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A frequency converter consists of four main parts: rectifier, DC link (intermediate DC circuit), inverter, and control circuit. The rectifier converts the mains AC into DC; the DC link filters and stores energy; the inverter, under the command of the control circuit, converts the DC back into AC with adjustable frequency and voltage for the motor.
The main circuit of a frequency converter can be divided into two types: voltage‑source and current‑source. A voltage‑source converter uses capacitors as the filtering element in the DC link, behaving like a voltage source. A current‑source converter uses inductors (reactors) as the filtering element, behaving like a current source. The choice between them depends on the load characteristics and application requirements.
The electromagnetic torque of any motor results from the interaction between current and magnetic flux. From the formula E = 4.44 × K × F × N × Φ, it can be seen that during variable‑frequency speed control, the magnetic circuit changes over a wide range with the operating frequency. This can easily cause severe magnetic saturation, leading to distorted excitation current waveforms and peak currents. Therefore, when changing the frequency, the output voltage must be controlled proportionally to maintain constant magnetic flux, avoiding field weakening or saturation. This V/f control method is widely used for energy‑saving loads such as fans and pumps.
Capacitors must never be connected to the output side of a frequency converter, nor should capacitor‑start single‑phase motors be used. The output of a converter is a PWM modulated waveform, and a capacitor would cause excessive charging current, leading to overcurrent tripping and even damage to the inverter module.
A frequency converter is essentially unsuitable for driving single‑phase motors. For speed‑control switch‑start single‑phase motors, the auxiliary winding will burn out below the speed‑control range. For capacitor‑start or capacitor‑run single‑phase motors, the capacitor may explode. Most frequency converters are designed for three‑phase input; only a few small‑capacity models accept single‑phase supply.
A variable‑frequency power supply outputs a pure sine wave and can provide any grid voltage and frequency required worldwide. A frequency converter (whose proper name is variable‑frequency speed regulator) outputs a pulse‑width‑modulated square wave with many harmonics; its voltage and frequency must change proportionally and cannot be adjusted independently. Therefore, a converter cannot serve as a general‑purpose power supply – it is only suitable for speed control of three‑phase asynchronous motors.
When selecting a frequency converter, the motor’s rated current should be the primary basis; the motor’s rated power serves only as a reference. When a mains‑rated motor is driven by a converter, the motor current increases by 10–15% and the temperature rise by about 20–25%. Driving high‑speed motors generates more high‑order harmonics, which increase the converter’s output current; hence, a converter one size larger than for a standard motor should be selected.
Signal and control cables should use shielded cables to prevent interference. When cable runs exceed 100 metres, the conductor cross‑section should be increased appropriately. Signal and control cables must not be placed in the same cable trench or tray as power cables to avoid mutual interference; it is best to route them in conduits. For signal transmission, current signals (e.g., 4–20 mA) are preferred because they are less prone to attenuation and interference.
Acceleration time is the time required for the output frequency to rise from zero to the maximum frequency, and deceleration time is the time to fall from maximum to zero. During acceleration, the frequency rise rate must be limited to prevent overcurrent; during deceleration, the frequency fall rate must be limited to prevent overvoltage. In commissioning, it is common to set longer acceleration/deceleration times initially, observe whether overcurrent or overvoltage alarms occur during start/stop, then gradually shorten the times until just before an alarm appears – that determines the optimal settings.
When a motor operates at very low speeds for extended periods, its cooling performance deteriorates significantly, which can lead to overheating and even burnout. In addition, the cable current also increases at low speeds, causing cable heating. Therefore, the minimum operating frequency must be set reasonably to avoid prolonged low‑speed operation.
At certain specific frequencies, the motor and driven equipment may experience resonance. The converter’s skip frequency (or frequency jump) function can bypass these resonant points, protecting mechanical equipment. When controlling compressors, care must also be taken to avoid the surge points of the compressor.
The protection functions of a frequency converter fall into two categories:
Automatic correction type – e.g., overcurrent stall prevention, regenerative overvoltage stall prevention. When an abnormality is detected, the converter automatically takes corrective action.
Emergency stop type – e.g., overcurrent trip, regenerative overvoltage trip, cooling fan overheat protection, instantaneous power‑failure protection. When an abnormality is detected, the PWM control signals are blocked and the motor stops automatically.
When a clutch is used to connect a load, the momentary inrush current may cause the converter to trip due to overcurrent. If a motor with power‑factor correction capacitors is connected to a converter, the charging current of the capacitors can also cause overcurrent tripping – the capacitors must be removed.
