Date: Wed, 1 Mar 2023 17:51:49 +0200
From: Victor Rosenthal 4X6GP <k2vco.vic@gmail.com>
To: amps@contesting.com
Subject: Re: [Amps] 10 KW CCS ON 6M...USING THE 3CX-6000A7... PART 19
<I don't see the point of paralleled contactors. The critical part is
<make and break, not steady state current. The danger is that contacts
<will be damaged or welded together by the arc that occurs on make/break.
<One of the contactors is always going to close/open before the other, so
<the probability of damage is not reduced by having two. If one of them
<welds itself shut (happened to me), then it doesn't matter if the other
<one doesn't!
<If one of these isn't big enough, then you need to replace both of them
<with a bigger one.
<73,
<Victor, 4X6GP
<Rehovot, Israel
<Formerly K2VCO
## nonsense, it's been done all the time for years now. There is a 20 ohm
step start resistor and a 3rd (2 pole) contactor still to be added to the
mix. At start up, max current is just 12 amps..... or 4 amps per
contact....that's dick. Those are 50 amp CCS rated contactors ( 50 amps
CCS per contact, 150 amps total). When shutting down for the night, the
primary magnetizing current (1.4 amps) is all that's being switched off.
The magnetizing current swamps out the B+ bleeder current too.
Trust me, it's a non issue. On buddy's 3x6 HF amp, he used NIB ( $50.00
each) surplus 3 pole contactors, 135 amps CCS per contact. plastic end
plates tossed, and all 3 x poles in parallel via a piece of 1" x 1" x 1/8"
thick angle aluminum. Ditto with each end of all 3 x 3 pole contactors..
One ( now 400 amp) contactor for each hot leg. 3rd ( now 400 amp)
contactor was bolted to the output of the 1st contactor, by bolting the 2 x
mating vertical angle aluminums together..with a pair of 1/4-20 SS bolts.
20 ohm ( later reduced to 10 ohms) was wired in parallel with the 3rd
contactor. 2 ga RW-90 was hardwired from main 200 amp panel ( 20' away)
to the input sides of contactors 1+2. That setup has been running
flawlessly since 2007.
RC snubbers across contacts is a wasted effort. Snubbers at best sorta
work. When they fail, the cap always fails open. MOV's across contacts
work superb, but when MOV's fail, they always fail shorted, which is damned
dangerous. IF MOV's are used, they HAVE to be fused.
In the case of B+ supplies, with step start, damaging contacts is a non
issue. It's not like we are turning on 3 phase 10-20 hp electric motors,
and slamming a huge load on the 3 pole contactor. Electric motor's, even 3
phase, typ draw a bit over triple the normal running current. And shutting
off any 3 phase electric motor is also an issue, since you are then
interrupting the normal running current.
Contactors are not relays. Contactor's are 2 x sets of contacts... in
series, sometimes known as a 3PST-DM. The potential difference is between
the input..and output. A bar comes crashing down onto the pair of
contacts. To speed up the release, a solenoid is used..and a very heavy
duty spring tension is also used. To overcome the spring tension, the
solenoid is used, but this requires a huge amount of magnetism. When the
bar opens off, it splits any arc, into 2 x arcs = zero arc.
To recap, we are not switching a load on..or off. Scott had plenty of 8
ga, silver plated, Teflon wire, so used it. ( I used 1-0 RW-90 and just 2
poles of a 300 amp, 3 pole contactor on my latest B+ supply).
Solid state relays, aka....SSR's are another wasted effort. They also have
to be supplemented by either a breaker, or a fuse, with fuses / breaker on
rear apron, for quick removal / shut down. The problem with SSR's is...
they are never quite shut off, and have a bit of leakage current when
...'off'. I went into SSR's in great depth years ago. They can be wired
in series for more voltage..and also paralleled for more current. P+B sold
em..and used em for an example in their catalog. 150 hp, 600 volt, 3
phase electric motor. They used a mess of series-paralleled SSR's to
handle the 600 vac, start up current, and normal running current. Then
protection galore, for the massive back emf when the load is shut off.
Their 90 and 110 amp CCS SSR's (single pole) each required a massive
heatsink, 12" square x 1/4" thick....and that's one huge alum plate per SSR
/ pole.
When you see the schematic for the entire mess of paralleled SSR's +
protection devices, toss in normal leakage current, it will make you gag.
If any device failed open, or shorted, it would be an absolute bitch to
trbl shoot. We went through all this at work, with HD commercial high
HP HVAC systems..and in the end, we went back to a dead simple 3 pole
contactor. Those 3 pole, 300 amp per pole, 600 vac rated contactors I
bought years ago at SSON (brand new in the box) were dirt cheap. The
solenoids have multiple taps and will run on anything from 110-250 vac.
The $50.00 surplus, NIB, 3 pole, 135 amp contactors came with just a 220
vac solenoid.... which will run on anything from 190-250 vac.
Jim VE7RF
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|