SCR Bridge Control - Is my Circuit Correct ?

[Okada]

I also tried with one voltage source in Proteus but not getting correct signals. In real hardware I will use one source only. I have not yet built the hardware. I am designing the PCB layout. I will post the circuit here in 1 hour. Please then check and tell me if circuit is ok.

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If I have to place RC snubber then how should I connect it. Should I connect one snubber across each SCR ? My load is inductive. How to calculate RC snubber component values ? My load is 1/12 HP motor.


I finally got output for non MOC3021 based SCR bridge rectifier in Proteus. See the attached simulation result. I don't know why signal is distorted. Firing is correct.

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Here is the simulation video.

 

[FvM]

I don't know why signal is distorted.

You are adding the trigger voltage to the output voltage, by using an unsuitable grounded trigger generator. Instead you should apply a trigger current between gate and cathode of individual SCRs. A trigger generator without current limiting would quickly destroy a real SCR, by the way.

You can use a floating trigger source in simulation, in real hardware you'll need a trigger transformer or similar isolation means.

There's however no advantage of using 4 SCRs in your circuit, 2 controlled rectifiers are sufficient for a one quadrant controller. Both SCRs can be connected with common cathode and a controlled by a single trigger source.

 

[CataM]

If I have to place RC snubber then how should I connect it. Should I connect one snubber across each SCR ? My load is inductive. How to calculate RC snubber component values ? My load is 1/12 HP motor.

With a freewheeling diode across de motor, you will be fine, no need for RC snubber.

I finally got output for non MOC3021 based SCR bridge rectifier in Proteus. See the attached simulation result. I don't know why signal is distorted. Firing is correct.

You are going to design the PCB. What part number are you going to use for the SCRs ? I see lots of simulations but no part number for the SCR and fired with ~5 V. Why do you think that will work in real life?

 

[Okada]

2FvM

Why Isolation is needed ?

Here

https://www.youtube.com/watch?v=odx9V9DIjgg

they have not used any isolation.

@CataM

Ok. I will use freewheeling diode across the motor. I have not been provided the specification of the motor.

I think it works becasuse it works in the above video.

 

[CataM]

With a freewheeling diode across de motor, you will be fine, no need for RC snubber.

Correction.

If you want it to work as in fig 10 to 12 in the schematic shown in post #1 called "ifraeuhfn-zewr" => no need of snubber, no need of diode.
If you do not want negative voltage across the inductive load => then freewheeling diode across the load.

 

[Okada]

Ok. CataM.

I posted the video in which the SCR firing was working but if I use the same circuit in another simulation where firing is done from SCR then the LOAD signal changes to pulse. It is not working . Why ?


See attached video. It is not working if I use PIC to fire the SCRs. Why. Why Load signal becomes pulse if I connect ground to bridge ?

 

[FvM]

Why Isolation is needed ?

Here

https://www.youtube.com/watch?v=odx9V9DIjgg

they have not used any isolation.

As you have stated yourself, they use MOC3021 for isolation...

 

[Okada]

Ok. FvM.

Sorry, I thought isolation is needed between anode of SCR and gate.

What will be the signal shape of gate signal in MOC3021 circuit ?

My motor is a 1/12 HP DC motor. Power source is 230V for SCRs. Now my client is saying they need direction control. What should I do to have direction control of motor. What changes I have to make ? Should I use two 4x SCR bridges ? How the bridges outputs are connected to motor ?

How to get negative cycle conduction in SCR ?

Should I use back to back scrs instead ?

 

[CataM]

My motor is a 1/12 HP DC motor. Power source is 230V for SCRs. Now my client is saying they need direction control. What should I do to have direction control of motor. What changes I have to make ? Should I use two 4x SCR bridges ? How the bridges outputs are connected to motor ?

First thing comes to mind is: Make a controlled AC/DC rectifier which would give the variable DC voltage at the bus capacitors for speed control, and then, add an H bridge for the motion control.

 

[Okada]

Ok. CataM.

In the video I mentioned, in that kit they have used 1 pair of 4x SCRs Bridge rectifier and they provide dc motor direction control. How they are doing it ? They are not using any H bridge.

 

[CataM]

How they are doing it ?

The dual converter is another possibility.

 

[Okada]

Thank you everyone. I completed the project. Here is the Simulation Video.

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When the load is inductive a part of the load voltage becomes negative. Is it due to back-emf ? Is it dangerous ? Will it destroy the SCRs ? My clients requirement is to show the working of SCR Bridge for R and RL loads. He has to show the signals. He cannot use snubbers are freewheeling diodes because they will remove the negative voltage. My source voltage is 230V AC 50Hz.

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In the video it shows the signals but I see for RL load the negative part of the signal is distorted when I simulate it on my core2 quad PC, but if I simulate it on core i7 PC then there is no distortion in the negative part of the signal. Why ? Is it PC problem ?

 

[CataM]

When the load is inductive a part of the load voltage becomes negative. Is it due to back-emf ? Is it dangerous ? Will it destroy the SCRs ?

Yes, back emf. It will make to still have 2 SCRs conducting at a time and hence more power dissipation.

In the video it shows the signals but I see for RL load the negative part of the signal is distorted when I simulate it on my core2 quad PC, but if I simulate it on core i7 PC then there is no distortion in the negative part of the signal. Why ? Is it PC problem ?

I was expecting to have some overshoot but now I do not know what to say to the difference between the results of the 2 PCs.

 

[Okada]

Hi FvM and other experienced people.

I have to demonstrate backemf effect when RL load is connected to Full bridge SCR circuit. Also motor will be connected but for demonstration RL load will be connected. AC source will be 230V 50Hz. Assuming max conduction angle what should be the value of R and L (series) I have to use for the load ? I think it is good to keep the current low. 230V AC will be converted to 325V DC (for full 180 degree conduction). Please tell me how to calculate R and L values for load. Also what should be the voltage rating of resistor and also wattage. I think if current is kept low then 1W or 2W resistor can be used. Am I right ?

With RL load and max conduction angle backemf will be more which appears at the output and which is negative.

Please provide formulas to calculate the values of R, L. The load voltage will not exceed 325V DC and so can I use 400V rated resistor ?

See the attached circuit. It is the final circuit. Only RL value has to be calculated.

For R load I have used 100k 2W

325V/100k = 3.25mA

3.25mA * 3.25mA * 100k = 1.05625W

So, 2W is enough.

325V DC appear across resistor for R load.

so should I use 400V rated resistor ? or can I use two 50k resistors in series so that 325V DC divides across each resistor ? 325 / 2 = 162.5V appear across each resistor (50k * 2) so can I then use 200V resistors ?

Are my calculations correct ?

 

[KlausST]

Hi,

R:
You said: 230V AC, Pmax = 2W.
Then use: P = U * U / R

This is true for both AC and rectified_AC, but only if R is the only load ... without any series L or C)
But it limits the max power dissipation in any case.

**

Yes, back emf.

One has to differ.

Back EMF voltage of an inductance may become negative.

But you talk about a motor.
Back EMF of a motor is the voltage that a motor generates (like a generator) when it is spinning.
The voltage has the same polarity as the driving voltage.

***
Driving an inductance with an SCR:
Since the SCR stays conductive until the current drops below a certain hold_current... there is only a small amount of energy remaining in the inductance.
Therefore the protection against negative voltage spikes is relatively easy.

Klaus

 

[CataM]

Assuming max conduction angle what should be the value of R and L (series) I have to use for the load ?

What kind of question is that ? It depends on what you want and I think the one that best models your DC motor.

The load voltage will not exceed 325V DC and so can I use 400V rated resistor ?

If you would have had a DC source yes. But you are using a simple controled full bridge. In so far, the sinusoidal steady state analysis would be more helpful.

 

[Okada]

Sorry, I forgot to attach the final circuit. Please answer after checking the circuit.

As AC is 230V and if full ac is converted to dc then v dc max across the R load will be 325V DC. So, I have to choose resistor value, wattage and current rating for this load voltage.

There are three separate loads.

R (a resistor)

RL (aresistor and inductor in series)

a DC motor

So, output signals have to be shown for these 3 loads.

For R and RL loads should I choose R vale high to keep current less so that lower wattage resistor can be used ?

200V rated resistors are available easily ? So, shall I divide the final resistance value / 2 and use 200V resistors ?

How to calculate L value ?

 

[CataM]

The attached circuit will work for the RL load without changing any value if alpha>45º.

 

[KlausST]

Hi,

How to calculate L value ?

What are the specifications for it?

The L will cause some current shift, but it won´t reduce the average current significantely, because your output voltage is not AC anymore.

Klaus

 

[Okada]

What specification you want ? I didn't get it.

Only one type of load will be connected at a time.

The project has to be completed in 2 weeks. This is not my project. I am doing it for someone else.

Actually they are referring the attached document and the same project in that document is in this book

https://www.lap-publishing.com/cata...ontrolled-rectifier-using-pic-microcontroller

the attached document is a part of the above book.

I have ordered that book but it will reach me after 4 weeks.

You can see in the attached document that they have used R and RL load for testing and also they have used 230V AC source.

Only in our circuit PIC is changed because lcd is added.

In the document it shows simple circuit (pcb). it contains a 4N37 for zcd, PIC12F675, scr bridge, moc3021 circuit.

I have drawn the circuit as they have also used the same circuit for R and RL loads but they have not used dc motor.

In our requirement 1/12 HP dc motor also have to be used as load.


I am not a power electronics expert. Also I don't have much experience with high voltage circuits and that is why I am asking.


I know that the circuit is just a controlled bridge rectifier but help me choose the right component values for the load.

First actually I will use 24V AC as source for scr circuit and do the testing. If it works then I will try with 230V AC.