248 Electrical Power Systems Technology
held closed by a ratchet wheel. At the current capacity of the device, the
fusible alloy melts, causing the ratchet wheel to turn. A spring then causes
the device to open the circuit.
Overheating Protection for Motors
Motors must be protected from excessive overheating. This protection
is provided by magnetic or thermal protective devices, which are ordinar-
ily within the motor-starter enclosure. Protective relays or circuit breakers
can also perform this function. When an operational problem causes the
motor to overheat, the protective device is automatically used to discon-
nect the motor from its power supply.
Undervoltage Protection
Motors do not operate efficiently when less than their rated voltage
is applied, and some types of motors can be destroyed if they are operated
continuously at reduced voltages. Magnetic contactors (see Chapter 15)
may be used effectively to protect against undervoltages. A specific level of
voltage is required to cause magnetic contactors to operate. If the voltage
is reduced below a specified level, the magnetic contactor will open, thus
disconnecting the circuit between the power source and the motor, and
stopping the motor before any damage can be done.
Figure 9-3. Block diagram of a motor-protection circuit