Soft starters for Induction Motors

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.