thyristor as ac voltage controller

[bagusdj]

hey guys, i done my small project. it is controller of ac fan using triac. i referred to this **broken link removed**

however, there are things that confuse me. so i done it by delaying tyristor firing at 3ms ( i got 188V), 4ms ( i got 154V), and 5ms (108.6V). i look for phase firing angle calculation in internet, and at 90 degree of delay angle, it is 155V. main is 220V and 50Hz. 5ms=90 degree.

someone said that the result that i got (188,154,108) is in average voltage. not rms. i use dvm to measures it. not sure it is specified for average or not.

then in pdf it says:

i dont get it, how load can give average value when being measured? knowing that source is rms.

by my data collection, mine is more to average value. is appears in this graph:

average graph tells that at 90 degree, load receives half max rms voltage (in this graph, main is 230V)
while rms graph is not showing half max voltage at 90 degree.

i dont get it, why my data collection is more to average value? i sourced it by 220Vrms main.

and also anybody know the equation of the graph?

please help me

 

[godfreyl]

i dont get it, how load can give average value when being measured? knowing that source is rms.

It is not correct to say "source is rms". Any AC voltage waveform has a peak value and an average value and an RMS value.

That's why the graph shows peak and RMS and average vs conduction angle.

e.g. The graph shows that for mains voltage = 230V RMS and conduction angle = 60 degrees:

• Peak voltage at the output = 280V.
• RMS voltage at the output = 105V.
• Average voltage at the output = 60V.

- - - Updated - - -

i use dvm to measures it. not sure it is specified for average or not.

Most multimeters are calibrated to show the RMS value for sine waves. For other waveforms probably not, unless it is a very expensive meter.

More likely the DVM will show either about 1.1 times the average value, or about 0.7 rimes the peak value. (For a sine wave, both of these equations give the RMS value. For other waveforms they don't.

 

[bagusdj]

so the voltage that goes out from outlet terminal in our house is not in term of rms?

my data collection as stated above, when i compared to graph it close to average value. why not in rms?

is there another term or name for controlling ac voltage using thyristor? as i know there are phase firing angle/phase angle control, and integral cycle control.

i dont know which one i used. i fired my triac by delays triac firing right after zero cross give (logic 1), then not giving logic 0. just let zero cross turn the triac off.

 

[godfreyl]

so the voltage that goes out from outlet terminal in our house is not in term of rms?

That is like asking if your computer screen is "in terms of height". The screen has a height and it has a width.

Similarly, the voltage that goes out from outlet terminal in our house has an RMS value and it has a peak value and it has an average value.

Actually the true average should be zero. What we may be interested in is the average rectified voltage. That is what is shown in the graph for "AVG".

my data collection as stated above, when i compared to graph it close to average value. why not in rms?

The output is not a sine wave. Therefore your DVM will not give an accurate RMS value, as explained in post 2.

is there another term or name for controlling ac voltage using thyristor?

Thyristors and triacs are not the same. I expect you are using a triac, not a thyristor.

as i know there are phase firing angle/phase angle control, and integral cycle control.

i dont know which one i used. i fired my triac by delays triac firing right after zero cross give (logic 1), then not giving logic 0. just let zero cross turn the triac off.

That is phase firing angle/phase angle control. The triac conducts for part of each cycle.

Integral cycle control conducts for one or more complete cycles, then switches off for one or more complete cycles. Thus it always switches at zero.

 

[bagusdj]

so if the output wave is not pure sine, when i measured using common dvm which calibrated to read rms, is the displayed number is in term of rms?

 

[godfreyl]

No. If the output wave is not pure sine, then the displayed number on a common dvm is not the rms voltage.

For pure sin wave it displays rms value. For other waveforms, it displays a wrong value.

 

[bagusdj]

so there is no chance dvm displaying average rectified value? so lets say that what i had done is wrong?

and whether it is display in term of rms or not, it is depend on the load i suppose?

 

[godfreyl]

so there is no chance dvm displaying average rectified value?

Quite likely it is displaying about 1.1 * average rectified value.

and whether it is display in term of rms or not, it is depend on the load i suppose?

No. It displays the voltage. That should not depend on the load current.

 

[The Electrician]

bagusdj, if you will go read this:

https://www.allaboutcircuits.com/vol_2/chpt_12/1.html

it will help you understand why meters may not read RMS. If your meter doesn't say "True RMS", then is is probably what is known as an "average responding" meter. It will read the RMS value of an AC voltage ONLY if the waveshape is a sine wave. The waveshape out of your triac controller is most definitely NOT a sine wave (except at 180 degree conduction angle).

If your meter is an "average responding" meter, then if you take the readings you get and divide by 1.1107 you will get the "average" value shown on the graph you posted.