On Sat, 13 Mar 2004 18:38:45 -0600, "Bob Swinney" wrote:
Since the rotor can't turn, I imagine the 1 ph current is more like locked
rotor current, considerably higher than normal inrush.

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.
This only lasts half a cycle, and motor rated breakers are designed to
tolerate this surge without the breaker contacts separating due to
magnetic force. Breaker thermal trip time as a function of overload is
as follows:

IEC 947-4-1
Tripping Time at multiple of Ie
Class 120 % 150 % 720 %
10A 2 hours 2 min. 2-10 sec.
10 2 hours 4 min. 4-10 sec.
20 2 hours 6 min. 6-20 sec.
30 2 hours 12 min. 9-30 sec.

So you can see that a class 30 breaker will allow the motor to remain
in the locked rotor state for up to 30 seconds before tripping. If the
windings are already hot from running, this is more than enough time
to burn out the windings and start a fire.

But it gets worse. Article 430 says motor breakers can be sized up
to 250% of the running current of the motor. This means a motor
installed in a Code compliant manner could remain in the locked
rotor state without tripping the breaker for up to about 5 minutes.

Gary