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AN INTRODUCTION TO SOFT STARTERS.
Reduced Voltage Starting of AC Induction Motors is normally used to reduce the starting current drawn by the motor.The Induction Motor has two prime functions to perform in industry:
- To convert electrical energy into mechanical energy in order to accelerate the motor and load to the operating speed (this is the starting function).
- To convert electrical energy into productive work output from the machine (this is the operating or work function).
Both the starting characteristics and the full load characteristics are very important in the selection and specification of the motors.
Motors consist of two major sections: The stator and the rotor.
In operation the induction motor performs as a transformer with current induced into the rotor by flux from the stator.
By variation of the winding configuration and the contour of the stator laminations, the full load characteristics are determined whilst the number of ‘poles’ governs the final speed of the motor at full voltage.
The rotor conductors often referred to as the ‘cage’ which combined with the conductor shape, material and position within the rotor, determine the starting characteristics.
At very low speeds the dominant impedance is that of the rotor whilst at full speed the stator impedance can become significant. Thus the rotor determines the starting characteristics of the motor, while the stator has influence on the full speed characteristics.
When voltage reduced starting is employed the torque available from the motor is reduced by the square of the current or voltage reduction. The initial starting torque requirement is the break away torque, whilst once the motor has begun to rotate the torque becomes the sum of the ‘work torque’ and ‘accelerating torque’. The ‘work torque’ is that required to overcome the mechanical work being done by the machine and includes frictional and windage losses, while the ‘accelerating torque’ is that which accelerate the load or machine to full speed.
When considering direct on-line starting it is a common misconception that starting the motor under light load will reduce the starting current. This is totally incorrect since the starting current is dependant on motor design, rotor speed and stator voltage. The load will only influence the time taken for the motor to reach full speed i.e. The current/speed curve of the motor is independent of all external influences other than stator voltage.
Thus the motor when started direct on-line will, initially, always draw locked rotor current and for the majority of squirrel cage induction motors this does not begin to fall significantly until the rotor speed is in the order of 70 – 80% of full speed.
A popular alternative to direct on-line starting is Star Delta starting which has been in use for many years as a reduced voltage starter.
The motor is initially connected in a star configuration and after a pre-set time, reconnected into a delta configuration. The current and torque in the star configuration are one third of full voltage current and torque but of course cannot be controlled.
It is not uncommon for the changeover from star delta to occur at some intermediate speed resulting in very high transients of both current and torque at the point of changeover, with the resultant problems such as disturbance on the mains network, and/or excessive wear on the mechanical transmission systems like couplings, belts, gear boxes etc.
