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© Semiconductor Components Industries, LLC, 2008
August, 2008 − Rev. 1
1 Publication Order Number:
AND8008/D
AND8008/D
Solid State Control
Solutions for Three Phase
1 HP Motor
INTRODUCTION
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.
DEFINITIONS
Three phase induction motor.
A three phase induction motor consists of a stator
winding and a rotor of one of the two following types: one
type is a squirrel−cage rotor with a winding consisting of
conducting bars embedded in slots in the rotor iron and
short circuited at each end by conducting end rings. The
other type is a wound rotor with a winding similar to and
having the same number of poles as the stator winding, with
the terminals of the winding being connected to the slip
rings or collector rings on the left end of the shaft. Carbon
brushes bearing on these rings make the rotor terminals
available at points external to the motor so that additional
resistance can be inserted in the rotor circuit if desired.
Three phase voltages of stator frequency are induced in
the rotor, and the accompanying currents are determined by
the voltage magnitude and rotor impedance. Because they
are induced by the rotating stator field, these rotor currents
inherently produce a rotor field with the same number of
poles as the stator and rotating at the same speed with
respect to the stationary rotor. Rotor and stator fields are
thus stationary with respect to each other in space, and a
starting torque is produced. If this torque is sufficient to
overcome the opposition to rotation created by the shaft
load the motor will come up to its operating speed. The
operating speed can never equal the synchronous speed of
the stator field.
The following figure shows a three phase 1HP motor
controlled through a conventional magnetic starter which
has an over−load relay for protecting the motor against
over−load phenomena.
APPLICATION NOTE
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