Electric Motor Control Basics - Book - Page 27
Industrial Control Basics: Part 2, Overload Relays
Overload Relay Tripping
The tripping time of an overload relay will decrease when the current increases. This
function is plotted on the inverse time curve below, and is termed as the trip class. The
trip class also indicates the time taken by the relay to open in an overload condition.
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
Trip Classes 5, 10, 20, and 30 are common. These classes suggest that the overload
relay will trip in 5, 10, 20, and 30 seconds. This tripping usually occurs when the motor
is running 720% of its full load. Trip Class 5 is suited for motors that demand fast
tripping, whereas Class 10 is usually preferred for motors of low thermal capacity like
submersible pumps. Class 10 and 20 are employed for general purpose applications,
whereas Class 30 is employed for loads with high inertia. Class 30 relays help avoid
nuisance tripping.
NEMA vs IEC Motor
Controls
Understanding
IP Ratings
27
