Emerson VSG & VSSG Specifications download pdf (Page 30)

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Section 2 • Operational Descriptions

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

OPERATING MODES

The loading and unloading of the compressor can be

conﬁ gured to operate in several ways, depending on the

needs of the site. This mode selection is made at the

“Start Menu” screen. There are four basic modes, de-

scribed below:

• Local-Auto: The compressor will adjust the position

of the capacity slide automatically using the control’s

capacity control algorithm. The target pressure or

temperature setpoint is set on the compressor’s lo-

cal HMI.

• Local-Manual: The operator is in control of the capac-

ity slide from the local HMI. When in Local-Manual

mode, the operator controls the capacity slide po-

sition using increase/decrease pushbuttons on the

compressor overview screen.

• Remote-Auto: The compressor will adjust the posi-

tion of the capacity slide automatically using the

control’s capacity control algorithm. The target

pressure or temperature setpoint is deﬁ ned by the

DCS/Central Controller via communications. (NOTE:

Control by Communications must be enabled to use

this mode.)

• Remote-Manual: The capacity slide position is con-

trolled by a DCS or Central Controller. There are four

selectable methods of using Remote-Manual Control,

settable from the HMI’s conﬁ guration screen.

• Caphold-Ethernet: Capacity slide valve position

commands are given by the central controller/

DCS via communications. (NOTE: Control by

Communications must be enabled to use this

method.)

• Caphold-4-20mA Hardwired: Capacity slide

valve position commands are given by the cen-

tral controller/DCS via a 4-20mA analog signal.

The scaling is 4mA = 0% Capacity Slide Position,

20mA = 100% capacity slide position. Control by

Communications does not need to be enabled to

use this method.

• Discrete Load/Unload – Ethernet: Capacity slide

valve increase and decrease commands are given

by the central controller/DCS via communica-

tions. (NOTE: Control by Communications must

be enabled to use this method.)

• Discrete Load/Unload – Hardwired: Capacity slide

valve increase and decrease commands are given

by the central controller/DCS via discrete hard-

wired inputs. Control by communications does

not need to be enabled to use this method.

In any of the above modes, all local safeties and load lim-

its still apply and will override external commands.

For safety reasons, remote mode is only enabled under

certain conditions. If these conditions are not met, the

machine will revert to local mode. To be able to enter

remote mode, the one of the following must be true:

• Auto Mode Selected, Control By Communications

Enabled.

• Manual Mode Selected, Remote Manual Control

Source is Hardwired

• Manual Mode Selected, Control by Communications

Enabled

In the event that the communications link between the

compressor PLC and the central controller/DCS is lost,

the action taken is selectable from the Conﬁ guration

screen. The machine will Trip or revert to Local mode

and continue to run depending on the selection.

The compressor PLC can be remotely commanded to

Local or Remote Mode if Control by Communications is

enabled.

NOTE

If a main motor VFD is being used for capacity control,

only discrete commands may be used in Remote-

Manual Control.

LOAD LIMITS AND FORCED UNLOADING

Reference example, Figure 2-5

To protect the compressor and process, the controller

will inhibit the compressor from loading or force it to un-

load if certain variables get outside of set ranges. Three

load limiting variables are continuously monitored:

• Low Suction Pressure

• High Discharge Pressure

• High Main Motor Amps

There are independent Load Limit Setpoints speciﬁ c to

“Setpoint 1” and “Setpoint 2” setpoint groups, allow-

ing load limits to be speciﬁ c to unique operating condi-

tions. For each of the three variables, three setpoints are

active:

• Inhibit Loading: when this setpoint is reached, the

compressor will not be allowed to continue loading.

• Unload at: when this setpoint is reached, the capac-

ity slide will move in the decrease direction continu-

ously (forced unload) until the “Unload To” setpoint

is reached.

• Unload To: this is the setpoint at which the capac-

ity slide will stop unloading from a forced unload

condition.

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Emerson VSG & VSSG Specifications Page 30

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