Appendix B: Soft Starter Application Considerations
Page B–11Stellar
®
SR55 Series Soft Starter User Manual – 1st Ed, Rev F – 09/18/2019
b.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 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. 22 kW [30 hp]), 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 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 recognized 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: Wye-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 wye-delta, or
star-delta starter.
Consisting of three contactors and a time switch (which can be mechanical, pneumatic,
electrical, or electronic), the wye-delta starter changes the motor winding configuration from
an initial wye connection to a delta connection 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 the wye connection 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 wye 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 very high-level short-duration “spikes”, are an increasing problem for 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.
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