Discussion in 'General Electronics Discussion' started by MadMechanic, Sep 7, 2012.

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Aug 28, 2012
Hello All, I am trying to completely wrap me head around a project of mine. Here is the scenario.

I have an engine and a generator. The Engine produces 25 watts power output at 30,000 rpm.

The generator will produce 15V, 1.5A, 22.5W 10ohm resistance. at 2832rpm.

I am still trying to check my logic, but if I were to bring this motor to the 30k rpm speed, then the potential for 15V 1.5A 22.5w is there. If I were to charge a phone at 5V 750mA, this should equal 3.75W correct? Should this system be able to maintain said charge without a problem? I kinda wanted to build the system to be able to handle most things up to 1 amp.

If I am using this mainly for the charging of batteries and devices, will power use (W) stay lower? And since the output of the engine is 25W and rpm were to decrease when loaded, would the rpm decrease cause loss of power? Just trying to wrap my head around it all. Thanks!

2. ### Harald KappModeratorModerator

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Nov 17, 2011
O.k.
First I don't think tht running the generator at 10* rated RPM is a good idea. Use a 1/10 gear to match the motor to the generator. This way the RPM of the motor will alo not decrease as much if loaded.
Second : If you use an efficient step down regulator to convert the generator's 15V to e.g. 5V you can use approx. 0.8*22.5W ~ 18W or [email protected]

48
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Aug 28, 2012
Yes, I definitely planned on gearing it down to match the Kv rpm rating of the motor required to meet the voltage I need. I mainly just wanted to make sure my rig would be able to produce over enough power to meet the requirements when loaded. I will fuse it accordingly.

Ive heard of people using stepper motors, such as the ones used in CNC machines and robots, as generators. They seem to produce lots of power at low rpm. Would there be an advantage to using a stepper motor vs. a brushless dc permanent magnet motor? The one I am currently experimenting with is an out-runner model airplane motor, 235Kv so 2820 rpm would give me 12V. Its got a quite a large max load of 60A and 1850W @ 37V. This is more than I need, and it requires quite a bit of torque to rotate without gears. Would a stepper motor be an option? Thanks again

4. ### Harald KappModeratorModerator

11,157
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Nov 17, 2011
Sorry, this is not my field of expertise.

5. ### duke37

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Jan 9, 2011
This is not my area of expertise either.
My impression is that a brushless DC motor is a stepper motor with internal circuitry to keep it going. It may be simpler to use a stepper motor.

What is an out-runner?
What is 235Kv, surely not 235kV?

6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
An outrunner is a type of motor. My best understanding is that the "rotor" is fixed and the stator is allowed to turn. Or something kinda like that.

I think the two hundred and thirty five kilovolts is a part number