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Should PWM signal be syncronized to the AC mains?

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y0hou002

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Hi, I really need some help here. I am using PIC16F628a's PWM signal to change speed of AC (120V, 60Hz) blower. At low speed, the blower tends to "hesitate". On the scope, I found that the chopped part of the AC wave is always changing, which makes the current fluctuate. I guess it is because my PWM is not synchronized to the AC frequency so that the duration from zero cross to PWM activation pulse for the Triac is changing. Can anyone tell me if it is required to synchronize them? Many thanks!
 
could you try increasing the frequency of the pwm? so that it is many times faster than 60hz meaning that the wave will be chopped at mltiple points?
 
Assuming you're trying to do phase shift power control?, and triggering a triac, then you obviously need to syncronise to the zero crossing point of the mains. You're confusing the issue by calling it PWM, which is really a technique for DC and not AC.
 
What sort of motor does your blower have?

If it's universal (with carbon brushes) then it should be no problem but if it's an induction motor (squirrel cage) then no wonder you're having problems.

You can't control the speed of induction motors using PWM or phas control, if you try to they will, loose torque at low speeds, stall and overheat, your triac will also be destroyed.
 
you need zero cross detection

This is what im working on in my project. Some simple circuitry is all thats required to plug into RB0 and configure it to be an interrupt.

Once zero cross occurs, the interrupt will be activated and then you can run you phase control routine.

Im using it on mine as i said and it all works perfect. Theres a few ZC detect circuits available... if you find 1 let me know as im looking for some more as mine isnt accurante on the way down...(during the cycle)
 
Thank you all for your reply. Yes I am using an induction motor (squirrel cage) . I did not know that PWM is a technique for DC and not AC. Increasing PWM frequency did not help because the triac operates only once in each half cycle of the AC mains.

So can anyone let me know what kind of technique people are using to control speed of induction motor? I did saw products that uses MCU to control speed of induction motor and the chopped part of the AC wave is consistent.

Thanks again.
 
That's good, suby786. I saw application notes AN521 ("Interfacing to AC power lines") which connects AC mains to the MCU pin through a resistor. It is a little scary and I am not sure if I should use it.
 
y0hou002 said:
Thank you all for your reply. Yes I am using an induction motor (squirrel cage).
Then don't expect it to work very well, it at all.

I did not know that PWM is a technique for DC and not AC. Increasing PWM frequency did not help because the triac operates only once in each half cycle of the AC mains.
PWM for AC is called burst fire control, it's always done below the mains frequency, (<0.5Hz is typical) and isn't suitable for controlling motors or lighting, it's only used for heating appliances.
 
Read pages 38.1 to 38.4 of this:
https://www.electro-tech-online.com/custompdfs/2006/12/hackar2.pdf

An excerpt:
Why should it be cheap, easy, and
trivial to regulate the speed of your
electric drill, yet super expensive and
next to impossible to control the speed
of a compressor or blower motor on an
air conditioner? After all, a motor is a
motor, isn’t it?
Sadly, the answer is no. Those AC
induction motors simply were never
designed to have their speed changed,
and they throw all sorts of really ugly
hassles at you when you try to do so.
Let’s see why this is so.
 
Maybe Tesla was wrong when he said AC was superior to DC =)
 
Maybe Tesla was wrong when he said AC was superior to DC =)
Yes, but it is less likely to kill a curious cat. :D
 
what if instead of phase delay w/c may have an effect the line frequency and do some damage on other frequency sensitive appliances, why not totally cut-off the entire wave or turn-on and off the motor during the entire 360 degrees of the wave. this way maybe torque will not be affected.
in short, to achieve 50% speed, for example, detect the zero crossing, turn on the triac, detect the 2nd zc but dont trigger the triac this time, so on and so forth. and perhaps to achieve a greater resolution grouping a number of zc in sequence for ex. to come up with 100 steps of speed control u nid 100 zc in group... or something like that.
 
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