In my last blog it was determined that the motor should ideally be faster than 400 RPM (for the impatient among us), and have a torque of 0.0115 N.m.

In DC motor language, 0.0115 N.m is equal to 118 g.cm, calculated using this app.

On ebay (my N# 1 place to go to), manufacturers are actually bloody negligent in adding information about their motors.They hardly ever mention the current usage of a motor (even without load), and if you're very lucky they'll mention torque.

The three most important factors that influence my choice of motor are:

- speed (rpm)

- torque (N.m, g.cm, ...)

- cost.

Basically, it all boils down to the motor power, which translates into torque and speed. Higher motor power: higher speed or torque. Cost is a factor as well, given that I don't want to spend more than approx £7.5 per motor.

Here's one geared motor in which they helpfully specified torque, and even current! :)

4300RPM Rotary Speed Vibration DC Geared Motor 16.8mN.m Torque 24V

Ok, torque: 172 g.cm. Check. (I hope this is correct).

4300>400 RPM. Check

£4.74 < £7.50. Check.

24 VDC drive voltage. Hmm. That's a bit more tricky. 6V or 12V would be easier. But with these motors, usually they start working at a much lower voltage anyway. And 1.2W of electrical power is not very hard for a step-up converter to source.

They didn't really specify motor volume in this case. I think in this case, what is meant with "gearbox" is: "The entire thing, gear box including the motor".

As for the "Vibration" part of this motor listing. I assume they are just trying to help me come up with possible uses...

Double check to see if same-ish motor has same-ish torque-speed product:

• Torque: 2.0kg cm
• Voltage: 12V DC
• RPM: 30RPM
• Diameter (top section): 40mm
• Diameter (body): 32mm
• Height (excl. shaft): 33mm

2000 g/cm @ 30 RPM = (2000*30) / 4300 @ 14 g.cm (calculating back to the first motor)

Hmm that's a factor 10 off. Oh dear.

Another one

Rated Voltage: 12V
Speed: 70 rev / min
Resist twisting: 7Kg/cm

motor volume: 30x20 mm

Assuming that "resist twisting" is actually the torque:

So that's 70 RPM * 7000 g/cm / 4300 = 114 g.cm.

Okay so this one has a slightly lower motor volume than motor candidate N#, but only a factor 2 off in torque. That verifies N# 1 somewhat. (actually, the torque for this one is exactly what I need for my mirror project. However RPM is a bit low).

Okay, so we could use that motor #1, but lets see if we can find one with a similar or larger motor volume, and lower price, and preferably lower driving voltage as well.

There is :

360-900R/min, 12v, 60x30 mm (L*D) £3.90

• Used in electric shaver, pager, and vibration toy.

(I don't know why they say 360/900R/min though. Under load? What load? Different voltages?)

... aaand that was about all I could find on ebay, everything else was more expensive or had worse RPM figures.

However, the last one seems to be exactly what is needed for this project.

The power was not rated in this ebay ad, but it has a slightly larger body than the previous motor in which it was defined, so probably higher motor power. Furthermore, the RPMs are lower, so it should have at least the same torque as N# 1, but probably higher, and the voltage is 12VDC, which is manageable.

Although this picture shows a motor with a pinion wheel, the listing doesn't mention this otherwise, so I'll assume that they've just been a bit careless there and used a generic picture.

If this doesn't work, I could always use it for a shaver, pager, or vibration toy. Perhaps as a surprise to my girlfriend? Although, what would she do with a pager? (...)

Anyway.

Next item on shopping list:

Some power driving electronics.

Requirements: 12 VDC, 1.2 Watt.