Best type of motor/generator to generate from a 10mm linear movement

Question

Firstly, this is not a homework etc question, but am trying to make a proof of concept for something. I have a mechanical force acting on a shaft (threaded bar) moving it in the direction indicated in the pic below:

Movement is roughly 10mm and force roughly 50kg/mm or 0.48 Nm. I an using this to drive a motor via a rudimentary crank. The movement is going from right to left in the pic above and is a one time thing, so results in about 10 to 20 degrees of rotation.

The first motors I tried were both brushed DC geared motors, 30:1 and 120:1, so this converted my small movements into a few turns of the actual motor. I also tried a stepper motor, I posted the results here.

My problem is, I'm not getting nearly the amount of power from the motor as I would like to see. I understand the power in watts is a factor of the time taken for the 10mm movement, and this is, in my setup, affected by the load, but by looking at the outputs on an oscilloscope I can view the duration of the 10mm movement.

The best result I got was 0.6W out for a theoretical mechanical input of nearly 5w (operation in roughly 100ms) which is ~10% efficiency. I think I'm loosing a lot in the gearbox, firstly having to accelerate it and the motor from a standstill (it's very hard to turn [get started] by hand with no load), then the frictional losses, and I think the fact I'm putting power in the "wrong way", into the output shaft is not helping.

Also, the speed of the motor is low, well below where it is designed to run as a motor.

The stepper motor seems to max out at ~0.6W even when driven by a mains drill @2,700 RPM - I salvaged it from a printer so no specs for it.

So my question is, what, if anything, might yield more power in my setup, where I can only turn the motor relatively slowly and for just a few turns at best. What will generate good power (in the 2-8 W range, so very small) at low speeds and has low frictional/inertial losses?

Maybe another stepper motor is worth a try? Or, I have not yet looked at BLDC. Would it be possible to make a linear generator of sorts?

UPDATE RE COMMENTS

3 March - Just to clarify some things from the comments so far (the first 4)

I think I confused things by saying I drove the stepper with a drill. This was just to test the stepper in isolation - I am not trying to generate electricity by using a motor to turn a generator.

Yes, there is a force acting on the rod. I should have used the word work in my figures. If you allow me to be a little lax with terms for a second, there is 5gk pulling on the rod and it moved 10 mm which is 50kg/mm of work, or 0.05kg/m *9.8 =0.49Nm of work. I measured this with a load cell.

If I can summarise my question, the DC motors I have are designed to run at roughly 4,500 rpm, even with the gearbox I'm not getting anywhere near that speed, and I believe the gearbox is causing huge losses. I have no specs on the stepper but it seems to top out at 0.6W regardless of how fast I run it.

So, what might be my best bet to generate 2W to 8W while running at a low speed (< 1,000 RPM)

Answer 1

there is 5gk pulling on the rod and it moved 10 mm which is 50kg/mm of work, or 0.05kg/m *9.8 =0.49Nm of work. I measured this with a load cell...

The best result I got was 0.6W out for a theoretical mechanical input of nearly 5w (operation in roughly 100ms) which is ~10% efficiency.

You are putting 5 watts of mechanical power into the mechanism, but not all of it is getting to the motor. Your lever arm has a high angle which is bound to cause losses due to the sideways force. You should offset the motor so the lever is as close as possible (on average) to horizontal.

I think I'm loosing a lot in the gearbox, firstly having to accelerate it and the motor from a standstill (it's very hard to turn [get started] by hand with no load),

Yes. Your gearbox ratio is too high, and it has a lot of gear stages with each one adding to the loss. With your lever setup you could probably use a gearbox with about 1/4 the ratio (and motor arm 1/4 the length) and still have an acceptably small angle of rotation.

Another problem is inertia of the motor and gearbox which stores kinetic energy as it winds up, some of which is lost when the mechanism hits its stops and/or changes direction.

Also, the speed of the motor is low, well below where it is designed to run as a motor.

This moves the best efficiency point to lower power, where mechanical friction losses (brush, bearing, gearbox, coupling mechanism) become more important than electrical losses. That's not necessarily a bad thing, but it does mean you need to keep mechanical friction losses low. On the upside the motor will put out higher voltage than a lower voltage motor would at the same speed, which could be important for efficiently utilizing the power produced.

I have no specs on the stepper but it seems to top out at 0.6W regardless of how fast I run it...

Maybe another stepper motor is worth a try? Or, I have not yet looked at BLDC. Would it be possible to make a linear generator of sorts?

Stepper motors generally have low efficiency. 'Small' brushed DC motors are typically 50-60% efficient at optimum loading. A coreless motor may be better because they have low friction and inertia. BLDC have the advantage of no commutator loss and potentially higher efficiency, but need a rectifier to generate DC.

However for your small travel a 'linear' motor might be the best option (if you don't mind building one).

So, what might be my best bet to generate 2W to 8W while running at a low speed (< 1,000 RPM)

You won't get 8 watts out of 5 watts, so I presume you want to make a unit that accepts higher power. 2 watts from 5 watts is 40% efficiency, which should be achievable with a good quality brushed or brushless motor if the mechanism is optimized for it.

At < 1,000 rpm you need a high quality gearbox and/or a motor that is designed to run efficiently at low rpm, and with only 100 ms operating time you also want low inertia (or some way to recover the kinetic energy when the mechanism stops). With 10 mm travel a 'linear' generator could be more efficient due to having fewer parts and more direct power production, but might have to be custom made.

You haven't told us the purpose of this device or what is applying the force. You talk about 'watts' but this doesn't make much sense if only a single stroke is involved. If that is all you want (2 W for 100 ms) then you might want to consider other ways of getting it.

Answer 2

trying to make a proof of concept

You have some evidence that the concept is difficult and inefficient at the scale implemented. It would probably work better at a larger scale.

rudimentary crank

"rudimentary" is part of the problem

I think I'm loosing a lot in the gearbox

That is to be expected even with the best (most expensive) gearbox. Efficiency is inversely proportional to speed reduction.

So my question is, what, if anything, might yield more power in my setup, where I can only turn the motor relatively slowly and for just a few turns at best.

The best alternative is likely to be a 3-phase, permanent-magnet, synchronous motor with as many poles as possible. Many poles means lowest speed for a given fundamental frequency. A brushless-dc motor without the electronic controller would probably be second best.

An arrangement using a solenoid coil with a permanent-magnet plunger might be worth investigating.