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In control applications for motors the choice of the solid
state switch to be used is always very important for the
designers: the cost, reliability, ruggedness, and ease to be
driven must always be kept in mind. This is especially
important in motor control circuits where the designer has to
optimize the circuitry for controlling the motors in the
correct and efficient way.
In the large family of electronic switches, the thyristor
must be considered as a low cost and powerful device for
motor applications. Thyristors can take many forms, but
they have certain features in common. All of them are solid
state switches which 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.