My circuit is unable to control dc motor

The purpose of my project is to design a speed control of separately-excited dc motor by using a SCR. The control is just half wave.The main block of my circuit is step-down transformer+voltage regulator+zero crossing detector+pulse transformer.

I have constructed the circuit. My step-down transformer+voltage regulator+zero crossing detector work perfectly. However, my pulse transformer circuit fails but my lecturer one works with my step-down transformer+voltage regulator+zero crossing detector.

I have connected it exactly the same way. The output waveform I get is exactly same the one works. It is around 10-12V at the primary of pulse transformer. The secondary side of pulse transformer is 0.3-0.4V. I just use Gate 1 and Cathode 1 for my connection to SCR. The pulse transformer model is Schaffner IT 133. The transistor model I use is cdil 2N2222A.

Please advise. Thanks

The gate drive of SCR will have a low impeadance. The work off of current to drive them not voltage like a FET. You should probably be hitting about 1-1.5 V peak on the gate drive of with the SCR connected. The attached drawing is one that I have done myself. I did neglect to put in all the gate pulses you would get from the zero cross ( just realized) However the basic principle is sound. If the motor is dc then full wave rectify the ac and delay your gate drive from zero crossing by the amount you desire. By changing the delay you can change where in the curve your SCR turns on and how much energy is delivered to the motor. It works like a light dimmer on a triac. If that helps at all. Good luck

I finally success to troubleshoot my circuit by changing the resistance value before Vin to a small value. Now, my speed control for the motor is not smooth. My lecturer told me that since our control is half wave so there is no ways to make the motor runs smoothly. Is he correct? If there is a way, please do share with me...thanks.

Glad you got it going. If the reason your motor is not smooth is due to the half wave rectification then you can do the same for a full wave and double the number of pulses being delivered to the motor. The other way to do the same thing is with an AC motor and a TRIAC or two SCR's in parallel facing in opposite directions. Good Luck...

dfullmer

Yes, I know that by doing full wave my motor will run more smoothly. The problem I face here is the cost. Only RM200 for my project. I have spent around RM80 for other part of project. Now left RM120 only. I hardly to do so since I need an extra SCR. Please advise on a better way which is cost saving. Thanks