Page 179 - Electromechanical Devices and Components Illustrated Sourcebook

Chapter 8 Circuit Protection 141

Contactor Overload Protection
Contactor
Heaters
Reset Button
220/480 VAC
Heaters Three-Phase
Coil
Terminals C
Overload M
Protector
Coil Jumper Switch Motor
Jumper Control Signal
Base
Figure 8-22 Schematic for a Motor Starter with Thermal
Heaters
Switch Terminals
Figure 8-20 Commercial Motor Starter

Power Input Contactor
Motor Terminals Heaters

Motor Terminals
Contacts Heating Element
Bimetal Strip
Closed
(Operate) Coil Terminals
Switch Terminals Figure 8-23 Commercial Motor Starter with Integral Thermal
Heater Protection

Heat
Open
(Over Load)
Switch Neon Lamp
Figure 8-21 Thermal Heating Motor Protection Assembly

M

Power Source Load
Figure 8-21. A thermal heater is matched to the operating
Figure 8-24 Glow Discharge Protection
voltage and horsepower of the motor. This heater is also con-
nected in series with the motor and mimics the basic heat pro-
file of the motor. The heaters are installed in close proximity
to a bimetal strip which operates a contact set. Under normal The neon lamp will turn on at a specific
conditions the heaters produce very little heat and the contacts voltage and while operating has a very
remain closed. Under overload conditions the heater gener- low resistance. If a transient is experi-
ates enough heat to cause the bimetal strip to deflect and enced, the lamp will flash on and suppress Schematic Symbol
open the contacts. The contacts are wired in series to the con- the excess voltage.
tactor coil and when open, the contactor opens. Thermal
starters will automatically reset when they have sufficiently
250 Volt
cooled. Figure 8-22 shows a schematic representation of a Metal Oxide Varistor
motor starter and Figure 8-23 shows a typical commercial
motor starter with thermal protection. (MOV)
To provide more precise transient control
Glow Discharge Protection a metal oxide varistor (MOV) as shown in
Figure 8-25, is generally used. These Figure 8-25
For applications that need a modest amount of transient pro- devices short at a specific voltage and are Metal Oxide
tection, a neon lamp may be used, as shown in Figure 8-24. quite effective in suppressing the inductive Varistor (MOV)