3 phase electronics

[triple_access]

Hello, i am studying 3 phase current and do not totaly understand it.

i have learned that 3 phase at a certain frequency causes harmonics??

i would like to take "3" transformers and have 3 phase as an output.


I can make many circuits, i can make anything if i have a schematic to go by. The problem is i cant find any circuits for driving the 3 3 phase transformer configeration.

i would like to be able to adjust the Frequency of the 3 phase transformers by just turning a pot.

any help will be greatly appreciated. even if you can't help maybe you can point me to a few 3 phase circuits just for driving transformers?

i really plane on building this circuit even if it takes me over a year of dedication, please take this post seriously because i really want to play with 3 phase frequencys electronicly.

Thank you

 

[triple_access]

i was also thinking, would the best way be to program a chip to drive each set of transistors with a sine wave? and add a pot to change the frequency threw the software on the chip?

i can also program chips, but id rather not, i was looking for something in a rather simple schematic.

 

[triple_access]

i have found this site to kinda help out **broken link removed**

 

[dknguyen]

So you want to drive a 3 phase transformer? Do you want to work with regular high voltage 3-phase? Or will low voltage 3 phase (like 12 or 24V) work for your learning.

You can build an 3-phase inverter that converts DC to 3-phase AC. It can run straight off a battery, or you can use a rectifier to convert wall voltage AC into DC. You use a transformer to step down the voltage, then run it through a diode bridge (full-wave) rectifier and have a giant capacitor in parallel with the output (maybe an inductor too in series with the output) to smooth the waveform.

Then you can use 3 pairs of HI/LO MOSFETs to take this DC and use it to make AC with each HI/LO pair producing the AC for each phase. You will need some output inductance to filter out the high frequency harmonics of the square waves being produced to get the fundamental sinusoid that you want.

To make AC with PWM, superimpose a triangle wave (with a frequency of the switching frequency of the MOSFETs) and overlap it onto a graph that has the AC sinusoid you want to output. Then whenver the triangle wave is larger than the sinusoid, you switch turn the HI MOSFET on, and whenever the triangle wave is lower than the sinusoid, you turn the LO MOSFET off. You do this for each phase and if you filter the high frequencies out square wave, you get the sinusoid. This would be done in a microcontroller by having a lookup table of various points on a sin-wave, and then comparing the current switching frequency times to the values on the lookup table to determine how you are switching the transistors.

To do this discretely you would need a sine-wave generator, a triangle wave generator feeding into a comparator...the schematic is simpler with a microcontroller but you have to program.

**broken link removed**

It's actually very similar to a brushless motor driver. I can give you my schematic so you can kind of see the inverter circuit but it will have a lot of other stuff on it that is needed to pull it all off.

 

[ljcox]

Did you do a search of this forum for "phase" or "3 phase"?

Here is one https://www.electro-tech-online.com/threads/3-phase-converter.5656/?highlight=phase

 

[triple_access]

i can see where this is going. the 3 phase i want must be from a 12 or 24 volt battery supply.

it has to replicate a 3 phase generator raking across fields, this way you will get harmonics. it is important that each transformer is driven at the proper time. so i feel this will be a rare circuit because of the fact i am wanting harmonics.

the below picture is what i have drawn up, i would assume i need 3 of these circuits all gated by seperate square waves? each gate has its on time to come on and go off.

again, i am not really sure how 3 phase works under the microscope, so this leaves me unsure of how to apply the 3 gated signals.

in the end, "one" potentiometer will control the frequency of all 3 gate signals. this way i can change the frequency to gain the harmonics i am wanting.

i am still puzzled. .

 

[dknguyen]

WHat you drew is a single phase, 4-switch rectifier (it is also an H-bridge used to drive brushed DC motors to allow reversing direction). (And you should have separate signals for the gates. Ideally you can drive a singlke P/N pair with the same gate signal, but in practicality bad things happen when you do this because devices are different and are not perfect).

THis is a 3 phase inverter/
https://en.wikipedia.org/wiki/Image:3-phase_inverter_cjc.png

You apply the PWM signal to drive a SINGLE phase in the method I Described above about turning on the high side transistor on whenever the triangle wave is higher than the sine wave, and to turn on the low side transistor whenever the triangle wave is below the sine wave. (THe sine wave and triangle wave being waves stored in memory and compared to each other) to produce a proper PWM signal whose fundamental is that of the sine wave stored in memory. To produce the other two phases, you just shift them by +120 and -120 degrees from the first.

I would also use MOSFETs rather than BJTs or IBGTs- much less losses at these power levels and easier to switch (and switch faster). THe MOSFET behaves as a very small resistor while the BJT behaves as a diode. At these low power levels, the current forms less of a voltage drop across the MOSFET's resistor than the BJT/IGBT's diode so you have less power loss which means less cooling and smaller devices. Only at higher current levels does the diode voltage drop become less than the voltage drop formed across the resistor by the current.

 

[triple_access]

oh my, you're so much more smarter than me! you're saying to use a gating for each fet, or a gate for each 2 fets?

i guess i have the rest of my life to figure this out... just to be sure, you're talking about a programmable chip here correct?

 

[dknguyen]

I am generally referring a programmable chip to drive the gates of the FETs.

"A gating for each FET"
??? I am saying that in a P/N FET pair, they both behave in predictable ways relative to one another. WHen one does one thing, the other one always does the opposite (ie. when one is one, the other must be off. you don't want both on at the same time or it makes a short circuit). THis means you ideally can control them with a single signal (you might need an inverter along the way or use a P-type and N-type device so they behave oppositely to the same gate drive signal.

But, in real life FETs aren't identical and turn off slower than they turn on. THis means that if you apply a signal for one to turn off and other to turn on at the same time, there will be a time period when they are both on = short circuit. YOu either need circuitry to deal with this, or use a programmable chip (a microcontroller) that basically controls the gates independently and adds in a delay between switchings to make sure both are never on at the same time.

BTW, when I talk I speak of just one phase. Since controlling all the other phases is pretty much the same thing, you just delay everything in the other phases by 120 degrees.

YOu might as well use a microcontroller for this because if you don't use one, you still need to find a way to generate a sinusoid and a triangle wave to feed into a comparator to produce a PWM gate drive signal that has a sinusoidal fundamental (to produce your AC output). Actually producing those two signals isn't nearly as easy or as flexible as just having it as a big table of lookup values in software.

If you don't understand how a sinusoidal PWM signal is generated, then look at this:
**broken link removed**
Notice that the triangle wave has the same frequency as the PWM signal (it's the carrier) and that whenever it is above the desired sinusoidal signal (stored as a lookup table or equation in memory) then the PWM is HI and when it is below, then it is LO.

 

[triple_access]

so if you can find the proper fets or transistors you can use a hex inverter to drive one of the the pair and there will be no short circuit? but are such drivers available?

 

[triple_access]

so i need 3 duty cycles

if this is so, i would rather use 3 555 timers all sharing the same circuit board.

so this way i would have 3 different gating outputs.

 

[Hero999]

Also please note that this isn't a project for beginners, you need a lot of experiance before yo build something like this.

 

[triple_access]

Also please note that this isn't a project for beginners, you need a lot of experiance before yo build something like this.

yes of course hero, i am no beginner in electronics, i have built several very complicated circuits. i just do not fully understand 3 phase yet.

i can say i have learned alot about short circuits and driver cut on/off speeds, i've learned alot from this thread.

thank you for that anyhow.

 

[ljcox]

so i need 3 duty cycles

if this is so, i would rather use 3 555 timers all sharing the same circuit board.

so this way i would have 3 different gating outputs.

The simplest way to generate these waveforms would be to use a Modulo 3 counter with gating to provide the outputs.

I designed the circuit in that attachment for someone in this forum some year or 2 ago. It is intended for a different purpose, but can be easily adapted to your purpose.

U1 is configured as a Mod 3 counter. You don't need U2.

The Phase 1 signal can be taken from the Q1 bar output of U1a and you can use 2 NOR gates to generate the other 2 signals.

You need to drive the counter from an oscillator whose frequency is 3 times the frequency that you want at the output.

 

[on1aag]

Something cheap and simple, what about this ?
No idea how it works though.

on1aag.

 

[triple_access]

thanks much, you guy's are so much help! i will also try to contact my internet friend on msn to help me get this assembled as he wants one to!

thanks guy's.

 

[ljcox]

Ignore my previous post. I forgot that the Mod 3 counter will give a duty cycle of 33/67%, not 50/50%.

Making it 50/50 would require some gates.

The attachment is an extract from a 3 phase meter that I designed and built for a friend some years ago. It generastes 3 pahes signal with a 50/50% duty cycle. IC1c is the "power on reset" which is necessary with tristed ring counters to prevent the possibility of it starting in one of the "trap" states.

Note that Q2 needs to be inverted in order to provide 120 degrees between each phase signal. Use one of the spare inverters.

 

[triple_access]

oh my is it that simple? that little thing lol, this is awsome..

i see to where you can set it to get 50 or 60 hertz so how do i make that more of a wider range? could i just use a 100k pot?

i assume that on some higher frequencys the caps would have to be changed?

 

[triple_access]

Oh, i think i see now! you just change the clocking to get whatever hertz you want?

oh yeah!!!! very nice and simple circuit!!! i was about to flash a chip..


EDIT, i see that the clocking for 50 to 60 hertz is 300MHZ.

i was wondering about slowing it down so you can get 60 hertz at say 20 MHZ, this way when i add a variable Clocking source i can easily scan threw the hertz for harmonics by changing the clock source.

 

[ljcox]

Oh, i think i see now! you just change the clocking to get whatever hertz you want?

oh yeah!!!! very nice and simple circuit!!! i was about to flash a chip..


EDIT, i see that the clocking for 50 to 60 hertz is 300MHZ. No! it is 300 Hz.

i was wondering about slowing it down so you can get 60 hertz at say 20 MHZ, this way when i add a variable Clocking source i can easily scan threw the hertz for harmonics by changing the clock source.

The counter is a Modulo 6, so it divides the oscillator frequency by 6!

From memory, a square wave signal has odd harmonics, for example, a 100 Hz signal has harmonics at 300Hz, 500 Hz, 700 Hz, 900 Hz, etc.