On 2005-07-28, Ignoramus9394 wrote:
Let's say, hypothetically, that I have a single phase mill. Power goes
out. The proper procedure is to turn off the mill and go do other
stuff.

If I forget to turn the mill off, it will turn back on again, with the
obvious consequences.

Now, let's say that I have a 3 phase mill. Power goes out. What are
the possible consequences of me forgetting to turn it (and RPC) off?

The answer is, the same as in the single phase example above. The phase
converter itelf will not be damaged by returning power, it will simply
restart.

If so, then, logically, I need to install similar relays on single
phase equipment as on 3 phase equipment. And yet no one does this with
single phase equipment.

Protection of RPC itself cannot be the reason, since it starts just
fine when power is restored.

What am I missing?

There are a couple of reasons to use a drop out contactor. One is to
protect the RPC and any other 3 phase motors you put on it; even though
your RPC is normally self starting, it might not start when there is an
additional 3 phase load on it. If the power goes out when the RPC is
running and some additional 3 phase machine is also running, and you
forget to turn either off, then when the power returns the RPC run
capacitors will be trying to start two motors simultaneously. You have
only verified that they can start the RPC by itself.

The other is simply that it is not generally a good idea to have machines
starting up unexpectedly, as they might when you forget to turn something
off when the power fails. This case isn't any different for single phase
machines, and they should have the same sort of relays. Of course as you
know they often don't.

allan

--
Allan N. Hessenflow