Synchronous motors tolerate only extremely short interruptions of supply without loss of synchronism. In practice, the dead time should be sufficient to allow the motor no-volt device to operate. Typically, a minimum dead time of 0.2-0.3 seconds is recommended.
General
Overload relays are intended to protect motors,
controllers, and branch-circuit conductors against
excessive heating due to prolonged motor
overcurrents up to and including locked rotor
currents. Protection of the motor and the other
branch-circuit components from higher currents,
due to short circuits or grounds, is a function of the
branch-circuit fuses, circuit breakers, or motor
short-circuit protectors.
Nameplate Versus NEC Full-load Current
The 1996 NEC in Tables 430-147, 430-148, 430-
149 and 430-150 lists full-load currents according
to motor horsepower and voltage. According to
Article 430-6, these full-load currents should be
used, rather than the motor nameplate full-load
current, to determine the ampacity of conductors,
ampere ratings of switches, or branch-circuit
overcurrent devices, etc. It is specifically stated in
Article 430-6, however, that “separate motorrunning overcurrent (overload) protection shall be
based on the motor nameplate current rating.”
Service Factor
NEMA standards for motors list service factors of
1.15 to 1.25 for general-purpose ac motors from
1/2 to 200 horsepower. Other motors, such as
totally-enclosed, fan-cooled, and motors over 200
horsepower have a standard 1.0 service factor.
Because of the way in which the standards are
established, there is no simple way of determining
the service factor without looking at the motor
nameplate. For this reason all of the thermal unit
selection tables in this catalog are designed for
1.15 to 1.25 service factor motors, except the
tables for Class 8198 High Voltage Starters.
These tables are designed for 1.0 service factor. It
is important to know when a motor has a service
factor of 1.0, because failure to recognize this fact
will result in an over-sized thermal unit selection
and reduced motor protection.
