Emerson 20/20 Control Mode, Stop Load and Force Unload, Capacity Slide Triggered Outputs

Section 6 • Compressor Control

grayed out and hence, the user will not be able to op- erate Pulldown feature. This is done to keep Pulldown & Pumpdown features mutually exclusive.

Control Mode

This drop down box gives the operator the ability to change the type of control mode such as suction pres- sure control or process temperature control. The opera- tor can also switch from setpoint 1 and setpoint 2 for each control method. What is available in this dropdown box is dependent on the number and type of control se- lected in the configuration screen.

Slide Valve Setpoint:

•Operator editable labels for the each output. Slide %:

•Indicates the capacity slide position where the digital output is triggered.

State Below Setpoint:

•Defines the state of the digital output when the slide position is below the “Slide %” setpoint. The operator can choose between “N.O.” or “N.C.”

Active:

• Check box to enable the digital output.

Stop Load and Force Unload

The stop load and force unload feature’s primary pur- pose is to attempt to prevent the compressor from tripping off due to particular instrument reading. For example, if the suction pressure drops too low, the com- pressor will trip off for safety reasons. However, the stop load & force unload algorithm recognizes a potential trip and either stops the compressor from loading up or even unloads the compressor to prevent the trip.

Stop load:

•When this value is reached, the capacity slide will not advance in any condition.

Force Unload:

•When this value is reached, the capacity slide posi- tion will decrease until the variable reading is below this value.

High Motor Amps:

•Motor current values for stop load and force unload. High Discharge Pressure:

•Discharge pressure value for stop load and force unload.

Low Suction Pressure:

• Suction pressure values for stop load and force unload

Capacity Slide Triggered Outputs

The Vission 20/20 offers two digital outputs that can be triggered at a specified capacity slide position. By de- fault, the outputs are preselected for economizer and hot gas bypass. However, these preselected outputs are customizable by the operator.

Volume Slide Position Offset

These setpoints offer the ability to alter the Volume po- sition table to take advantage of potential energy sav- ings. Since the volume position is a function of the ca- pacity position, the offset to the volume is based on the position of the capacity slide. The volume offset can be applied to the entire capacity slide range or just a por- tion using the Capacity Range minimum and maximum setpoints.

Volume Slide Adjustment %:

•The value in percentage of the volume slide offset. Capacity Range:

•Defines the range that the volume position slide off- set will be applied.

Soft Load

This setpoint is used to slow the loading of the compres- sor. In some refrigeration systems, a loading compres- sor can have dramatic effects on the system parameters. This setpoint allows an operator to reduce the continu- ous load pulse as defined in the proportional control sec- tion to a percent duty cycle.

Soft load %:

•Defines the duty cycle of the continuous load pulse. At 100%, the continuous pulse will truly be continu- ous. At 50%, the continuous pulse would be reduced to half time on and half time off in the time interval defined in the proportional control section.

6 – 6 Vission 20/20 • Operation and Service Manual •Vilter/Emerson • 35391SC 1.8.5153