CB-110-436
6 Control functions
Graphical explanations of the control functions de-
scribed in the following sections, see figure 18, page
37.
Compressor start and STO
The compressor can only be started when it is released
(STO input closed), the start command is given (DI1
closed) and the "start to start interval" (timer P0-17)
and the "stop to start interval" (timer P0-18) (5s) of the
compressor have elapsed.
For control by means of external setpoint signal and
control characteristic ''0...Max'', a setpoint >1% must
also be present.
As soon as the start command is cancelled (DI1), the
compressor is stopped via the ramps.
If the release is cancelled (STO), however, it is directly
switched off.
"Ramps"
To optimise the operating behaviour, the speed ramps
of the compressor can be adjusted (ramp up P1-03 and
ramp down P1-04). For slow reacting systems such as
water chillers or refrigeration systems with only one
evaporator, we recommend selecting long ramp times
while compound systems may require shorter ramps.
Apart from the limitation of the external setpoint, these
ramps define the frequency change for the "Force"
function.
To start and shut off the compressor, i.e. below the
minimum frequency, the optimum ramps are fixed in
the VARIPACK frequency inverter and therefore ensure
a safe and reliable compressor start.
"Start to start interval" (P2-21) (cycle time between
two compressor starts)
The compressor start activated by a VARIPACK fre-
quency inverter results in a lower mechanical stress on
the compressor drive gear and less heating of the mo-
tor compared to a direct-on-line start. A greater number
of compressor starts per hour are permissible this way.
An excessive number of compressor starts, however,
should be avoided.
The "start to start interval" limits the maximum number
of compressor starts. If the defined time since the last
compressor start has not yet elapsed, the compressor
start is delayed. If the compressor is operated for a
longer period than the "start to start interval", it can be
restarted immediately after the shut off. As standard,
the minimum "start to start interval" recommended for
the compressor is set. It can be extended to any de-
sired period in order to improve the operating perform-
ance of the system and to increase the system effi-
ciency by avoiding very short cycles. If required, shorter
time values than the preset and recommended ones
can be configured.
Relay "Compressor running" (RL2)
The relay "Compressor running" indicates the operating
condition of the compressor. It can be used, for ex-
ample, for switching off the oil heater or as a signal for
the electronic expansion valve. The relay is switched
on after a successful compressor start and after having
reached the minimum frequency, and is switched off as
soon as the frequency falls below the minimum value.
"Hold time on hold frequency after start" (P2-04
und P2-26)
The feature "Hold time on hold frequency after start"
can be used for different purposes.
Ensuring the oil delivery after the compressor start:
• For reciprocating compressors for transcritical ap-
plication and scroll compressors, it is recommended,
to operate with a higher frequency after the com-
pressor start to ensure an immediate oil delivery
within the compressor.
• Recommended configuration for transcritical recip-
rocating compressors: Hold time on hold frequency
after start ≥ 10 s, hold frequency after start ≥ 40 Hz
• Recommended configuration for scroll compressors:
Hold time on hold frequency after start ≥ 60 s, Hold
frequency after start ≥ 50 Hz
Optimizing the operating behaviour:
• In temperature-controlled systems with only one
compressor, the frequency of the compressor gets
often increased fast after the start due to the high
setpoint deviation.
• This increase of the capacity might be unnecessary
for covering the cooling demand and lead to short
and inefficient operating cycles. This can be avoided
by enforcing the compressor to run with the min. fre-
quency after start. In case of low load conditions, the
min. capacity of the compressor might be sufficient
for reaching the target temperature. This allows to
optimize the operating duration as well as temperat-
ure differences of the heat exchangers. Only in case
the target temperature isn't reached after the "Hold
time on hold frequency after start" counted down, the
capacity of the compressor will be increased.
• Another issue might be, that a fast power up leads to
significant pressure drops in the low-pressure circuit
when the pressure cannot be regulated quickly
enough by the expansion valve. Also that can be
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