SCR Gating Circuit for Soft Starter
Posted 06 March 2005 - 07:38 AM
How can I use this pulses to trigger the SCRs for driving a 3phase 5.5KW induction motor?
Posted 12 March 2005 - 02:50 AM
Posted 12 March 2005 - 06:46 AM
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.
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.
Mark Empson | administrator
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