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Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
yah the push-pull couplier so fare is is a nathan special. I wanted to add as much ferite as pausible. With all those wires I can do 9:1, 18:1, 36:1. the mmc on the top has a ESR of 4m ohm at 3uf. it may not be necisary at all I just want about 2k amp circulating current.it is in paralell wit he celem's for more circulating current The green current transformer is a person transformer. I think that it is .1v per amp. I am not shure of its construction. I'll just use it for diagnotics and tuning, it will tell me the current wave form for my o-scope.
the compresion fittings are 1/2"swagelock and should be enough for the moment. I agree that a flaired coupling alows for better contact and I my go with at a later date
Registered Member #4254
Joined: Sat Dec 10 2011, 09:36PM
Location: Dhaka, Bangladesh
Posts: 27
Hey, your project is the most neat and powerful i've ever seen.
I am very much interested to build a similar one of 15KVA. I've made a capacitor bank capable of 120/150 amps. The combined rating is 3.8 uF 1000V. Just need to get those bigAss IGBT's, Coupling transformer. I have a AVR development board to try the pll self locking and soft start. The current limiting switching is just AWESOME!! Which eliminates the requirement of a large Transformer or Variac !!!!
In my country the mains are 240 V ac. Can i hook up the mains directly to the bridge rectifier???? For snubber cap what kind of capacitor should be used???
Registered Member #5258
Joined: Sun Jun 10 2012, 10:15PM
Location: Missouri - USA
Posts: 119
mohiuddinHimel wrote ...
Hey, your project is the most neat and powerful i've ever seen.
I am very much interested to build a similar one of 15KVA. I've made a capacitor bank capable of 120/150 amps. The combined rating is 3.8 uF 1000V. Just need to get those bigAss IGBT's, Coupling transformer. I have a AVR development board to try the pll self locking and soft start. The current limiting switching is just AWESOME!! Which eliminates the requirement of a large Transformer or Variac !!!!
In my country the mains are 240 V ac. Can i hook up the mains directly to the bridge rectifier???? For snubber cap what kind of capacitor should be used???
Thanks in advance :)
Hi Mohiuddin, thanks for the complements. There is plenty of room for improvement, namely in the tank setup, driver section and cleaning up the control layout.
I was looking at your capacitor bank the other day when you posted it, very creative! You will want to make sure your connection to the bank right in the center of each side so the reactive load will be distributed properly. (Unlike mine where I have an inline setup: cap > cap > coil. Although those big caps handle it fine at the relatively moderate power levels I'm running.)
Do you have a target resonant frequency?
The AVR controlled PLL is pretty simple to setup, and if you do it cleanly it even works on a breadboard. Let me know if you hit any snags and I'll do what I can to help out.
I really wasn't happy with the closed loop current limiting set up I had. Which was basically this: line current read with a closed loop ratio-metric hall effect sensor, integrated, rectified, and converted to a dc voltage. Having a response time of a couple hundred uSeconds after conditioning. The dc voltage was then read by the processor at the same time that the phase value was read from the PLL comp output. In the control loop where the processor would adjust the PWM output for the VCO voltage if the current was equal to or above where the software limit was set it would not allow the VCO to be lowered. If it was above the limit it would force a positive offset, raising the VCO frequency regardless of whether or not the phase was above resonance. I had issues with oscillation that were difficult to deal with because it's behavior different when there was a work piece in. The size and temperature of the work piece also affected it. So I'm going another route...
Rather than limit the current with an above resonance offset. I'm going to use a variable duty cycle to drive the IGBT's while keeping it at resonance. This way the tank is always pulling it's maximum current draw and is always at resonance. So it's maximum current will be a celling set by the duty cycle of the inverter rather than trying to back down from a celling so oscillation will be impossible.
There are a few changes that need to be made first which I'm working on now. The PLL's VCO can not be used because it is locked into 50% duty cycle operation. The driver setup I'm using will not work either since it is also setup for use with a 50% duty cycle waveform. I'm changing over from the easy MosFET driver/pulse transformer type to a Hybrid-IC opto-isolated driver. This will allow me to set the on and off time independently and will also greatly increase my frequency range since I will not be dependent on a pulse transformer. Another benefit here is that I will be able to add in desaturation detection to detect/stop short circuits. Lots of other cool stuff that I'll go into once it's done.
Yes you can connect your mains directly to the bridge rectifier as long as it is properly rated. Right now my IH is running on 240V using a 100A 1600V 3-Phase rectifier.
As far as the snubber cap goes you generally want a low henry cap very close to the DC bus (right on the IGBT preferably). It should be rated nearly 3-4 times the maximum bus voltage. It's value can range from .1 to 20uF depending on your bus voltage, ripple, inverter dv/dt, and current draw.
This defiantly is not as easy to build as some of the self resonant IHs out there but I have always had trouble getting those to resonate cleanly at high current. Sacrificed my share of FETs to the trash. (Until canceling the antiparallel diode and replacing it with a faster one.) :)
Registered Member #4254
Joined: Sat Dec 10 2011, 09:36PM
Location: Dhaka, Bangladesh
Posts: 27
hey bro, thanks a lot for the feedback
The duty cycle current limiting topology sounds damn good to me...it's even less complicated to establish as the whole thing will be done inside the uC. But in that case as you've said, the gate drive transformer cant be used.
I've made a circuit for variable duty cycle switching. Its a full bridge inverter. Take a look if it can be used. It needs some modification to drive large IGBTs.
But i'll finish my ZVS first. This way i'll be able to test my work coil and capacitor bank.
After the ZVS i'll go straight for the high power unit. If lucky i'll get some celem/ eurofarad caps for the tank.
Registered Member #5258
Joined: Sun Jun 10 2012, 10:15PM
Location: Missouri - USA
Posts: 119
Hey Mohiuddin, make any progress?
I tried pulse skipping as a power control method again this time with a better zero cross detector and a bigger TRIAC. I still did not like the results though. With or with out a filter cap on the HV rectifier the control was not very linear. I have settle on phase shifting one side of the full bridge (see update in OP).
I basically divided the 60Hz pulse change up into 8 pulse sections. I had a byte who's bits controlled if a pulse was on or off during each of the 8 steps. So the byte would just be continually rotated, if the sampled bit was a 1 the DIAC would turn the TRIAC on, if it was a 0 it would leave it off.
The 9 different power levels were: If Pwr_level = 0 Then Pwr = &B00000000 If Pwr_level = 1 Then Pwr = &B00000010 If Pwr_level = 2 Then Pwr = &B00010001 If Pwr_level = 3 Then Pwr = &B01001001 If Pwr_level = 4 Then Pwr = &B01010101 If Pwr_level = 5 Then Pwr = &B10111010 If Pwr_level = 6 Then Pwr = &B10111011 If Pwr_level = 7 Then Pwr = &B11111011 If Pwr_level = 8 Then Pwr = &B11111111
So each level would just add one more cycle to the 8 bit group, keeping them somewhat evenly spaced.
Video showing each of the 9 steps -
Another concept I tried:
Using a varying duty cycle PWM I disabled the output of the gate drives. I tried centering the disabling pulse in the middle of each phase, at the end, and beginning. None of which produced satisfactory results. The power level was again choppy and very non-liniear. The phase shifting method is definitely the way to go for smooth linear control.
Registered Member #4254
Joined: Sat Dec 10 2011, 09:36PM
Location: Dhaka, Bangladesh
Posts: 27
hey josh, i couldn't do any progress for a long time as i had a shoulder surgery. I've seen your latest video on youtube and that was just brilliant. Achieving such thing is only a dream for the time being.
After being able to work again i completed the 2nd prototype inverter and a transformer isolated gate driver board. Frequency tracking was done by 4046 PLL with simple current limiter circuit. The old capacitor bank worked just fine. I was able to push 4~5KW of power (25 amps, 220V mains).
But the current limiter circuit seems a poor decision for power level above 2KW. I've lost dozens of IGBTs withing a week. I've kept a cycle by cycle enable option at driver stage to implement pulse skipping power control scheme. I applied a low frequency signal 1/2 KHz into the "enable" port of the driver board. I assumed that by adjusting the dead time i would be able to regulate the power. Everything worked as planned but the PLL failed to lock on whenever the deadtime is below 50%. Moreover the tank and coupling transformer made loooots of noisy sound.
I still didn't use any micro controller or DSP chip to track the resonance as my knowledge is as little as a noob. Before using any IGBT brick i need a proper way of regulating power.
any suggestions on power control??
the pll circuit is similar to uzzor's with a addition of current limiter
Registered Member #5258
Joined: Sun Jun 10 2012, 10:15PM
Location: Missouri - USA
Posts: 119
Signus wrote ...
Hi Josh, any updates to GitHub/ReactorForge?
Hi Signus,
I am working this week on incorporating a list of 20-30 hardware modifications and tweaks after the first board run, that will be my first public hardware release for MKIII. I have a few software tasks that need completed as well before I release that, but I've already started on those. I'm also going to release a small tutorial a key building block in the MKIII, the micro controller PLL.
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Induction Heater - Series Resonant - uC/PLL Tuning [ReactorForge]
