Electric Motor Control Basics - Book - Page 22
Industrial Control Basics: Part 2, Overload Relays
How an Overload Relay Works
The overload relay is wired in series with the motor, so the current that flows to the
motor when the motor is operating also flows through the overload relay. It will trip at
a certain level when there is excess current flowing through it. This causes the circuit
between the motor and the power source to open. The overload relay can be manually
or automatically reset after a predetermined time duration. The motor can be restarted
after the cause of the overload has been identified and rectified.
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
Types of Overload Relays
1. Bimetallic Overload Relay
Many overload relays include bimetallic elements or bimetallic strips, also referred to
as heater elements. The bi-metallic strips are made of two types of metals – one with
a low coefficient of expansion, and another with a high coefficient of expansion.
These bimetallic strips are heated by a winding around the bimetal strip, which
carries the current. Both of the metal strips will expand due to the heat.
Applying Motor Control
in a Wastewater
Application
NEMA vs IEC Motor
Controls
Understanding
IP Ratings
22
