Wye-Delta Starting
General:
Wye-Delta starting is a method of increment starting used with a three phase A.C. induction motor to
reduce the initial values of motor starting current and torque compared to those values obtained with
across-the-line starting.
Both end of each phase winding are brought into the motor conduit box. The starter is designed to
connect these windings in Wye on the first step. After a preset time delay, the starter will disconnect from
the Wye configuration and reconnect in Delta for continuous operation.
The voltage impressed across each phase of the motor winding during the first step (Wye connection) of
a Wye-start Delta-run motor is lower than the voltage which would be impressed across each phase of
the motor if across-the-line starting were used. This lower voltage results in lower starting current and
torque (Refer to Figure 1).
Operation:
Ingersoll-Rand uses the open-circuit transition type. The other type is a closed-circuit transition. Both
types connect the motor windings in Wye on the first step. After a predetermined time interval, the timer
causes the starter contactors to reconnect the motor windings in the Delta or "run" connection.
Advantages:
1. Starting torque per starting amp ratio equal to that of the across-the-line starter.
2. Starting current is reduced to approximately 33% of the values encountered, if the motor were started
across-the-line.
3. Soft start - Starting torque is approximately 33% of the value encountered if the motor were started
across-the-line. This low starting torque is often desirable to softly accelerate loads having high inertia and
low retarding torque (Example - centrifuges and unloaded compressors). The low starting current during
the starting period allows the motor to withstand a longer acceleration time than an equivalent sized acrossthe-line start motor. The Wye-Delta motor, however, will accelerate only slightly more inertia than the
across-the-line motor, since the thermal capacity of both motors is the same. To accelerate a specific
inertia, the Wye-Delta motor will produce a lower temperature rise within the motor than an equivalent size
across-the-line motor because the longer acceleration time allows the heat in the motor to be efficiently
dissipated to the housings and surrounding atmosphere.
4. Can be adapted to most 2, 4, 6 and 8 pole motors.
5. May be started as frequently as an across-the-line starter if the retarding torque of the load is negligible.
6. Limited motor noise and vibration during starting.
7. On the open-circuit transition type, no resistors are required.
8. On the closed-circuit transition type, voltage fluctuation during the transition period is minimized.
Caution:
1. Motor started on Wye connection and operated on the Delta connection must be specifically designed for
Wye-Delta. Motors rated 4000 volts and higher are usually not suitable for Wye-Delta starting.
2. The will not start if the torque demanded by the load exceeds that developed by the motor on the Wye
connection. When the motor is connected in Delta, normal starting torque is available to start the load.
3. The transfer from Wye to Delta should be delayed until the motor sped is high enough to ensure that the
current change during switching will not exceed power source requirements. Generally, the starter timer
should be set so that switching from Wye to Delta occurs at 80-90% of full load speed.
4. On the open-circuit transition type, line voltage fluctuation can result during the transition period due to
sudden current changes.