Airedale OptiChill FreeCool 500kW - 1365kW - Manual (Page 20)

OPTICHILL
™
Chillers
20
Chillers
Technical Manual : 6530166 V1.6.0 02_2013
Design Features & Information
STAR/DELTA STARTING OF 3 PHASE COMPRESSORS
Open Transition Starting In conventional open transition Star/Delta starting, a run up time of 2 seconds is required,
however, a changeover delay of approximately 50 ms is usually observed when switching
over from Star to Delta motor connections.
During this switch from Star to Delta, the motor is completely disconnected from the
supply and, depending on the counter torque, its speed decreases to some extent.
When the Delta step is switched on, it may be that the supply phase angle and the
magnetic field in the motor are in opposition to each other, which will lead to transient
phenomena causing high changeover current peaks.
Typical current waveform for standard screw compressor Star Delta start:
Star 50ms Delta
Sta
rting
Curren
t F
a
c
tor
2.5
1.5
0.5
-0.5
-1.5
-2.5
Start Up Time
Closed Transition
Starting In closed-transition Star/Delta starting, the motor is Delta-connected to the network via a
transition contactor and resistors after the run-up in Star of 0.7 seconds switching over to
the normal Delta step without disconnection from the supply. Thus no switching interval
during which the motor is disconnected from the supply occurs, and high transient
changeover current peaks are avoided.
Typical current waveform for closed transition start of a screw compressor:
Star 50ms Delta
Sta
rting
Curren
t F
a
c
tor
2.5
1.5
0.5
-0.5
-1.5
-2.5
Start Up Time