Operating instructions
60
The compressor speed is regulated by the control of the
voltage/frequency values applied to the motor. The voltage
applied, generated by pulse width modulation (PWM), is
proportional to the frequency.
Decreasing the compressor speed makes the machine more
susceptible to surging; the prevention and protection
algorithms against this phenomenon interact with the
control of the speed-vane couple.
Deviation of the water temperature parameters measured,
from the assigned “Control point” value and the“Chilled
water deadband” parameter is calculated in the “Guide Vane
Delta” parameter. This parameter leads to an adjustment of
the machine capacity by a simultaneous and coordonated
action on the guide vane opening and the compressor speed
(“Target VFD Speed”).
“Normal” operating mode: The capacity is primarily
controlled by the variation of the compressor speed within
the limits defined by the “VFD Minimum Speed” and
“VFD Maximum Speed” parameters (default settings 70
and 100%). If the capacity reduction objective permits, the
guide vane opening is maintained at 100%. If the“VFD
Minimum Speed” is reached, capacity reduction is obtained
by reducing the guide vane opening.
Operating mode to prevent surging: If the PIC III detects a
surge risk, the prevention logic requests an increase of the
compressor speed up to the risk zone outlet defined in the
dP-dT diagram. The capacity reduction is controlled by
reducing the guide vane opening.
Operating mode to protect against surging: Effective surging
is detected by the compressor motor current fluctuations.
In these conditions the compressor speed is increased. The
logic that requests the compressor safety shutdown in case
of prolonged surging is the same as the one for a standard
machine without variable-frequency drive. Please also
refer to the 19XR PIC III controls manual.
The power module of the variable-frequency drive is
equipped with a heat exchanger to provide cooling, using
the refrigerant in the machine. The PIC III control regulates
the temperature by ON/OFF change-over of the valve (35)
on the cooling pipes.
ATTENTION: The typical diagram is not contractually
binding: refer to the machine PID.
The reading reported by the relative humidity sensor is
interpreted by the PIC III in order to maintain the tempe-
rature set point at a value that prevents condensation
formation in the power module.
1. FLASC chamber
2. Condenser water
3. Condenser
4. Condenser isolation valve
5. Transmission
6. Diuser
7. Guide vane motor
8. Motor
9. Guide vanes
10. Impeller
11. Compressor
12. Cooling line isolation valve
13. Oil cooling
14. Oil lter
15. Oil pump
16. Stator
17. Rotor
18. Motor cooling valve
19. Linear oat valve chamber
20. Filter drier
21. Oriced tting
22. Moisture/ow indicator
23. Oriced tting
24. Thermostatic expansion
valve (TXV)
25. Distribution pipe
26. Cooler isolation valve
27. Evaporator
28. Chilled water
29. Refrigerant liquid
30. Refrigerant vapour
31. Refrigerant liquid/vapour
32. Oil
33. Cooling line isolation valve
34. Heat exchanger, VFD
cooling
35. Solenoid valve, VFD
cooling control