Thyristor based dc motor controller

So, i made a triggering of thyristor in full bridge. THe problem is i get a very small amperes at the output about 4mA... can anyone explain what probably is the reason?

Here is the simple schematic:

I get very small amperes at the load part..... anyone can explain?

What is the input voltage and the load resistance?

But I'm pretty sure that the problem is most likely that the triggering between complementary SCRs in the bridge is not synchronized, or is not long enough to allow everything to latch?

Would you kindly show your WHOLE trigger circuit, including the means for line synchronization?

Speaking of which, in a SCR bridge you only require 2 devices, not 4. the other two SCRs are replaced with plain diodes.

okay.. Here it is...

Load part. I basically short the anode of the optocouplers and wire it from the output of the 3rd LM324.
https://obrazki.elektroda.pl/9246884200_1454293033.jpg

This is the Trigger circuit. Basically, the first op amp is the zero cross, 2nd op amp produces the ramp and the 3rd produces the triggering pulse and the BJT(not used in the wiring yet but i know somewhere this can be used) provides the inverted triggering pulse
https://obrazki.elektroda.pl/8724662100_1454293034.jpg

These are the waveforms produced at the trigerring part:
https://obrazki.elektroda.pl/9800951100_1454293035.jpg
This is the output waveform at the load:
https://obrazki.elektroda.pl/2302147900_1454293036.jpg

Basically, i used 0.96 ohms resistive load of the motor(assumed) since i want 25A and 24V at the output

uhmmm i used 220vrms, basically the outlet sorry for this late reply

Even if the transistor optocoupler were capable of withstand the peak voltage of a 220 volt sinewave (which it won't by a factor of at least 10), it does NOT provide a HARD TRIGGER, which is absolutely necessary for the SCR to latch.
You doubly compound the problem by using a four SCRs, where for the application you only require two plus two diodes.
This is best accomplished via pulse transformers, like so (the schematic is only partial):



SCR motor controls have been used for at lest 40 years, without requiring a single IC, only a unijunction transistor, like these one which also does not require a pulse transformer, because it only uses two common-cathode SCRs:

schmitt trigger said:

Even if the transistor optocoupler were capable of withstand the peak voltage of a 220 volt sinewave (which it won't by a factor of at least 10), it does NOT provide a HARD TRIGGER, which is absolutely necessary for the SCR to latch.
You doubly compound the problem by using a four SCRs, where for the application you only require two plus two diodes.
This is best accomplished via pulse transformers, like so (the schematic is only partial):

View attachment 125724

SCR motor controls have been used for at lest 40 years, without requiring a single IC, only a unijunction transistor, like these one which also does not require a pulse transformer, because it only uses two common-cathode SCRs:
View attachment 125725

Click to expand...

Hello everyone.

I'm trying to do something similar and learned a lot from your comments.

What I need to do is to control the speed of a 200V 15A DC Motor, using 220V AC and a PWM from a PCL at 20MHz (I can change the frequency up to a 100MHz).

Before reading this post I was thinking on using an H-Bridge rectifier and a Mosfet transistor, with the PLC pulse at the gate. The PLC pulse would switch the Mosfet on, allowing the rectified current to reach the motor. Then, changing the PWM duty cycle would allow me to control the motor's speed.

In this scenario what would be the best approach, using the Thyristors or the rectifier/Mosfet? What are the factors to consider for choosing either?

Thanks a lot!

One does not do PWM with a Triac or SCR. One does phase control.

This means a couple of things:
-You have to detect the AC main's voltage zero crossing, to generate a sync pulse.
-You delay the Triac trigger, based on that sync, a certain amount of milliseconds. Where that delay varies between 0 and 10 ms (at 50 Hz) or 0 to 8.333ms (at 60 Hz)

In this scenario what would be the best approach, using the Thyristors or the rectifier/Mosfet? What are the factors to consider for choosing either?

Click to expand...

Since your motor is DC, DC voltage should be applied so I would say 220V AC rectified and filtered to DC and applying PWM to gate of Mosfet.

As schmitt trigger shows in post #5, the motor is not a resistive load, but a reactive load, meaning for calculation purposes you should model it like so and place a Snubber circuit or Freewheel circuit as shown in post #5.

- - - Updated - - -

on using an H-Bridge rectifier and a Mosfet transistor, with the PLC pulse at the gate. The PLC pulse would switch the Mosfet on, allowing the rectified current to reach the motor

Click to expand...

Sorry but I do not see how a H-Bridge would rectify the AC voltage..