Lesson 4: Diagrams and controls for Reduce Voltage Starting 

Wye/delta

 Wye/delta: this starter consists of three contactors – main (for isolation); wye (lower voltage for start); and delta (higher voltage for run) – which provides for initial starting of the motor via a wye (“star”) connection, then a timed transition to a “delta” connection to produce motor full-rated speed. The wye connection provides for 58% of line voltage (V-line/1.73) across each winding, and since current varies as the square of the voltage and torque is proportional to current, current and torque are reduced to 33% during starting. A simple schematic is below: 

Auto-transformer 

Auto-transformer: in this method, an autotransformer supplies the motor via adjustable tap settings (typically 50%, 65%, and 80% of line voltage). This results in reduction in current at the motor terminals by the same amount. Line current is further reduced by the transformer tap ratio, ensuring that during starting the line current is always less than the current seen by the motor. The motor thus produces a level of torque consistent with a higher amperage than is actually drawn by the line, giving it a relatively high torque/amp rating. An example schematic is shown below: