Need advice on very low power project

Recursive said:

I've been thinking about building various battery-powered tools. Some that need to be in a much higher power class than what this post is about. I have a few conditions to work with. 1st that I have the Hart 40-volt batteries which are not being used often enough & a pair of 3D-printed holders for those with 12 gauge pigtails(maybe a bit light?). 2nd, I want something in a power range of 1/2~3/4 horsepower.

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I don't want to describe the more interesting project ideas, as they might be worth developing later into a business. But, one example of the sort of thing I would like to do is a Dremel type tool, with a tethered battery on a short cable. Something that's intended to be carried on a shoulder sling, in a large pocket or set on a workbench. Keep the tool head as compact & lightweight as possible by having the motor controller located at the battery. The tool would have a trigger control, so possibly the cable will have three main power leads, and three light gauge wires to connect power,ground & signal. The trigger could be a potentiometer.

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Additionally, I would like something that is capable of controlling the battery amp draw - I'm new at this so I'm assuming there should be many different ways to do that. I also want a controller that can control how quickly it ramps-up/ramps-down on motor speeds & be reversing

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amberwolf said:

Recursive said:

I've been thinking about building various battery-powered tools. Some that need to be in a much higher power class than what this post is about. I have a few conditions to work with. 1st that I have the Hart 40-volt batteries which are not being used often enough & a pair of 3D-printed holders for those with 12 gauge pigtails(maybe a bit light?). 2nd, I want something in a power range of 1/2~3/4 horsepower.

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How much power you really need will be determined by the specific work you need it to do. Some power tools need speed, and torque is less relevant. Some need torque, and don't want speed. Some need both. Power is speed x torque. So if you can determine how much of each you need, you can then get a motor suitable for that for each tool, or one that is a good compromise of each if you are only making one tool (or that can handle the worst-case torque load at the highest speed).

I don't want to describe the more interesting project ideas, as they might be worth developing later into a business. But, one example of the sort of thing I would like to do is a Dremel type tool, with a tethered battery on a short cable. Something that's intended to be carried on a shoulder sling, in a large pocket or set on a workbench. Keep the tool head as compact & lightweight as possible by having the motor controller located at the battery. The tool would have a trigger control, so possibly the cable will have three main power leads, and three light gauge wires to connect power,ground & signal. The trigger could be a potentiometer.

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If you want the tool head to be lightweight, putting the entire electric system in the "base" and then using a spinning cable (like quite a few tools use, such as my Ryobi 125r weedeater, etc) to drive the head, will make a less efficient but much lighter tool head unit. The few powered dental tools I've run across as "scrap" over the years are also designed this way (some of them are air-driven, some are electric-motor-driven). The catch with this method is there is a limit to the bend curve of the drive cable, and the more it's bent the worse it's efficiency is (more friction against it's casing, etc). So it takes a higher power drive system, and more battery capacity/capability, to do the same work as if the motor is in the tool head.

But you can put a much bigger motor, with much better cooling for it, in the base than you can in the head.

Additionally, I would like something that is capable of controlling the battery amp draw - I'm new at this so I'm assuming there should be many different ways to do that. I also want a controller that can control how quickly it ramps-up/ramps-down on motor speeds & be reversing

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The motor controller does both of those.


The normal steps for creating a system to do a job using an electric motor, controller, and battery would be:

Pick a motor that can handle the worst-case load your usage will ever put on it, continuously, forever, without overheating. (ideally)

Pick a controller that can supply the power necessary for that load, continuously, forever, without overheating.

Pick a battery that can supply more than that power (because systems are nowhere near 100% efficient, probably more like 70-80% under heavy loads, or worse).


It's a bit more complicated since you are starting with specific batteries (the opposite of how one would ideally want to do this). You must determine what current output they are capable of, worst case, continuously, and then limit yourself to using controllers that will limit to less than that. Then pick a motor that will still do as much of what you want as possible given the power limitation of the battery you've chosen.

The motor must spin at the required RPM at the voltage you've already chosen, so you'll have to find one with the right kV (RPM/volt) to do this, that also fits in the space you have, and can handle the power you'll need for the job, and is also capable of the torque you need to do the job.

The controller will then follow the command you give it (via throttle, buttons, or computer control, depending on the control system you choose / design) to cause the motor to spin as much or little as you need it to. The typical ebike controller uses the control input to vary the PWM'd voltage to the motor, so it just controls it's speed. Without knowing exactly how you want your system to work, I'd say it's very likely that you would be better off with torque control, which uses the control input to vary the *current* to the motor instead; true FOC controllers do this. Some FOC controllers can operate sensorless, which would make your system simpler, as you only need the three phase wires from controller to motor, and there are no sensors in the motor to fail.

Whatever controller you use, if you choose one with a programmable current limit, you can then increase or decrease it's capabilities by changing this, to match the battery being used to power it. If you start with less-capable batteries, you can limit it to protect them, then get better batteries and increase the limit to match, and thus increase the capability of your system with a simple setting change. (assuming you've chosen a motor that can take the extra, too).

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