Starting method significantly
affects the starting current, torque, mechanical stress and hence the life of
an induction motor. The soft starter electronically controls the starting
torque and current of an Induction motor. Their field of application includes
HVAC fans and pumps, industrial fans, pumps, conveyors and other processing
equipments etc. Soft starters can be used with both 3-phase and 1-phase
induction motors. The following material is intended to acquaint the reader
with the theory and operation of solid state soft starter and motor controller.
It is necessary to understand the
load characteristics requirements and the motor capability when used with a soft
starter and controller. Induction motors can be classified based on the locked
rotor torque and current, breakdown torque, pull up torque, and the percentage
slip. General purpose Induction motors has the highest share in terms of sale.
They have a typical slip of 3 to 5% and are used in applications requiring low
starting torque such as industrial fans, blowers, centrifugal pumps,
compressors etc.
Starting the induction motor with Normal
motor starter:
The rotor of an induction motor can be squirrel cage or wound rotor. The wound rotor induction motor allows controlling the speed and torque and is generally started with a secondary resistance in the rotor. As the resistance is reduced, the speed of the motor increases. Thus the motor can develop substantial torque while limiting the locked rotor current.
It is common to start an
Induction motor using a motor starter which directly connects the motor to the
utility supply causing the motor to draw a high starting current or inrush
current. The inrush current drawn by a motor when started with a normal starter
is of apprehension as it causes the supply voltage to dip causing an impact on other
sensitive electrical loads. When the starting current is large, the magnetic forces to which the motor winding is subjected, are also large. The mechanical shock thus created
can damage the winding insulation, motor shaft, belt etc. leading to premature
failure of motor and the associated system.
Theory and Operation of solid state soft
starter:
A soft starter is a form of
reduced voltage starter used for starting of Induction motor. These starters
are similar to resistance or reactance starter and are connected in series with
the motor. These starters use solid state switching devices such as TRIAC or
SCR, to control the voltage/current fed into the motor. The TRIAC or SCR are
turned ‘ON’ for a part of each cycle. The average voltage is controlled by
varying the conduction angle of these SCRs/TRIACs. Thus the voltage to the motor
can be easily changed according to the required starting conditions and this
can be done automatically with the help of a control circuit. The control
circuit can be pre-programmed to provide a particular output voltage profile based on a time sequence. The circuit can also dynamically control the
output voltage to get a voltage profile based on the measurements of current
and speed of the motor. The earlier type of controlling is an example of 'open loop control' while the later is called ‘closed loop
control’.
Advantages of solid state soft starter:
Solid state soft starters and
controllers can control the starting characteristics such as-
Ø acceleration
and deceleration time,
Ø starting
and overload current and
Ø motor
torque, to match the load requirements.
Suppose a motor takes a starting
current of 6 times the normal current. The soft starter, when used, can be set
to limit this current up to 3 times (as shown in fig.1). Reduction in current also reduces the
torque (as shown in fig.2) which in turn reduces the mechanical stress. The torque available from
the motor is proportional to the square of the current. As the starting
current, in this case, is reduced to 50%, the torque reduces to 25% of the
value, produced when the motor is started with a normal starter.
Fig.1: Motor current-speed characteristics for Normal/ Soft starter
Fig.2: Motor torque-speed characteristics for Normal/ Soft starter
The SCR or TRIAC must be able to
control the current applied to the motor at line voltage. To get a high degree
of reliability, these SCRs or TRIACs must be rated 3 times the line voltage.
Thus the SCR-diode or SCR-SCR combination is usually used. The SCR-SCR method
provides a symmetrical output and thereby reduced harmonics, whereas the
SCR-diode combination gives an inferior output, but is cheaper in cost and easy
to implement. The 3-pulse technology uses a SCR-diode combination whereas a
6-pulse technology uses a SCR-SCR combination.
