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SCR Gating Circuit for Soft Starter

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#1 Iced_tepanyakii


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Posted 06 March 2005 - 07:38 AM

I am currently facing problem in designing a SCR gate triggering circuit for a soft starter. I have worked out a soft start pulses at 800mV (low) 4.1V (High) at 100 Hz.

How can I use this pulses to trigger the SCRs for driving a 3phase 5.5KW induction motor?

#2 jraef


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Posted 12 March 2005 - 02:50 AM

Well, perhaps you could enlighten us on why you need to know. If you are a potential competitor, good luck. Everyone else had to learn the hard way or buy their way into the market, join the club. If you are a student, learn to do your own research, it will serve you better in the long run. If you are just curious and want to see if you can do it on your own, you might want to think about buying a pre-made trigger card from Phasetronics or Enerpro. They are most likely much less expensive than designing one from scratch, and already proven. There are several ways of doing it that work, but getting there is far too complicated to post in a forum like this IMHO. Marke, the superadmin of this forum, is very knowedgeable on this subject and may want to offer his expertise, I don't know.
Good luck.
"He's not dead, he's just pinin' for the fjords!"

#3 marke


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Posted 12 March 2005 - 06:46 AM

Hello Iced_tepanyakii

There are a number of different ways of achieving this and you will find quite good information available from the SCR manufacturers such as Semikron.

There are three main strategies in common usage.
1. Single Pulse trigger
2. Picket Fence trigger
3. Hard fire.

1. Single pulse trigger is where a pulse is applied to the gate at the instant that the SCR is required to turn ON. This pulse is relatively short (typically in the order of 100uS) The pulse would typically be applied to the gate via a pulse transformer.

2. A series of pulses is applied to the gate of the SCR. This reduces the potential of premature commutation of the SCRs.

3. Hard Fire. A continuous DC gate drive is applied to the SCR for the required period of SCR conduction. - you treat the SCR like a bipolar transistor.

The selection is dependent on your requirements and characteristics.

There are a number of important characteristics of gate drive circuits that you must consider.
a) The gate current must be high enough to saturate the gate region to ensure that the whole gate region of the SCR is turned ON. It is possible to turn ON part of the SCR with a low gate drive. Once the SCR is conducting, the charge will spread through the gate region but this is slow and can result in SCR failure.
B) The leading edge of the gate pulse must be very fast.
c) The isolation between the gate circuit and the driving circuit must be adequate for the application. It is preferable that the capacitance is very low to prevent accidental pulses coming from dv/dt on supply or load.

As Jraef mentioned above, there is more potential complexity here than it would appear. The gating circuit and the timing of the pulses is critical to the performance and reliability of the SCRs.

If you are looking for a commercial solution, you may be best to either buy in an existing solution, or to contract to someone with the required experience and expertise.

Best regards,

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