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Auto Transformer Starter
Operation
At starting, the auto transformer reduces
motor terminal voltage to 50, 65, or 80
percent of line voltage. For a timed inter-
val, a segment of the transformer coil is
connected in series (as an inductor) with
the motor. Then a contactor automatically
connects the motor across the line and
shorts out the inductor.
Wye Delta Starter
Operation
Initially the starter connects the motor
on a Wye connection, reducing voltage
across the winding to approximately
58 percent. Starting torque and current
is about 33 percent of across the line.
After a time delay the motor is
connected Delta.
Part Winding Starter
Operation
Part winding starters apply starting
current in timed steps to part winding
motors. Two step controllers apply volt-
age through one starter to one motor
winding followed by the second starter
which connects voltage to the second
winding. The timer may be adjusted to
automatically close the second controller
at the most favorable time to minimize
voltage fluctuations.
306 CSI Section 16160
Siemens Electrical Products and Systems
Specification Guide
AC Controls
15
General
AC Controls
Reduced Voltage Starters — Electromechanical
Typical Speed Torque Curves
Auto Transformer Starter
Wye Delta Starter Part Winding Starter
Table 15.35
of Reduced Voltage Motor Starting
Comparison of Various Electromechanical Methods
50%
Tap
25%
First in order of
smoothness
30 seconds
Based on NEMA
medium duty
transformers
65%
Tap
80%
Tap 2 Step Wye(Star) Delta
Reduced Voltage Starting
Autotransformer
ቢ
Part Winding
Characteristics
Starting current drawn from line as % of
that which would be drawn upon full
voltage starting
ባ
Starting torque developed as % of that
which would be developed on full
voltage starting
Smoothness of acceleration
ብ
Allowable accelerating times (typical)
ቢ
Closed Transition.
ባ
Full voltage start usually draws between 500% and 700% of full load.
ቤ
Approximate values only. Exact values may be obtained from motor manufacturer.
ብ
Solid state starters provide the smoothest starting characteristics.
42% 64% 33%
33%
64%
ቤ
45%
ቤ
Third in order of
smoothness
2-3 seconds
Limited by
motor design
45-60 seconds
Limited by
motor design
Second in order of
smoothness
Increased slightly with
speed