Electronic Versus Contactor Switching
Posted 16 October 2006 - 08:45 PM
Posted 16 October 2006 - 10:06 PM
When you switch the capacitors with a contactor, you will get an inrush current to charge the capacitors. It is common to use some form of impedance to reduce this inrush current. Capacitor contactors have early make contacts with series resistance that close before the main contacts.
Solid state switching of capacitors is designed to switch at the voltage zero crossing.
This eliminates the charging current to the capacitors and allows far more frequent operation.
With sold state switching, it is important to ensure that a hard fire technique is used for the SCRs, otherwise, the SCRs will effectively switch ON a few cylces after the zero crossing and there will be a current transient that could damage the SCRs, plus it tends to defeat the whole idea of the solid state control.
In most situations, because the tarrif is based on a holf hour integral, rapid switching is not really required, so little is gained from the use of the solid state.
One disadvantage of the solid state is that the capacitors will cause a high current transient current to flow if there are any voltage transients on the supply. This transient current is passed through the SCRs and can damage them. The addition of detuning reactors will limit the transient currents.
I would recommend that detuning reactors are used when solid state switching is used.
for high speed switching, use solid state plus detuning reactors.
for standard switching, the normal contactors are OK except for the transient when they switch which will reflect as a voltage transient on a high impedance system.
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Posted 17 October 2006 - 12:02 AM
Posted 17 October 2006 - 06:37 PM
Posted 18 October 2006 - 04:59 AM
I recall doing a job for a manufacturer of ovens many years ago. The application was a hand held trigger operated spot welding gun. Electro-mechanical contactors were failing every month and a half due to the 3000 operations per hour experienced during peak production times. Unfortunately the customer expected the cause of failures to be electrical and as a result of that kept increasing contactor size without any joy. He persisted for many years.
When approached on the matter I offered a solid state contactor ex-Payne Engineering US. From memory it was from their 11C series. That SS contactor provided troublefree operation for a period of 12 years before the plant finally shut down.
Through the implementation of solid start control (in this case) the customer experienced substantial savings in term of cost of replacment product and labour, but more importantly they were able to maximise uptime and productivity. I can only guestimate that that 11C contactor paid for itself thousands of times over the years.
The point that I was making in my previous response was simply that the introduction of solid state contactors is not always pursued for electrical reasons. In fact I would go so far as to say that noise reduction is driving SSR and SSC business in many regions around the globe. We know of individual companies in Singapore taking up 40,000 SSR's per year for that reason alone.
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