PWM for the trolling motor?

[kofkorn]

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.

 

[nlester]

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

 

[kofkorn]

From Minn Kota's FAQ page:

https://www.whitworths.com.au/pages/adviser/minnkota_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.

 

[nlester]

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.

 

[kofkorn]

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

 

[nlester]

Enjoy