Motor protection
breakers as a form
of effective drive
protection
Electric drives require effective protection against overloads and short circuits.
Both phenomena cause motor disruption and create a risk of damage to the
motor. This could lead to a potentially dangerous situation for people in contact
with the devices. Modern motor breakers are the current remedy for both threats,
and are a part of multi-level drive protection systems together with circuit
breakers, RCD breakers, contactors, detectors (sensors) of phase failure and
phase sequence, and undervoltage protection.
What are motor breakers and what
do they do?
In short words: motor protection breakers
are connectors for connecting, protecting
and separating current circuits with load.
Moreover, these connectors protect
drives from damage due to start-up block,
overload, short circuit or one phase failure
in 3-phase mains. They are characterized
by the presence of a thermal and
electromagnetic release and the possibility
to connect to them undervoltage release,
shunt release, auxiliary contacts and releasesignalling
contacts.
The above description is very concise
and needs to be further explained. It is worth
mentioning that motor breakers enable
the protection of drives without additional
fuses, which means that they are a compact
solution that saves space and reduces
costs. The importance of these modules is
great. They react in milliseconds and protect
the most valuable element of any electrical
device: the motor that drives it. Moreover,
they protect the power supplying mains
from the impact of short circuits, overloads
or phase asymmetry. However, the most
important task of motor breakers is a quick
power supply shutoff in case of detection
of short circuit or overload current, so their
use is recommended e.g. wherever large
inrush current occurs. Each motor breaker is
therefore equipped with an electromagnetic
release which is responsible for short circuit
response, and a thermal release that is
responsible for switching off the circuit
when overload is detected (motor winding
protection), and for reacting in case of one
phase failure in 3-phase power mains. Phase
control and response to increased circuit
temperature is very important. Sometimes,
after a power failure, the local electricity
supplier turns on only two phases first, which
can result in damage to the drive. These
two releases (thermal and electromagnetic)
constitute “the heart of motor breakers”
and protect not only the motor, but also the
contactor. However, one condition must be
met: the breaker must be installed before the
contactor.
The diversity of drives used currently in
the industry and manufacture is huge, just
as is the diversity of motor breakers. When
choosing a motor breaker for any drive,
follow a simple rule: the set rated current
of the breaker must not be lower than the
rated current of the motor operating normally
under optimal conditions. In order to set it
correctly, it is sufficient to follow a simple
formula Ir x 1,1 = Ibr in which Ir is the rated
current the motor, and Ibr is the target
current of the motor breaker setting.
It is rarely mentioned that motor breakers
also have the function for manual switching
the motors on and off. Various switches may
serve the purpose, including rocker switches
and rotary switches. In simple words, they
enable manual control of the motor.
Basic types of motor breakers and
their complementary equipment.
The most typical breakers classification is
based on the current and mains parameters.
Therefore, there are single-phase breakers
(cage, serial, etc.) and three-phase breakers
(cage, linear, annular, etc.). By analogy,
there are also switches requiring AC or
DC power supply. Popular classification
includes also classic breakers, which use
the properties of bimetal, and electronic
ones with an electronic overload module
and a control unit. Another way is to
classify them with regard to polarity (2-
pole, 3-pole). Further classifications into
groups, types and subtypes are based on
specific parameters of the breakers. Each
major manufacturer classifies their products
according to their own criteria.
Motor protection breakers can be
additionally equipped with multiple
accessories, including undervoltage
releases and shunt releases. In the case of
the former, the breaker is triggered when
the voltage is decreased (fading) in the
connected circuit. The latter (shunt trip)
triggers the breaker when receiving an
impulse signal or continuous voltage.
The second important group of
accessories for motor breakers are
auxiliary contacts. They are used to transfer
information about the breaker operation
and (among other things) to implement
control systems. Release-signalling contacts
constitute an important group, although the
leading manufacturers of motor breakers
increasingly often equip their products
with a built-in accessory that signalizes the
activation of each protective module. Often,
the position of the knob or the rocker switch
itself indicates e.g. whether the overload
release has been activated. Very often, the
coloured or mechanically moved indicators
can be described as the user-friendly
features. They indicate with their colour (e.g.
red) whether the short-circuit breaker has
been activated. The solution was developed
to help the operators supervising the
motor-breaker-equipped drives, who often
had trouble distinguishing which type of
protection was activated at any given time –
the short-circuit or the overload release.
16 22 September 2020 www.newelectronics.co.uk
/www.newelectronics.co.uk