On Mon, 25 Nov 2013 04:42:43 +0000 (UTC), wrote:

Jeff Liebermann wrote:
On Fri, 22 Nov 2013 16:14:02 +0000 (UTC), wrote:

There are two relays; the "little" SPST one that operates first has
about a 10 ohm ceramic resistor in series with the contacts. The
"big" one that operates second has no resistor, and is DPST, for both
sides of the DC bus.

That's the same idea as the one I described.

The more I think about it (and look at the wiring diagram), the more I
think the "big" relay is really two SPST relays operated at the same
time.

It's useful for power supplies that have relay driven power on/off
circuits, where one doesn't want the full AC power drain going through
the on-off switch as in TTL/CMOS on-off.

In this case, the operator position is not convenient to where the
relays need to live, so the relays get switched by 12 VDC control
signals. There are some other safety functions between the operator and
the relays as well; the computer can decide that bad things are
happening and drop out the control signals to the relays.

As mentioned, the problem is when the contacts short, the full inrush
current appears on turn-on, blowing the fuse.

I think this system is set up to fail safe if one set of contacts gets
welded. The normal startup sequence is something like

1. Close main relay in - line.
VFD should not yet have DC.
2. Close pre-charge relay in + line.
VFD has current-limited DC. Input caps charge.
3. Close main relay in + line.
VFD has current-unlimited DC.
4. Open pre-charge relay in + line.

In the Lambda LFS-48-24 power supply I mentioned, only one set of
contacts are required. The sequences is roughly:
1. Apply AC power (there's no on-off switch in the PS) with a 10 ohm
resistor in series with the AC line. BFC's (big fat caps) charge
slowly, limiting inrush current.
2. When the voltage across the BFC's is at full charge and stable,
short the 10 ohm resistor with an SPST (actually DPST with both
contacts in parallel) to provide full power.

Shutdown is something like:
1. Remove AC power. Relay remains closed until BFC's are nearly
discharged. This is to prevent momentary power glitches from
producing much longer duration power drops at the output.
2. Relay opens when BFC's are discharged.

and the normal shutdown sequence is something like

1. Open main relay in - line.
The input caps should start discharging, and the voltage at the VFD
should start dropping.
2. Open main relay in + line.

Ummm... my guess is that it should be:
2. Open main relay in + line only when caps are fully discharged. In
this case, the idea is to NOT open the contacts under load to prevent
arcing.

If the pre-charge relay contacts or main relay + line contacts are
welded, then the VFD will see DC as soon as it does startup step 1. It
can then open the main relay - line and be safe.

If the main relay - line contacts are welded, it probably can't detect
it at startup, but it can detect it at shutdown when the voltage fails
to drop off; it can open the main relay + line and be safe.

If there was something to detect input current, it could detect a much
faster than normal rise in input current, and use that to shut down
the PS. The problem is that by that time, it would be too late, and
with welded contacts, there would be no way to shut it down. Argh.

There is also a 125 A fuse on the DC input. It is rated at 100 sec
max at 250 A (2x rated), 15 sec max at 375 A (3x), and 1 sec max at
625 A (5x).

Yikes, that's big.

Roughly 34.5 kW at nominal voltage. The motor that this thing drives
is rated at 33 kW, or just a touch under 25 hp.

Ummm... that's a bit larger than the typical commonly available power
supply. Well, it's larger than anything I've ever seen. My closest
experience is with solar powered battery controllers and inverters.

Physically, the fuse is only about 1.5" (38 mm) long and maybe 0.75"
(19 mm) diameter, with big lugs sticking out of the ends to bolt it
down. In use, it lives behind a polypropylene cover that you have to
remove an interlock and a screw to get at.

Is the fuse cartridge filled with sand?

I don't think a fuse that big actually "blows". More like "melts".

Well, technically, all fuses just "melt".

I think of melting as a slow motion explosion. I was playing with a
5V 300A computer power supply. It had a 250A fuse in the output. So,
I decided to see what would happen if I shorted the output with a
screwdriver. (Do not try this at home). There was very little spark
as the screwdriver welded itself to the copper bus bar contacts. I
watched the fuse slowly melt into a puddle of tin(?). It took about 5
seconds. The screwdriver was quite hot and had to be removed from the
contacts with a cold chisel and hammer. Apparently, the screwdriver
was too high a resistance to "blow" the fuse and NOT trigger the over
current detection circuit. I later tried to reproduce this with a
similar power supply, but couldn't find the correct resistance
screwdriver.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558