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Registered Member #514
Joined: Sun Feb 11 2007, 12:27AM
Location: Somewhere in Pirkanmaa, Finland
Posts: 295
There was a thread about this a while back, but I don't feel like digging it up, so here's a new one.
I bought a large variac (15A, 230V in, 260V out). Today I decided to test it, but upon plugging in the slow-blow 10A fuse popped. Am I correct to assume it's the large inrush current?
I thought of a scheme for a soft-starter. Basically, I'd have a contator supply power to the variac and have a resistor (1k, 500R?) that supplies power while the core is magnetizing. I'd use a rotary switch with two positions: First position would turn power on through the resistor, and the second would turn on the contactor, shorting the resistor.
Just thought I'd ask, in case anyone has any suggestions or sees some problem in my thinking
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Here was the thread..
Startup through resistors works well.
At school I used one of those lecture time-relays set at like 1 second or so to short out the bank of 1500ohm resistors (3 phase system)
One of problems with hand operated switch is that your resistors will cook if you have a load on the variac and resistors are switched for too long. Unless you use something like light bulbs.
So in order to keep worst-case dissipation low you'd have to use rather large resistors.
Time relay has them only momentarily switched, and only little heat is produced.
The other more complicated way of startup could be use of a triac and peak voltage detection circuit which would always turn the system on as the voltage reaches peak, the flux would start from zero at right time and core would never saturate.
Load on the variac also calms down the startup current (try connecting some light bulbs at the output and then plugging it in).
Registered Member #514
Joined: Sun Feb 11 2007, 12:27AM
Location: Somewhere in Pirkanmaa, Finland
Posts: 295
I just thought I'd use the rotary switch, since I have one that has been laying in my junk box for years. I'd make it so that the contactor also disconnects the output of the transformer while it's being fed off the start-up resistor. Don't know how much idle current this thing will draw though, so heat dissipation might be a problem.
Meh. I'll probably some sort of automatic soft-starter and use a regular switch. The two stage swich would be so much simpler, though.
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Variac and tripping breakers
