The manufacturer makes available coils of
practically any desired control voltage. Some
starters are designed with dual-voltage coils.
NEMA standards require that the magnetic
switch operates properly at varying control voltages
from a high of 110 percent to a low of
85 percent of the rated coil voltage. This range of
required operation is then designed by the manufacturer.
It ensures that the coil will withstand
elevated temperatures at voltages up to 10 percent
over rated voltage and that the armature
will pick up and seal in, even though the voltage
may drop to 15 percent under the rating. Normally,
power company service voltages are very
reliable. Plant voltages may vary due to other
loaded, operating machines and other reasons
affecting the electrical distribution system.
If the voltage applied to the coil is too high, the
coil will draw too much current. Excessive heat
will be produced and may cause the coil insulation
to break down and burn out. The magnetic
pull will be too high and will cause the armature
to slam in with too much force. The magnet pole
faces will wear faster, leading to a shortened life
for the controller. In addition, reduced contact
life may result from excessive contact bounce