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Section 2 • Operational Descriptions

Single Screw Compressor PLC Operation Manual • Vilter/Emerson • 35391CM

AUTO START-STOP

When enabled, the Auto Start-Stop feature will allow

the compressor to cycle on and off based on the con-

trolled pressure or temperature. The Anti-Recycle timer

still applies when Auto Start-Stop is enabled. Additional

variables that factor in to Auto Start-Stop are described

below:

• Auto Start Pressure/Temperature: Pressure or

Temperature at which the control will start the

compressor.

• Auto Start Delay: Amount of time that the Auto Start

Pressure/Temperature must be met before the com-

pressor is commanded to start.

• Auto Stop Pressure/Temperature: Pressure or

Temperature at which the control will stop the

compressor.

• Auto Stop Delay: Amount of time that the Auto Stop

Pressure/Temperature must be met with the capac-

ity slide at or below “Minimum Slide Position” before

the compressor is commanded to stop.

• Minimum Slide Position: Position the capacity slide

must be at or below before the compressor is com-

manded to stop.

When Auto Start-Stop control is enabled, it will function

as follows:

• When Auto Start Pressure/Temperature is met for the

Auto Start Delay Time, the compressor will be com-

manded to start.

• When the Auto Stop Pressure/Temperature is met

and the capacity slide has decreased to the Minimum

Slide Position for the Auto Stop Delay time, the com-

pressor will be commanded to stop.

ANTI-RECYCLE

After the compressor main motor stops, it is not allowed

to re-start again for a settable time. This is to protect the

compressor from damage by allowing oil to drain from it

before the next start. If the Anti-Recycle timer is active,

a banner will appear on the overview and menu screens

that shows the remaining time. If a start is initiated, the

compressor state indicator will indicate “Standby” until

the Anti-Recycle timer is done. If all other permissives

are met, the compressor will re-start.

CAPACITY SLIDE CONTROL

Reference examples, Figures 2-2, 2-3 and 2-4

While the compressor is running, the controller will au-

tomatically adjust the position of the compressor capac-

ity slide valve to hold the desired pressure or tempera-

ture. Three control methods are available:

• Discharge Pressure Control: Compressor loads to in-

crease discharge Pressure to desired setpoint

• Suction Pressure Control: Compressor loads to lower

suction pressure to desired setpoint

• Process Temperature Control: Compressor Loads to

lower process temp to desired setpoint. An instru-

ment to monitor the Process Temperature is required.

The capacity slide control of the PLC operates by periodi-

cally calculating an adjustment to the capacity slide po-

sition based on where the selected process variable is in

relation to the target setpoint. When the process pres-

sure or temperature is far away from the setpoint, capac-

ity slide position is adjusted by the deﬁ ned maximum.

As the control pressure or temperature gets closer to

setpoint, smaller adjustments are made to the capacity

slide position. All of the variables that factor in to this

calculation are described below:

• Target pressure or Temperature – the desired suction

pressure, discharge pressure, or process temperature

that the machine will try to maintain.

• Upper Proportional Deadband – Range above Target

pressure or Temperature where no capacity slide ad-

justment will be made.

• Lower Proportional Deadband – Range below Target

pressure or Temperature where no capacity slide ad-

justment will be made.

• Upper Proportional Band – Range above Target pres-

sure or Temperature where adjustments to capacity

slide position are made proportional to the differ-

ence between Target pressure or Temperature and

Actual. Outside this range, the adjustment made will

be the set maximum.

• Lower Proportional Band - Range below Target pres-

sure or Temperature where adjustments to capacity

slide position are made proportional to the differ-

ence between Target pressure or Temperature and

Actual. Outside this range, the adjustment made will

be the set maximum.

• Load Interval Time – Cycle time for calculating ca-

pacity slide position adjustment when the controller

is calling for the machine to increase capacity slide

position. Where the Load Interval Time is T1, the

control will make an adjustment calculation every T1

seconds.

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Emerson VSG & VSSG Spécifications Page 26

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