Comparing Geared motors

[ge_rik]

I've decided to bite the bullet and produce my own motor blocks / powered chassis - after struggling to find commercially available units of the right dimensions. However, when testing my first chassis with a (c£20GBP) Polulo geared motor, I was disappointed to find that it doesn't seem to want to run at slow speed. It jolts to a start when the voltage is relatively high. By comparison, a similar, much cheaper AliExpress alternative (c£4GBP) is much happier to start running almost as soon as the voltage is applied.



Have those with more knowledge of the performance of 12v geared motors any ideas?

I've tested them both with the PWM output from a Deltang Rx and the performance is better for both - but the cheaper motor is still the better performer at low revs.

I've now ordered a couple more AliExpress motors in preference to the Polulo version with a view to producing my own motor blocks. Will the cheaper motor prove to be less powerful do you think?

Rik
PS - Also, as you can see (and hear) the AliEpxress motor is much quieter.

[Phil.P]

Rik, there is a lot to work on here..

Firstly, I assume (by the tone) that the apparent 'pulsed' rotation, is an artifact of frame-rate and video compression?

These motors may quieten down, and run better, if run for an hour or so, in each direction.

Different motors 'prefer' different PWM frequencies.
Personally, I like to run at as high a frequency as possible. - The 'PWM whine, does my head in!
But some motors will heat up at higher frequencies, or just perform' better' at a different frequency.

Back EMF, and inertia control, still require some work (even in the new Micron receivers) but this is a complex issue, and there just aren't the hours in the day (or night) at the moment.

One way to get 'better' low-speed performance, is to use a higher voltage motor, but a lower voltage battery-pack (within reason, obviously).
Using a 12-15V motor, with a 9.6V battery-pack, is one option.

Phil.P

[ge_rik]

Thanks Phil
The pulsed nature of the speed change is accurately portrayed in the video but might have something to do with the LGB transformer/controller I'm using. Must admit though, I've not noticed the step change in speed with LGB motor blocks previously, but I'll do a bit more experimentation.

Interestingly, the motor is rated at 12v, the transformer is 0-24v and the battery pack I usecwith the Deltang Rx was 9v.

Even more experimenting required....

Rik

[Trevor Thompson]

While I don't pretend to have much experience of geared motors - I am just a few steps ahead of you.

I have had success with a similar motor from China that I have bought via eBay. The smooth slow speed running in that video of Moelwyn which I shared recently was achieved with a motor which LOOKS identical to the one you show. I have not altered any of the Deltang receiver settings.

The motor I used on the model of Palmerston was an "N20" type motor from China again bought via eBay.

Both were 6 volt motors running from a nominal 6 volt battery pack, and both are proving to be powerful( for each ones size), and quiet. Of course the bigger motor is much more powerful.

trevor

[ge_rik]

Thanks Trevor. I'm still in the process of tinkering with the design of the motor block, but it's reassuring to hear the motor works well in practice.

Rik

[GTB]

However, when testing my first chassis with a (c£20GBP) Polulo geared motor, I was disappointed to find that it doesn't seem to want to run at slow speed. It jolts to a start when the voltage is relatively high.

I'm currently working on a steam loco build, so my mind is in a different space. However today was housework day, so I tested a motor while waiting for the washing machine to cycle .......

FWIW, the spare Pololu 25D 34:1 gearmotor in my motor box has never been run, until today.

Straight out of the bag, it started at <1V and ran smoothly at 10rpm, current draw running light was 30mA. Even at 1V it generated enough torque that I couldn't stall it by gripping the drive shaft with my fingers.

The only noise at 1V was the 8kHz whine from the bench power controller. The gearbox noise didn't become audible until about 7V. At 12V the gearbox noise drowned out the controller whine.

The voltage rating of an electric motor is nominal and this '12V' one is rated to operated at up to 18V. I only use this size motor in large heavy models and use it with a 14V battery. Most of the time they run at half speed (7V), so there is little noise from the gearbox. The ESC I use in heavy locos runs at a high enough frequency that I can't hear it.

Pololu import their gearmotors from China. The 25D motor has a Mabuchi part no. on it, but no Mabuchi logo, so I presume it is a clone. I've noticed other chinese motors with a Mabuchi part no. printed on them, so they probably have similar specs. to the Mabuchi original.


I've not struck the problem you have. If the motor/gearbox is mechanically tight, it may improve with running in. If it has an electrical fault, it won't get better. In either case, it will use more current than the spec., which is 50mA running with no load.

The chinese factories make these small motors in industrial scale quantities and there will always be duds, but in my experience Pololu (or their supplier) have a reasonable quality system.


Power output of electric motors is a function of voltage and current. watts = volts x amps. It's easy to measure the input power, but output power will be lower and it's harder to measure, unless you build a miniature dynamometer. How much lower will depend on the efficiency of the motor. If two clone motors from different suppliers with the same part no. draw the same current at the same voltage, odds are they have the same output power.

Regards,
Graeme

[ge_rik]

Thanks, Graeme
I did wonder if I'd got a rogue motor - but I actually bought two at the same time and both behave in exactly the same way as in the video. I bought them from a reputable supplier (PiHut), so I don't think I've been sent cheap clones. The fact that the cheap AliExpress version performs better suggests the clones would be comparable (to yours) anyway.

No idea why they behave like this. I did try taking one apart but chickened out when I saw the number of gears inside the housing.

As you say, maybe it will improve once it's run in.

Rik

[ge_rik]

I've noticed there are low-power, medium-power and high power versions of the Polulo gear motor:
https://www.pololu.com/file/0J1829/polo ... motors.pdf

I bought the 12v high power version, assuming it would be more powerful. I wonder if the low power versions run better at low revs??

Rik

[GTB]

As you say, maybe it will improve once it's run in.

It is certainly worth trying. Run one in at different speeds, as well as changing direction now and again and see if it improves.

I've got a vague memory that the Pololu 25D motors in my big diesel became very free running after the test runs they went through as I developed the power bogie design.

Two bought together acting the same suggests there is a dodgy batch out there. Even if the supplier changes them over, the rest of the box in stock may be the same as these two.

Graeme

[GTB]

I bought the 12v high power version, assuming it would be more powerful. I wonder if the low power versions run better at low revs??

What rating are the generic gearmotors? If they have the same motor revs. and stall current rating as the Pololu high power motor then the different running characteristics aren't due to the power rating.


I used medium power 25D motors in the big diesel and the spare motor I tested yesterday was a Pololu medium power one.

I didn't bother with high power motors, as the medium power ones are able to slip the wheels of the big diesel if run up against the buffers at full power. Fitting the high power motors wouldn't give the loco any more drawbar pull to move heavier trains than I already get from the medium power motors.

The small N20 gearmotors also come in various power and voltage ratings. I use the 6V high power versions as a standard. They are overkill in the small rail motors, but were necessary to get the required performance in the Harman loco.

One thing to watch is that the different power versions of the 25D gearmotors run at different revs. for a given gear reduction, with the high power ones being faster. Also the stall current of the high power version is significant and limits ESC choices.

Graeme

[ge_rik]

This is interesting in relation to the above.

https://electronics.stackexchange.com/q ... ors#264641

I think they're saying much the same as Graeme, but it looks like the final paragraph is the most significant for my problem.

This only goes to prove what I've always maintained, that garden railway modelling should be introduced as a curriculum subject in schools and universities. It involves applied knowledge and understanding of a wide range of topics.

Rik