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.
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.
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.
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.
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.
I don't think a fuse that big actually "blows". More like "melts".
Well, technically, all fuses just "melt".
Matt Roberds