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I'm designing a generator using a nitro engine and a motor. My design is similar to the YouTube video that I linked below. My main concern is to generate 12 V of electricity.

DIY 220v Dynamo Generator Using Nitro 2-stroke Engine

Nitro engine : Nitro Engine Specification

The nitro engine specification is in the description box of the YouTube video. But there is not information for the motor used.

The nitro engine RPM is 28,000. Do I need to find a DC motor which has lower or higher RPM than the nitro engine? I don't really understand what the relation is between the parameters of engine and motor. It is quite difficult to find a DC motor which operates at more than 28000 RPM. In my case, the shaft of the engine drives the shaft of the motor.

How is the RPM of the engine and motor related? With the specification of engine and motor, how do I ensure that I produce electricity at 12V? Is there any formula that I can use to calculate?

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    \$\begingroup\$ You could use a gear box... \$\endgroup\$ – Voltage Spike Aug 6 '19 at 3:48
  • \$\begingroup\$ How are you going to handle load changes on the generator side? Dipping the RPM will change the frequency, which can damage whatever you are running. \$\endgroup\$ – Ron Beyer Aug 6 '19 at 3:53
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    \$\begingroup\$ In the video, the motor/generator is directly connected to the engine shaft so it will be turning at up to 28000 RPM - if it produces AC, it won't be anywhere near 50 or 60 Hz. \$\endgroup\$ – Peter Bennett Aug 6 '19 at 4:02
  • \$\begingroup\$ Possible duplicate of Type of DC motors for generators \$\endgroup\$ – Chris Stratton Aug 6 '19 at 4:12
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    \$\begingroup\$ 28000 RPM is too much for mechanical commutators. I recommend an 1:2 or 1:3 gearbox between engine and generator. \$\endgroup\$ – Janka Aug 6 '19 at 5:05
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It is not clear if the engine has a nominal speed of 28krpm or is this max speed when unloaded.

I presume the 28 krpm is max speed and nomimal is somewhere 14 rpm. I would choose such motor/genenerator that produces 36V at 28 krpm, so KV=28/36approx 800KV. The rotor has to be very thin and long, to not choose pancake type or outrunner motor like for quadcopter.

Example of long rotor BLDC:

enter image description here

You connect motor leads to the six pulse rectifier preferably with high power schottky diodes. Then you will need a buck converer that will regulate higher DC voltage to suit constant 12VDC output.

schematic

simulate this circuit – Schematic created using CircuitLab

EDIT:

If you try to make a drone, then you could use ESC drivers that accept 3-6S voltage. In this case max voltage shall be not more than 29.6 at 28krpm, so a bldc of approx 1000KV is suitable. Instead of using extra buck converter, the ESC shall do the same job.

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  • \$\begingroup\$ 28krpm sounds about right for max power. These things really scream! rcuniverse.com/forum/car-nitro-gas-engines-269/… \$\endgroup\$ – Bruce Abbott Aug 7 '19 at 1:26
  • \$\begingroup\$ @BruceAbbott The question is, for how long they scream? \$\endgroup\$ – Marko Buršič Aug 7 '19 at 6:17
  • \$\begingroup\$ About 10 minutes on a 4oz tank, 20 hours total run time before it wears out. \$\endgroup\$ – Bruce Abbott Aug 7 '19 at 6:30
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What is the power rating of the engine? Roughly speaking, you would choose a '12V' rated motor of similar or larger power rating. So if the engine makes about 2HP as stated in its datasheet, you'd choose a motor of at least 1500W, about 125A@12V which seems kind of insane to me. With those kind of currents, using a 24 or 48V motor might be more realistic.

The example you show direct-couples the generator (motor), but you have the option of choosing a lower-speed higher-torque motor with a belt or gear reduction too. If you do use reduction you have more flexibility in choosing an optimal shaft speed for your motor, and choosing bigger motors with higher outputs.

Either way, take the raw generator output and post-regulate it as needed using one or more DCDC converters.

More: a hybrid approach might allow a smaller gas power unit. Example / inspiration: https://www.vice.com/en_us/article/wnxjaq/a-canadian-startup-wants-to-replace-drone-batteries-with-a-gas-engine-pegasus

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  • \$\begingroup\$ Weight is my concern because I will place this generator with my drone. \$\endgroup\$ – Samala Dewi Aug 6 '19 at 4:28
  • \$\begingroup\$ I am using brush dc motor. But I just don't know what is the suitable rpm should I choose for my motor. \$\endgroup\$ – Samala Dewi Aug 6 '19 at 4:29
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    \$\begingroup\$ @SamalaDewi Pick a motor that spins at the same speed as your engine when the same voltage is applied to it that you want to produce. You can use a DC brushed motor, but most people will use an brushless motor which won't produce AC, but instead produce a 3-phase trapiezoidal or sinusoidal output which is then rectified with diodes and smoothed with a capacitor to make DC. \$\endgroup\$ – DKNguyen Aug 6 '19 at 4:47
  • \$\begingroup\$ @DKNguyen I'm using DC brushed motor because it produces DC without any conversation. It's quite difficult to find brushed DC motor that has rpm at 28 000 rpm. But I found a 12V brushed DC motor that has a speed at 22000 rpm when no load is applied. Is this motor suitable to generate a 12V electricity? \$\endgroup\$ – Samala Dewi Aug 6 '19 at 5:42
  • \$\begingroup\$ Another consideration is to tune the engine for optimal efficiency vs. max power. You want to keep it in the sweet spot that gives the best BMEP, that is, the torque peak. \$\endgroup\$ – hacktastical Aug 6 '19 at 20:44
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An alternator will produce a lower voltage at a given RPM than the voltage required to operate it at the same RPM and power level.
This is because when in motor mode the alternator STILL acts as an alternator but the voltage produced opposes the applied voltage and is called the "back EMF".
The motor current is (Vmotor-Valternator)/Zmotor at the load and RPM concerned.

SO to produce 12VDC you will need more RPM than the motor runs at when powered by 12 VDC and with a power level the same as you wish to achieve.

The above applies to brushed or brushless motors - to DC output motors and to AC output alternators. you COULD try to use a brushed motor but a brushless motor will be generally superior in a number of areas including cost, availability, reliability (no brushes or commutator), choice, ... . AC output from a 3 phase (by far the most common) BLDCM can be converted to DC by using 6 diodes and some DC filtering.

Modern BLDCMs (Brushless DC motors) have a "KV" specification. This is the RPM they run at per volt applied. So eg a 2000 KV motor runs at about 2000 RPM per Volta applied etc.

Assume for the moment that motor and alternator speed are the same at a given voltage. In your case you want a motor that requires 12V to run at 28000 RPM.
The KV of this motor is thus KV = speed/volt = 28000/12 = 2300.
However, because Valternator will be lower than Vmotor, to get 12V you will need a higher KV motor - say around 2300 x 1.25 ~~= 3000.
Motors are available with KVs from under 100 (rare) to over 8000 (also rare but far from unknown).

Motor wattages vary very widely, but a say 50 to 100W 3000 KV BLDCM will be readily available.

Examples only:

2300 KV, 12V, 80 Watt, about $US11 here

15V (as motor) at 28,000 RPM around 150W+ - here about $US13.

Many more here

enter image description here

enter image description here

Note: As you load the nitro motor down or throttle it back its RPM will decrease, so a lower KV motor will be able to be operated at around the desired output voltage.

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