18 Questions on Basic Knowledge of Inverters

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1. What is an inverter? An inverter is a power control device that uses the on-off action of power semiconductor devices to convert an industrial-frequency power supply into electrical energy of another frequency. 2. What is the difference between PWM and PAM? PWM is the abbreviation for Pulse Width Modulation; it is a method of regulating the output level and waveform by changing the pulse width of a pulse sequence according to a certain pattern. PAM is the abbreviation for Pulse Amplitude Modulation in English; it is a modulation method that varies the amplitude of pulses in a pulse sequence according to a certain pattern, in order to adjust the output value and waveform. 3. What is the difference between voltage-type and current-type? The main circuit of inverters can be broadly divided into two categories: the voltage-type inverters are those that convert direct current from a voltage source into alternating current, with capacitors being used for filtering in the DC circuit; the current-type inverters are those that convert direct current from a current source into alternating current, with inductors being used for filtering in the DC circuit. 4. Why does the voltage of an inverter change proportionally to its current? The torque of an asynchronous motor is generated by the interaction between the magnetic flux in the motor and the current flowing through the rotor. At the rated frequency, if the voltage remains constant while only the frequency is reduced, the magnetic flux becomes too high, causing the magnetic circuit to saturate; in severe cases, this can lead to the destruction of the motor. Therefore, the frequency and voltage must change proportionally; that is, while changing the frequency, the output voltage of the inverter is controlled to keep the magnetic flux of the motor constant, thereby preventing magnetization weakening and magnetic saturation. This control method is commonly used in energy-saving frequency converters for fans and pumps. 5. When a motor is driven by a power supply at the mains frequency, a decrease in voltage results in an increase in current; for drives powered by inverters, if the voltage also decreases when the frequency drops, does the current increase as well? When the frequency decreases (low speed), if the same power is output, the current increases; however, with a constant torque, the current remains almost unchanged. 6. When operating with an inverter, what are the motor’s starting current and starting torque? Operation is carried out using an inverter, which increases the frequency and voltage as the motor accelerates, thereby keeping the starting current below 150% of the rated current (ranging from 125% to 200%, depending on the model). When starting directly with a power frequency supply, the starting current is 6 to 7 times higher, thus causing mechanical and electrical shocks. Using an inverter for drive allows for a smooth start-up (the start-up time is prolonged). The starting current is 1.2 to 1.5 times the rated current, and the starting torque is 70% to 120% of the rated torque ； For inverters with a torque automatic enhancement function, the starting torque is 100% or more, allowing them to start under full load. 7. What does V/f mode mean? When the frequency decreases, the voltage V also decreases proportionally; this issue has been explained in Answer 4. The proportional relationship between V and f is predetermined taking into account the motor characteristics; usually, several such characteristics are stored in the controller’s memory device (ROM), and can be selected using switches or a dial. 8. How does the motor’s torque change when V and f are modified proportionally? If the voltage decreases in perfect proportion as the frequency drops, then since the AC impedance becomes smaller while the DC resistance remains unchanged, there will be a tendency for the torque generated at low speeds to decrease. Therefore, at low frequencies, given a specific V/f ratio, to increase the output voltage somewhat in order to obtain a certain starting torque, this type of compensation is known as enhanced starting. It can be achieved using various methods, such as automatic methods, selecting the V/f mode, or adjusting potentiometers. 9. The manual states that the speed range is 60–6 Hz, which corresponds to a ratio of 10:1. So, is there no output power below 6 Hz? Power can still be output at frequencies below 6Hz, but depending on factors such as the motor’s temperature rise and starting torque, the minimum operating frequency is around 6Hz; at this frequency the motor can deliver its rated torque without causing significant heating problems. The actual output frequency (start-up frequency) of the inverter ranges from 0.5 to 3 Hz, depending on the model. 10. For combinations with ordinary motors, it is also required that the torque remain constant at frequencies above 60 Hz – is this possible? Generally, it’s not possible. Above 60 Hz (there are also modes above 50 Hz), the voltage remains constant, resulting in a roughly constant power characteristic; when the same torque is required at high speeds, care must be taken in selecting the capacity of the motor and the inverter. 11. What does \"open loop\" mean? When a speed detector (PG) is installed in the motor device used to feed back the actual rotation speed to the control unit for regulation, this is called a \"closed-loop\" system; operation without a PG is referred to as an \"open-loop\" system. Most general-purpose frequency converters operate in an open-loop mode, although some models can utilize an option for PG feedback. 12. What to do when the actual rotational speed deviates from the specified speed? In open-loop mode, even though the inverter outputs a specified frequency, when the motor operates under load, its speed varies within the range of the rated slip rate (1%–5%). For applications that require high speed control accuracy and need to operate at a speed close to the set value even when the load changes, an inverter with PG feedback functionality (an optional feature) can be used. 13. If a motor with PG is used, can the speed accuracy be improved through feedback? Inverters with PG feedback function achieve improved accuracy. However, the speed accuracy depends on the precision of the PG itself and the resolution of the frequency output by the inverter. 14. What does the stall prevention function mean? If the given acceleration time is too short, the change in the output frequency of the inverter exceeds by far the change in speed (electrical angular frequency); as a result, the inverter trips due to excessive current flow and stops operating, a condition known as stall. To prevent stalling and keep the motor running, it is necessary to detect the current level and perform frequency control. When the acceleration current is too high, appropriately reduce the acceleration rate. The same is true when decelerating. Combined, the two form the stall function. 15. What is the significance of machines where the acceleration time and deceleration time can be specified separately, as opposed to those where both acceleration and deceleration times are specified together? Models in which acceleration and deceleration can be specified separately are suitable for situations requiring short acceleration times and slow deceleration, or for small machine tools where it is necessary to precisely set the production cycle time. However, in applications such as fan drives, the acceleration and deceleration times are relatively long, and both times can be specified together. 16. What is regenerative braking? If the command frequency is reduced while the motor is running, the motor operates in a non-synchronous generator mode and functions as a brake; this is known as regenerative (electrical) braking. 17. Can greater braking force be achieved? The energy regenerated by the motor is stored in the filter capacitors of the inverter. Due to the relationship between the capacitance value and the voltage rating of these capacitors, the regenerative braking force of a standard inverter is approximately 10% to 20% of the rated torque. If a selected brake unit is used, 50%–100% can be achieved. 18. Torque enhancement issue: The setpoint signals of an automatic control system can be used to adjust the frequency flexibly through an inverter, thereby controlling various process parameters. For example, in the tobacco industry, during the processes involving sugars and flavors, the flow rate signal from a belt scale can be used to control the frequency of the inverter; this allows the pump speed to change automatically in response to the flow rate signal, thus regulating the amount of material added and ensuring even distribution of flavors and sugars. The start/stop signals from the production line can also be used to control the start/stop and forward/reverse operation of the inverter through the positive and negative terminals, thus making it part of an automated assembly line. Furthermore, on an assembly line, when the equipment ahead fails, the equipment behind should shut down automatically. The emergency stop terminal of the inverter can achieve this function. In the SANKEN, MF, FUT, and FVT series of frequency converters, three to four or even up to seven frequencies can be preset, and automatic production processes can be set up using these values in some devices. After setting the operating frequency and time, the inverter enables the motor to run at different speeds at various times in sequence, thus creating an automated production process.