KT-420-4 // PDF Download
38
!
!
NOTICE
Compressor and motor damage!
Do not combine a frequency inverter with mechanical capacity control of the compressor! Especially at low
speed, adequate motor cooling is not guaranteed because refrigerant mass flow is heavily reduced.
Electrical power consumption at full load is slightly higher than when operating the compressor directly on the
mains supply. This is due to losses in the frequency inverter – caused by the losses of individual electronic com-
ponents for power conversion and for cooling the frequency inverter. Another source of motor heat-up and reduced
motor efficiency are harmonics: The higher the quality of the frequency inverter and the better it is configured, the
lower the harmonic distortion factor in the output signal.
There are several variables involved in the operation of the inverter which affect the running and starting of the
compressor:
• The voltage characteristic limits and regulates the current supplied to the motor,
• the switching frequency of the converter in the frequency inverter regulates the motor performance and reliabil-
ity,
• the start sequence and voltage boost control the starting process of the compressor.
In general, however, losses caused by the frequency inverter are normally offset by gains in system efficiency by
operating at a more efficient cycle through matching compressor capacity to system load requirements. Inverter ap-
plications will thus usually increase overall system efficiency under “real world” conditions.
In order for the motor to always operate at its nominal operating conditions, a control mode with constant voltage-
frequency (U/f) ratio is chosen at the frequency inverter.
3.2
Application range
For a safe compressor operation with frequency inverter, the following limiting factors must be strictly observed:
• minimum and maximum frequency (see below)
• maximum motor temperature
• maximum discharge gas or oil temperature and/or pressure difference (p
C
– p
O
)
• maximum and minimum discharge pressure
• maximum operating current of the compressor
• maximum evaporating temperature
• minimum pressure difference (p
C
– p
O
)
• minimum suction pressure (should be slightly higher than atmospheric pressure)
• sufficient additional cooling
These limits define the application limits and can vary according to frequency ranges and operating conditions.
Speed and frequency ranges
Compressor
Frequency range [Hz]
Speed range [min
-1
]
Standard motor
Semi-hermetic reciprocating compressors
ECOLINE
2KES .. 2FES
30 .. 70
900 .. 2050
40S
2EES .. 2CES
30 .. 70
900 .. 2050
40S
4FES .. 4BES
25 .. 70
750 .. 2050
40S
4VES .. 4NES
25 .. 70
750 .. 2050
40P
4VE .. 4NE
25 .. 70
750 .. 2050
40P
