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).

 

[HMEL]

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.

No, in simple terms the higher the Kv the more rpm and the more power the motor has. Has to do with the windings. With a generator you want the windings to be able to conduct the current the magnetic field will generate. Being of old school I hate when naming conventions conflict with standard terms like kilo-volt. Not sure how the K represents rpm but it does. They certainly could have used Rv rpm-volt. But there are a number of brushless generators in actual use on various type of engines. They can be a little hard to regulate power output as the magnetic field is a constant value and so output is based on rpm.

 

[Steamchick]

Hmmm. The only brushless generators I had anything to do with are Crankshaft end alternators on motorcycles, or separate self-contained alternators on cars. The output from both was rectified and controlled for 12V DC system use.
Earlier motorcycles tended to have no control system just windings sized to nearly suit the load... and eventually fail to keep the battery charged if you changed to higher wattage bulbs! Car devices were controlled. Because they had excited rotors, early ones were controlled with mechanical regulators, like the dynamos before them, but eventually had electronic controls (field switching) to regulate the voltage. - Then came complexities like "load sensing control", "Battery terminal voltage control", and even later systems that used the generator to assist braking (By dumping a lot of power into a large battery - which battery was used for re-starting the engine after traffic-light engine idle stops). A racing friend (prompted by me) set a disconnect switch on his alternator field line so when at full throttle, the alternator did not generate (releasing a part of a horse-power for speed!), but when the throttle wasn't full open (braking and cornering) the alternator would work hard re-charging the battery. - He reckoned it made a second or three on lap-times: In 24 hour races that was a lot! He also cut off 2/3rds of the fan blades cooling the alternator, but ducted air to blow through for cooling. Another small saving at high rpm.
Lesson. Learn about the demand for power, as well as the prime mover, to deign the generator to suit.
K2