Author Topic: A.C. Generator  (Read 4955 times)

Offline airmodel

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A.C. Generator
« on: July 22, 2016, 02:30:20 AM »
Hi everyone

As I really like to build model generators I decided to modify a small induction motor from a bread maker. My last model was a small D.C. generator which worked really well but D.C. can be very difficult to change the voltage output. So the reason I went with A.C. this time because it is so easy to alter the voltage output with a transformer. It is always interesting for me to experiment with the generator when completed to see what will happen. Have a look at the  video. 

Offline yogi

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Re: A.C. Generator
« Reply #1 on: July 22, 2016, 04:42:38 AM »
Very interesting! Thanks for sharing.  :ThumbsUp: :ThumbsUp: :ThumbsUp:

Offline Admiral_dk

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Re: A.C. Generator
« Reply #2 on: July 22, 2016, 11:32:43 AM »
The voltage of your generator depends on magnet strength, distance between poles, number of poles, number of windings on poles, rpm and load.
Your first run is without a load = much higher voltage than with a load.

Example ; all my modern motorcycles has a three phase generator and the no load voltage at midrange rpm must be higher than 90 Vac. (other wise it is defect) - the same generator connected to the rest of the wiring harnes will measure about 16Vac. on it's leads and around 14.4Vdc. on the battery at the same rpm.

The 10W 12V bulb has a much lower resistance than the 150W 240V bulb = higher load = lover voltage ....

26V^2 / 10W = 14.4 Ohm.  -  240V^2 / 150W = 384 Ohm.  ..... simple laws of physics really ....  ;)
« Last Edit: July 22, 2016, 11:52:35 AM by Admiral_dk »

Offline PStechPaul

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Re: A.C. Generator
« Reply #3 on: July 23, 2016, 04:20:28 AM »
Admiral_dk provided a quick explanation. I posted the following more detailed analysis on your YouTube page:

Quote
A motor/generator (as well as a transformer, or other electromagnetic device), has a characteristic impedance, which can change with RPM or frequency. It appears that this motor/generator is well matched to the 150W 220V lamp, which has an incandescent resistance of 220/0.68=323 ohms. At the same RPM, the generator probably puts out about twice the voltage, or 440 VAC. Thus the generator has an internal impedance about the same as the lamp. Maximum power transfer occurs when the impedance of the source equals that of the load - this is known as impedance matching.

The maximum current in this case would be 440/323, or 1.4 amps. But it may be higher, as the impedance is an estimate, and may vary with RPM. Ultimately it will be limited by winding resistance at low RPM and inductance at high RPM. A 12V 24W lamp has an incandescent resistance of 6 ohms and nominal current of 2 amps. It probably has a cold resistance of 1/10 that, or 0.6 ohms. So it is practically a short circuit, and the generator will be limited to 2 or 3 amps. The lamp probably saw power of 20V * 3A = 60W for a short time, and survived. Also note that the load caused the motor to slow down to perhaps half RPM, also reducing the power delivered.

Offline Ian S C

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Re: A.C. Generator
« Reply #4 on: July 23, 2016, 12:13:13 PM »
I converted a six pole ,shaded pole fan motor by making a claw type armature with two speaker magnets in it. This is for use on one of my  hot air engines.  Ian S C
« Last Edit: July 23, 2016, 12:19:21 PM by Ian S C »

Offline Rustkolector

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Re: A.C. Generator
« Reply #5 on: July 23, 2016, 08:34:55 PM »
Airmodel,
Very interesting work. I agree with you on the simplicity and versatility of the PM AC alternator for model engine applications vs DC designs. As you have warned about the danger of high voltage, I would further encourage those interested in building model alternators to limit the potential of higher voltages in their alternator designs either by using lower voltage stators, weaker magnets, or by increasing the rotor gap. I prefer an alternator with a maximum AC output of about >20vac in the operating range. I rectify to DC and control voltage output at 12vdc using an inexpensive in-line voltage regulator or converter. This allows me constant DC voltage output with variable engine speed since my gas engines dont have governors. The video below is a 12vdc/ac alternator running at 600 RPM and demonstrates the ability of the DC regulator to maintain voltage as the engine slows with load.

My efforts have focused on direct connected alternators for my slow speed (500-600 RPM) gas engines. I would be very interested to know what voltage and wattage you can get from your alternator at about 500 RPM. How much does your rotor cog?
Jeff


Offline airmodel

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Re: A.C. Generator
« Reply #6 on: July 25, 2016, 05:22:14 AM »
PStechPaul

I agree with your analysis, especially where you said that the generator would be limited to 2-3 amps.

 I have learned that I cannot see the electrons flowing so meters are needed  to tell me exactly what is going on. So I hooked up two multimeters to the output of the A.C. generator, one measuring volts another measuring amps.

With the drive motor running at full speed the A.C. generator puts out 22.5 volts, 0.967 amps with a 10 watt 12 volt globe.

A 5 watt globe produced 20 volts, 0.94 amps.

 I then tried the 150 watt 240 volt globe, it measured 243 volts, 0.616 amps.As it is a resistive load it worked out to be 0.617 amps.

Now I know what the maximum current flow is from my A.C. generator at maximum speed from the drive motor. So another method is needed to find out how much current is needed to destroy the filament in the globe.

A large variac was used to find that out. A 10 watt 12 volt globe blows at 32 volts, 1.173 amps and a 5 watt 12 volt globe blows at 32 volts, 1.19 amps.

Now I can see why my A.C. generator would not blow the 12 volt globe even though the voltage was very high. It just does not produce enough amps.
I can also now see why the motor protested and slowed down, it takes a lot of extra power to produce the amps to blow that globe.

Your statement about the globe filament resistance being very low when cold helped explain why my driving motor was so overloaded when trying to heat up that piece of steel wire from room temperature. As the steel heated up there was more resistance so the drive motor was less stressed. I watched the video again and when the steel heated up the volts rose at the same time.

Rustkolecter

I measured what my A.C. generator puts out at 500 RPM, 37 volts no load so I connected a tiny incandescant Christmas tree globe. The output would be well under 1watt as there was about 3 volts at the globe and it flashed a lot because the frequency was only 17 Hz. That is  why electricity is generated at 50 Hz, it is so the human eye cannot see a light flashing at 50 Hz. Yes it does cog a lot because of the rare earth magnets I used.

I did measure RPM for three outputs. 5 watt 3190 RPM, 75 watt 3920 RPM, 150 watt 5290 RPM.



Offline Rustkolector

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Re: A.C. Generator
« Reply #7 on: July 26, 2016, 04:30:24 AM »
Airmodel,
Your results compare closely to what I have found in my efforts to build model PM alternators. That is, with low pole count stators, you need high speeds to produce wattage. To get to lower speeds and/or more wattage, higher pole count stators are needed along with a very strong magnetic rotor. The latter causes significant cogging, however higher pole count stators make it possible to minimize the effects of excessive cogging. The small diameter high pole count stators needed for model alternators are not easy to find, as you may know, but they can be found.
Jeff

Offline Ian S C

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Re: A.C. Generator
« Reply #8 on: July 26, 2016, 11:38:33 AM »
The little alternator in the photo above is normally run at about 700 rpm, the maximum voltage I can get is 36 V, under load it runs nicely at about 12V, enough to run a 12V fan for engine cooling, and a transistor radio, to annoy the neighbours, with a bit left over to light some LEDS.  I was playing around with it a couple of weeks ago and the best power I got was 4Watts at 680 rpm.   isc

Offline airmodel

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Re: A.C. Generator
« Reply #9 on: July 28, 2016, 06:57:23 AM »
I just had a idea, ceiling fans have multiple poles so they run at a slow speed. They should be really good for slow speed alternators.

Offline Ian S C

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Re: A.C. Generator
« Reply #10 on: July 28, 2016, 01:51:28 PM »
I think it was a ceiling fan that my 6 pole motor came from.  The case is about 4" diameter.
The motor I'm currently trying is a 35v brushless DC motor that I got with a whole lot of old printer parts, it's about 60 mm dia x 120 mm long, with an 8 mm shaft, but it doesn't matter what size it is, you only get out what you put in minus a bit, and its reducing that minus bit as much as you can is what I am looking for,  so a shelf full of generators, and alternators.
Ian S C

Offline Admiral_dk

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Re: A.C. Generator
« Reply #11 on: July 28, 2016, 01:54:14 PM »
Quote
I just had a idea, ceiling fans have multiple poles so they run at a slow speed. They should be really good for slow speed alternators.

Maybe .... how you wind the poles has an influence on the motor speed, but I'm not sure that applies to generators. All the generators discussed here so far has one coil round each pole in series with the next - made from one long wire. So starting winding around the first pole until it is filled up, then to the next and fill it up, etc.
If you instead wind the wire around two poles at once - leaving a space between the two poles, a kind of interleave - round 1,2 next round 2,3 etc. - you are gearing the motor down to half the speed and double the torque (if memory serves ...). I'm not sure this kind of motor is a good generator too, but what I'm trying to say is that how you wind the poles gives different speeds in a motor, but not necessarily in a generator .

Offline Rustkolector

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Re: A.C. Generator
« Reply #12 on: July 28, 2016, 07:59:16 PM »
I have experimented with both ceiling fan stators and automotive type alternator rotors, all having high pole counts. It the first case it was a 2 phase 152 mm diameter stator from a center rotor 18 pole ceiling fan (most fans now have outside rotors and are not suitable). The two phases were used separately to keep the voltage as low as possible. I was able to get 100 watts @ 90v and 700 RPM out of each phase using neodymium magnets. Note that these magnets require cautious use because they are very powerful and are very brittle. The video below shows this model alternator running at ~600 RPM with a 28 watt load.


The black B&E engine alternator in the previous video (I'd post a photo, but doesn't seem to work) required at smaller diameter stator to be closer to scale. I used a small tractor 12v, 12 pole 93 mm diameter 3 phase alternator stator. After rectification this stator produced good wattage but low voltage at 500 RPM. I wanted a controlled 12vdc output at 500 RPM. The stator was rewound with 3x turns of finer wire. Neodymium magnets were again used. This provided 18vac and 25vdc no load voltage after rectification.  A 3A rated in-line voltage regulator controls the dc output to 12v, so 3A x  12vdc = 36 watts is the design output limit of the system which works the little 2 cylinder B&E engine much harder than I normally allow it to work. A variable speed lathe was used for testing the alternator. The output was:
   RPM   VDC   Amps   Wattage
   350   12.0   1.0   12   (max output)
   400   12.0   1.9   23   (max output)
   450   12.0   2.7   32.4   (max output)   
   500   12.0   3.0   36+   (volt reg limited*)
*The max 12vdc output is estimated at 50 watts, 500 RPM and 60 watts, 600 RPM if a larger voltage regulator is used.

The excitation strength of the very strong neodymium magnets is imperative to the output of both of these alternators. Both had significant cogging if all rotor magnets were placed directly adjacent to respective stator poles. The cogging can be minimized by tweaking the magnet orientation or spacing. Jeff

Offline airmodel

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Re: A.C. Generator
« Reply #13 on: July 29, 2016, 06:32:45 AM »
Rustkolector and Ian S C

Thankyou for the information for your slow speed alternators, it made me go out to my workshop to do some testing at low voltage. Here is what I came up with.

780 RPM 5 watt 24 volt globe
920 RPM 5 watt 12 volt globe
500 RPM 0.52 watt using two Christmas globes
600 RPM 0.93 watt    "     "          "            "

Automotive alternators are supposed to put out a reasonable output at engine idling speed (500RPM) but they seem a bit large in size for model engines.
I was hoping that using powerful magnets would make it generate at a lower speed but anything under 1000 RPM it has very little output.

Offline Ian S C

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Re: A.C. Generator
« Reply #14 on: July 29, 2016, 12:43:41 PM »
One thing you don't need is an automotive alternator,.  With luck you can get an alternator to strike without a battery.  You need at least a couple of HP, and 2500/ 3000 rpm at the alternator shaft.  An old auto generator is ok, but still best with a battery in the circuit.
A good low speed alternator is a cycle hub dynamo, I have a Sturmy Archer one that produces 8 volts at 120 rpm.  My one was part of a 3 speed dyno hub, the gears were taken out and the hub  reconfigured to suit a wind turbine.
Here is my portable power supply, it uses a 3hp Kawasaki motor, and a 45W(I think) Lucas auto alternator.  The frame is in the form of a sack barrow, and rigged to be towable with my bicycle (I have other trailers for the bike).  I could do with fitting a pulley about 2" larger on the motor.
Ian S C       
« Last Edit: July 29, 2016, 12:50:26 PM by Ian S C »