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Some notes on the frequency converter for the condensate pump motor: The frequency converter for the #1 condensate pump motor of Unit 6 in our plant is a Beijing Legrand A06/130 model. The capacity of this frequency converter is 1350 KVA, with a rated output current of 130 A. When the load on the frequency converter exceeds 20% of its rated load, the input power factor is 0.95. The output frequency range is 0–50 Hz, and the acceleration/deceleration time ranges from 0.1 S to 3000 S. It consists of a phase-shifting transformer, power units, and a controller. There are 21 power units; every 7 of them are connected in series to form one phase. Each power unit has exactly the same structure and electrical properties, allowing them to be interchanged. The circuit structure of the power unit is shown in Figure 2; it is a basic single-phase AC-DC-AC rectifier-inverter circuit. The rectifier side uses three-phase fully controlled diodes, while the IGBT inverter bridge is controlled via sine PWM. 1. Frequency-closed-loop speed with open-loop control results in a deviation between the calculated speed and the actual speed. The “actual” speed of the motor is displayed on the CRT; it is determined by calculating based on the operating frequency and the motor’s load conditions. The synchronous speed of the motor is proportional to its operating frequency, while the slip rate s is essentially proportional to the motor’s load current. The motor’s speed is calculated using the following formula: n = (60f/p) * (1-s) = n0 * (1-s), where p represents the number of pole pairs, s is the slip rate, n0 is the synchronous speed of the motor, n is the actual speed of the motor, and f is the current operating frequency of the motor. The frequency in the formula is the actual output frequency of the inverter, as measured by the inverter itself, while the slip rate s is determined by the function s=f(i); here, the current i represents the current output by the inverter. The function s=f(i) is a downward-sloping line with the output current of the inverter on the horizontal axis and the slip rate on the vertical axis, as shown in Figure 1. The \"actual\" speed thus determined is close to the actual speed when the motor is operating under normal load, and it can basically serve as a substitute for the actual speed to meet the needs of operation monitoring. However, this approach is not effective in special situations, such as when the frequency converter is brought online while the motor has not yet started. In such cases, the bias frequency set by the frequency converter is 0HZ; once the frequency converter receives a command to adjust the frequency, the motor receives a command to operate at a low frequency. As a result, the starting torque is insufficient, causing the motor to stall. Although the output voltage of the frequency converter is not very high during the initial startup phase due to the constant voltage-frequency ratio constraint, the stalling current of the motor is usually quite high. The slip rate calculated based on this stalling current will be much less than 1, and the speed calculated using the speed formula will be much higher than 0 rpm. Moreover, as the frequency converter accelerates and the output voltage and frequency increase, the stalling current becomes even greater, the calculated slip rate decreases further, and the calculated speed increases still more. This creates an illusion of operation for remote monitoring, giving the impression that the motor has already started. Therefore, when a variable-frequency drive drives a motor to start, the acceleration time should be short, and the motor should not remain in the low-frequency range for too long. To determine whether the motor has started from a distance, a comprehensive assessment based on thermal parameters is necessary; changes in outlet pressure, flow rate, and water level can all indirectly indicate whether the motor has started. 2. Regarding the offset value of the inverter’s output frequency, a too high output frequency makes smooth starting difficult. The regulations state: “Check that the opening degree of the inverter-driven condensate pump is 0% (to prevent high temperatures in the thrust bearings; an opening degree of 20% is also acceptable).” #The bias frequency for pump #1 of the 6-pump set is set at 0.5 Hz. Once the inverter receives the operation command but the operator does not send a frequency adjustment signal, the motor controlled by it will continue to receive the command to operate at 0.5 Hz and thus cannot start. The stall current of the motor should not be sustained for too long, as this could very likely trigger the inverter’s overload protection mechanism. Therefore, the operator should send the frequency adjustment signal quickly, avoiding prolonged operation at low frequencies; it is best to set the inverter’s command for the pump’s flow rate to 20% before starting the inverter. 3. Regarding the closed-loop control of the frequency converter with a constant voltage-to-frequency ratio (U/f = constant), the output is calculated based on the formula for the induced electromotive force in the motor: E = 4.44fNΦm. When the number of turns in the motor stator coil, N, remains constant, the amplitude of the magnetic flux, Φm, is proportional to E/f. If the voltage drop due to stator impedance is ignored, then the stator voltage U ≈ E. If the electromotive force E remains unchanged, the voltage U also remains unchanged; however, when the frequency f decreases, the amplitude of the magnetic flux Φm increases. This causes the motor stator core to become oversaturated, resulting in a sharp increase in the excitation current, a significant rise in iron loss, a decrease in the motor’s power factor, as well as increased mechanical and electromagnetic vibrations and noise. Therefore, the frequency converter adjusts both the voltage and current while regulating frequency, in order to maintain a relatively constant voltage-to-frequency ratio and flux saturation level. Closed-loop control of the output voltage, current, and frequency is employed to improve voltage control accuracy and stability under dynamic loads; this also leads to an improvement in the current waveform to a certain extent, and essentially solves the problem of smooth speed regulation for asynchronous motors. Another advantage of this control method is its significant ability to suppress the overvoltage and overcurrent caused by regeneration, thereby enabling rapid acceleration and deceleration. 4. Regarding overcurrent during acceleration, the output capacity of the inverter should be sufficient to enable the motor to start properly and operate at full capacity. When overcurrent occurs in the inverter during startup acceleration or normal operation, it cannot be simply assumed to be a fault of the inverter; instead, it is necessary to consider the possibility of a motor fault, and the motor can be checked by disconnecting the connection between the inverter and the motor. 5. Input power factor and energy-saving effect of the inverter: The input power factor of an inverter is the ratio of the active power to the apparent power on the input side. The reasons for the decrease in the power factor λ are current waveform distortion and phase lag of the input current. The power factor λ = ξ COSΦ, where ξ is the distortion factor and COSΦ is the displacement factor. Waveform distortion is caused by higher harmonics, and the currents associated with these harmonics are almost always reactive currents. Appropriate measures have been taken in the design and manufacture of large frequency converters to address this issue; for example, phase shifts are created using the star and delta connections of phase-shift transformers, and the control method of connecting 7 power units in series increases the number of pulses in the rectifier circuit thereby reducing reactive power. The closer the current waveform is to a sine wave, the closer the distortion factor ξ is to 1. The power factor λ ≈ COSΦ, where Φ is the phase difference between the fundamental wave of the primary phase voltage and that of the primary phase current, and COSΦ is the cosine of this phase difference. During off-peak periods, when the inverter operates at a reduced capacity, the conduction angle of the insulated gate bipolar transistors (IGBTs) decreases while the control angle α increases. As a result, the phase difference Φ between the input current waveform and the voltage waveform increases, causing the displacement factor COSΦ, or power factor λ, to decline. When the load on the inverter exceeds 20% of its rated value, the input power factor is above 0.95, and it can be considered a constant of 1. However, during off-peak operation with reduced capacity, the input current I decreases significantly; therefore, the active power P∝UICOSΦ is essentially proportional to the current I, meaning that the active power declines in proportion to the current, resulting in significant energy savings. 6. PWM control technology has always been one of the core technologies in frequency conversion. Since PWM can simultaneously achieve frequency and voltage conversion as well as harmonic suppression, it is widely used in AC drive systems and other energy conversion systems. PWM control techniques can be roughly divided into three categories: sine PWM (including various PWM schemes in which sine is the target for voltage, current, or flux; multiple PWM also falls into this category), optimized PWM, and random PWM.