A variable-frequency drive (VFD) is a power control device that uses frequency conversion technology and microelectronics to control an AC motor by changing the frequency of the power supply. The switching power supply circuit provides the power for the entire VFD control system and is a prerequisite for its normal operation. To repair a VFD, one must first understand the switching power supply circuit.
The switching power supply circuit used in frequency converters is a DC-AC-DC inverter circuit, a voltage and power converter that transforms DC voltage and power into pulse voltage, and then rectifys it into another DC voltage. The input and output voltages are isolated by a switching transformer, which performs power transfer and voltage/current conversion. The switching transformer is a step-down transformer. The characteristics of a switching power supply are as follows:
1) The oscillation and voltage regulation of switching power supplies are achieved by changing the pulse width or period to adjust the output voltage, which is called time proportional control. It is further divided into two control methods: PWM (pulse width modulation) and PFM (pulse frequency modulation).
2) From the perspective of energy conversion characteristics, the circuit can be divided into two operating modes: forward and flyback. When the switching transistor is saturated and conducting, the rectifier connected to the secondary winding is reverse biased and cut off, and current flows into the primary winding of the switching transformer to store energy (electromagnetic conversion). When the switching transistor is cut off, the secondary winding releases electrical energy through the load circuit (magnetic-electric conversion). The forward converter operates in the opposite way and is not commonly used in practice.
3) From the perspective of the primary circuit structure of a switching transformer, there are two circuit forms: those composed of discrete components and those composed of integrated oscillator chips. Therefore, from the perspective of the source of the oscillation signal, they can be divided into self-excited (discrete components) and externally excited (IC circuit) switching power supplies. Both circuit structures are used.
4) Switching transistors include bipolar devices and field-effect transistors.
5) Low-power frequency converters use single-ended forward converters, while high- and medium-power frequency converters often use double-ended forward converters. The switching power supply of a typical frequency converter usually provides the following voltage outputs: +5V for the CPU and auxiliary circuits, control circuits, and operation display panel; ±15V for fault detection circuits (current, voltage, temperature, etc.) and control circuits; and 24V for control terminals and working relay coils. Four isolated approximately 22V power supplies for the drive circuits are also provided. These four power supplies are often further regulated by voltage regulators to provide +15V and -7.5V positive and negative power supplies to the drive circuits, providing excitation current for the IGBT inverter output circuit.
In any electronic device, the power supply circuit always has a relatively high failure rate because it provides the power supply for the entire machine and bears the heaviest burden. The switching power supply circuit of a frequency converter is relatively simple in form and structure. However, even simple circuits can produce difficult-to-diagnose faults. Troubleshooting a switching power supply is not as intuitive as troubleshooting a linear power supply. Any abnormality in any small component of the circuit—oscillation, voltage regulation, protection, load, etc.—can cause various fault phenomena in the circuit.
After powering on, there is no response; the operation display panel shows nothing, as if the inverter is not powered on. Measuring the control voltage and 10V frequency adjustment voltage at the control terminals both show 0, and the resistance at the inverter's main terminals is normal. Therefore, it can be roughly determined that the problem lies in the switching power supply circuit.
