"gradstdnt" wrote in message
om...
I am working on building a 7.5 hp rotary converter. I plan on using a
start circuit with start cap that drops out after a potential relay
senses enough voltage in the generated leg. Grainger has a listing of
potential relays but I really don't like dealing with them as I am not
a business and their past stock levels leave much to be desired. I
preferr McMaster Carr for service and ease of ordering. Strangely
enough, McMaster has zero listings for "potential relay" but has
plenty of double throw relays.

It appears to me, a double throw relay, one that can be wired either
normally off or normall on, should be able to work just fine as long
as the voltages for the relay cutout are appropriate. Is a potential
relay simply a "normally on" relay with a certain or adjustable cutoff
voltage?

I've got most all the hardware to start putting together this
converter except for the start cap and relay. I know what I need for
a start cap. Any help on specs for a relay that will do the job is
greatly appreciated.
....

After some searching, Grainger is the only place I found that sells
potential relays. They aren't normal relays. Your best bet for an
alternative source is a commercial air conditioning / refrigeration shop as
that is where these are normally used. (Starting single-phase hermetic
motors)

A potential relay differs from a normal relay in that it has a precise spec
and some hysteresis over the opening and closing voltages. A normal relay
will not predictably pull in and firmly engage the contacts as the coil
voltage slowly increases. The potential relay stays open until a high
threshold is reached and then stays closed until a low threshold is reached.
Your mileage may vary... maybe you can find a 240V coil DPDT relay that
will work in your application.

If you don't use a potential relay I suggest using a different start
circuit - either a one-shot timing relay that is adjustable down to the
sub-second range, or the "Jim Hanrahan" pushbutton start circuit which I
don't like for safety reasons but that is OK if you're the only one using
the shop.

If you or a friend can get into some experimenting, you can build hysteresis
into a DPDT relay by using one set of contacts and some power resistors
wired to add a "boost" to the coil voltage once the relay pulls in.
Resistance and power rating would depend on the particular coil and current.
Seems like something that ought to work but that is only ni theory.

Bob