## 13.34. NA7.10.4 Compressor Variable Speed Controls

At-A-Glance

NA7.10.4 Compressor Variable Speed Controls
Use Document NRCA-PRC-07-A
Purpose of the Test
The test ensures that the applicable compressors control compressor speed in response to the refrigeration load, per §120.6(a)5B.
Instrumentation
None
Test Conditions
To perform the test, it will be necessary to override the normal operation of the controls. The control system for the compressor must be complete, including:
Variable speed drive on all applicable screw compressors. Controls to control the compressor motor speed. Document the initial control settings before executing system overrides or manipulation of the setpoints. The compressor control system must be returned to normal operation at the end of the test.
Estimated Time to Complete
Construction inspection: 1 hour (for one compressor)
Functional testing: 2 hour (for one compressor)
Acceptance Criteria
Compressor speed decreases with decrease in load, and the slide valve (or other unloading means) are held at 100 percent capacity until the compressor speed reaches the minimum allowable setpoint.
With an increase in load, the compressor slide valve (or other unloading means) should load to 100 percent capacity, and then the compressor speed should start in increase.
Potential Issues and Cautions
Coordinate test procedures with the refrigeration or controls contractor, or the facility supervisor since they may be needed to assist with the manipulation of the control system.

A. Test Application

Newly Constructed Buildings: Applies to functional testing and verification of compressor variable speed controls.

B. Construction Inspection

The field technician should check the following:

• All applicable single open-drive screw compressors dedicated to a suction group have variable speed control.

• All pressure and temperature sensors have been calibrated and read accurately.

• All sensor readings used by the compressor controller convert or calculate to the correct conversion units at the controller (e.g., saturated suction pressure reading is correctly converted to appropriate saturated suction temperature (SST)).

• All compressor motor speed controls are operational and connected to compressor motors.

• All speed controls are in “auto” mode.

• Compressor panel control readings for “RPMs”, “percent speed”, “kW”, and “amps” match the readings from the PLC or other control systems.

• Compressor data is correctly entered into the PLC or other control system, to the extent required for proper control (e.g. minimum speed)

• Records showing calibration was performed, what offsets or control system calibration values were used, and documentation of the instrumentation used for calibration.

C. Functional Testing

The system cooling load must be sufficiently high for the test, but the compressor should be not operating at fully capacity. Artificially increase the load by decreasing the zone setpoint, or decrease the load by increasing the zone setpoint or turning off evaporators as needed to perform the Functional Testing.

Step 1: Override any floating suction pressure functionality before performing functional tests.

Work with the refrigeration contractor or facility staff to disable controls that may interfere with the Functional Testing.

Step 2: Document current operating conditions. Note these may be the same as the current setpoint.

• Current suction pressure

• Current saturated suction temperature

Step 3: Document current setpoint: Suction pressure setpoint or saturated suction temperature setpoint.

Program into the control system a target setpoint equal to the current operating condition measured in Step #2. Allow 5 minutes for system to normalize. This will be referred to as the “test suction pressure/saturated suction temperature setpoint”.

Step 4: Using the control system, increase the test suction setpoint in small increments until the compressor controller modulates to decrease compressor speed. An increase of 1psi or 1°F SST will be appropriate. The increase will need to consider any control dead band or time delay that is in place.

Verify:

• Compressor speed decreases

• Compressor speed continues to decrease to minimum speed

• Any slide valve or other unloading means does not unload until after the compressor has reached its minimum speed.

Step 5: Using the control system, decrease the test suction setpoint in small increments until the compressor controller modulates to increase compressor speed. A decrease of 1psi or 1°F SST will be appropriate. The decrease will need to consider any control dead band or time delay that is in place. You must wait a sufficient amount of time so that any timer or delay can expire.

Verify:

• Any slide valve or other unloading means first goes to 100 percent before compressor speed increases from minimum

• Compressor begins to increase speed

• Compressor speed continues to increase to 100 percent

Step 6: Using the control system, program the suction pressure or saturated suction temperature setpoint back to original settings as documented in Step #3.

Confirm that the control system is restored back to correct control setpoints.

Step 7: Restore any controls that were disabled in Step 1.

Verify that the control system is restored back to original conditions.