On 03.05.16 11:42, andy pugh wrote: > On 3 May 2016 at 10:41, Nicklas Karlsson <nicklas.karlsso...@gmail.com> wrote: > > In other words i/C should be less than maximum allowed du/dt. > > I am not sure how to determine the i for this calculation. I guess I > can find the dv/dt from the component numbers on this page: > http://www.instructables.com/id/The-inner-workings-of-Counterfeit-FOTEK-SSRs/ > The servo drives have so far been limited to 4A, but whether that > translates to 4A at the input to the rectifier I am not sure. > Section 6 on page C-343 here: > https://www.omron.com/ecb/products/pdf/precautions_ssr.pdf shows a > square-wave with zero time at 0 current. But I don't understand how > that is possible with a sinusoidal input voltage. Surely the current > is zero for all the period of the cycle where the input voltage is > less than the capacitor voltage?
Spot on, but the circuit in section 6 doesn't include a capacitor, that I can see. > I wonder whether I would be better with DC-rated SSRs on the DC side > of the rectifier? The link to SSR precautions indicates that the SSR already includes a snubber. But page 3 of the notes says that is often not sufficient, and suggests a 440 - 470 volt varistor (MOV) as protection in a 200 - 240 volt application. That clearly assumes the use of e.g. a 600v SSR, not a mere 400v unit. > > Problem might also be because you discharge a voltage into a capacitor and > > get a very large inrush current but this require a different solution. > One of the failed SSRs is meant to be that solution, it is used to > bypass a current-limiting resistor once the bus voltage reaches 300V. > > In case it helps, this is the input circuit. > https://imagebin.ca/v/2fs1b7szpsMy The SSRs are controlled by LinuxCNC > individually. IIUC, there's also a bridge rectifier between the SSRs and the capacitor. But the inductance being switched is the power transformer on the input. If the rectifier were not there, you'd have an LC circuit which would oscillate at its resonant frequency if the current were interrupted mid-cycle, rather than at a zero crossing. In such a situation, several times the 340v mains peak voltage could appear in the circuit, to the distinct detriment of the SSR. The bridge rectifier prevents that resonance. It is tempting to move to the DC side, and use a 600v DC SSR, or just a high voltage MOSFET. (N-channel, in the -ve lead has lower RDSon, and is easier to drive.) The section 6 that you mention recommends using a MOSFET SSR on the DC side. Do you see a 600v one of those on ebay? Regards, Erik ------------------------------------------------------------------------------ Find and fix application performance issues faster with Applications Manager Applications Manager provides deep performance insights into multiple tiers of your business applications. It resolves application problems quickly and reduces your MTTR. Get your free trial! https://ad.doubleclick.net/ddm/clk/302982198;130105516;z _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
