In all kinds of manufacturing, it is very common to have
equipment that has three phase motors for doing different
work functions on the production lines. These motor
functions can be extruders, fans, transport belts, mixers,
pumps, air compressors, etc. Therefore, it is necessary to
have equipment for controlling the start and stop of the
motors and in some cases for reversing them. Actually, one
of the most common solutions for performing this control
functions is by using three phase magnetic starters. It
consists of a block with three main mechanical contacts
which provide the power to the three main terminals of the
motor once its coil is energized. However, the magnetic
starter has a lot of disadvantages and the most common
appear when they are driving high current levels that can
cause arcing and sparks on their contacts each time they are
activated or de–activated. Because of these kind of effects
the contacts of the magnetic starters get very significantly
damaged causing problems in their functionality. With time
it can cause bad and inefficient operation of the motors. This
is why, thyristor should be considered as a low cost
alternative and indeed a powerful device for motor control
applications. Thyristors can take many forms but they have
certain features in common. All of them are solid state
switches that act as open circuits capable of withstanding the
rated voltage until triggered. When they are triggered,
thyristors become low impedance current paths and remain
in that condition (i.e. conduction) until the current either
stops or drops below a minimum value called the holding
level. Once a thyristor has been triggered, the trigger current
can be removed without turning off the device.