Thyristor Control Motor

[kirlcheah]

Currently I am a college student with a project. I need to build a thyristor control voltage feed to power up a dc motor of 24V. i have basically build the power supply, which have 24V with 7824 and without overcurrent protection and have a 2N2955 bipolar transistor to provide more current to the circuit. Then i have another 7809 with plus minus voltage. This plus minus voltage will be used for opamp use for the pulse width modulation. generating a gate signal for the thyristor.. the problem is that i need the value for the commutation circuit and also for the generating pulse for the gate. how much value should i use for the capacitor, resistor and also inductor for me to switch of the thyristor. I need help fast. thanks for replying... i will post the layout or schematic fast. i need some protection for the thyristor such as dv/dt and di/dt protection also. (snubber circuit) what value to use? my thyristor is o.8A. 30V. gate current is 350uA and 1.2 Volt gate.

 

[FRED6298]

I do not understand why you are using a thyrister or SCR. Once you fire an SCR, the SCR remains 'ON' till you remove power.
If you are doing PWM, then use a FET or BGT to control the power to the
motor.
Does the motor run both forward and backward ?

 

[Russlk]

Use the FET to drive the motor and put a reverse diode across the motor to catch the inductive kick back.

 

[Nigel Goodwin]

I do not understand why you are using a thyrister or SCR. Once you fire an SCR, the SCR remains 'ON' till you remove power.

Presumably it's required by his teacher?, it's commonly used for high power DC motor control, fork lift trucks, dodgem cars, things like that. You use a second thyristor and a capacitor to turn the first one off.

 

[Styx]

I do not understand why you are using a thyrister or SCR. Once you fire an SCR, the SCR remains 'ON' till you remove power.

Presumably it's required by his teacher?, it's commonly used for high power DC motor control, fork lift trucks, dodgem cars, things like that. You use a second thyristor and a capacitor to turn the first one off.

Yep Thyristors are not old tech. Check out the power handling of controlle dswitches - thyristors are at the top.

They are used in the high voltage DC-link that joins Europe to Britain - the 3rd 1/4 of 2003 saw the first time that is accully exported power to europe, it has always drawn power from Europe.

A better way to fire a thyristor (and standard) is via a pulse transformer. You get the isolation that is required from power side to controller side, and since it is pulsing the power the XFMR is small.

Thyristors are used intensively in current source drives where natural commutation turns the thyristors off.

 

[kirlcheah]

it's actually a requirement from my lecturer. i need to use commutation circuit, which is forced commutation to shut the thyristor off. i am planning to do class A and class B DC chopper. Class A is quite normal to drive a dc motor and class B is used to return energy back to the system where currents are redirect back to system. I need to exactly calculate the resistor,inductor and capacitor values so it can turn off the thyristor so that i can use the pulse on the gate so that i can control the speed of the motor. I am using M.H Rashid book, Power Electronics and it seems that i can't get the right value. I don't know what happened? I have designed the power supply to power up the pulse generator (something like PWM) and the circuit for the commutation circuit with thyristor circuit. I tested it on breadboard and the circuit seems to be switching the power supply on and off. i changed the frequency on the frequency generator and it seems that there is not much increased in the power. please help on the calculation. thanks.

 

[Nigel Goodwin]

Try posting the circuit you are using, changing the frequency won't affect the power much - you need to change the mark/space ratio. Depending on the design, altering the frequency 'could' do this as well - which is why you need to post your circuit.

 

[Styx]

Forced commutation. Yep tricky with Thryistors (soo much easier using tyristor drives with Induction machines/synconous machines)

All I remember is there is loads of caps line-line and line-star on the output side.

Increase in switching frequency will not increase power output (duty will though). All increase in switching frequency will do is cause less current ripple but at teh expense of more switching losses in the Thyristors.

 

[kirlcheah]

yeah will load the files as soon as possible. thanks to all for the help. I appreciate it guys. need it in what form ? circuit maker,proteus or orcad? i find orcad kinda hard to use. circuit maker is easy to draw diagram and proteus is one of the best i have tried but i can't use it cause demo version. downloaded lite version also but kinda have to wait for that. i am a student only and kinda no money to buy the software. it's good and able to simulate pics in progress. good buy. but the software is kinda steep for me. 30 pound is RM240. so if buy whole version, cost thousands here in malaysia.

 

[kirlcheah]

here is the power supply and also type a dc chopper.the thyristor is 350mA current or 1.2 V to switch on and can handle up to 0.8A. can somebody help me to calculate the snubber circuit and also the ringging circuit? commutation. switch off the thyristor so that i can control the pulse on the gate. thanks for the help