually not variable. There are usually two or three different speed coils in the motor housing, each with a different set amount of windings. The windings provide the magnetic field for the motor armature, and the magnetic field controls the speed of the motor. The switch at the top usually stacks these three coils in different combinations to get varying windings, and therefore varying speeds. It's essentially having varying amounts of magnetic field based on how many times the motor is wrapped. However, you're applying a full 12V every time.

So bAcKpAiN, in theory, your idea would be OK and the simplest way to control the speed to the motor. The challenge is that a typical trolling motor draws a VERY large amount of current. @12V ~1A per lb of thrust. So a 50lb thrust motor is drawing about 50A at 12V. Putting that together means 12V x 50A = 600 watts of power. The potentiometer that you listed is for signal voltages and would fry in a matter of seconds the first time you used it if placed inline with the motor. Now if you go looking for a 1K 600W rheostat, you're going to find they are in the $200-$300 range. The rheostat would need to be able to dissipate up to 600w of power to work properly.

As nlester stated, the PWM unit does this far more efficiently so it doesn't need to dissipate nearly as much power. This means that you will get more out of your battery.

You just need to make sure that you are sizing your components properly for the application.

Good luck!

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bAcKpAiN
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PWM for the trolling motor?

Post by bAcKpAiN » 16 Apr 2015, 19:27

I understand what you are saying but aren't the speed control switches wired into a foot pedal where that kind of juice isn't flowing? I know my battery life varies greatly depending on my speed settings. Don't all trolling motors already have some sort of variability built into them?

If you were building one from scratch (plain dc motor) I would see the need, but not a comercial trolling motor with speed settings already built into the design. If i understand correctly the OP was looking to replace the canned speed settings with a variable linear speed control.

I guess they have different types of speed control all together ?

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kofkorn
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PWM for the trolling motor?

Post by kofkorn » 20 Apr 2015, 09:05

There are generally two different types of speed control available now. The "5-Speed" or multi-Speed types are still using speed coils to control the speed of the motor. They don't use the same amount of power in low as they do in high, but they do use a lot more in low than a unit with a PWM controller.

The units that have a digital speed control (I-pilot, Co-Pilot) or a variable speed control (Maximizer) are using various types of PWM to control the motor speed.

Currently the speed coils are still cheaper to produce than the PWM electronics. I wouldn't be surprised to see that change in the future, and to have speed coils go away, but for now, they're still commonly used.

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nlester
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PWM for the trolling motor?

Post by nlester » 20 Apr 2015, 16:12

Question - How do the speed coil motors work to control the speed?

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kofkorn
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PWM for the trolling motor?

Post by kofkorn » 20 Apr 2015, 16:36

From Minn Kota's FAQ page:

https://www.whitworths.com.au/pages/adv ... faq.asp#hs

How does Maximizer compare to a speed coil?

How Speed Coil Motors Work

Electrical loads are hooked up in series with the motor such that power from the battery is split between supplying the motor and supplying the additional loads (speed coils). Power from the battery is fairly constant regardless of the speed setting. Therefore, on speed 1 for example, more power is diverted to the speed coil than is supplied to the motor. This makes the overall motor assembly less efficient (vs. variable speed/maximizer). Extra power consumed by the speed coil produces heat, much like an electrical heating element. The heat is then transferred through the motor housing to the water. That is why speed coils are located in the lower motor unit.

Speed Coil Analogy

You are driving a car at full speed. To slow down, you brake with one foot while continuing to keep your foot down on the acceleration pedal. By doing this, you continue to burn petrol (power) while causing the brakes to heat up. Same theory applies with a speed coil motor. The power supply is still constant, but is diverted to the speed coils for the lower speeds which reduces efficiency and run time.


How Maximizer Works

Since speed coils reduce the power going to the motor, the Maximizer was designed to, eliminate this inefficient speed control method by using rapid on/off switching. The Maximizer is simply a switch that turns the connection to the battery on and off 20,000 times per second.
If “on” time is longer compared to “off”, the motor runs receive more power. If “off” time is longer, the motor gets less power. Since there is minimal power lost to accomplish this, the overall motor is very efficient.

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nlester
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PWM for the trolling motor?

Post by nlester » 20 Apr 2015, 21:14

It has been 55 years since I have tried it think this through but if the switched speed coils are in series with the motor's armature and the resistance of the armature does not change, then a higher resistance speed coil has to switched in to the circuit causing a higher total resistance with a greater amount of amperage being drawn for each step the speed is dropped. The amperage draw and total resistance for a speed coil motor will highest the lowest speed.

I had forgotten that power (watts = volts * amps) will vary as the amps vary with the total resistance.

I assume that physical conditions, current flow, wind and weeds will cause small variations in the amperage draw.

Will the constant stopping and starting of the motor with a PWD cause a higher power draw as you run the motor at lower speed?

It's been a long time since high school.

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kofkorn
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PWM for the trolling motor?

Post by kofkorn » 21 Apr 2015, 08:38

nlester wrote:It has been 55 years since I have tried it think this through but if the switched speed coils are in series with the motor's armature and the resistance of the armature does not change, then a higher resistance speed coil has to switched in to the circuit causing a higher total resistance with a greater amount of amperage being drawn for each step the speed is dropped. The amperage draw and total resistance for a speed coil motor will highest the lowest speed.

I had forgotten that power (watts = volts * amps) will vary as the amps vary with the total resistance.
You're on the right track, but a little backwards. P=VI as you stated above (I=amps). But also V=IR. So if the voltage from the battery remains the same, then if you increase the resistance, the amps will drop. But motors throw a little bit of a curve at you. The windings of the motor have a fixed resistance, but they also develop an additional resistance from the reactive power of the magnetic field. As the motor increases speed, this reactive resistance increases. So a motor will decrease resistance when it is under load or starting up.

In the end, the speed coils are burning off some of the voltage that would normally have been used by the motor and effectively lowering the voltage the motor sees. Essentially dumping the extra energy into the water as heat.

I'm not exactly sure why the PWM circuit doesn't develop the same high current draw at slow speeds. The motor is seeing the full 12V in very small pulses, and it does something to the magnetic field that is better for the motor. I know with my PWM controller, I could slow the motor prop down to a single revolution every 2 seconds. Even if I stopped it at that speed and turned it on again, it would maintain the very slow speed. It was wonderful, because I could creep VERY slowly along the shoreline all day long.

If I'd lowered the voltage (or used resistance) to slow the motor down that much, it would have been stalled and would have quickly burned the windings.

So however it works, I'm glad I can use it :)

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PWM for the trolling motor?

Post by nlester » 22 Apr 2015, 13:13

Enjoy

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