Electric Motor Control Basics - Book - Page 23
Industrial Control Basics: Part 2, Overload Relays
However, the metal with a high coefficient of expansion will expand more in
comparison to the metal with a low coefficient of expansion. This dissimilar expansion
of the bimetallic strips causes the bimetal to bend towards the metal with a low
coefficient of expansion.
As the strip bends, it actuates an auxiliary contact mechanism and causes the
overload relay normally closed contact to open. As a result, the contactor coil circuit
The amount of heat generated can be calculated by the Joule’s Law of Heating. It is
expressed as H ∝ I2Rt.
• I is the overcurrent flowing through the winding around the bimetal strip of the
• R is the electrical resistance of the winding around the bimetal strip.
• t is the time period for which the current I flows through the winding around the
The above equation defines that heat produced by the winding will be directly
proportional to the time period of the flow of overcurrent through the winding. In
other words, the lower the current, the longer it will take the overload relay to trip
and the higher the current, the faster the overload relay will trip, in fact it will trip
much faster because the operation of the relay is a function of the current squared.
Bimetallic overload relays are often specified when automatic reset of the circuit is
required, and occurs because the bimetal has cooled and returned to its original state
Part 1, Contactors
Basics: Part 2,
Basics: Part 3,
The Benefits of Type E/F
Basic Pump Control:
Three Phase DOL Starter
Control of an Electric Pump
Applying Motor Control
in a Wastewater
NEMA vs IEC Motor