### Author Topic: need help with a ac-dc conversion  (Read 2180 times)

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#### apc3161

• Jr. Member
• Posts: 36
##### need help with a ac-dc conversion
« on: May 28, 2009, 07:35:37 PM »
Hello all,

I signed up for this forum for robotics purposes (want to get started in it as a hobby) and have already started building the \$50 robot. But I am a mechanical engineer, and have a question from another project about electricity production (not my strong point) . I along with others am designing/building a micro turbine. It will be about the size of a AA battery, but have a power density of basically it's fuel, ethanol, which has a power density (joule/gram) around 10x that of batteries. Hence you can see the attraction (you can also "recharge" your power source instantly, just like a car). I actually hope to test the product on a robot one day.

Anyways, being that I am a mechanical engineer, I don't know much about electricity, and need help converting an AC current from our generator to DC current.

Here is the generator (it is a DC stepper motor that we plan to use in reverse as a generator)

I was thinking of using a rectifier made up of diodes to solve our problem.

http://en.wikipedia.org/wiki/Rectifier

as in this instructable

http://www.instructables.com/id/S4K4UDCF3KRXJ08/

The problem is that the details are fuzzy. I was wondering if anyone out there had any experience with converting AC to DC and electrical motors.

If so, could you recommend what diodes specifically to use, what capacitors to level out the DC output, and any other details. I would follow that instructable verbatim, I just don't know if the diodes in it are good for what I need.

Price isn't really a big deal as long as its not too astronomical.

Any help appreciated.

#### billhowl

• Supreme Robot
• Posts: 376
##### Re: need help with a ac-dc conversion
« Reply #1 on: May 29, 2009, 02:40:46 AM »
Parts are listed in instructable step 1.
8 numbers of 1N4001 Diodes which operate at 50V 1 Amp is good for your use.
As for capacitors you can use anything from 220uf to 470uf will do.

You cam follow the power Plant Schematic Circuit Diagram in the instructable.

But look at your spec on the generator, you can't get high current from that, the most is 100 to 200mA, can't power a robot.
BTW what are you want to operate with it?

#### apc3161

• Jr. Member
• Posts: 36
##### Re: need help with a ac-dc conversion
« Reply #2 on: May 29, 2009, 08:12:12 AM »
Parts are listed in instructable step 1.
8 numbers of 1N4001 Diodes which operate at 50V 1 Amp is good for your use.
As for capacitors you can use anything from 220uf to 470uf will do.

You cam follow the power Plant Schematic Circuit Diagram in the instructable.

But look at your spec on the generator, you can't get high current from that, the most is 100 to 200mA, can't power a robot.
BTW what are you want to operate with it?

Thanks for the help. That generator is just for testing, but our actual turbine has an output between 5-10 watts when running on ethanol. Alright, I will try and follow that instructable, hopefully it will work. Thanks for the help!

#### Soeren

• Supreme Robot
• Posts: 4,672
##### Re: need help with a ac-dc conversion
« Reply #3 on: May 29, 2009, 09:11:12 AM »
Hi,

Being of such low power (the motor/generator), you want as small losses as possible and that means that you should choose your rectifiers with a lot of care. 1N400x is absolutely not suited here! Your best bet is getting Schottky diodes or similar low-loss diodes (or even MOSFETs used as diodes).

I'm, on my way to a party in an hours time, so I'll look at it in more depth tomorrow (or Sunday, as there's a dinner party that I have to attend Saturday), if you need to be run through the construction.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Engineering is based on numbers - not adjectives

#### apc3161

• Jr. Member
• Posts: 36
##### Re: need help with a ac-dc conversion
« Reply #4 on: May 29, 2009, 04:18:16 PM »
Thanks Soeren. Any help is much appreciated. Here is what I think is going (although please feel free to correct me as I'm sure I've made a lot of mistakes)

Ok, so our generator (motor in reverse) is this one.

I think that is a 3 phase motor. We plan to rotate it at 60,000 rpm. So because it's back EMF constant is 0.159 mV/RPM, I think we will get a voltage from it of 9.54 Volts from each of the 3 leads coming off of it.

Did I do that right? Note: I'm not sure whether that voltage represents RMS voltage or peak voltage.

Then, I think we can use these diodes, the type you recommended in your last post Soeren, Schottky: 1N5817

Now, our turbine outputs roughly 5 watts. So if our voltage is roughly 10 volts for each phase, I think we should see a current output of roughly 0.1667 amps (=5 watts/ 3 leads/ 10 volts ) from each of the three leads. And that PDF claims that "Average Rectified Forward Current" is equal to 1 AMP. Assuming "Average Rectified Forward Current" refers to the average current the diode can handle, we are less than its rated value of 1 A so we should be ok.

I'm also confused about what I should do with the 3 DC outputs from the three different diode bridges I make. Can/should I hook them up in series to get roughly a 30 volt/0.1667 amp output? Or should/can I put them in parallel to get an output of roughly 10 volts / 0.5 amp.  That one is confusing me.

Again, any and all help/clarifications are much appreciated. If I've made any mistakes, please let me know. Thanks so much.

apc

#### Soeren

• Supreme Robot
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##### Re: need help with a ac-dc conversion
« Reply #5 on: May 30, 2009, 07:49:38 AM »
Hi,

I think that is a 3 phase motor. We plan to rotate it at 60,000 rpm. So because it's back EMF constant is 0.159 mV/RPM, I think we will get a voltage from it of 9.54 Volts from each of the 3 leads coming off of it.

Did I do that right? Note: I'm not sure whether that voltage represents RMS voltage or peak voltage.
For the 12V motor, it is. The measure is probably unloaded, so may be lower in use. I'd assume it's RMS (so expect a rather larger peak voltage).

Do you have the motor at hand now?
The datasheet mentions pin 1 as "Star point", which indicates a "Y" connection (opposite "Delta" connection), but each phase has got 2 pins in the connector, perhaps just for load sharing in the connector - if you could measure if eg. pin 2 and pin 3 is connected directly, or if it's each end of a winding, we'd know for certain.

Then, I think we can use these diodes, the type you recommended in your last post Soeren, Schottky: 1N5817

I'd go for the 1N5819, as they'll be able to handle higher voltage spikes. Otherwise, they should be quite fine.

Now, our turbine outputs roughly 5 watts. So if our voltage is roughly 10 volts for each phase, I think we should see a current output of roughly 0.1667 amps (=5 watts/ 3 leads/ 10 volts ) from each of the three leads. And that PDF claims that "Average Rectified Forward Current" is equal to 1 AMP. Assuming "Average Rectified Forward Current" refers to the average current the diode can handle, we are less than its rated value of 1 A so we should be ok.
The diodes will be fine since the current will be well with limits, but you cannot get 5W out of that motor/genny, even if you turned it with a 5 MW turbine - it's the motor that limits the output!
According to the datasheet, you'll be able to get slightly below 0.4A at 60k RPM

I'm also confused about what I should do with the 3 DC outputs from the three different diode bridges I make. Can/should I hook them up in series to get roughly a 30 volt/0.1667 amp output? Or should/can I put them in parallel to get an output of roughly 10 volts / 0.5 amp.  That one is confusing me.
You shouldn't use 3 full diode bridges, just 6 diodes like this:

That's assuming that the motor really is star wound.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Engineering is based on numbers - not adjectives

• Jr. Member
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