106
agility
TM
Series Soft Start User Manual
MAN-AGY-001. Version 04. 21/09/2018
reduced transmission 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-
Applications (continued)