1. What is a frequency converter?
A frequency converter mainly consists of a rectifier (AC to DC), a filter, an inverter (DC to AC), a braking unit, a drive unit, a detection unit, and a microprocessor unit.
We know that the expression for the synchronous speed of an AC motor is:
n=60 f(1-s)/p (1)
In the formula
n — the speed of the asynchronous motor;
f — the frequency of the asynchronous motor;
s — Motor slip;
p — Number of pole pairs of the electric motor.
As shown in equation (1), the rotational speed n is directly proportional to the frequency f. Changing the frequency f changes the motor's rotational speed. When the frequency f varies within the range of 0–50 Hz, the motor's speed adjustment range is very wide. A frequency converter achieves speed regulation by changing the motor's power supply frequency, making it an ideal, highly efficient, and high-performance speed control method.
Frequency converters adjust the voltage and frequency of the output power supply by switching their internal IGBTs, providing the required power voltage according to the actual needs of the motor, thereby achieving energy saving and speed regulation. In addition, frequency converters have many protection functions, such as overcurrent, overvoltage, and overload protection. With the continuous improvement of industrial automation, frequency converters have been widely used.
2. Functions of frequency converters
2.1 Variable Frequency Energy Saving
The energy-saving benefits of frequency converters are mainly seen in the application of fans and water pumps. To ensure production reliability, various production machines are designed with a certain margin of safety in their power drives. When the motor cannot operate at full load, the excess torque beyond meeting the power drive requirements increases the consumption of active power, resulting in wasted electrical energy.
Traditional speed control methods for equipment such as fans and pumps involve adjusting the opening of inlet or outlet baffles or valves to regulate air and water flow. This method has high input power, and a significant amount of energy is consumed in the flow throttling process of the baffles and valves. When using variable frequency speed control, if the flow requirement decreases, the requirement can be met by reducing the speed of the pump or fan.
The purpose of using a frequency converter (VDC) for an electric motor is to regulate its speed and reduce starting current. To generate variable voltage and frequency, the device first converts the alternating current (AC) to direct current (DC), a process called rectification. The scientific term for the device that converts DC to AC is "inverter." Generally, an inverter converts DC power to a power supply with a fixed frequency and voltage. Inverters that convert DC to AC with adjustable frequency and voltage are called frequency converters. The output waveform of a frequency converter is a simulated sine wave, primarily used for speed control of three-phase asynchronous motors; it is also called a variable frequency drive (VFD). For variable frequency inverters used in instrumentation and testing equipment with higher waveform requirements, the waveform needs to be shaped to output a standard sine wave; this is called a variable frequency power supply. Generally, a variable frequency power supply costs 15-20 times more than a frequency converter. Because the main device in a frequency converter that generates changing voltage or frequency is called an "inverter," the product itself is named "inverter," i.e., a frequency converter.
Inverter technology doesn't save electricity everywhere; in many situations, it doesn't necessarily save power. As an electronic circuit, the inverter itself consumes power (approximately 3-5% of its rated power). A 1.5 horsepower air conditioner consumes about 20-30W, equivalent to a light bulb left on continuously. It's true that inverters operate at the mains frequency and have energy-saving functions. However, this is conditional:
First, it must be a high-power load, specifically a fan/pump type load;
Second, the device itself has a power-saving function (software supported);
These are the three conditions that demonstrate energy-saving effects. Beyond these, whether or not an inverter saves energy is meaningless. Claiming that an inverter saves energy when operating at power frequency without considering the underlying conditions is exaggeration or commercial hype. Knowing the facts will allow you to skillfully utilize it to your advantage. It is crucial to pay attention to the application environment and conditions to ensure correct application; otherwise, you risk being blindly followed, easily misled, and "deceived."
2.2 Power Factor Compensation Energy Saving
Reactive power not only increases line losses and equipment heating, but more importantly, the reduction in power factor leads to a decrease in the active power of the power grid. A large amount of reactive power is consumed in the lines, resulting in low equipment efficiency and serious waste. After using a variable frequency drive (VFD), the reactive power loss is reduced and the active power of the power grid is increased due to the effect of the internal filter capacitor of the VFD.
2.3 Soft start energy saving
1. Hard starting of a motor causes a severe impact on the power grid and places excessive demands on its capacity. The large current and vibration generated during startup cause significant damage to baffles and valves, severely impacting the lifespan of equipment and pipelines. However, using a variable frequency drive (VFD) energy-saving device utilizes the VFD's soft-start function to ensure the starting current starts from zero, with a maximum value not exceeding the rated current. This reduces the impact on the power grid and the demand on power supply capacity, extends the lifespan of equipment and valves, and saves on equipment maintenance costs.
2. Theoretically, frequency converters can be used in all mechanical equipment with electric motors. When a motor starts, the current is 5-6 times higher than the rated current, which not only affects the motor's lifespan but also consumes more electricity. While system design includes a certain margin in motor selection and the motor speed is fixed, in actual use, it is sometimes necessary to operate at lower or higher speeds. Therefore, frequency conversion retrofitting is essential. Frequency converters can achieve soft starting of motors and compensate for power factor errors.
3. Classification of frequency converters
3.1 AC-AC frequency converter
The development of frequency converters has also been a gradual process. Early frequency converters did not use the AC-DC-AC topology (AC to DC then back to AC), but rather a direct AC-AC topology without an intermediate DC link. This type of frequency converter is called an AC-AC converter, and it is currently used for ultra-high power and low-speed regulation. Its output frequency range is 0-17 (1/2-1/3 of the input voltage frequency), which cannot meet the requirements of many applications. Furthermore, at that time, there were no IGBTs, only SCRs, so its application range was limited.
The working principle of a frequency converter is to directly generate the required voltage and frequency power from a three-phase mains frequency power supply through several sets of phase-controlled switches. Its advantages are high efficiency and easy energy return to the grid. Its biggest drawback is that the highest output frequency must be less than 1/3 or 1/2 of the input power frequency; otherwise, the output waveform will be too poor, causing the motor to vibrate and malfunction. Therefore, AC-AC frequency converters are currently limited to low-speed speed control applications, thus greatly restricting their scope of use.
I. Circuit composition of a switching power supply:
The main circuitry of a switching power supply consists of an input electromagnetic interference (EMI) filter, a rectifier filter circuit, a power conversion circuit, a PWM controller circuit, and an output rectifier filter circuit. Auxiliary circuits include input over/under voltage protection circuits, output over/under voltage protection circuits, output overcurrent protection circuits, and output short-circuit protection circuits.
II. Principles and common circuits of input circuits:
1. Principle of AC input rectifier and filter circuit:
① Lightning protection circuit: When lightning strikes and generates high voltage that is fed into the power grid, the circuit consisting of MOV1, MOV2, MOV3, F1, F2, F3, and FDG1 provides protection. When the voltage across the varistor exceeds its operating voltage, its resistance decreases, causing the high voltage energy to be dissipated in the varistor. If the current is too large, F1, F2, and F3 will burn out the protection circuit downstream.
② Input Filtering Circuit: The double π-type filter network composed of C1, L1, C2, and C3 mainly suppresses electromagnetic noise and spurious signals from the input power supply, preventing interference to the power supply and also preventing high-frequency noise generated by the power supply itself from interfering with the power grid. When the power supply is turned on, C5 needs to be charged. Due to the large instantaneous current, adding RT1 (thermistor) effectively prevents surge current. Because all the instantaneous energy is consumed by resistor RT1, after a certain period of time, as the temperature rises, the resistance of RT1 decreases (RT1 is a negative temperature coefficient element). At this point, the energy consumed is very small, and the subsequent circuits can operate normally.
③ Rectifier and filter circuit: After the AC voltage is rectified by BRG1, it is filtered by C5 to obtain a relatively pure DC voltage. If the capacitance of C5 decreases, the output AC ripple will increase.
2. Principle of DC input filter circuit:
① Input filter circuit: The double π-type filter network composed of C1, L1, and C2 mainly suppresses electromagnetic noise and spurious signals from the input power supply to prevent interference with the power supply, and also prevents high-frequency noise generated by the power supply itself from interfering with the power grid. C3 and C4 are safety capacitors, and L2 and L3 are differential-mode inductors.
② R1, R2, R3, Z1, C6, Q1, Z2, R4, R5, Q2, RT1, and C7 form a surge protection circuit. At the moment of startup, Q2 is not conducting due to the presence of C6, and the current flows through RT1, forming a loop. When the voltage across C6 reaches the regulated value of Z1, Q2 conducts. If C8 leaks or there is a short circuit in the subsequent circuit, the voltage drop across RT1 increases at the moment of startup, causing Q1 to conduct and Q2 to not conduct due to the lack of gate voltage. RT1 will burn out in a very short time, thus protecting the subsequent circuit.
III. Power Conversion Circuit:
1. Working principle of MOSFET: The most widely used insulated gate field-effect transistor is the MOSFET (Metal-Oxide-Semiconductor Transistor), which operates by utilizing the electroacoustic effect on the semiconductor surface. It is also called a surface field-effect device. Because its gate is in a non-conductive state, the input resistance can be greatly increased, up to 10⁵ ohms. The MOSFET uses the magnitude of the gate-source voltage to change the amount of charge induced on the semiconductor surface, thereby controlling the magnitude of the drain current.
2. Working principle:
R4, C3, R5, R6, C4, D1, and D2 form a buffer connected in parallel with the switching MOSFET to reduce voltage stress on the switching transistor, decrease EMI, and prevent secondary breakdown. When the switching transistor Q1 is turned off, the primary winding of the transformer is prone to voltage and current spikes. These components, combined, effectively absorb these spikes. The peak current signal measured from R3 participates in the duty cycle control of the current operating cycle, thus acting as a current limit for the current operating cycle. When the voltage across R5 reaches 1V, the UC3842 stops operating, and the switching transistor Q1 immediately turns off.
R1 and the junction capacitances CGS and CGD in Q1 together form an RC network. The charging and discharging of the capacitors directly affect the switching speed of the switching transistor. If R1 is too small, it is prone to oscillation and electromagnetic interference will be significant; if R1 is too large, it will reduce the switching speed of the switching transistor. Z1 usually limits the GS voltage of the MOSFET to below 18V, thereby protecting the MOSFET.
The gate voltage of Q1 is a sawtooth wave. The larger its duty cycle, the longer the conduction time of Q1, and the more energy the transformer stores. When Q1 is turned off, the transformer releases energy through D1, D2, R5, R4, and C3, which also achieves the purpose of magnetic field reset, preparing the transformer for the next energy storage and transfer. The IC constantly adjusts the duty cycle of the sawtooth wave at pin 6 according to the output voltage and current, thereby stabilizing the output current and voltage of the entire device.
C4 and R6 form a peak voltage absorption circuit.
3. Push-pull power conversion circuit:
Q1 and Q2 will be switched on alternately.
4. Power conversion circuit with drive transformer: T2 is the drive transformer, T1 is the switching transformer, and TR1 is the current loop.
IV. Output rectifier and filter circuit:
1. Forward rectifier circuit:
T1 is a switching transformer, with its primary and secondary terminals in phase. D1 is a rectifier diode, D2 is a freewheeling diode, and R1, C1, R2, and C2 form a peak clipping circuit. L1 is a freewheeling inductor, and C4, L2, and C5 form a π-type filter.
2. Flyback rectifier circuit:
T1 is a switching transformer with its primary and secondary terminals out of phase. D1 is a rectifier diode, and R1 and C1 form a peak clipping circuit. L1 is a freewheeling inductor, R2 is a dummy load, and C4, L2, and C5 form a π-type filter.
3. Synchronous rectifier circuit:
Working principle: When the upper end of the transformer secondary winding is positive, the current flows through C2, R5, R6, and R7, turning on Q2 and forming a circuit loop. Q2 is a rectifier diode. Q1 is cut off due to reverse bias. When the lower end of the transformer secondary winding is positive, the current flows through C3, R4, and R2, turning on Q1, which is a freewheeling diode. Q2 is cut off due to reverse bias. L2 is a freewheeling inductor, and C6, L1, and C7 form a π-type filter. R1, C1, R9, and C4 form a peak-shaving circuit.
