Electrical Control for Air Compressors Equipped with Frequency Converters

The electrical‑control system of air compressors fitted with frequency converters consists of two subsystems: an internal control system and a starter panel section. The starter panel adopts the common Y‑Δ starting control for general‑purpose motors. The control section features electronic control, whose internal circuits and control logic are relatively complex.

Important Notes for Frequency‑Converter‑Driven Operation

1. Variable‑frequency start‑up of the screw air compressor The controller commands the frequency converter to ramp its output frequency from 0 Hz up to 50 Hz, and the motor speed rises from zero to the rated speed. During this process, the intake valve opens, the blow‑off valve closes, the blow‑off solenoid valve is energized, and the pressure inside the oil‑gas separator tank rises.

2. Variable‑frequency operation When the pressure inside the oil‑gas separator tank rises to a certain value, the pressure‑sustaining valve opens and air is delivered. Based on the set‑point pressure and feedback of output pressure, the control system performs PID closed‑loop control on the output frequency of the frequency converter, so as to stabilize the discharge pressure of the air compressor accurately.

3. On‑load / Off‑load / Sleep mode When the output frequency of the frequency converter drops to the minimum preset frequency of the air compressor yet the pressure keeps rising until it reaches the preset unloading pressure, the blow‑off solenoid valve actuates to start venting. The intake valve closes, air flows in through the bypass pipeline, and pressure inside the oil‑gas separator tank begins to release. At this instant, the pressure‑sustaining valve closes rapidly to isolate system pressure. The pressure inside the oil‑gas separator tank continues to drop to the off‑load pressure, which supplies power required for lubricating‑oil circulation.

4. Auto‑shutdown system for prolonged off‑load operation and restart‑time setting If the system air consumption decreases, the compressor will run under off‑load condition. Should the off‑load running time exceed the preset duration, the air compressor will shut down automatically and the motor stops. When system air consumption goes up and system pressure falls, the air compressor will start automatically to replenish air supply. Apart from the setting for auto‑shutdown upon prolonged off‑load operation, the air‑compressor controller has an additional parameter: the compressor cannot be restarted until five minutes after motor shutdown.

5. Fault reset Once a fault occurs on the frequency converter, relevant fault codes will be displayed. Users may perform fault reset via the <kbd>Stop</kbd> key on the keypad or terminal functions (Group F4). After fault reset, the frequency converter enters standby status. If the fault is not cleared, the frequency converter stays in operational protection mode and cannot run.

Frequency‑Converter Maintenance

1. Routine maintenance for screw air‑compressor frequency converters Ambient temperature, humidity, dust and vibration can accelerate component aging inside the frequency converter, trigger potential faults and shorten service life. Therefore routine and periodic inspection & maintenance for the frequency converter are mandatory.

WARNING: High voltage remains on filter capacitors even after power disconnection. Do NOT perform maintenance immediately after power‑off. Wait until the display on the operation panel goes blank, then verify with a multimeter that the bus voltage is no higher than 36 V before any servicing work.

2. Routine inspection items

1. Abnormal noise from the motor during operation;

2. Vibration generated by the running motor;

3. Changes in the installation environment of the frequency converter;

4. Normal operation status of the frequency‑converter cooling fan;

5. Overheating of the frequency converter.

3. Routine cleaning

1. Keep the frequency converter clean at all times;

2. Thoroughly remove surface dust to prevent dust (especially metallic dust) from entering the converter interior;

3. Remove oil contamination from the cooling fan of the frequency converter.

4. Periodic inspection Carry out periodic checks on locations inaccessible during normal running. Inspection items are as follows:

1. Inspect and clean air ducts on a regular basis;

2. Check for loose screws;

3. Check whether the frequency converter suffers corrosion;

4. Inspect connection terminals for arc‑burn marks;

5. Insulation test for the main power circuit.

NOTICE: When measuring insulation resistance with a megohmmeter (DC 500 V recommended), disconnect main‑circuit wiring from the frequency converter. Do NOT test insulation of the control circuit with a megohmmeter.