Part winding start (PW start, Y/YY or Δ/ΔΔ)
Part winding start is possible with motors whose stator winding is divided by design into 2 halves with 3 windings each. Both winding partitions lie parallel in the stator slots. During start, they are switched on one after the other, with a time delay of e.g. 0.5 .. 1 s.
2 contactors are required in the main circuit of the motor, which are switched on one after the other:
- contactor for 1. part winding (PW1)
- contactor for 2. part winding (PW2)
In contrast to a star-delta start, there is no transition break in which the motor is voltage-free.
If start unloading is used, it can be switched off after the 2. part winding has been switched on.
Typical winding partitions (given by design) are
- 60% / 40% (starting current is only 75% of that for direct-on-line start) or
- 50% / 50% (starting current is only 65% of that for direct-on-line start).
The winding partitions can further be connected internally in different ways (winding type). This can no longer be changed in a given motor and has no practical effects on connections, arrangement of the wiring, etc.
- YY: Each winding partition is connected in itself in star wiring (Y), the part winding start is represented as Y/YY.
- ∆∆: Each winding partition is connected in itself in delta wiring (∆), the part winding start is represented as ∆/∆∆.
Comparison of starting currents
In contrast to a star-delta start, the voltage is not interrupted during transition, neither is the torque, and a further current peak is largely prevented. The starting current indicated in the Bitzer Software is the root mean square (RMS) value of the current. To derive this from the oscillogram, the current must be divided by √2.
For Bitzer CS. compressors, the starting currents can e.g. be as follows.
Part winding start:
![Starting current (peak value) for a compressor model CSH8593-180Y with part winding start (winding partition 50% / 50%, winding type Δ/ΔΔ) and start unloading (i.e. slider position at 25%).](../Images/png/9007199662869003__Web.png)
Star-delta start:
![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%).](../Images/png/9007199662870923__Web.png)
Another advantage of a PW start compared to a star-delta start: Only two smaller contactors are required, which significantly reduces expense and space required for the electrical system. As with star-delta start, additional start unloading is usually required (Additional start unloading (SU)).