Makings of a generator

TonyM said:

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.

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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).

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.

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

Steamchick said:

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

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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

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).

TonyM said:

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.

Click to expand...

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.

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

Did I post this curious arrangement?

Seems to use the inductive rotor to cause a high frequency fluctuation in the magnetic field when the rotor is turned, by linking and disconnecting the magnetic circuit. This fluctuation is picked-up as a high voltage low current and illuminates filament bulbs. At low rpm... because the prime mover is his finger n thumb providing low torque. I think the motor windings are then acting as a transformer primary coil, and the new windings he is adding acts as a transformer secondary coil to give the power at the load (bulbs) . BUT we don't know what AC voltage he develops, what bulbs he has (voltage wattage, series or parallel connection, etc.), etc.
I don't have a suitable motor to play at this game.
K2

So a cheap chinese motor works. This is 24V 350W. It was taken from an electric scooter and is about 100mm Dia so suited to a larger sized model. Unfortunately the bearings in this are shot so I am going to make new end caps to look like an old style generator with new bearings. I may also move the brushes outboard as well.

Driving the 12V 21W bulb it gives 6+ volts 1.2amps @ around 900RPM. and 13.5 Volts 1.8amps @ 1800
One unanswered question is the power required to drive it.

Apologies about the video quality/editing.

Back to the thread..."something that could perhaps charge a small battery that in turn would supply power for a couple of diodes plus the engine ignition.". A small motorcycle uses 6 V and 4A = 24 W of power. I don't know what small ignition modules for model engines use, but I guess about 5V~6V and 2A? - So that's about 12W. - The question becomes "Can to keep a 6V 12W bulb fully illuminated with your motor, at maybe 900rpm?" (Answer is "NO".) - And does the proposed engine develop perhaps 50W of power to power the generator plus losses for belts, bearings, etc.?
Simple sums can tell you a lot, and you have "13.5 Volts 1.8amps @ 1800rpm" = 24W at 1800rpm ( ) and only 7W at 900rpm. ( )- That means you need to double the speed via a Pulley or gear or something IF your engine runs at 900rpm...
K2

Steamchick said:

Back to the thread..."something that could perhaps charge a small battery that in turn would supply power for a couple of diodes plus the engine ignition.". A small motorcycle uses 6 V and 4A = 24 W of power. I don't know what small ignition modules for model engines use, but I guess about 5V~6V and 2A? - So that's about 12W. - The question becomes "Can to keep a 6V 12W bulb fully illuminated with your motor, at maybe 900rpm?" (Answer is "NO".) - And does the proposed engine develop perhaps 50W of power to power the generator plus losses for belts, bearings, etc.?
Simple sums can tell you a lot, and you have "13.5 Volts 1.8amps @ 1800rpm" = 24W at 1800rpm ( ) and only 7W at 900rpm. ( )- That means you need to double the speed via a Pulley or gear or something IF your engine runs at 900rpm...
K2

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Thanks Ken.
I assume either option could increase the operational life of a battery whether the ignition is 1 amp or 5 amps. Certainly a 2:1 drive belt ratio is not a concern.
I was just trying to get a feel about what size of model would be needed to drive something like that. I will still give it a try but I could end up with something more ornamental than useful.

Sorry. I am a pedant. I realise the forum is for Model Engine Machinists, of which I am not a "model", but a poor representation. But to respect the excellent machinists on this forum, I throw in a bit of Engineering where I can, having been a professional Engineer for 45 years.
In your case, I simply feel that correct design will balance the engine with the generator, and with the electrical load of ignition, etc. Otherwise it is simply a battery powered model.
But that is simply my opinion, you can do what you like. I have simply tried to help you "do it right".
K2

There are certainly some model engines running on magnetos so well able to produce enough electricity for a spark, should make no real difference if it is going straight to the plug or keeping a battery topped up. Some ignition modules will run of a single AA size 1.5V battery which reduces the required voltage.

A hit and miss engine would be an ideal choice particularly if it has a spark saver and then only sparks on the hits rather than wasting the sparks if it is firing when the cycle is missing.

Your input is always appreciated Ken, Although I think wires are getting crossed here.(pun intended)
I started this thread because I came across some old motors that I thought could be converted to "period" generators.
Not for any specific application, but to be driven by an engine and do some useful work other than just light a few diodes. Keeping the battery charged was one of those thoughts.
The first tries were not successful. This is the first one that I have got a reasonable output from. Unfortunately it is very modern and needs a lot of modifications to make it look period.
Having also been a mechanical engineer for very many years, I have limited knowledge of all things electrical, I know even less about generators. I am hoping to be pointed in the right direction so that my experiments might come to fruition and help others in the process. So far so good. I have learned quite a lot from the comments on this thread and internet searches.

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