The compressor life time test system consists of 15 temperature controlled cabinets. Inside each cabinet a compressor can be positioned on which three different types of life time tests can be performed:
- On/Off test: Test in which the compressor is turned on/off within a certain time interval.
- OLP test: Test normally performed during the on/off test once every 250 cycles. In this test only the main compressor winding is powered (compressor does not start) and system waits until overload protector trips.
- Wear test: Test in which the compressor is running continuously at a high pressure ratio.
- High temperature test: Test in which the compressor is running continuously at a high winding temperature.
The cabinets are positioned in 3 rows of 5 compressors. Each row of 5 compressors functions completely independent.
- Each cabinet contains a gas-loop for the compressor. Basically the gas that is compressed by the compressor, flows from the compressor discharge (high pressure) back to the compressor suction (low pressure) via an automatic expansion valve. The expansion valve controls the compressor discharge pressure. The compressor suction pressure is fixed by the amount of gas in the loop (this is manually set when starting up the life test).
- The compressor suction and discharge pressure are measured.
- Several temperature measurements are performed for each compressor: Tsuction, Tdischarge, Tshell_bottom, Tshell_top, Tambient and there is a spare temperature sensor.
- For each compressor the electrical input current is measured.
- The compressor supply voltage + frequency can be set for each row of 5 compressors (on 230V 50Hz, 230V 60Hz or on 115V 60Hz). These voltage and frequency values are also measured for each row of compressors. The power supplies present in the laboratory are applied for this, the test system contains a switch to select between the different connected power supplies.
- It is possible to measure the main winding resistance of each compressor (from which the winding temperature is. This is possible for Variable speed as well as Normal (single speed) compressors.
- The compressor winding temperature can also be controlled by the system. For this purpose an inteliggent control algorithm is applied that relates the winding temperature to the discharge temperature and in that way can control the winding temperature on a stabil value.
- The temperature of each cabinet is controlled using a variable speed fan for cooling and an electrical heating element for heating.
- The system distinguishes 3 compressor types:
- Normal compressor
- VCC freq
- VCC main
- A normal compressor has an electromotor with a run and a start winding. Normally for a compressorthe PTC controls the amount of time the start winding is powered. In an On/off test the compressor would need a certain off-time to let the PTC cool down. In order to avoid such a long off-time, for this specific test, the PTC is not used and instead the system powers the main and the start winding of the compressor individually. (So for example at the start both windings are turned on and after 0.5 sec the start winding is turned off.)
- VCC freq and VCC main: VCC stands for variable capacity compressor. This type of compressor has a motor that can change speed and is controlled by an inverter. In the life test systemthis type of compressor can be started and stopped in two different ways, namely by turning on/off the main power supply to the inverter (“VCC main”)changing the control frequency to the inverter (“VCC freq”).
- Each cabinet is placed on a rail system and can easily be completely removed from the system for service activities (all electrical connections to the cabinet are via connectors). This makes it possible to install the system directly against a wall, minimizing the required space.
Basic layout of single compressor cabinet
Inside the temperature controlled cabinet the compressor can be connected using a flexible suction and discharge hose.
- Automatic expansion valve: The hose on the discharge side leads to an automatic expansion valve placed on top of the cabinet.
- Discharge pressure sensor: The discharge pressure sensor is placed on top of the cabinet and measures the discharge pressure via a capillary tube leading inside the cabinet
- Filter: After the expansion valve the refrigerant flows through a filter
- Gas cooler: The gas cooler functions to cool down the gas to a value slightly higher than the ambient temperature.
- Suction line to inside of cabinet: After the gas cooler the refrigerant flows into the cabinet again and via the flexible suction hose arrives at the compressor suction
- Suction damper vessel: A suction damper vessel is also connected after the gas cooler, this vessel serves to stabilize the suction pressure and to make the system less sensitive to the exact refrigerant charge.
- Suction pressure sensor: The suction pressure sensor is also connected after the gas cooler (before the suction line goes into the cabinet to the compressor).
- Oven fan (variable speed): At the backside of the cabinet a variable speed fan is used to suck air from the cabinet (air is entering the cabinet via the “Oven air inlet valve”). In this way the cabinet can be cooled
- Suction and discharge pressure display: On the front side of the cabinet the digital displays of the suction and discharge pressure sensor are installed. (pressures are also displayed in the software)
- Quick connector for evacuation and charging: A quick connector is present at the front of the cabinet for evacuation and charging the gas loop with refrigerant
- Electronics cool fan: To cool all the electronics (e.g. pressure displays, winding measurement relais, automatic expansion valve, temperature sensor connections etc.) a small fan is installed in the electric compartment above the cabinet.
- Winding resistance relais: The “winding resistance relais” interrupts the power to the compressor (variable speed or single speed) and connects the main winding (or 1 of the windings for variable speed) to the winding resistance measurement device. For the winding temperature measurement the compressor is always shortly stopped.
- Inverter box: in the inverter box the inverter for variable speed compressors can be installed
Compressor electrical connections
Directly on the left of all compressor cabinets there are 3 small cabinets (1 for each row) to install the electrical compressor connections (e.g. PTC with overload protector, or inverter for variable speed compressors). It has to be noted that the long distance between this location for the inverter and the compressor sometimes give problems in operating the variable speed compressor inside the cabinet. For this purpose there is an alternative position to install the inverter on the front of each cabinet.
|Example of standard electrical compressor connections (e.g. PTC + overload protector) for 5 compressors
|Example of inverter connection for 5 variable speed compressors
The control software basically consists of a general overview sheet, showing the status of all 15 compressors. The following is indicated in this overview:
- Which kind of test is running (wear, high temperature or on/off)
- Any possible alarm messages (e.g. if a compressor stopped running)
- The test progress using a slide bar showing the percentage of the test finished (some tests have a duration of over 3 months)
- A warning if operator action is required
For each compressor in test a “show”-button is available, when pressing this button a separate program is opened in which all details of the tests can be seen:
- Test overview showing an overview of all measured parameters over the complete test period (complete test period can be over 3 months)
- Pressure control graph (showing the control process of the discharge pressure and the suction pressure)
- Temperature graph (with control of ambient temperature or winding temperature)
- Fast graph (showing the pressures and compressor current with a high sample rate, which is especially usefull in the on/off test)
- Settings: showing all test settings
The next figure show an example of the “fast graph” in an on/off test of a variable speed compressor. The compressor start delay caused by the inverter as well as the motor positioning can clearly be seen in the graph (the white line shows when the on-signal is given, the green line shows the inverter input current, the red line shows the discharge pressure and the blue line the suction pressure).
The program automatically saves data files of all tests. All these data files can be loaded in a general data base that contains all information from all tests performed. The data base shows a general overview of all parameters measured in each test (e.g. average values and standard deviation for pressures, temperatures, current, voltage etc. as well as test conditions, test duration etc.) For more detailed test information a specific test from this database can be selected and al details are presented in a graph.
Finally also an automatic test report containing the main data of for example 5 compressor samples of the same type can be generated.