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.