Electric Motor Control Basics - Book - Page 24
Industrial Control Basics: Part 2, Overload Relays
(form). Once this happens the motor can be restarted. If the cause of overload is not
rectified, the relay will trip again, and reset at predetermined intervals. It is important
to be careful during the selection of an overload relay, because repeated tripping and
reset can reduce the mechanical life of the relay and may cause damage to the motor.
In many applications, the motor is installed at a location with a constant ambient
temperature, and the overload relay and motor starter may be installed in a different
location, which experiences different ambient temperatures. In such applications, the
trip point of the overload relay can vary depending on multiple factors. The current
flow through the motor and the temperature of the surrounding air are two factors,
which may cause premature tripping. In such cases, ambient compensated bimetallic
overload relays are used. The relays of this type feature two types of bi-metal strips
– a compensated bi-metal strip and a primary non-compensated bi-metal strip. At
ambient temperatures, both these strips will bend equally, thereby preventing the
overload relay from nuisance tripping. However, the primary bi-metal strip is the
only strip that gets affected by the current flow through the heater element and the
motor. In the condition of an overload, the trip unit will be engaged by the primary bimetal strip.
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
24
