Reduced Voltage Autotransformer Type Starting
General:
The autotransformer starter is classified as a reduced voltage starter. It is a device with which the applied
motor voltage can be reduced below that of the line voltage. Both motor starting current and torque,
therefore will be reduced below those values obtained with across-the-line starting.
Any standard three phase induction motor may be used with an autotransformer starter. The starter
portion of the autotransformer start connects the motor leads to the reduced voltage output winding of the
autotransformer. After a pre-set time delay (normally 10 to 20 seconds) the starter connects the motor
leads to the full line voltage.
Operation:
Ingersoll-Rand uses the closed-circuit (Korndorfer) transition type. For closed circuit operation, a voltage
is continuously applied to the motor terminals from the moment of reduced voltage starting and during the
switching to full line voltage operation.
Advantages:
Starting torque per starting amp ratio equal to that of across-the-line starter.
The most desirable starting current and starting torque can be selected by means of reconnecting the
motor leads to the 50%, 65%, or 80% output taps of the autotransformer. The characteristics of the motor
load and allowable accelerating times establish the best connection.
% Tap % LT %LRA
50
25
27
65
42
45
80
64
66
Where:
%LT= Starting torque expressed as a percentage of the value encountered during across-the-line
starting.
%LRA= Starting current drawn from the power lines expressed as a percentage of the value encountered
during across-the-line starting. This value includes the approximate required autotransformer
magnetization current.
NOTE: Both % locked torque and % locked rotor current vary approximately as the square of the voltage
Limited motor noise and vibration during starting.
On the closed circuit transition type starter, voltage transients during the transition period are minimized
which reduces the possibility of unacceptable performance of other electrical components within the same
plant.
Caution:
The autotransformer output tap may have to be changed to a higher percentage voltage value if the load
torque and/or inertia exceeds that which the motor can accelerate within the required starting period.
The transformer from reduced voltage to full voltage operation should be delayed until the motor speed is
high enough to ensure that the current change during switching will not exceed power company
requirements.
The starter as well as the motor should be evaluated for applications requiring frequent starting. For
autotransformer starters, NEMA states that one 15 second starting period for every four minutes for a
total of four per hour is acceptable. The majority of standard induction motors are capable of four 15
second starting periods per hour.
On the open-circuit transition type, line voltage transients can result during the transition
period due to sudden current changes.