GENERAL Assist Technical Reference

VRF VII - Compressor Operation and Control

Compressor operating conditions, capacity control, speed limits and sequencing
Overview: VRF VII compressor operation is controlled according to the cooling or heating load of the connected indoor units. The system combines inverter compressor speed control and constant-speed compressor staging to match required refrigerant circulation while maintaining target system pressures.

1. Compressor Operation / Stop Conditions

When either cooling requirement capacity or heating requirement capacity is received from an indoor unit within the same refrigerant circuit, the compressor operates.

When all indoor units have a cooling requirement capacity or heating requirement capacity of 0, the compressor is stopped.

The compressor may still operate during the following control conditions:

  • 3-minute restart prevention operation
  • Icing protection
  • Failure / protection operation
  • Oil recovery
  • Electronic expansion valve initialisation
  • Protective operation
  • Emergency stop processing
  • Defrost operation
  • Peak cut stop operation

2. Capacity Control

By combining operation of the DC inverter rotary compressor and constant-speed scroll compressors, the system supplies the required refrigerant circulation according to the cooling or heating load.

The inverter compressor provides fine capacity control by varying rotational speed. As system load increases, additional compressor capacity is staged in.

Target Low-Pressure and High-Pressure Control

Cooling: compressor capacity is controlled using the outdoor-unit low-pressure sensor so that indoor-unit evaporation pressure is maintained at the required level.

Heating: compressor capacity is controlled using the outdoor-unit high-pressure sensor so that indoor-unit condensing pressure is maintained at the required level.

Note: Target low-pressure and high-pressure temperature depends on system capacity, compressor operating capacity, pipe length and capacity-shift switch settings.

3. Inverter Compressor Speed Range

ConditionSpeed
Stop mode0 rps
Operating mode30 - 100 rps
Multiple outdoor unit systemMaster and slave inverter compressors are controlled at the same rotational speed.

Cooling Starting Process

For cooling operation, the upper compressor speed limit at start-up is 50 rps and is increased in 10 rps increments every 60 seconds.

Elapsed TimeUpper Limit
Start50 rps
60 sec60 rps
120 sec70 rps
180 sec80 rps
240 sec90 rps
  • If target speed is higher than the current upper limit, the compressor operates at the upper limit.
  • If target speed is lower than the current upper limit, the compressor operates at target speed.

Heating Starting Process

At the start of heating, all compressor start-up operation is controlled in association with four-way valve changeover. Normal target high-pressure control resumes after the starting process is complete.

Outdoor UnitStarting Process 1Starting Process 2
O2 compressor
12, 14, 16 HP		30 rps fixed, with constant-speed compressor ON70 rps fixed
O1 compressor
8, 10 HP		70 rps fixed70 rps fixed

Starting Process 1 ends when:

  • 7 minutes have elapsed, or
  • 1 minute has elapsed and high pressure of all outdoor units is at least 2.50 MPa, or
  • Compression ratio of any outdoor unit exceeds 8.

Starting Process 2 ends when:

  • 30 minutes have elapsed, or
  • High pressure of all outdoor units is at least 2.63 MPa, or
  • Discharge superheat of all operating compressors exceeds 12°C.
Heating starting process is not performed when:
A. Outside air temperature is above 20°C, or
B. The heating starting process, including thermostat-OFF stop, has occurred within the previous 30 minutes.

When these conditions apply, the inverter compressor of all outdoor units starts at 40 rps and normal target high-pressure control begins immediately.

High-Pressure Speed Limiting

High PressureMaximum Speed
Below calculated threshold [a]100 rps
Approx. 3.00 MPa95 rps
Approx. 3.30 MPa90 rps
Approx. 3.80 MPa80 rps
[a] = 1.25 × Low Pressure + 2.125

4. Compressor Sequence Operation

The system creates a starting sequence and starts and stops compressors according to that sequence.

  • Starting sequence 1: compressor starts first and stops last.
  • Starting sequence 2: compressor starts second and stops second from the end.
  • Inverter compressors are started before constant-speed compressors.
  • Operating inverter compressors are controlled to the same speed where possible, subject to upper and lower speed limits.

Starting Sequence Rotation

The compressor starting sequence is rotated during:

  1. Defrosting
  2. Oil recovery
  3. High cooling discharge temperature
Service note: A change in which compressor starts first, or a change in the number of compressors operating, may be normal system control rather than a fault. Always consider load, pressure control, oil recovery, defrost and compressor sequence before diagnosing a compressor as failed.

5. Key Diagnostic Points

  • A compressor not operating does not automatically indicate a fault; first confirm current system load and sequence.
  • Cooling start-up intentionally limits inverter speed for the first several minutes.
  • Heating start-up can hold compressor speed at fixed values while target high pressure is established.
  • Rising high pressure can intentionally reduce maximum inverter speed.
  • Defrost and oil recovery can change normal compressor sequence.
  • In multi-outdoor systems, inverter compressors are normally controlled at the same rotational speed.
This article is intended as a technical reference for qualified service personnel and is based on VRF VII compressor operation and control information.