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building a generator

 
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nuttaphong191



Joined: 31 Mar 2007
Posts: 22

PostPosted: Fri Apr 20, 2007 12:31 pm    Post subject: building a generator Reply with quote

how do I use a standard 240v single phase ac motor driven by a petrol engine to create 240vac between the active (hot) and neutral wires of the motor?
if i got a 2400w motor and drove it at the appropriate speed would it push out the full 10 amps that it would normally draw. (maximum capacity of australian powerpoints in the house, except for 15a air con / specialty sockets)

is there any components i need to make it work and is there anything i need to watch out for?
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awright



Joined: 10 Aug 2005
Posts: 20
Location: Oakland, California

PostPosted: Sat Apr 21, 2007 6:07 pm    Post subject: Reply with quote

Well, yeah, you are messing with relatively high power equipment and can get into embarrassing situations. It's par for the course.

A common induction motor drives power back into the power lines if it is driven at above synchronous speed and draws power from the lines if it drives a load at below synchronous speed. But it needs the power line to excite the motor. You can't just take an induction motor and drive it with a gas engine and generated power in an off-line situation unless you have some source of line frequency power to excite the motor and some load to absorb the generated power.

Windfarms almost always use large induction motors because they are very economical machines for the alternator function, but windfarms are always connected to the grid. A gas-powered AC generator will have an excitation source built in or a permanent magnet excited alternator or a wound-rotor alternator with a small exciter generator, not a plain induction motor.

To summarize, while not impossible, making an engine and induction motor into an AC alternator takes more design than simply hooking up a motor to an engine.

Warning! I am a hobbiest, not an alternator expert.

awright
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nuttaphong191



Joined: 31 Mar 2007
Posts: 22

PostPosted: Thu Apr 26, 2007 5:22 am    Post subject: Reply with quote

awright wrote:
Well, yeah, you are messing with relatively high power equipment and can get into embarrassing situations. It's par for the course.

A common induction motor drives power back into the power lines if it is driven at above synchronous speed and draws power from the lines if it drives a load at below synchronous speed. But it needs the power line to excite the motor. You can't just take an induction motor and drive it with a gas engine and generated power in an off-line situation unless you have some source of line frequency power to excite the motor and some load to absorb the generated power.

Windfarms almost always use large induction motors because they are very economical machines for the alternator function, but windfarms are always connected to the grid. A gas-powered AC generator will have an excitation source built in or a permanent magnet excited alternator or a wound-rotor alternator with a small exciter generator, not a plain induction motor.

To summarize, while not impossible, making an engine and induction motor into an AC alternator takes more design than simply hooking up a motor to an engine.

Warning! I am a hobbiest, not an alternator expert.

awright




Thank you for idea
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RODALCO



Joined: 28 Oct 2006
Posts: 30
Location: Auckland - New Zealand

PostPosted: Sun Apr 29, 2007 9:37 am    Post subject: Reply with quote

You can try to put a 400 Volts rated capacitor across the 230 Volts motor terminals. (50 to 80 µF.)
It may create enough reactive power to excite the motor to work as a generator.

I use caps in my workshop to start a 3Ø motor from a single phase 230 Volts supply, when motor is up to speed, then have a 3Ø supply to use to start an other 3Ø motor from.

Good luck. Raymond
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PostPosted: Wed May 02, 2007 2:00 pm    Post subject: Reply with quote

Problem is... how to control the output voltage under varied load conditions.. you may well end up with massive overvoltage if you remove the load all at once, or conversly have a huge drop in voltage when yo connect something. you really need a voltage dependant speed control on your engine...
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awright



Joined: 10 Aug 2005
Posts: 20
Location: Oakland, California

PostPosted: Wed May 02, 2007 5:38 pm    Post subject: Reply with quote

You've also got the problem of how to control the frequency if you are not exciting the motor(now generator) with a fixed excitation frequency. Remember that an induction motor is basically a rotary transformer with the coupling between the rotor and the stator being the motor slip frequency, i.e., the difference between the synchronous speed and the actual speed. Using a capacitor to excite the motor (if it worked at all) leaves you without any frequency control except the resonant frequency of the winding with the capacitor. Where do you see the frequency control within the engine-alternator system with capacitor excitation?

I have seen demonstrations of this type of resonant excitation with a large induction motor. I don't remember exactly, but probably a three-phase motor due to it's size. You could spin it up by hand freely until the resonance got stimulated, then you would suddenly feel strong resistance to rotation and a lamp connected as a load would start to flicker. Very interesting demonstration, but I'm not sure it has practical application as a home power generator due to absence of voltage and frequency control. And, don't forget, if it was that easy, engine-generator manufacturers would have adopted it long ago. Probably an interesting place to start experimenting, though.

Use of a capacitor to get a three-phase motor started is a totally different matter. In this case you are using the capacitor to cause a slight phase shift in excitation of one winding relative to the directly powered winding, thus causing a slight rotation of the magnetic field. This gets the rotor spinning in the desired direction, albeit with negligible torque. Once the rotor gets up to a speed sufficient to induce its own rotation to the magnetic field, the capacitor can be switched out and usually is because motor STARTING caps cannot withstand continuous motor running current. The motor then continues to run as a single-phase motor at significantly reduced power rating and generates three-phase power for other tools.

awright
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