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Section 2 • Operational Descriptions

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

VFD CAPCITY CONTROL

When a VFD is used on the compressor main motor to

control capacity, it works in conjunction with the ca-

pacity slide valve to maintain the Target pressure or

Temperature. At start, the compressor main motor

will run at 50% speed until the capacity slide reaches

its maximum position. At this point, the VFD will begin

adjusting the speed of the main motor to maintain the

target pressure or temperature. The speed adjustments

made by the controller follow the same algorithm as the

capacity slide. Additional variables that factor into the

operation of a main motor VFD are described below:

• Max Speed Increase per Interval – Maximum mo-

tor speed adjustment that can be made when the

control is calling for the machine to increase motor

speed. Where the Maximum Speed Increase is S

the main motor will speed up by S

% every cycle of

the Load Interval Timer when outside the bandwidth

range. When inside the bandwidth range, the motor

speed adjustment will be a percentage of S

• Max Speed Decrease per Interval – Maximum mo-

tor speed adjustment that can be made when the

control is calling for the machine to decrease motor

speed. Where the Maximum Speed Decrease is S

the main motor will slow down by S

% every cycle of

the Unload Interval Timer when outside the band-

width range. When inside the bandwidth range, mo-

tor speed adjustment will be a percentage of S

VOLUME SLIDE CONTROL

When the compressor is running, the volume slide valve

adjusts automatically to operating conditions for opti-

mal efﬁ ciency. The volume slide position is determined

by a lookup table that takes into account the size and

model of compressor, type of refrigerant or gas being

compressed, capacity slide position, and pressure ratio.

An offset can be applied to the volume slide’s calculated

target position to adjust it for unique running condi-

tions. In Compressor Control Setpoints page 2, the set-

points below are available:

• Volume Slide Adjustment Factor: amount of offset

that will be applied to the calculated volume slide po-

sition target.

• Capacity Slide Range Adjustment Factor is Applied:

The volume slide adjustment factor or offset will be

applied when the capacity slide is within this range.

The volume slide may be manually controlled, although

it is best to leave it in Auto Mode. Manual volume slide

control may be used when calculating an offset to apply

to the volume slide calculation. The volume slide mode

is settable in the “Volume Slide Details” screen.

When the compressor is stopped, the volume slide valve

is commanded to go to its minimum position.

SAFETIES

The compressor controller continuously monitors oper-

ational and process data and annunciates an alarm and/

or stops the machine if any condition becomes abnor-

mal. Two levels of safeties exist when an abnormal con-

dition is detected.

• Alarm: If active, alarms are annunciated on the com-

pressor HMI. When activated, a popup screen show-

ing the date and time of the alarm and alarm message

will appear. Alarms are also logged in the Event List.

An alarm serves only as a warning to the operator; if

an alarm is active the machine is still allowed to run.

• Trip: If active, trips will shut the machine down or not

allow the compressor to start. Trips are annunciated

on the compressor HMI. When activated, a popup

screen showing the date and time of the trip and

trip message will appear. Trips are also logged in the

Event List.

The “Alarm Reset” pushbutton on the overview screen

will reset any active alarms or trips if the abnormal condi-

tion has been removed.

For a comprehensive list of alarms and trips and possible

causes, see the troubleshooting guide in this manual.

EMERGENCY STOP

Reference Figure 2-6

The Emergency Stop circuit in the compressor control

panel energizes the Master Control Relay, which pro-

vides power to PLC outputs that control actuators, heat-

ers, motor starters, valves, etc. The Master Control Relay

may be energized by pressing the “Control Power On”

illuminated pushbutton on the door of the compressor

control panel. When the Master Control Relay is ener-

gized, the “Control Power On” pushbutton will illumi-

nate. The following conditions must be satisﬁ ed to en-

ergize the Master Control Relay:

• Emergency Stop pushbutton on the door of the com-

pressor control panel must be pulled out.

• Any additional Emergency stops or safety devices

tied in to the Emergency Stop circuit must be reset.

• The compressor PLC must be booted up and

operational.

• The 24-volt DC power supplies in the compressor

control panel must be powered up and OK.

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Emerson VSG & VSSG Spécifications Page 32

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