# Thyristor phase control

Discussion in 'The Projects Forum' started by noh, Sep 26, 2008.

1. ### noh Thread Starter New Member

Sep 26, 2008
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I want to use 6 back to back thyristors to control the voltage to the motor. but I don't know how to calculate the firing angle and just even how to determine the rating of the thyristors for the application. i just need steps i can follow to achieve that. how can i fire them using the phase control method?

2. ### thingmaker3 Retired Moderator

May 16, 2005
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6

What kind of thyristors are you using? For the ratings of the thyristors, simply read the data sheet.

Is this an AC motor or a DC motor?

If you can provide a schematic, even a partial one, we can all begin talking about the same thing instead of making random assumptions.

3. ### mik3 Senior Member

Feb 4, 2008
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Why dont you use a triac and you want to use back to back thyristors.

4. ### DickCappels Moderator

Aug 21, 2008
2,746
665
The granddaddy of the thyristor is the thyratron, a gas filled tube. Triacs and SCRs are both thyristors.

At this point, its not clear what Mr. Noh intends to accomplish. It appears that he needs to use some sort of thyristor, and he does not know enough about the topic to describe his needs (he doesn't know what he doesn't know). This is not his field, and he wants help. Perhaps, Mr. Noh, if you explain what you are trying to do, we can give you some helpful advice.

In the mean time:

"...but I don't know how to calculate the firing angle."
The greater the conduction angle, the more power you will supply to the motor (and the higher the peak voltage, to some extent).

"...and just even how to determine the rating of the thyristors for the application."

Basically, you an go by the supply voltage and the maximum current draw of the motor.

"i just need steps i can follow to achieve that."
I doubt you are going to get anybody to write a short course on the use of thyristors for motor control on this forum, but there are many application notes on the web that address this topic, and more than enough circuits to give you some ideas. Do a search if you have not already, and I think you will be pleased at what you can find.

"...how can i fire them using the phase control method?"
The application notes address this. You can use a diac, a microcontroller, or a "firing circuit", which can be quite sophisticated.

So... if you still have a question, please post details of what you are trying to accomplish.

5. ### ali dabo New Member

Feb 9, 2009
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pleas i want a trigger circuit for the thyristor using 380 volt ac atthe input

6. ### ali dabo New Member

Feb 9, 2009
2
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DEAR MY FRIEND.
for DC motor control circuit using 380v ac , I NEED THE TRIGGRING CIRCUIT DIAGRAM FOR THE SCR.
BEST REGARDS

7. ### eblc1388 Senior Member

Nov 28, 2008
1,542
102
Although you have not mentioned it explicitly, the AC would be three phase with 380V line voltage.

Am I correct?

This makes generating the firing phases a little bit more difficult than the case of single phase.

8. ### mik3 Senior Member

Feb 4, 2008
4,846
63
A better solution is to use a 3-phase rectifier to convert the AC into DC. Then use a BJT, MOSFET, IGBT or GTO to PWM your motor and control it.

9. ### hoven New Member

Jan 30, 2009
2
1
i have an issue to control the six back to back thyristor to control the volatage to the motor,basically iam designing a three phase indcution motor soft starter bt ia wnt to do simulation using matlab please help