Speed Control of Motor using SCR

[leena2k6]

Hi,

Does anyone know a good link explaining how the circuit shown in the attached document works?
It is used to control the speed of motors using Diac and Triac.

Many Thanks
Leena

 

[uncle betty]

OK mate, I'll give you this one, but if you're preparing for an exam you need to start knuckling down to be honest.

In a nutshell - The diac acts like a switch, and isolates the Triac from the Capacitor C1.

At this point the motor will not be running.

As the C1 charges, at some point it will have enough charge to break through the Diac. This is called the ''breakdown voltage'' of the Diac.

So the Diac is now like a closed switch, and allows voltage to go onto the Gate of the Triac.

This switches the Triac on, and allows voltage to the motor.

Adjusting the Variable Resistor changes the length of time it takes to charge up the Capacitor C1.

High resistance set at VR1 means a longer portion of the ac cycle to charge C1. Therefore a shorter period of time for the Diac to conduct; therefore a shorter period of cycle-time that the Triac is getting voltage; therefore shorter period of voltage to motor. So the motor runs slowly.

Reduce VR1 to speed up charge of C1, the Diac conducts for more of the cycle, the Triac is on for longer, the motor gets more voltage and goes faster.

R2 and C2 are called a ''snubber'' circuit. They take away supply voltage spikes, which could trigger the Triac. (They can come from the supply itself, or from the motor as it is switched off).



By the way - your pdf link is calling it an SCR (silicon-controlled rectifier). But there's no SCR in that circuit and you should have spotted that.

An SCR is also called a Thyristor. It can only conduct in one direction. The Triac will conduct in both directions.

So you need to start getting to grips with your component symbols as well ;-)

 

[BlueToBits]

Excellent explanation uncle betty. I've often wondered myself

 

[leena2k6]

Thank you so much uncle Betty.

 

[uncle betty]

Thank you so much uncle Betty.

You're welcome.

 

[leena2k6]

One more thing - What happens if C1 is shorted?
Thanks a lot!

 

[leena2k6]

I think the motor will never turn ON if the capacitor C1 is shorted as the diac will remain off. Could you please confirm this uncle betty? Thanks!

 

[uncle betty]

I think the motor will never turn ON if the capacitor C1 is shorted as the diac will remain off. Could you please confirm this uncle betty? Thanks!

Deleted reply, as you have me confused now...

 

[leena2k6]

Or the motor will always be running as the voltage across the resistor keeps the Diac ON. I am confused

 

[uncle betty]

Or the motor will always be running as the voltage across the resistor keeps the Diac ON. I am confused

If the cap shorts out, does that not mean that the current has a much easier path rather than trying to breakover the Diac voltage ? So the Diac would just stay out of the circuit altogether.

The motor runs, but you have no speed control.

Anyone else any thoughts ? This is all just theoretical stuff for me, have no practical experience unfortunately.

 

[leena2k6]

Hi, any inputs on how does this circuit(this actually uses SCR) is used for speed control of motor?

Thanks much.

 

[Inteificio]

What exam is this for?

 

[uncle betty]

Hi, any inputs on how does this circuit(this actually uses SCR) is used for speed control of motor?

Thanks much.

The supply is 3-phase AC, and it's a DC motor, and you should know that SCR stands for Silicon Controlled Rectifier.

The Speed of a DC motor is proportional to the Voltage applied.

The Voltage across the Field is constant. Therefore so is the current, and so you get constant torque.

But the SCR varies the voltage to the Armature and therefore controls the rpm.

The SCR/Thyristor can only conduct in one direction, ie half the sine wave. If you want to change the speed of the motor, you change the point in time along the half-sinewave at which the SCR's are triggered. The earlier in the sine wave you trigger, the more voltage is passed to the Armature, and the faster the motor goes. And vice versa.

Obviously the SCR is off at the zero crossings.