Can a PWM generated from the PIC microcontroller to control TRIAC? Need help. urgent

[Chee Hoe]

currently I'm doing a project of Single phase Induction motor speed control by using microcontroller. My idea is to generate a pwm by the microcontoller and this pwm will then be fed to the gate for the Triac, so that the speed of the motor can be controlled. Will it be worked? any circuits can be provided for this project? If the triac not the right component, what are the other components can be used? MOSfet? IGBT? pls advice ... need help...

 

[johansen]

induction motors cannot be controlled with a triac on a single phase supply unless you are willing to accept very low motor efficiencies and other issues, such as rather low torque at low speeds.

 

[Chee Hoe]

Hi, Johansen, thank you for your reply, so for your opinion, if i replace the triac with mosfet, will it be better?

 

[johansen]

you can't pwm an induction motor either....

If you want speed control and full torque you have to build a variable frequency drive.

what's your budget for this?, and what is the application? search ebay for "VFD". if you need less than 1/2 hp you may want to just buy a new.
https://www.driveswarehouse.com has 1/4th hp drives starting at $140.

 

[tcmtech]

The closest you are going to get to speed controlling a single phase motor is with a very limited selection of motors such as the small fractional HP shaded pole types and the capacitor run types that most fans and blowers under one HP use.
Those can be speed controlled by phase angle regulation of the input AC input power but if any level of constant speed stability is required you will need a feedback circuit that reads the motors RPM and adjusts the phase angle firing rate of the triac to compensate for it.

Anything that uses a centrifugal switch to change it from start mode to run mode will have a very limited speed range it can work in without burning out.

 

[MrAl]

Hi,

As others have pointed out, you need a variable frequency drive to power the motor.
Luckily, if you have the same voltage input power available (at say 50 or 60Hz) you can get away with an H bridge drive with no output or input transformer. This would mean an input rectifier and filter, H bridge made from appropriate transistors, and a control circuit to allow variation of the voltage AND frequency simultaneously. To get the variable voltage you cut back the pattern to get lower voltages, and to get the frequency variation you simply vary the local oscillator frequency. The 'pattern' here is usually a pseudo sine pattern which doesnt bother the motor like a square wave might, although you may get away with a square wave where the full output is always a cut back pattern if i remember right it is set to cut the 3rd harmonic. All you do then is make this pulse shorter (PWM). If it doesnt work then you'll have to go to the more complex sine pattern. It will still be PWM but of a more complex nature.

If your input and output are not the same but the output is lower, then you can still do it that way. But if the output has to be higher than the input voltage, then you need a transformer on the input to step up the main power to a higher peak level before rectification.

I should caution you though this isnt a beginners project.

 

[Chee Hoe]

thank you tcmtech and MrAl , i will try it out . by the way, as i'm using single phase AC motor, can i just simply use mosfet as a switch to control the on/off state of the AC source to it so that the speed can be controlled?

 

[tcmtech]

As stated earlier without knowing what type of induction motor you have there is now way to say what is the correct way to if possible control its speed.

As far as just switching the AC power on and off like a DC PWM circuit does with Mosfet no you cant. Study up on AC current theory and how its switched and controlled paying close attention to power factor principals and you will understand why.

After that read up on how the voltage current and frequency are highly dependant on each other and the load itself in a induction motor as well. You have to keep all four ballanced or you will have problems.

 

[Chee Hoe]

@tcmtech, thanks for your advice. actually the induction motor that i'm using is the one from the table fan. How if the Power factor (PF) and the level of constant speed stability and speed of the motor are not concern in this case, i means i just want to increase/ decrease the speed of the motor through the PWM? or that is no way for an AC motor controlled by PWM?

ok. i will go through the AC current theory

 

[MrAl]

Hi,


You can use PWM but you would also use an H bridge setup which requires four transistors. You then PWM the transistors in the H bridge and that gives the motor the correct average voltage and the correct frequency. The input gets rectified and filtered into DC, then the DC gets chopped up into a pseudo sine (or special pulse width). This is the essence of a transformerless output controllable frequency drive.

 

[Chee Hoe]

@MrAl , really thank you for your reply. i got read through about it. but i couldn't find the complete circuit for it as a starting reference for me as this is my first time doing this project. besides, it is very hard get the a 12-230ac transformer at my place. do u got the schematic circuit ? do you know what model is the transformer for this application? really thanks for your guidance.

 

[johansen]

If you can't get a 12 to 230vac transformer you're not going to be able to find 2 half bridge drivers, 4 igbt's, 2 opamps, two current sense resistors, a microprocessor to generate a sine wave at arbitrary frequency, and a 5v and 12 volt power supply to drive the whole thing.
you'll also need at least a couple thousand microfarad capacitor (400v for 230vac line) and a bridge rectifier.

if you really want to save some money, buy a 1/8th hp drive off ebay and take out the igbt module, replacing it with a larger unit, discrete igbts or larger discrete mosfets.. there's been a few people who have successfully converted a 1 hp vfd into a 100 hp vfd using this method. the only thing you have to do besides that is change the current transformer.

 

[MrAl]

@MrAl , really thank you for your reply. i got read through about it. but i couldn't find the complete circuit for it as a starting reference for me as this is my first time doing this project. besides, it is very hard get the a 12-230ac transformer at my place. do u got the schematic circuit ? do you know what model is the transformer for this application? really thanks for your guidance.

Hi again,

You're saying that you need a 12v to 230vac transformer, so are you planning to go from 12v to 230v or from 230v to 12v, and also what output current do you need?

Part of the schematic is the H bridge. You could look on the web to find out how to build one of these.
The control part could be a circuit with two outputs that drives each half of the H bridge. For a simple pulse pattern you'd use this to turn on one half of the bridge for a max time period of 0.707 of the total half cycle time, and turn on the other half for the same time period max. To get lower outputs you cut back this pulse width to a shorter time period.

You may also want feedback in which case you need to measure the output and feed it back to an error amplifier and after integration feed it to the control circuit. The control circuit then decides how width to make the control pulses.

There's quite a bit to it so im not sure if you want to get this involved or not.

 

[tcmtech]

@tcmtech, thanks for your advice. actually the induction motor that i'm using is the one from the table fan. How if the Power factor (PF) and the level of constant speed stability and speed of the motor are not concern in this case, i means i just want to increase/ decrease the speed of the motor through the PWM? or that is no way for an AC motor controlled by PWM?

Those types of motors are shaded pole types which can easily controlled by simple voltage regulation or phase angle adjustment with a simple light dimmer circuit.

Basically you are complicating what should be and is a simple subject.

 

[Chee Hoe]

@tcmtech can this circuit (**broken link removed**) to be used? are we just need to adjust the 250k pot to control the phase angle? how to relate the resistance(potentiometer voltage) to the firing angle ? or maybe do you got any reference or formula? i can go through myself ... thank you

 

[tcmtech]

You are over analyzing and over complicating a simple device and it s application.

Just buy one at the local hardware store for $7 and be done with it or build it and turn the knob and see what happens.

Either way at most you may need to add a small AC capacitor of 1 - 5 uf across your fan motor to balance out the power factor so that the dimmer circuit can work most efficiently.

 

[Chee Hoe]

hehe.. tcmtech, I'm living in asia country, my place got no such hardware store to buy the ready circuit. so can i build the same circuit as given by this **broken link removed** website? is there any precaution steps need to be taken when building this circuitry?

 

[MrAl]

You are over analyzing and over complicating a simple device and it s application.

Just buy one at the local hardware store for $7 and be done with it or build it and turn the knob and see what happens.

Either way at most you may need to add a small AC capacitor of 1 - 5 uf across your fan motor to balance out the power factor so that the dimmer circuit can work most efficiently.

Hi,

You arent suggesting that he should drive a capacitor with a triac that has a variable conduction angle are you?

 

[tcmtech]

I have used off the shelf light dimmers to run shaded pole motors on small fans and whatnot a number of times. The capacitor goes in parallel with the motor. What I have found is that many light dimmer circuits don't work smoothly on small induction motors so a little bit of capacitance to cancel out the motors inductance is necessary.

 

[MrDEB]

here is a speed control using a 555, an opti couplier a triac and a zero crossing detector
https://www.electro-tech-online.com/custompdfs/2012/04/AN-3006.pdf