# Motor as Generator + Bridge Rectifier issues!

Discussion in 'Sensors and Actuators' started by Moha99, Nov 18, 2011.

1. ### Moha99

261
0
Nov 18, 2011
Hallo!

I have this problem with my bridge I'm trying to convert AC voltage to DC generated out of a small 12V/DC motor.

I generated almost 8.4V/AC. I'd like to convert it using my bridge,so I made a simple rectifier using this schematic.

Added a 1000uF capacitor at 16 volts to smooth out the DC output and I followed this formula to calculate the expected voltage Vin/AC x 1.4 = Vout/DC.

And here's the result:

http://imageshack.us/f/855/img2011111801154.jpg/

*Here's the problem... I made the "full wave bridge rectifier" and tested out the product I had 8.4V/AC in and I got almost 3.4 Volts DC out, I thought the voltage would be 11.76V/DC than "3.4" according to that smoothing formula "V/ACin x 1.4 = V/DCout".

What do you all think is the problem?

I used another capacitor 4700uF at 16 volts and the output was almost 2.6V/DC... What am I doing wrong? Or is it just normal?
lets say even if there was a voltage drop I thought it would be some simple number not that much of a drop .

Here are some pictures so you guys can see what I'm dealing with.

capacitor( 1000uF at 16V) + bridge:

http://imageshack.us/f/404/img2011111801155.jpg/

*Bridge(Close up):

http://imageshack.us/f/651/unled1ayk.jpg/

*Capacitors"1000 uF at 16V & 4700 uF at 16V":

http://imageshack.us/photo/my-images/196/img2011111801152.jpg

Thanks.

2. ### davelectronic

1,087
12
Dec 13, 2010
Hi Moha99.
Possibly your motor is not producing the force of voltage you expect, those small units will produce something, your AC x 1.41 is ok, but there is emf to consider, the little motor will only produce a peak maximum output, but the emf and voltage are two small due to losses, if it was say 4 x or more the size you might get your volts you hoped for, and again current will be low due to size and losses from the motor magnetic force to produce energy and power, there are also volts drop across your bridge rectifier, about 1 volt or so.

You could spin it faster, or use pulley's to gear it up, but with a small unit its not worth it, as you will ever only get a modest power level and voltage from it. What you can take away from it is you produced your own electricity, be it on a small scale.

Other members will have ideas as well, but if it where bigger it would have been more useful, try and get something bigger, wind or water flow to power it, and a battery plus small charging circuit and you could power lots of light duty electricals at low voltage.

3. ### Moha99

261
0
Nov 18, 2011
hey davelectronic!

Well I'm experimenting on something with my motor and thanks for that information but what is the highest expected DC voltage I can generate out of this motor? Im just trying to make at least at least! 4.5 volts DC I can't spin it faster...

But generally my converter is working fine right? I know the energy from this motor is nothing but I'm just experimenting.

Thanks for that davelectronic!

4. ### Moha99

261
0
Nov 18, 2011

+ What wound you recommend for a motor? I'm just trying to get that much voltage in DC! So what you recommend? Something small and where do you think I can find it?

5. ### davelectronic

1,087
12
Dec 13, 2010
Your definitely making DC electricity coveting AC to DC, small is only worth pursuing if small is all you want to power, off the top of my head i cant think of another small unit with more power to offer, i think its the motors winding configuration that determines if its any good as a generator, there are three types i know of, series/ shunt/ compound wound motors, ive a book lying around somewhere, i cant remember which makes the best generators, permanent magnet motors have a self generating field, but wound stators need a way of creating the field energy before they can generate power, a lot of people try old car alternators as wind generators.

You should read up on it, to be honest i need a refresher as i am rusty, ive been so engrossed in power supply construction ive neglected other things, one of them is motor theory and how small motors can be used for little generators, or tachometers for measurements speed etc. Maybe another more experienced kind member could look in and help us both out, or i will need some serious revision.

6. ### Harald KappModeratorModerator

9,864
2,084
Nov 17, 2011
Hi,
as far as I can see from the photos it looks like you hooked up the capacitor the wrong way. The "-" wire (the one where the capacitor's housing has the white strip) goes to the negative output of the rectifier (where the diodes have the white strip, too).

An electrolytic capacitor used with the wrong polarization can draw huge currents (and can explode, if the current source can deliver enough energy). This huge current cannot be supplied by your motor/generator so the volatge drops.

Try reversing the capacitors(s).

Regards,
Harald

7. ### davennModerator

13,552
1,850
Sep 5, 2009
Yes apart from the capacitor back to front. You have to consider the effect of a load on the output of the motor. It may well be generating 8.4VAC open circuit, but its loaded voltage is likely to drop considerably, especially since the current generated will be very very small.

For some serious experimenting connect up an old car generator or alternator to a windmill etc and start generating some useable power

The same thing happens with guys playing around with solar cells/panels when then discover the loaded voltage is quite a bit below the open circuit voltage

D

Last edited: Nov 18, 2011
8. ### Moha99

261
0
Nov 18, 2011
I just noticed that the photo's were wrong... I didn't realize that. But I wired it correctly when I tried it out. Same problem no change.

I'd like to use a car generator but the key of my experiment is to use a small motor smaller than the size of my hand. I do not require much current or voltage... I just need 4.5V/DC and 0.7A that's enough for my experiment!

Do you all have any suggestion of how I can generate 4.5V/DC?

Thanks.

9. ### Resqueline

2,848
2
Jul 31, 2009
What kind of a motor is that (type, purpose, voltage, power, RPM), and what speed can you attain driving it?
Do you want to use a brushless motor or can you accept using a brushed motor?
Anyway, the motor you want is rated for at least twice the wanted voltage and twice the wanted current, at an RPM half of what you can attain driving it.

10. ### Harald KappModeratorModerator

9,864
2,084
Nov 17, 2011
Hi,

I suggest using a bicycle dynamo. It should generate enough power for your requirements.
Add a rectifier, some fiilter caps and a simple voltage regulator (7805 will give you 5 Volt, if that's o.k., otherwise use an adjustable regulator).
Don't forget an overvoltage protection (zener diode) at the input of the voltage regulator as the voltage of the dynamo can rise to tens of volts when not loaded (the classical bicycle dynamo can be considered as a kind of current source).

Regards,
Harald

11. ### Moha99

261
0
Nov 18, 2011
Well I'm using this 12V/DC old cassette player motor.
I'm trying to find a bigger motor but It's pretty hard too...
I found this bigger motor from a printer at my house but someone blew it up lol connecting it with 220Volts and its just 12

Do you think that might work?

69
20
Dec 2, 2010
There are a few things that haven't been mentioned that need to be noted:

1) A permananent-magnet brushed DC motor also generates DC when rotated. It has a commutator. The direction of rotation will determine polarity.

2) Multiplying RMS AC voltage by 1.414 to predict smoothed DC voltage only applies to sinusoidal input. It is unlikely that a small DC motor will generate a sine wave.

3) Many inexpensive digital multimeters use DC coupling on their AC ranges, and will incorrectly indicate the presence of AC given a DC input. To test, set your meter to AC volts and probe a small battery. It should read 0V, or close to it.

My suggestion would be to forget the bridge, put the cap directly across the motor, set the meter to measure DC volts, and see what happens. Determine the correct polarity quickly and avoid reverse-biasing the cap.

13. ### Resqueline

2,848
2
Jul 31, 2009
Ok, so it's a direct drive 12V capstan motor, but what type of motor is it, what are the connections like, and where are the electronics driving it located? Pic's?

14. ### Moha99

261
0
Nov 18, 2011
I've decided to get a better motor with a higher voltage...
But if I could not find one I have to make this bridge work!

15. ### davelectronic

1,087
12
Dec 13, 2010
Thats a good response from other members Moha99.
A lot of posative info there, you can put these motors to work, its just selecting the right one so you get a really good output for a given size.

These and most motors have different property's in function, Some thing unrelated to generators got my attention once, vacuum cleaner motors run flat out, but derated they run less RPM this i tried it worked, but i was at a loss for a practical application, motor function is fascinating, as you can apply so many ideas to there uses.

But the most practical is wind water solar ideas, battery bank storage through a regulator / charging circuit, but the small stuff can be pressed into service to power small low voltage current applications.
Have fun with it, i will watch this space, interesting.

16. ### Moha99

261
0
Nov 18, 2011
Thank's anyways from the help!

I have a 12 volt brushless motor... And I'm trying to find a proper Stepper motor to try and do this:

So If I can't find a stepper motor what should I do? What if the diode is not fit for the job the one I used on the picture right there.
I still don't understand why my rectifier didn't work as this guy's video.

NOTE* I toke the photo after this problem and installed the "Capacitor wrong by mistake"

Thank you hope you can help clear out the reason my bridge didn't work well.

17. ### Moha99

261
0
Nov 18, 2011
Ok, I found a 24Volts DC Stepper motor
and I found another 36Volts Dc Stepper motor

Do you think they will do the job? Of generating high dc power "5 to 7Volts DC"?

Will it be enough? I mean its double the 12 volts and tripe it.. so it should make a good output under the same RPM speeds right?

18. ### davelectronic

1,087
12
Dec 13, 2010
I cant work out what the green thing is spinning, it looks like a desk fan opened up for the motors drive, not really the way to go one powered motor driving the other, kind of defeats the object of free power, i would go with wind and solar maybe, your bridge rectifier can be 4 diodes or a package rectifier imagine 4 diodes in two lines 2 diodes per line, connect the anodes and cathodes of each pair of two, each end is + volts and minus volts, between the two pairs AC goes in, the filter capacitor goes across the terminals of the DC output from the diodes, the correct way round, but if you use a meter and your motor is producing DC from its circuit already you dont need a rectifier any way, as adding one will drop the voltage, its only if your motor is producing AC at its output do you need a rectifier.

Your capacitor goes across the + volts - volts of the rectifier.

Last edited: Nov 18, 2011
19. ### davelectronic

1,087
12
Dec 13, 2010
Give it a try, but under load the voltage will drop some, you wont know how much by until you test and try it.

20. ### Moha99

261
0
Nov 18, 2011
I will but don't you recommend me getting a better motor before trying this out? I mean i think the problem is all about the motor right?

Last edited: Nov 18, 2011