Makings of a generator

[peterl95124]

Just out of curiosity I ran up a couple more small motors today. Both with the same 21w 12v bulb for load. One was 14v from an old drill(so probably high RPM.) It barely illuminated the bulb and was showing 4.5v the other was marked as 12V showing 10V brightly illuminating the bulb. Very similar physical sized.

A question for the more knowledgeable in the subject. - Brushless motors are rated in Kv which is how many RPM per volt. How might this be applied to using the motor as a generator? Say for example we had two motors that were similar in physical attributes but one was rated at 400Kv and one was rated at 100Kv. My guess is the 100Kv would be higher torque as it is lower speed for the same voltage. Would it be correct in assuming this is therefore the better option.

similar sized motors can have very different armatures, especially wire size and number of turns, which greatly affect output as a generator, more turns will equal higher output voltage (just like more turns in a transformer secondary equals more output voltage). you can sometimes count the number of turns, but you'll have a hard time measuring the magnet strength or the magnet to armature air gap, so you'll have a hard time doing the math. predicting the output voltage will be difficult, you're probably stuck with trial-and-error.

you could see what I mean by getting three standard sized RC-car motors from the same manufacturer with different numbers of armature turns, for example 8, 16, 32 or there abouts, and you'll see that you probably won't be able to spin the 8-turns motor fast enough to generate the same voltage as the 32-turns motor (if all the rest of the internals are the same then you'd have to spin it 4 times as fast).

 

[TonyM]

Thanks Peter.

From the net.
As a general rule, as the number of windings in the coils increases, the Kv of the motor decreases. Mechanically speaking, low Kv motors have a higher number of windings of a thinner wire and the thin wire carries more volts at lower current.

So from what you say then it seems the lower Kv is better.

 

[Steamchick]

Never worked with that sort of kV. Only 33kV to 500kV VOLTAGE of electrical switchgear, busbars, etc. for cities and power stations.
Rotating machines were only something I played with a little, to get better lights on my 1950s ~70s Motorbikes, 1965 van, etc.
All I really know is Maxwell's fundamental stuff. B = nI, etc.
K2

 

[TonyM]

Never worked with that sort of kV. Only 33kV to 500kV VOLTAGE of electrical switchgear, busbars, etc. for cities and power stations.
Rotating machines were only something I played with a little, to get better lights on my 1950s ~70s Motorbikes, 1965 van, etc.
All I really know is Maxwell's fundamental stuff. B = nI, etc.
K2

I am sure they could have used a different designation to avoid confusion. Kv is RPM per volt used in brushless motors. As opposed to kV kilovolt.
Just checked wiki and it has loads

• Kv (flow factor), a measure of the flow factor of a liquid
• Kilovolt (kV), a unit of electric potential
• Motor velocity constant (Kv), of an electric motor
• Voltage-gated potassium channel (Kv), in cell biology
• Kv, a programming language implemented in Kivy

 

[Steamchick]

Probably because SI units are not really global standards. Korean inches bear no resemblance to metres! (Close to 3cm, but that's all you can say).