Using RSlogix PLC to Gate Trigger to a TRIAC or Thyristor

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sahilgupta1527

sir

i m using rockwell RSlogix 1400 with analog module. i already get analog output from analog input in this plc. how do i use this analog output and trigger a TRIAC or thyristor by this analog output?

one of my sir reply me that.

You will need to use a discrete (on/off) output to trigger the SCR. In your PLC program, set a timer that resets itself when its preset is reached. Calculate the analog output's percentage of its maximum value. Also calculate the timer accumulator's percentage of its maximum value. While the timer's percentage of its maximum value is less than the analog output's percentage of its maximum value, turn on the discrete output that triggers the SCR. When the timer's percentage of its maximum value is greater than the analog output's percentage of its maximum value, turn off the discrete output that triggers the SCR.

so please tell me that how can i make this ladder diagram of this?
 
Hi Sahilgupta,

Did you mean AB Micrologix 1400 PLC with RSLogix 500 programming software?

If so, to control power with SCR, you will need to implement pulse width modulation (PWM) strategy. with this strategy you will control firing rate of SCR using pulse train that the pulse width vary with power need to be delivered.

Simplest and proven method is to use PTOX/PWMX instruction.

Ikhtiander
 
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sahilgupta1527

>Simplest and proven method is to use PTOX/PWMX instruction.

can you send me example of this?
 
The PTO instruction is used to generate input to a servo with a pulse train input. The profile works sending this pulse train to implement a simple servo motion profile. I don't think this is suitable for your purpose. There's a PWM instruction, I don't know how you'd synchronize it with the power waveform. Keep in mind these instructions are scan independent, so there's not much you can do to this purpose. You'd be better off using power modulator SCR packs, I have used one made by crydom (PCV series), very simple to use, because it only needs an analog output to obtain power modulation from 0 to 100% easily. They handle up to 40A loads single phase at up to 240 VAC. Very reliable too. I hope this helps.

Best of luck
 
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sahilgupta1527

i want to make a project which is like that. actually i want to make a speed control of induction motor using Allen bradley micro logix PLC. for this i place a sensor (temp sensor , air flow sensor, pressure sensor, humidity sensor etc.) which give as analog output; now i gives this analog output at the analog input terminal of AB PLC..

my question is this how can i now this PLC will give some (at analog output terminal) gate Signal to the TRIAC or THYRISTOR for change the load voltage so that we can change the speed of motor?
 
Changing the load voltage will only get you a burned motor. Speed is not voltage dependent in any induction motor; it is frequency dependent. You should use an AC Drive and a 3 phase motor; you can easily get them off the shelf. My previous suggestion would only apply for resistive or transformer loads that require power modulation.
 
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