Author Topic: Brushless DC motor as a alternator  (Read 1667 times)

Offline airmodel

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Brushless DC motor as a alternator
« on: January 07, 2014, 11:54:01 PM »
I made a compressed air model turbine to run a brushless DC motor as a alternator. I was surprised that it put out 60 watts.

Have a look at the video, http://youtu.be/iTgcHp582rE   

Offline Don1966

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Re: Brushless DC motor as a alternator
« Reply #1 on: January 08, 2014, 12:03:50 AM »
Awesome I like.......... :ThumbsUp:


Don

Online b.lindsey

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Re: Brushless DC motor as a alternator
« Reply #2 on: January 08, 2014, 12:09:39 AM »
OK, I admit I don't do sparky parts so someone please explain to me how a motor designed to run off a DC input can generate an AC output. Even so, I can still appreciate the model work :)

Bill

P.S.  I need the electricity  for dummies version :)

Offline steamer

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Re: Brushless DC motor as a alternator
« Reply #3 on: January 08, 2014, 12:10:28 AM »
Cool!...,,Any idea of rpm?

Dave
"Mister M'Andrew, don't you think steam spoils romance at sea?"
Damned ijjit!

Offline Don1966

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Re: Brushless DC motor as a alternator
« Reply #4 on: January 08, 2014, 01:55:13 AM »
OK, I admit I don't do sparky parts so someone please explain to me how a motor designed to run off a DC input can generate an AC output. Even so, I can still appreciate the model work :)

Bill

P.S.  I need the electricity  for dummies version :)
Bill the motor stator has windings on it and the rotor has magnets. If you remember for science class that when you pass a wire through a magnetic field it produces electricity. The current flows in one direction when the wire goes through the field and reverses as the wire is pull back through the field. This action produces AC Voltage or alternating current as it is called.
Now by rotating the magnets around the field coils you do the same thing and produces AC voltage. Which here is pass through a rectifier to produce DC.  The rectifier acts like a commutation device to keep the positive going pulses to flow one direction and the negative going pulses to flow the same direction producing pulsating DC current. Now by adding the proper size capacitor across the DC. Terminals with the load produces a time constant. Which is R* C, this time constant will smooth out the pulses to a pure DC by charging and discharging across the load. Like I  said with the right value it will smooth the pulsing DC to almost pure DC.
Now he also produce High Voltage AC also at the end. This AC is useless to run Regular appliance with unless the proper frequency is produces. It is good to light lamps and heating elements that don't rely on frequency of 50 or 60 HZ.
The brushless DC motor is designed different than it counterpart the DC motor. Because it use magnets on the rotor and transistors and a controller to make it run. While the Brush type has a wound armature with commutator bars and can have magnets for stator or winding.
I hope this helps. Just voice yourself if you don't.

Regards Don

Online b.lindsey

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Re: Brushless DC motor as a alternator
« Reply #5 on: January 08, 2014, 02:01:26 AM »
Thanks Don, that helps a bit.

Bill

Offline airmodel

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Re: Brushless DC motor as a alternator
« Reply #6 on: January 08, 2014, 06:11:42 AM »
Don

Thank you for giving the detailed info on how AC works. I will add that brushed DC motors/generators to produce AC and the commutator rectifies it to DC.

steamer

Offhand I cannot remember the exact rpm but is about 6500rpm under full load. At 800rpm it will light up a low voltage Christmas tree light. This one of the most interesting and satisfying models I have ever built. I used to work in a coal fired power station so building a model of a working turbine and three phase alternator was high on what I wanted to build.

Offline ths

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Re: Brushless DC motor as a alternator
« Reply #7 on: January 08, 2014, 07:56:47 PM »
Fantastic work there. Hugh.

Offline Admiral_dk

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Re: Brushless DC motor as a alternator
« Reply #8 on: January 08, 2014, 09:42:32 PM »
The electronic controller inside the motor (not shown in the video, isn't used now), converted the 12VDC to 3 phase AC in order to turn the motor - So it ran on AC inside before and delivers AC out now.

Offline Bezalel

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Re: Brushless DC motor as a alternator
« Reply #9 on: January 10, 2014, 12:17:47 PM »
Hi Bill
 
Just as the Admiral explained a brushless DC motor does not actually run on DC.
 
It's the controler that runs on DC.  The controller, (goes under a lot of other names like inverter, VFD, ESC, processor controlled IGBT, motor speed controller - the list goes on depending the group your talking to) chops up the DC and breifly feeds it to the motor windings in pulses.
 
The pulses are not all the same polarity some are "inverted" thats where the name inverter comes from.  It is actually a form of AC but the waveform is very complex and made up of multiple odd harmonics of the primary AC frequency. Unlike the mains power AC which is sinusoidal at a single frequency (superimposed with a little bit of noise in most cases).
 
It is very likely many are thinking that VFDs are AC input only, which they are, but the first componant in the circuit is almost always a 6 or 4 diode full wave bridge rectifyer. So you could cut out the rectifyers and run the VFD directly off the solar panels on your roof if you wish.   
 
If you want to put DC directly into the motor ( not recommended) the motor will lock up and will require a very large external force to rotate the motor shaft at all. Brushless DC motors can pull a lot of current  so after just a short time with DC connected, the smoke will escape and the motor becomes easy to turn by hand again.  It is recommended you allow it to cool down first, of course.   ;D
 
 
Bez
 
« Last Edit: January 10, 2014, 12:28:53 PM by Bezalel »
Queensland - wet one day, humid the next

Offline Don1966

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Re: Brushless DC motor as a alternator
« Reply #10 on: January 10, 2014, 03:06:07 PM »
Bez when a member ask a question like Bill did, he doesn't understand none of the engineering language that we know. I tried to keep the answers in simple terms so that he can understand. I realize you are trying to help and that's a good thing, but please let's keep it to a level where other members that don't know what we know can understand. And Bez no disrespect intended. I can see from your answers you know what you are talking about.

Don

Offline Bezalel

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Re: Brushless DC motor as a alternator
« Reply #11 on: January 11, 2014, 01:17:29 AM »
Point taken Don
 
Sufice to say the current required to drive a brushless DC motor must regularly change direction. (i.e an Alternating Current)
 
So when the motor shaft is driven externally the current flowing through the windings when a load is connected,  also changes direction regulary (i.e an Alternating Current)
 
 
Cheers
 
Bez
Queensland - wet one day, humid the next

Offline tvoght

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Re: Brushless DC motor as a alternator
« Reply #12 on: January 11, 2014, 01:35:51 AM »
It could be argued that even a "brushed" DC motor is an alternating current motor. As the brushes contact adjacent segments of the commutator, they serve to alternate the direction of current flow through the armature winding.

As a generator, the current in the armature is alternating, but is alternately directed to the correct output terminals so that they are always at the same polarity. You might say in this case that the brushes and commutator act as a mechanical rectifier.

--Tim

Online b.lindsey

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Re: Brushless DC motor as a alternator
« Reply #13 on: January 11, 2014, 01:57:53 AM »
Guys, thanks for all the explanations, it does make a bit more sense to me now. I admire those of you that know this stuff as well as you obviously do... unfortunately for me, despite having to take a basic AC/DC course way back in engineering school, this is one light bulb my brain apparently wasn't wired to turn on...lol.

Bill

Offline Ian S C

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Re: Brushless DC motor as a alternator
« Reply #14 on: January 11, 2014, 12:32:08 PM »
Thought I put in a question last night, what sort of voltage are you getting with your 60Watts?    Ian S C