On Tue, May 3, 2016, at 06:42 AM, andy pugh wrote: > 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?
That picture shows an inductive load. And the text refers to solenoids which are inductive. A sufficiently large inductor will draw a nearly constant current with modest ripple at double the line frequency, which becomes a more-or-less square wave on the line side of the rectifier. > I wonder whether I would be better with DC-rated SSRs on the DC side > of the rectifier? > In case it helps, this is the input circuit. > https://imagebin.ca/v/2fs1b7szpsMy The SSRs are controlled by LinuxCNC > individually Is there any particular reason you are using SSRs instead of regular contactors? I like contactors because they are rugged and pretty much immune to overvoltage, dv/dt, and di/dt problems. Contactors (or electromechanical relays) also provide an air-gap between line power and the load, which means I'm comfortable using them in an e-stop circuit. SSRs (or any other semiconductor) are far more likely to fail shorted than open, and can't safely be used to disconnect power. Also, is there any particular reason you are running the full load current through both relays? I design large AC motor drives that rectify the AC line to make a DC bus. (Large is hundreds of kW). It would be prohibitively expensive to use two contactors sized for the full rated power. The precharge resistors are fed through a small contactor, rated (and fused) for the precharge current only (line voltage divided by resistor value). The bypass contactor is rated for full load current and bypasses the resistors. See the attached drawing "dc-supply-threephase.pdf" for an example. (The three 1.5uF caps are to absorb line-to-ground transients that couple across the distribution transformer from HV switching.) The other attached drawing is for a single-phase home-shop scale project. For home stuff. I use two poles for both precharge and bypass. American 240V consists of two lines, each 120V from ground, and for safety I need to disconnect both. I use the outer two poles of a three-pole contactor or relay. If British 240V has line and neutral then in theory you could use only one pole in the line side - but I personally would still use two. Most contactors have three poles anyway, might as well use them. At home-shop power levels, only the bypass device needs to be a contactor. The charging device can be a simple two-pole relay, like the ubiquitous ice-cube or KUP. -- John Kasunich jmkasun...@fastmail.fm
dc-supply-precharge.pdf
Description: Adobe PDF document
dc-supply-threephase.pdf
Description: Adobe PDF document
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