Electric Motor Control Basics - Book - Page 21
Industrial Control Basics: Part 2, Overload Relays
What is Overload Protection?
When the motor draws excess current, it is referred to as an overload. This may
cause overheating of the motor and damage the windings of the motor. Because of
this, it is important to protect the motor, motor branch circuit, and motor branch
circuit components from overload conditions. Overload relays protect the motor,
motor branch circuit, and motor branch circuit components from excessive heat from
the overload condition. Overload relays are part of the motor starter (assembly of
contactor plus overload relay). They protect the motor by monitoring the current
flowing in the circuit. If the current rises above a certain limit over a certain period of
time, then the overload relay will trip, operating an auxiliary contact which interrupts
the motor control circuit, de-energizing the contactor. This leads to the removal of the
power to the motor. Without power, the motor and motor circuit components do not
overheat and become damaged. The overload relay can be reset manually, and some
overload relays will reset automatically after a certain period of time. After which, the
motor can be restarted.
Industrial Control
Basics:
Part 1, Contactors
Industrial Control
Basics: Part 2,
Overload Relays
Industrial Control
Basics: Part 3,
Starters
The Benefits of Type E/F
Motor Protection
Circuit Breakers
Basic Pump Control:
Three Phase DOL Starter
Control of an Electric Pump
Applying Motor Control
in a Wastewater
Application
NEMA vs IEC Motor
Controls
Understanding
IP Ratings
21
