Author Topic: Brushless DC motor as a alternator  (Read 9673 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

Offline 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
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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

Offline 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

Offline 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

Offline Pete49

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Re: Brushless DC motor as a alternator
« Reply #15 on: January 12, 2014, 01:44:52 AM »
my interpretation of the answers taking into account I'm nowhere near the level on electrics you all :Jester: are is:
blah blah blah blah blah blah blah blah blah smoke will escape blah blah blah etc.
But I have been following as it helps to expand what little mind I have left  :old: That's the good thing about this forum, we seem to cover all topics involved with model engineering :ThumbsUp:
Pete
I used to have a friend.....but the rope broke and he ran away :(

Offline airmodel

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Re: Brushless DC motor as a alternator
« Reply #16 on: January 12, 2014, 11:49:08 PM »
Ian S C

With that 60 watts I get 14 volts DC or AC because it is a resistance load. If the load has is inductive the power output is more complicated to work out. Also with that 60 watts I use a home made three phase trans former to step up the voltage to 240 volts. This load is also resistive but with the transformer it could be slightly less.

 Another factor is the multimeter I am using, measuring voltage with the higher frequency that the alternator is producing can lead to errors is the readings. A way to get around that is to compare two light globes, one run from mains frequency and the other from the alternator.  The light globe run from the alternator will always be slightly dimmer even though the meter reads 240 volts AC. The same comparison can be done with a 12 volt battery and using 12 volt globes.

Bezalel

I did have a go at putting 12DC on two of the output terminals. The motor does not lock up it moves to the next pole and stops. Because there are not enough turns on that pole the wire gets hot very quickly. It would be possible to have mechanical switches to make the motor run but would be very complicated. I can see why electronics is a much better choice for these to work.

Bill.

Offline Bezalel

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Re: Brushless DC motor as a alternator
« Reply #17 on: January 13, 2014, 04:55:15 AM »
Thanks Bill / Airmodel
I notice I omitted a few things in my post, not the least of which was my appreciation for you sharing your model with us.
I must admit I spent a few hours working my way though your youtube channel.
Thouroughly enjoyed all of it.
Lots of good tips and ideas hidden away in there, and lots not so hidden.
 
The other bit, you're quite right, I omitted mentioning the initial pole alignment which occurs if they aren't already aligned.
 
Switching mechanically, I think its ok to call such a mechanical switch a cummutator if it is part of, or attached to the rotor.
Just as Don describbed at then end of his Post. ( but I guess it wouldn't be a brushless motor then)
I don't know what you would call it if you used micro switches operated by cams.   :shrug:
 
A future project ???? any one??   :zap: 
 
Bez
Queensland - wet one day, humid the next

Offline Ian S C

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Re: Brushless DC motor as a alternator
« Reply #18 on: January 13, 2014, 12:47:58 PM »
Here is a shaded pole motor, converted into an alternator, with a claw type armature, with permanent magnet in it.  This has been seen before, but since then I managed to cut the shading turns, thereby improving the performance. Ian S C
« Last Edit: January 13, 2014, 12:51:16 PM by Ian S C »

Offline airmodel

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Re: Brushless DC motor as a alternator
« Reply #19 on: January 14, 2014, 01:56:20 AM »
Ian S C

I didn't think too many model makers were into making their own alternators so I was pleased to see your conversion. Any idea of output voltage, amps? I have never tried to put magnets into a ceiling fan motor. They would be good for models because they rotate so slow and would output a high voltage at a slow speed.

Offline airmodel

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Re: Brushless DC motor as a alternator
« Reply #20 on: January 14, 2014, 03:09:26 AM »
Just in case someone would like to experiment with brushless motors as alternators, I made a video about where to find them and how to dismantle the appliance to get the motor out.

Have a look at the video  http://youtu.be/RYCWY52yVVQ 

Bezalel

Did you see my video on how I machined all the parts for the air turbine/alternator?

Offline Ian S C

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Re: Brushless DC motor as a alternator
« Reply #21 on: January 14, 2014, 10:57:55 AM »
I fiddle around with motors/ generators/ alternators for my hot air engines, trying to get the maximum out put, with a minimum in put.
I ran this alternator with the Stuart S9, I think I got some thing like 80V, and used a 40W, 230V light bulb as a load, it glowed a bit more than just a glimmer, didn't measure it, but probably around 10W, S9 running on 30psi air, It might do better now with the shading loops cut.
Sorry it's a bit fuzzy, but here's the armature, it has two magnets from the back of a speaker.   
The original motor was 5W, 230V, 0.3A, 850rpm, 50Hz.  It is about 4" in diameter.
                                Ian S C
« Last Edit: January 14, 2014, 11:09:15 AM by Ian S C »

Offline Bezalel

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Re: Brushless DC motor as a alternator
« Reply #22 on: January 14, 2014, 11:26:57 AM »
Yep! Sure did.
 
There is only a handful of your vids I am yet to see.
 
It looks like there is a lot I can learn from your posts and build logs, I'm loking forward seeing more if you are happy to indulge us.
 
 :cheers:
 
Bez
Queensland - wet one day, humid the next

Offline airmodel

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Re: Brushless DC motor as a alternator
« Reply #23 on: January 14, 2014, 11:21:51 PM »
Ian S C

Thank you for the photo. I used the same idea with a 35 amp car alternator using microwave oven magnets. It only put out 6 amps. I am not sure why it gave such a terrible output. Maybe the magnets were not powerful enough?

Bezalel

I am glad that you enjoy my models and workshop tinkering. You seem to know a lot about electricity, is that your full time job?

Offline Bezalel

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Re: Brushless DC motor as a alternator
« Reply #24 on: January 15, 2014, 07:35:24 AM »
Yes, normally  high power electro magnetic transmission
 
Queensland - wet one day, humid the next

Offline Ian S C

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Re: Brushless DC motor as a alternator
« Reply #25 on: January 15, 2014, 11:39:51 AM »
Bezalel, you might call electricity a hobby/interest, I'v been playing around with it for over 50 years.  I started as an aircraft engineer, first in the RNZAF, then for a short time with the New Zealand Cessna agents, on becoming redundant there, I went nursing(quite often got the job of fixing anything broken, other than human as well).    Ian S C

Offline dieselpilot

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Re: Brushless DC motor as a alternator
« Reply #26 on: January 15, 2014, 02:24:17 PM »
I used the same idea with a 35 amp car alternator using microwave oven magnets. It only put out 6 amps. I am not sure why it gave such a terrible output. Maybe the magnets were not powerful enough?

First, you have to have enough mechanical power to generate the electrical output you're trying to achieve. Then the voltage and current output will be dependent on the characteristics of the motor/generator. Matching the generator to the engine and load is important for efficiency. Depending the type of electrical load it also must be controlled in order to achieve the most output from the engine. None of the details are really important unless you're trying to be efficient, but then you'd probably would just buy a generator off the shelf. Any text about motors discussing motor characteristics would give the required information.

Greg

Offline Rustkolector

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Re: Brushless DC motor as a alternator
« Reply #27 on: January 17, 2014, 05:39:12 AM »
I'm glad to see a generator topic here. I build slow speed early 1900 vintage model IC engines and finding capable alternator designs in the 300-600 RPM range has been difficult. I finally built one of my own for a 4 cylinder IC engine I built a few years ago. I used an 18 pole, 100 watt 120v ceiling fan motor stator. I got the idea from the windmill guys. The first rotor used 18 magnets aligned with the rotor poles. The cogging torque was so great I couldn't get it to turn without a wrench on the shaft. I tried skewing the magnets and that eventually allowed fairly light starting torque. The cogging is quite mild, but the cogging still causes a humming noise when it runs. The stator is 6" diameter (152 mm for you metric guys) and I fashioned it after a Westinghouse alternator I had seen. The stator was a two phase design. The original voltage output was 240v at about 650 RPM. Way too high! I split the phases into two separate 100 watt coils and got a more respectable 120v at 600 RPM. I now use one coil for AC lighting demonstration load and the other coil is connected through a multi voltage DC converter to drive a computer cooling fan for the engine. I run the engine with 3 or 4 7 watt light bulbs at 100-110v which puts a light running load on the engine. The alternator and engine have demonstrated 90 watts at about 750 RPM. It struggled at that load but did carry it with a little more throttle left. I don't normally torture my models like that but, the temptation was just too great. I have no idea of efficiency and don't really care. I wanted useful accessory for my engine to drive and now I am hooked on alternators and generators.

The second electric plant I built was smaller engine and in the absence of finding a suitable motor to cobble up, I used a PM Research Inc. 12 vdc 10 watt generator kit. I requires very high speed and gets rather noisy. I had to exchange the generator bushings for ball bearings as they didn't like the belt load. The generator drives a few 2 watt lights through and external voltage regulator which makes for a much steadier light.

I am still looking for a 4" dia short stack 12-18 pole stator for an engine I have just started building. I would like a more modest 30-50v output in the 400-600 RPM range. The only thing I see so far are motorcycle alternator stators which I would have to spin inside a outer magnet ring. Any suggestions on something this size would be appreciated.

I hope to see more posts and ideas on model engine driven alternators and generators. Here are links to a couple video's of my engine electric plants mentioned above. 



Jeff

Offline Roger B

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Re: Brushless DC motor as a alternator
« Reply #28 on: January 17, 2014, 09:34:45 AM »
Very nice looking engines Jeff  :ThumbsUp:  :ThumbsUp:

I also used a small permenant magnet motor as a generator for my first engine. It powered four 6V 0.5A bike headlamp bulbs.
Best regards

Roger

Offline Ian S C

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Re: Brushless DC motor as a alternator
« Reply #29 on: January 17, 2014, 11:39:08 AM »
I was test running an alternator I have rigged up, driven by a 3hp Kawasaki industrial engine, so I got the little alternator(see above), put a belt on the 30 mm pulley, to the pulley on the Kawasaki running at 3000rpm, this worked out at 7500rpm at the alternator, this came up as 320 volts no load, the motor then ran out of fuel, experiment ended.
For a low speed(ready built) alternator, a hub type dynamo for a bike, I'v got a Sturmy Archer hub dyno that I prepared for use as a wind turbine power unit, about 12V at 200rpm.        Ian S C

Offline airmodel

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Re: Brushless DC motor as a alternator
« Reply #30 on: January 19, 2014, 06:38:23 AM »
If you have looked at my video about locating brushless DC motors, I will take it one step further and show how to make the necessary connections to make it work as a alternator.

 Before the PCB is unsoldered look for groups of three connections and mark all these connections so you know where to reconnect them. The first is a star connection, look at the "star PCB" photo. Those three have to be soldered together when the PCB has been removed. These connections are labeled " SA,SB,SC" s means star.

 Next have a look at the output PCB photo. There are another group of three connections, these are the output wires. They are labeled "A,B,C". connections so you know where to reconnect them. Have a look at the star motor photo, it shows how I reconnected the star connection. Then have a look at the output motor photo, these three wires are connected to the load (lights).

  There are two ways to connect the lights (Load) to the alternator, star or delta.

 Have a look at the drawing to see how this is done. Instead of having a three wire output, it can be reduced to two wires by changing the AC output to DC. Have a look at the diode photo. Two bridge reticfiers are used. The yellow wires connect to the three output wires on the alternator. The red wires are positive and the black wires are negative. On the lefthand top of the photo you will see a red and black wire going out of the photo, they are now your output wires to the light or other load. The bridge reticfiers have + and - marked on the sides, also there are two ~  these are where the AC input wires go to. Because there are three output wires one ~ AC is not connected on the righthand bridge reticfier.

Offline airmodel

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Re: Brushless DC motor as a alternator
« Reply #31 on: January 19, 2014, 06:42:54 AM »
Here are the rest of the photos