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Synergy Series Soft Start User Manual
MAN-SGY-017. Version 01. 27/06/2016
189
8.3.4 Electro-Mechanical Methods Of Starting (continued)
equipment life and higher risk of motor failure, particularly when frequent starting and stopping is needed.
In larger sized motors special strengthening is necessary, at higher cost, before they can be safely used with
direct-on-line starting. However, the shortcomings of the direct-on-line starter have been recognised ever
since motors have been used and alternative systems have been developed over the years to
reduce the damaging effects of this form of control.
Method B: Star-Delta and other Reduced Voltage Starting Systems
Reduced voltage starting makes use of the fact that motor torque is proportional to the square of the
terminal voltage; the most familiar type of reduced-voltage starter is the star-delta starter. Consisting of
three contactors and a time switch (which can be mechanical, pneumatic, electrical or electronic), the star-
delta starter changes the motor winding configuration from an initial star connection to a delta as the
motor accelerates. The change-over or transition point is controlled by the time switch and is usually
arranged to be approximately at 80% of full speed. The effect of starting in star is to alter the voltage across
each stator winding to 58% of normal. This reduces the starting torque to a third of locked rotor torque
(LRT) with a consequent reduction in starting currents and acceleration forces.
Although an apparent improvement over the direct system, significant disadvantages still remain. The
transfer from star to delta momentarily removes the motor from the supply. During this time the motor is
under the mechanical influence of the rotating load and, at the instant of disconnection, current will still
flow in the rotor bars due to the time delay necessary for the magnetic flux to die away. Therefore, there is
a residual flux “frozen” on the surface of the rotating rotor, which cuts the stator windings, generating a
voltage whose frequency depends on the rotor speed. If the load inertia is small, such as in a pump, or if
the friction is high, there could be a significant loss of speed during the time the supply is disconnected.
In this case, when the reconnection to delta is made, a large phase differential can exist between the supply
and the rotor fluxes. This can give rise to very large current surges (as much or more than full-voltage
locked rotor current), together with massive transient torque oscillations, which can peak at levels in the
region of fifteen-times full-load torque. Although the effects described are only present for a very short
period of time (about one fifth of a second), they are sources of great stress and damage to the whole drive
system, and where frequent starting is necessary, invoke high maintenance costs. The current surges, in the
form of a very high level short duration “spikes”, are an increasing problem in these days of computer
control systems and other “sensitive” electronic equipment. The voltage disturbance on the supply is very
difficult to filter out and can cause severe problems, especially when larger motors are involved.
There are methods of control, for example, the Wauchope starter, which eliminate or reduce the
reconnection transients. However, such starters are expensive and have reliability implications; for these
reasons they are not widely applied.
The star-delta starter also has disadvantages due to the restricted starting torque available (if you need
40% LRT to break-away, you can only increase the motor size, or revert to direct-on-line). Combined with
the severe effects of the re-switching surges, and the additional costs of bringing six conductors from the
motor to the starter instead of only three, star-delta only offers an imperfect solution to the problem of
starting the induction motor.
Method C: Primary Resistance Starter
It has long been recognised that the transition step in the star-delta system was a source of problems such
as welded contactors, sheared drive shafts etc., and for many years a method of stepless control has been
available in the form of the primary resistance starter.
8. Applications (continued)