Here it is simple and quick from an electrician.
There are two types of current for motors, full load amperes and
nameplate amperes. Full load current (FLA) is found in the National
Electrical Code Tables located in Article 430. There are tables for
single phase, three phase, direct current, and wound rotor motors.
Nameplate current is found on the motor nameplate. Nameplate current
is used to size motor overload current protective devices. FLA or
nameplate horsepower is used to size conductors, motor short circuit
and ground fault protective devices (SCGFPD) , controllers, and
disconnects. A motor circuit generally consists of a short circuit and
ground fault protective device, disconnect, conductors, controller, and
overload devices. A motor circuit is unique because the motor
starting current is high generally 6 times the FLA so the SCGFPD has to
handle this high current for a short time duration and therefore is
sized several times larger than the FLA. This device does not protect
the motor conductors or the motor from overload because it is sized too
high, but it does protect from ground faults and short circuits that
cause very high currents for a short duration of time. Protection from
long term over current and over heating is the function of the overload
devices that are generally set just above the nameplate current. The
overloads are slow acting and do not trip during the starting of the
motor but serve as long term protective devices for over current.
There are rules in the NEC for sizing these devices for safety
purposes. For the USA, the NEC is not an engineering standard, but a
safety standard that establishes minimum safety rules for electrical
design, installation and manufacturing.
Now about induction. The stator winding is wired to the supply. When
the stator is energized it induces a magnetic field into the rotor.
This induced magnetic field and resulting counter electromotive force
(EMF) acts as part of load and limts the current in the stator.
Consequently, if the applied voltage is too low the current will
increase in the Stator because of too low of a counter EMF and cause
the motor overloads to trip. This problem can be caused by too high
of a voltage drop in the conductors as happened at Pump Station 8 on
the trans Alaska pipeline. We overcome this problem simply by
increasing the size of the overloads. The opposing magnetic poles
cause the rotor to rotate.
This is very brief and to the point and serves for newsgroup discussion
purposes. There are many sources of information on the Internet. Try