Star-delta start (Y/Δ)

Star-delta start is possible with motors that can be operated in delta connection at the nominal supply voltage. In addition, the motor windings must be open, i.e. the ends of the windings must be accessible at the terminal plate. 3 contactors are required in the main circuit of the motor.

The motor windings are operated in star connection during compressor start and in delta connection after a certain time delay. The start is divided into 3 phases:

  1. Star phase: Time in which the compressor motor starts in star wiring. This time from switching on the compressor to switching to delta operation should be sufficient to reach 75 .. 80% of the nominal speed.
  2. Transition break: Time during which the motor switches from star to delta operation. During this time, the compressor is briefly voltage-free and its speed decreases. The transition break influences the position of the voltage vectors in the motor and thus directly the height of the switching current peak. The duration of the transition break is either determined by the reaction time of the contactors or set by programming. The transition break must not be too long, otherwise the compressor speed will reduce too much. If, on the other hand, it is too short, the switch-off arc of the star contactor will not be extinguished and a short circuit may occur.
    In order to reduce this second current peak,
  3. Delta operation: The compressor motor continues to run in delta wiring.

At standard conditions and with locked rotor, star-delta starting reduces the starting current to approx. ⅓ compared to direct-on-line start in delta wiring, as the voltage at the windings is smaller by a factor of √3.

Starting current (peak value) for a compressor model CSH8593-140Y with star-delta start (IEC connection) and start unloading (i.e. slider position at 25%).
Starting current (peak value) for a compressor model CSH8593-140Y with star-delta start (IEC connection) and start unloading (i.e. slider position at 25%).

Since the torque is proportional to the square of the voltage, the starting torque for star-delta start also drops to ⅓. If the motor is to start in this wiring, the load torque must not exceed ⅓ of the torque for direct-on-line start. Therefore, additional start unloading is often necessary (Additional start unloading (SU)).