Fairford synergy, User Manual

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Synergy Series Soft Start User Manual
MAN-SGY-017. Version 01. 27/06/2016
188
Figure 8.3.4: Torque/Speed Curve – High Starting Torque
However, most induction motors are designed to have a “standard” characteristic that provides a
compromise between starting torque and operating efficiency. To summarise, an induction motor will only
start and accelerate when it produces more torque than the connected load absorbs. This is true for all
speeds - including standstill and full speed.
8.3.3 Starting Induction Motors
Starting a de-magnetised induction motor from standstill is a demanding and complex process. At the
instant of switching all the energy necessary to magnetise the motor, to provide the acceleration force, and
to supply the kinetic energy of the rotor and load, must be present together with the energy to overcome
the mechanical and electrical losses. To do so at full supply voltage places considerable stresses on the
supply, the motor windings, and the iron cores of the stator and rotor. Excessive acceleration of a rotor
when the mechanical load is small can produce torque oscillations in the shaft causing severe wear to
transmissions, gears and drives. Excessive acceleration when the load inertia is high such as in centrifugal
fans, causes belts to slip in the pulleys, producing rapid wear and early failure.
8.3.4 Electro-Mechanical Methods Of Starting
Method A: Direct-on-Line
The most simple means of controlling energy flow to an induction motor is to interrupt the power supply by
a single, solenoid operated, 3-phase switch, known as a contactor. Very widely applied, the method is
known variously as “direct-on-line”, “across-the-line”, “direct” etc., and is the usual form of control where low
cost is the first, and most important consideration. As a result, it is most often used on small motor sizes
(up to approx. - 22kW), or where the supply is strong enough to withstand the inrush and starting current
surges without causing unacceptable voltage drops.
The harsh, damaging effects described earlier are all imposed by direct-on-line starting and, as a control
method, it is the most destructive of equipment. Its simplicity and apparent low cost, although attractive at
first sight, hide large cost penalties in the shape of increased maintenance, reduced transmission
8. Applications (continued)