There are two very basic ways of determining the correct point at which
the control gear should change the winding configuration from 'star'
(starting mode) to 'delta' (running mode). Both methods determine the
point at which the rotor has achieved it's maximum speed in star, and
therefore the point at which the starter should change to delta.
One
is by current measurement: Firstly, set the delta timer to it's maximum
setting. Put a clip on (tongue tester) ammeter on one of the lines
FEEDING the starter. Set a stopwatch going when you press the start
button. Watch the ammeter - it will peak immediately on start up, then
the current will drop off as the load accelerates. As soon as the
current steadies off, stop the watch. At this point (approximately 85%
full load speed) the motor can achieve nothing more by remaining in
star, and this is the latest point in time that delta changeover should
be made.
The other simple way to determine optimum change over
time is to use a tachometer on the motor shaft (mechanical or optical).
Again, use a stop watch. Observe the Tacho. Acceleration characteristics
will vary dependent on the driven load, but the speed will settle out
(again at approx. 85% full load speed. Stop the watch at this point. Set
the timer to the time recorded on the stop watch.
In star Delta
usually we consider that the load and motor torque are equal when the
motor reach 75-85% of the speed. At this time you stop the star
connection to go with delta connection, the time is around 30 to 50
milliseconds, to avoid short-circuit. But when the motor is connected in
delta, at start you've got a temporary current peak due to back
electromotive force.
So to avoid this you have different option,
by these, you can delay of 1 or 2 seconds the time between star and
delta, but it depend on inertia of the load, another solution is to put a
resistor in series with the delta connection for 3 seconds to limit the
current peak, another is to put the resistor directly in series with
the motor at star disconnection.
These methods determine the
LONGEST period a motor can be left in 'star'. High inertia loads should
be allowed to reach this point.
Different applications i.e fan
motors or centrifugal pump motors which run up to speed with closed
dampers or valves can be switched into 'delta' much earlier, as the only
load being accelerated is the physical mass of the impeller or blade.
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