Electronic – Understanding a motor’s specs

Rushing.
More later maybe ...

Note that "stall torque" is often used to mean locked rotor 0 RPM torque BUT you use it in the sense "dropout torque at a given speed". That's entirely fine as long as you note that some references will mean the former and not the latter.

Criticism (kind / constructive) welcome.
Written at a rush and unchecked. Better is possible.

See writer "Toper925" comment here

He notes:

• There really is no single equation that fits all states of a PMSM but this one works in general:

  • Te = 1.5p[λiq + (Ld - Lq)idiq]

Where:

• p is the number of pole pairs

• λ is the amplitude of the flux induced by the PMs in the stator phase

• Lq and Ld are the q and d axis inductances

• R is the resistance in the stator winding

• iq and id are the q and d axis currents

I'd need to read more on what he said to make total sense.

Stall torque is when torque is not sufficient to "pull in" the next rotor pole piece using the available magnetic field.

SO, I'd expect

• More pole pairs better. I'd expect better than linear gain as distance halves with doubled pairs BUT magnetic force at worst falls as distance cubed (only a considerable number of magnetic pole diameters away so not in most sensible motors), comes closer to falling with distance squared as gap falls to near pole width and at best can only approach linear at close proximity. SO more ples should give less interpole distance so ... (but pole sizes are down so ...).

• Torque = power per rev. If the power falls faster than RPM your margin is dropping until you reach the point of no pull in. At a quick glance I think that this is what this man here is alluding to about half way down below the graph. Leading to ...

• If you have a power curve you also have a torque curve as the two are related by motor rpm. (Torque = k x Power / RPM). If you have a speed-power plot for you load you should be able to overlay this on torque curve and see where load torque is > generated torque. This will be better than real world (probably).

• Lowest R should help as it allows greatest I but this is really a secondary effect for two motors with the same power at the same RPM.

• Induced flux should play an immense part. I'd expect non saturating magnetic (eg steel) cores to provide superior results EXCEPT if you can get all gaps so small that field is well maintained by magnet. Rule of thumb is you can get about 0.5 Tesla at an airgap of 1/2 a magnet diameter using a top class NdFeB magnet. Say N52? N45 won't be too bad.

• Note that the US process NdFeB magnets are cast but ground and sintered subsequently and are inferior in max possible flux to the Japanese versions. This should all be covered in the flux spec.

This is a bit off-the-cuff mathematically just to see if things stack-up: -

Your mass is 100 kg and your top speed is 20 kmph (5.56 m/s).

You want to accelerate at 0.4 m/\$s^2\$ - this means it'll take about 14 seconds to get to full speed.

The energy attained by 100 kg at 5.56 m/s is \$\dfrac{mV^2}{2}\$ = 1546 J and this equates to a power during acceleration of \$\dfrac{1546}{14}\$ = 110 watts or about 28 watts per motor.

Am I looking for something feasible by a dc brushless motor?

It definitely sounds feasible - I don't see why you shouldn't find a brushless motor like this but it'll be easier to find a brushed one for sure.

What Voltage/Amperage values am I to expect from a motor of these characteristics?

I think you can be looking at 80% efficiency so each motor will consume about 35 watts. 4 x 12V, 3A full-load rated motor should work but maybe I'd play conservative and go for something like 50 watts: -

I know these aren't cheap but it was virtually the first hit when I searched for "50W brushless motor". There should be plenty that are a bit cheaper.

Try this too - maxon are the supplier.