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Vacation Break for Electrical Machines
By Thomas Yoon
Are you overloaded? Have you become a part of the highly stressed modern
working environment? You may need a break - a vacation, a change in environment,
a change of pace...
Similarly, machines can be overloaded too. Those that were designed for a
particular system are often pushed to their limits when new requirements
emerge.
Just like the final straw that breaks the camel's back, electrical machines
too, can reach their breaking point. If there is no vacation for them, they
will burn.
Fuses and circuit breakers are designed to stop the current in a circuit
before any damage can occur - create a vacation or a break for them.
A circuit breaker is one of the ways to prevent motors (or other electrical
machines) from burning due to overloading. All electrical motors produce
heat when running. At the time of a fault, this heat builds up at a much
higher rate than it can be dissipated. This can result in damaged equipment,
fire or even explosion.
The electrical protective device like the circuit breaker is designed to
de-energize the circuit before this heat level becomes dangerous.
There are basically 2 means of activating a break - thermally (heat) or
magnetically.
Thermal circuit breakers make use of bimetallic strips that will bend due
to the different expansion rates of each of the metals in the strip. The
bending of the strip will move a tripping mechanism latch that will then
open up the contacts of the circuit breaker.
Because bimetallic strips take time to heat up and bend, this type of tripping
device is used for tripping prolonged overloaded situations. The setting
for breaking the circuit at a particular overload value can be adjusted.
Magnetic circuit breakers make use of the field strength of magnetic coils
to trip a latch that will then open up the contacts of the circuit breaker.
This method is used for high current faults, like in a short circuit. This
method is designed to trip instantly.
A thermal-magnetic circuit breaker incorporates both a thermal strip and
a magnetic coil. This type of circuit breaker is able to protect against
momentary overloads as well as high or short circuit current faults.
What happens if the circuit breaker is placed in a high temperature environment?
The thermal bimetallic strip bends on high temperature. In this case, the
current is not causing the high temperature in the bimetallic strip, but
the ambient air is! The current may still be below the safe operating rated
value for the motor.
In this case, a compensating bimetallic strip may be incorporated into the
circuit breaker to eliminate this type of error. These circuit breakers are
often used in hot areas like furnace, oven, and boiler rooms.
Circuit breakers can be reset very quickly, once they have tripped. Can your
break or vacation be reset very quickly too? Don't reset. Take a long break!
Until next time...
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