Please note that compressors with additional steps do not start totally unloaded but
on one and two cylinders respectively.
3.4.5 Oil heating
Oil heating is not a configuration point, but it has various functions depending on whether the
compressor in question is configured as a reciprocating or a screw compressor.
For a reciprocating compressor the heating element output is powered by decreasing oil tem-
perature. This function is active both at compressor standstill and when the compressor is in
operation.
The set point of this value is set in the picture Control values/Oil/Temperature/Control, Set
point 2. The set point has a fixed hysteresis of 5K.
SMC/CMO: Set point 2 = 35°C: Heating starts at 35°C and stops at (35+5) = 40°C.
HPC/HPO: Set point 2 = 55°C: Heating starts at 55°C and stops at (55+5) = 60°C.
3.4.6 Rota (VSD)
A reciprocating compressor can be fitted with a frequency converter (VSD) for additional ca-
pacity control. In Johnson Controls Denmark documentation this unit type is called a RecipRo-
ta. The cooling capacity is a combination of the number of engaged steps and the rotation
speed set by the frequency converter.
A RecipRota can be controlled in two ways:
• With limited frequency control where all steps are engaged before the frequency con-
trol is enabled, i.e. increased from minimum speed.
• With full frequency control on (between) all steps.
In the Setup/Configuration/Drive menu (see subsection 6.4.1. Drive) the rota method can be
configured to either the step (limited) or speed (full) control method.
Note: The Speed configuration is only recommended for compressors with Centa coupling.
For compressors with other coupling types VSD operation might cause torsional vibration
problems at some part load frequencies (especially when not all steps are engaged).
The Speed configuration is not available for TCMO and TSMC reciprocating compressors.
With VSD operation the capacity step in/out is controlled by the PID or I capacity regulator
following the set point signal. The Delay Up and Delay Down timers are not used. To make
sure that the compressor cannot engage the cylinders too fast, a ramp function will limit the
maximum capacity (Yield) change to 2.0%/sec for step up and 3.0%/sec for step down. This
means that a yield increase from 0% to 100% will take minimum 50 seconds, and a yield de-
crease from 100% to 0% will take minimum 33 seconds.
The step method is the standard Rota control method for reciprocating compressors. It
works this way: The compressor starts with the first step engaged then it ramps up to mini-
mum speed. It remains at minimum speed until all steps are engaged and not until then, the
speed can be ramped up to maximum.
With a setting where minimum speed is set to 700 [rpm], motor rpm min. 2 is set to 700
[rpm] and maximum speed is 1500 [rpm], the reciprocating compressor will be step regulated
up to 47% and then speed regulated between 47% and 100%.
With a setting where minimum speed is set to 500 [rpm], motor rpm min 2 is set to 700
[rpm] and maximum speed is 1500 [rpm], the reciprocating compressor will be step regulated
up to 47% and then speed regulated between 47% and 100%. But in this case the compres-
sor can run at a lower minimum capacity.
Compressor control and surveillance
118/319
Engineering manual - Unisab III 1.10
001930 en 2014.09
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