Running a 24V motor on a 12V boat system

Question

I'm trying to design a jigging machine for a fishing boat, using a RaspPi as the controller. (A jigging machine jerks a 1kg lure up and down near the sea bed, and pulls in a fish when it's hooked.)

The motor I'm hoping to use is here: 3336_0 - 86STH156 NEMA-34 Bipolar Gearless Large Stepper and the supplier recommends I recommends I use a 24V supply.

However, the boat I want to fit the machine to has only 12V. I'm trying to assess the pros and cons of different approaches.

The simplest solution is to just use a 12V supply to the motor. I understand I'll need thicker wires to supply this. I'm thinking that using 12V will generate more heat and produce less torque. There will be times that the machine will need to pull up several fish at a time from 50m deep - say a 20-30 kg load. And sometimes the hook will snag the bottom. I'll include a current monitor, so I could limit or cut the supply when there is too much load. I don't know how often this might happen till I try it though, and how inconvenient it would be.

I've ruled out having 2 batteries, in parallel when the engine is running and in series when the engine is stopped, using solenoids to switch - the engine is often left running while the boat is fishing, as it needs to continually drift with the tide/wind, then motor back up to the spot where the fish are.

So I think the only other option is to buy a second battery and wire in series, swap the 12V alternator for a 24V alternator, and have something that supplies 12V from the 24V for the GPS, radio etc.

Naturally, I'd favour the first option, but I wonder how important a 24V supply would be for the jigging motor, and how much we'd lose out going with the 12V option.

Thanks for any further input on my inputs quandary.

John

Answer 1

Lots of issues here. Firstly, a stepper motor requires a current-controlled driver, which is effectively a buck-mode switching converter using the motor's winding inductance as the output inductor. What happens is that while stationary, the current will be at some chosen level (max 3A/winding on this one); your selection of which defines the holding torque. Note the coil resistance is 0.9R, which means there will be only 2.7V across each winding, hence the need for the switch-mode driver. The rest of the voltage is expressed across the winding inductance with a duty cycle of approximately 2.7/Vin, e.g. 11% duty-cycle for a 24V supply.

If you directly connect the windings of this motor to a 24V DC supply, it will draw 24/0.9 = 26.6A => 640W per winding and rapidly decompose into a nice little puddle of smoking char and molten copper.

The reason people use high supply voltages for steppers is that dI/dt = V/L and if you want to step the motor, you need to reverse the current in one of the coils. The faster you can reverse the current, the quicker you can step the motor. If you don't need to step quickly, you don't need a high supply voltage. When you're reversing the current in a winding, the full supply voltage is applied, causing the current to change rapidly to the opposite direction. Once it has reached the appropriate peak value in the opposite sign, the controller resumes switching to limit the current.

If you use a 12V supply instead of 24V, your max stepping rate (max speed) will be approximately halved (because the current rate-of-change is halved), and the torque available at high speeds will also be reduced.

Assuming you use a proper current-controlled driver, the heat dissipated by the motor will be the same regardless of supply voltage, it is R*I^2 in the windings. A lower supply voltage means that the switcher operates at a higher duty-cycle. The same current will flow through the windings but twice as much current will be sourced from the power supply. In other words, I'm sure that this motor will be just as good for your purposes on 12V as it would be on 24V... but it is not an appropriate motor.

You should use a normal series-wound or permanent-magnet DC motor instead. No need for a big, expensive and delicate stepper controller, plus the motor will should be cheaper and definitely more efficient than a stepper.

Windscreen-wiper or car window-winding motors might be a good place to start looking for cheap options. In general, look for a gear-motor with integrated gearbox, you'll find one with plenty of torque if you look.

Answer 2

https://forums.lr4x4.com/topic/20463-12v-winch-24v/

The 4WD boys run 12V winches on 24V and they are fine.