SW workround update [RE-wrenches]

[R. Sparks Scott]

>My question is about transferring loads that may(?) be out of phase.

	Kurt;

	Yes, this can be a big problem, particularly when inductive loads
(transformers, motors) are involved. If your relay transfers a load from
one phase to another, or from one source which is significantly
out-of-phase with another, you'll end up blowing breakers, frying
contactors, and possibly cooking the equipment as well.
	What happens is that any inductive loads will have a magnetic field built
up around the ferrous core/armature. This field will begin to collapse as
soon as the relay disconnects, inducing counter EMF into the AC line. If
the (new) incoming AC is out-of-phase, you'll have *twice* the single-phase
potential across the relay contacts at lots of amps instantaneous. Arc-city
big time!
	What you may want to do is use two relays, one for one AC supply, and a
second one for the other. Relays with a built-in time delay are available
from Grainger, etc. You may need to control the large AC contactor with
these smaller relays. Even a .5 second delay before the AC transfer takes
place will be enough for the magnetic flux to collapse and stabilize.

>Question 2?  I still can't find a good supplier for 24VDC contactors in
>the 60-100 amp range.  What is Trace using for their 60 amp, an open
>framed power relay, or a contactor?  Any thoughts?

	Trace is using double pole 30 amp enclosed relays with the poles
paralleled. I tend to lean towards solid-state relay devices.
Zero-crossover switching, no contacts to arc or pit, no moving parts,
operate from a variety of DC voltage sources, etc...

	-S

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