Johnson Controls SABROE Unisab III Control User Manual

3.3.24 Rotatune (VSD)
A screw compressor can be fitted with a frequency converter (VSD) for additional capacity
control. In documentation from Johnson Controls Denmark, this unit type is called a “Rota-
tune”.
The cooling capacity can be a combination of the rotation speed set by the frequency convert-
er and the capacity slide position. This is the case if the compressor is equipped with capacity
slide control. For compressors not equipped with capacity slide control, the cooling capacity is
given only by the rotation speed set by the frequency converter.
See the compressor descriptions in section 3.3. for more information about the screw com-
pressor types covered by Unisab III.
Most screw types have capacity slide control for regulation purposes and for start-up torque
relief. Some types do not have a capacity slide but e.g. a relief valve handling start-up torque
relief.
Check the following parameters to set up a screw compressor for VSD operation
The following Rotatune parameters must be configured in the Setup/Configuration/Drive
menu (see subsection 6.4.1. Drive):
• Rotatune and Drive Com is set to define VSD and operation.
• Motor rpm min and Motor rpm max are set to define minimum and maximum motor
speed.
• Forced speed is set to ensure sufficient oil pressure at compressor start-up.
• Drive cap limit is only set in special cases.
The minimum slide position during operation must be configured to ensure sufficient oil pres-
sure at low speed (see subsection 6.4.3. Compressor block).
• Manual zero - is set automatically for some compressor types.
Set the Ramp function in the frequency drive in a proper way.
Set the capacity regulator parameters in Unisab III to values which are appropriate for the
actual process (see subsection 3.1.7. Regulator settings).
• Decrease or increase the proportional band - to make the regulator respond faster or
slower.
This will affect both the capacity slide and the speed control.
Adjust the throttling valves in the capacity block in a proper way (see subsection 3.7.1. Ca-
pacity slide).
• Adjusting slide speed - to fit the capacity regulator Run time settings.
Unisab III can handle VSD operation in both manual and auto/remote operation
In manual operation, when the start button is pushed the compressor will start and the
drive will jump to 'Forced Speed' to achieve oil pressure. At the same time the capacity slide
valve will move up to the manual zero point which is, say, mechanical 30%.
When main oil pressure has been achieved, the speed will fall back to minimum speed. The
slide valve should now have increased to the set Manual zero point and the display will show
0%. When the 'Cap Up' button is pushed, the slide valve will start to move up against maxi-
mum capacity slide position. The speed will stay at minimum until the capacity slide is at
maximum, then the drive will start to increase speed.
Compressor control and surveillance
Engineering manual - Unisab III 1.10
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Johnson Controls SABROE Unisab III Control Specifications

General

Brand	Johnson Controls
Model	SABROE Unisab III Control
Category	Control Systems
Language	English