Looking ideas about voltage chopper

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dsanthosh67

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Hi all,

I am looking to control a heating element at various voltages from 0 to 230VAC(50Hz) to maintain a specified heat.I don't have any idea about this,so kindly me suggestions and schematic for controlling at higher frequency(more than 50KHz).
 

Hi,

mains frequency = 50Hz,
Why do you ask about switchng with 50000 Hz?

Klaus
 

Hi Klaus,

Main frequency is 50Hz only, i need to chop this frequency with 50KHz signal due to fine variation of the input supply voltage. Here i attached image like this.
 

How big is the heating element? I suspect you do not need to chop using 50KHz PWM, the far simpler method is phase control as already mentioned.

The chop rate is only relevant if the thermal mass of the heater is small enough that it responds within 1/50KHz seconds = 0.00002 seconds, if the mass is bigger the chop rate can be reduced. The thermal mass 'averages' the power applied to it so if the mass is big enough you can use phase control instead, effectively chopping at 0.02 seconds rate. Phase control is simple and cheap, chopping mains voltage at 50KHz is complicated and expensive!

Crude check - if switching the heater on from cold to it reaching full temperature takes more than about half a second, use phase control.

Brian.
 

Also the EMC ramifications from this approach are concerning me too. If your element time constant is not quick then even using burst firing would be preferable as this does not present much of a harmonic footprint. Chopping at 50kHz certainly would!

Rob.
 

hi,
What about the efficiency when using phase control method and also give me any schematic for both chopping and phase control method,here i attached the heating element datasheet.we are using 650W FTE, give me any suggestions to control this device.View attachment heating_element.pdf
 

The efficiency of using a Thyristor based phase angle controller will probably be better than a 50kHz chopper because of switching losses. The thyristor only suffers switching losses when it turns on, plus the conduction losses. Like I said the Crydom phase angle controllers will probably give you what you need.

**broken link removed**

You can then directly connect these to a temperature controller with either 0-10V or 4-20mA output. Very easy solution.

Regards,

Rob.
 

Thanks for the reply,

Ya absolutely crydom is the right solution for my application,but anyway kindly give me schematic i need to understand the operation of phase angle controlling and voltage chopping.
 

Recent power quality regulations don't allow phase control for heating applications, it's only permitted for lighting and electrical tools.

The time constant of the shown infrared heater is so large that you can achieve perfect control with full wave switching.
 

Now we know the type of heater the optimal solution is easier to see. As FvM explained, you don't even need phase control, the thermal mass (time constant) of that heater is so large that you can control it by simply switching the power on and off in bursts. For example, for 100% power, turn it on all the time, for 50% power, turn it on for one mains cycle then off for the next, for 33% power turn it on for one cycle and off for two. Phase control may be legal in India but in some countries it is frowned upon because it distorts the mains waveform and can upset the electricity company. Full cycle control lets you turn it on and off in multiples of complete cycles so it is 'clean' and if switched at the AC zero crossing point, causes no inteference.

Brian.
 

As far as i know heaters are pwm around 3k to 5khz as far back as the sixtys.
 

Thanks for your valuable comments,Is the full wave switching has same as the voltage chopping?. Could you share me any circuit for full wave switching.
 

No, phase control is when you fire a thyristor towards the back of the cycle and then advance it to get more power. Burst firing is when you turn on the thyristor for a few cycles at a time, in order to preserve mains harmonics, but achieve control, based on the long thermal mass time constant of the heater.

**broken link removed**
 

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