On 14 Mar 2004 08:44:27 -0800, jim rozen wrote:
In article , Gary Coffman says...

Locked rotor current is typically about 6 times running current. Starting
surge for direct-on-line (DOL) starting is up to 18 times running current.

Still, pony motor started converters don't have that 18 times
surge on startup. As I mentioned, mine starts up just fine
with overcurrent protection that will trip out in less than
a second for locked rotor condition.

I use a pony started converter to reduce startup surge to something
my electrical service can stand (big converter, ordinary 200 amp service).
But the load motors started by the converter are still DOL, and the
surge when they start still requires breakers sized for DOL starting.

I wonder if the capacitor start converters should maybe have two
levels of protection, larger fuses in series with the start
circuit, and then smaller ones once the start contactor drops
out.

Capacitor start converters depend on impedance limiting to
reduce starting surge. But if the timing of contactor closing
is wrong, they can produce nearly double the ordinary DOL
starting surge. For a big capacitor start converter, popping
breakers is a way of life, unless you use delta-wye starting,
and even then bad timing can still cause excessive surge
when you switch.

But in any case, what we're concerned about here is what
happens when there is a momentary power failure. Then
the start circuit does not engage, and the motor sits at
locked rotor current until either the breaker trips, or the
motor burns up.

There are motor protect circuits which can prevent this.
The simplest is the dropout contactor.

Gary