On Sat, 03 Apr 2010 09:10:11 -0800, Robert Baer wrote:

snip
That is LTSpice aka SwitcherCAD i(if i remember correctly). Anyway,
the transfer pump is a small one, prolly 1/4HP at the most
but the main motor can run from 1/4HP to perhaps 2HP at the top end (but
not for a few years).
Am using optically isolated triac switching circuitry to drive
relays, where the relays do all of the work including turning on and off
that resistive heater mentioned.
**
On rare occasion i hear some buzzing of a relay when it is on - any
ideas?


AC powered relays and contactors do occasionally buzz. It's usually
because the magnetic circuit isn't fully closing because of dirt on the
pole faces or just that the pole faces aren't accurate enough. Some sorts
seem to be worse than others. The buzz is harmless (but can be annoying).
On some gear (usually contactor panels for switching interior lights) we
use rectified AC feeding DC contactors. That gives a nice silent system,
but costs more. BIG contactors (100A plus) are often DC operated anyway
as the magnetic system is more reliable that way.

Just make sure that your relays are definitely rated for your motor load,
otherwise their life will be *very* short! Generally relays are rated for
resistive loads, whereas contactors are rated for inductive loads. A
typical 10A relay will only switch about 7A inductive (max). For motor
starting you should look for a class AC3 rating. You should be ok using
relays at 1/4HP, but 2HP won't do it. That's a small contactor. Actually,
the difference in price between a good quality plug-in relay plus base
and a small DIN rail contactor is minimal. You may as well use contactors
- but they don't fit on PCBs.

--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info
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