Three phase PWM for AC motor with PIC18F4431

Scarface · Joined: 03 Dec 2013 Posts: 5

Hi,
I'm making a project using pic18f4431 which drive three phase ac motor. I put pwm outputs in complimentary mode and everything is OK. But to drive the ac motor I need 120 degree offset between each inverter arm. So, can someone help me with code?

Mike Walne · Joined: 19 Feb 2004 Posts: 1785 Location: Boston Spa UK

We need lots more information about what you are trying to do.

1) What is the adc reading supposed to control?
2) Are you wanting to control motor speed?
3) What have you already tried?
4) What searches have you done?
5) ...............................

Mike

temtronic · Posted: Tue Dec 31, 2013 12:39 pm

There's a Microchip application note about doing it ! Had it on my desktop..opened it ,read a bit then ..decided to clean up the PC...now it's in 'some folder, somewhere' on this silly PC. Shouldn't be too hard to find at the uChip site though,faster than me trying to figure out just where I put it this morning.

hth
jay

+++++++++++++++++
Jay,
A reminder. Give a useful answer - not that you lost an appnote.
It's a help forum. Not a chat forum.

- Forum Moderator
+++++++++++++++++

Scarface · Joined: 03 Dec 2013 Posts: 5

Mike,
adc reading in code is for variable speed control. The idea in this project is exactly motor speed control. I have also current,voltage and temperature feedbacks and LCD display and there is no problem to write their source code. I want to generate three phase pwm which will drive my ac motor. What I've done is to set pwm in complimentary mode (T1, T3, T5-high; T2, T4, T6-low), but this iя not correct for three phase inverter - all pwms have same phase angle. So I need 120 degree offset between each inverter arm. I read CCS manual but I don't find any function for this.

T1....T3....T5
.|......|......|.
T2....T4....T6

Applications which I use are:
AC Induction Motor Fundamentals http://ww1.microchip.com/downloads/en/AppNotes/00887a.pdf
Controlling 3-Phase AC Induction Motors Using the PIC18F4431 http://ww1.microchip.com/downloads/en/AppNotes/00900a.pdf

Ttelmah · Joined: 11 Mar 2010 Posts: 19513

The point about Jay's post, is that there _is_ an app note.
He has lost it, but is pointing out that a quick search of the MicroChip site should find it, and this ought to be the starting point.

I think it is probably AN900 that he is thinking of (there are actually quite a few more for more complex control), which the original poster has already found.

In general the 'flowchart' of such notes gives a great key to how it needs to be done.

However I think there is a fundamental misunderstanding about how you do this from the original poster. You don't use the pulses of the PWM to directly drive the individual phases with just on/off. You synthesise a sinusoidal waveform by varying the mark/space of the PWM. You run the PWM, at something like 10KHz, and are changing the value fed to each of the three outputs on every cycle of this. Study how AN900 updates the pwm ratios....

Best Wishes

Mike Walne · Joined: 19 Feb 2004 Posts: 1785 Location: Boston Spa UK

Having found AN900 you need to read it carefully.

Ttelmah has drawn attention to the fundamental error in your understanding of what's required.

Each of the PWMs is varied to synthesise a sine wave.
It's these three sine wave which are 120 degrees out of step with each other.
The principal determinant of motor speed is the frequency of the sine waves, not the PWM ratio as you seem to think.

It's all in the app note.

Happy new year.

Mike

Scarface · Joined: 03 Dec 2013 Posts: 5

I know that synthesise a sinusoidal waveform is by varying the space of the PWM. Varying duty cycle of PWM I control sine waves and respectively induction motor. But to work three phase inverter you not have to allow transistors T1,T3,T5 or T2,T4,T6 to work at the same time. If they work at the same time they will burn. What I mean by wanting to put PWM offset like a picture I posted you can see this in this app http://www.tntech.edu/files/cesr/StudThesis/asuri/Chapter4.pdf . App explains fundamental princip of inverters. Look at from point 4.3 three phase inverter to the end of pdf and you'll understand what I want to do.

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Search the Microchip forum. Use Google to search for this:

Ttelmah · Joined: 11 Mar 2010 Posts: 19513

The only FET's that mustn't/won't be on at the same time, are the second one in each pair. So, using the numbers from Fig1, in AN900, PWM2 must not be on at the same time as PWM1. Similarly 2/3, and 4/5. However PWM3, and PWM1, _will_ be on at the same time, as will PWM1, and PWM4, at different parts of the cycle etc..
The point is that the individual PWM waveforms _won't_ display the relationship you are looking for. It is the combined 'effect' out, from the selection of mark/space ratio and which pairs are being driven, which has the 120 degree relationship....

Best Wishes

Mike Walne · Joined: 19 Feb 2004 Posts: 1785 Location: Boston Spa UK

Scarface · Joined: 03 Dec 2013 Posts: 5

Ok let's say that I'm wrong and you're right.
This is ASM source code used in AN900 app. As you see there is PWM offeset exactly how I want to do.

temtronic · Posted: Wed Jan 01, 2014 9:52 am

comment: OK, I finally did find the apnote and it is AN900 and managed to loose 27,000+ PIC CCS files in the meantime,so I'm not exactly happy this new year...
Providing the asm code you show is compatible with your PIC, you code add it using the CCS instaruction..#asm..... something that CCS does allow and I've used in the past.
Another option is to 'decode' what it's doing and create similar C code.Good news is uChp has pretty good comments on what's going on. Yes, not exactly what a rookie programmer can easily do, but can be done,just takes some time.
Yet another option is to use additional hardware to make the 3 phase outputs.If you start with a 6* clock signal then feed that into a /6 counter, you'll get 3phases 120* apart.Sorry I don't have apart number or schematic to provide but it's an 'old school' way of doing it.You only need one PWM signal(6* desired frequency).

hth
jay

Mike Walne · Joined: 19 Feb 2004 Posts: 1785 Location: Boston Spa UK

Yes. The offset is forcing each PWM channel to look up a sine table value which is 120 degrees phase shifted from the other channels.

In the microchip code all three PWM channels are being set edge aligned.
There's nothing to stop T1, T3 & T5 turning on together. Nor for that matter T2, T4 & T6.

It's important T1/2 don't turn on together, (and of course T3/4 & T5/6).

If you try to prevent T1,3,5 coming on simultaneaously you'll likely end up with a huge DC component across at least one winding with equally disastrous results.

Mike

Scarface · Joined: 03 Dec 2013 Posts: 5

Is there any way this code to be modified from ASM to CCS because I have no idea of assembler programing. Or my code to be written with offset.

The reason why T1,T3 and T5 haven't work together at the same time is because then the electrical circuit can't be closed. The Circuit always is between Power supply/Signal and ground but this case you haven't it. The second thing is that in this mode 100% you will burn out some of the electronic elements. Some is with T2,T4 and T6. I've seen blown inverters for this reason. You are right when you say that T1 and T2(3/4 or 5/6) don't be turn on together. The correct way of working is: two transistors up(T1,T3 or T5) and one down(T2,T4 or T6) or one up and two down at the same time. This is the reason why is set 120 degree offset. It keeps transistor from wrong work. You can see it and from the picture I posted.

Ttelmah · Joined: 11 Mar 2010 Posts: 19513

The code is not exactly hard to modify. 50% of it is remmed out, and the values are just a matter of array lookups, multiplication, and loading the registers.
The point though is that the hardware does not ensure the transistors don't overlap. It is just the code that does this.

Best Wishes