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agility
TM
Series Soft Start User Manual
MAN-AGY-001. Version 04. 21/09/2018
proportional to the square of the applied voltage, the starting torque follows the same pattern
giving the characteristic smooth, stepless start of the soft-starter.
Running Induction Motors
Once a start has been completed the motor operating efficiency becomes of interest. When
working at or near full load, the typical 3-phase induction motor is relatively efficient, readily
achieving efficiencies of 85% to 95%. However, as shown below, motor efficiency falls dramatically
when the load falls to less than 50% of rated output.
In fact, very few motors actually experience consistent fully rated operation, the vast majority
operate at much lower loads due to either over-sizing (a very frequent situation), or natural load
variations. For Fan and Pumping applications, the affinity laws will allow the inverter drive to show
very considerable energy savings over virtually all other methods of control through varying the
speed of the motor in response to changes in load. Where motor speeds cannot be varied, an
optimising version of semiconductor motor controller, such as agility
TM
will also produce energy
savings in lightly loaded motors. Less sophisticated systems of soft-starter remain at full
conduction and the motor then behaves as if it were connected directly to the mains supply.
However, at light loads and mains voltages, induction motors always have excess magnetic flux,
and efficiency loss and power factor degradation result. By detecting the load at any instant, and
adjusting the motor terminal voltage accordingly, it is possible to save some of the excitation
energy and load loss, and improve motor power factor when the motor is running inefficiently at
light loads.
All agility
TM
soft-starters are microprocessor controlled, and this gives them a number of
advantages. Firstly, there are no adjustments to be made for the energy saving function: all
calculations necessary to find the best degree of phase-back of the thyristors for any load
condition is made by the microprocessor. Secondly, the start always synchronises with the supply
voltage and a special structure of turn-on pulses virtually eliminates the inrush currents
Applications (continued)