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#1 Fiso

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Posted 07 February 2007 - 06:04 PM

sad.gif Hie guys,

l am new to this forum and to electronics. l am trying to build a three phase ac soft start for an induction motor using components which are readilly available on the market and would greatly appreciate your help.

Preferably l would like to build this circuit using a phase control ic, and would like to find out which one is the easiest one to use.
Or to use a circuit which will make it possible for zero volatage detection
l would also like to use an opto-triac for isolation and would like to know which one l can use for this particular circuit.

Best regards

Fiso

#2 Tua

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Posted 08 February 2007 - 02:02 AM

Hi Fiso!
Excuse me, but I don't see any questions...
Of couse you have to specify exactly the begining of each phase, I mean when voltage is zero and direction of growth for correct open your thyristor.

#3 marke

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Posted 08 February 2007 - 07:56 AM

Hello Fiso

Firstly, if you are not experienced in high voltage power electronics, then you not find the soft starter easy to build reliably.
The basic concept appears simple enough, but there are a number of issues that you would need to address in order to have stable operation, plus the interfacing of high voltage to low voltage controls can cause a number of issues as well.

There are a number of ICs etc that are designed for phase controlling a triac into a resistive load on a single phase supply. Unfortunately, these components are not suitable for use on three phase control. - The voltage is too high, and they are not designed to control three phases symmetrically.

If you are doing this project as a means to learn about power electronics etc, then you will have quite a lot of work to do. If you are doing this project as a means of getting a lower cost soft starter, forget it as it will cost you more to buy the SCRs and heatsink on a one off basis than to buy a complete unit!!

The modern soft starter is quite a sophisticated system with a lot of research going into achieving good control and stability.

I do not think that you will find a "free" design easily, or any one who will design the unit for you by remote control.
If you have particular questions that you want some advise on, feel free to ask them and you may get the answers that you are looking for, but you will have to do the work.

Best regards,

#4 Fiso

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Posted 08 February 2007 - 03:10 PM

Thank you guys for your responses, what l need at this moment is information on ic's. The thing is l've tried two different ic's withno positive results. One is a u2008 from atmel and the other is a tca 785 to drive the triac. The tca 785 gave me some good results at first but seems to be unstable as is fires the triac at certain times and doesnt the next, so l need information on alternative ic's l can use.

Kind regards

Fiso

Thank you guys for your responses, what l need at this moment is information on ic's. The thing is l've tried two different ic's withno positive results. One is a u2008 from atmel and the other is a tca 785 to drive the triac. The tca 785 gave me some good results at first but seems to be unstable as is fires the triac at certain times and doesnt the next, so l need information on alternative ic's l can use.

Kind regards

Fiso

#5 marke

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Posted 08 February 2007 - 06:33 PM

Hello Fiso

QUOTE
There are a number of ICs etc that are designed for phase controlling a triac into a resistive load on a single phase supply. Unfortunately, these components are not suitable for use on three phase control. - The voltage is too high, and they are not designed to control three phases symmetrically.


In addition to the lack of three phase ICs, there are problems with triacs and opto triacs for three phase usage.

The triac is OK for single phase resistive loads, but has issues with high rates of dv/dt, di/dt and voltage blocking. You will find that three phase soft starters use SCRs, not triacs for good reason.

Best regards,

#6 Fiso

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Posted 09 February 2007 - 03:48 PM

Thank you man, can you suggest any ic's l can use besides the u2008 and the tca 785?




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