Frequency conversion starting of asynchronous motor

Generally starts from 0HZ

It is best to start adjusting from 10HZ and set the starting frequency to 10HZ.

The general startup process is from low to high.

They all start from 0HZ and run to the lowest frequency (usually the lowest frequency is set at 10HZ), and then adjust to the required frequency.

For v/f control, it is better to set it higher and the starting torque is relatively small.

1. The concept of frequency converter is derived from the speed formula of asynchronous motor n=60f (1-s)/p (where: n-motor speed, f-power frequency, p-number of pole pairs of the motor, s-slip), it can be seen that when p remains unchanged and the frequency of the power supply is uniformly changed, the synchronous speed of the motor can be continuously changed. This speed regulation is called variable frequency speed regulation. 2. Frequency Converter Classification and Principle The key equipment of the frequency conversion speed regulating device is the motor frequency conversion speed regulator, also called a frequency converter. The frequency converter includes an AC-AC frequency converter, which is composed of a three-phase reverse parallel thyristor reversible bridge type and adopts the natural commutation principle of the power grid to form a frequency converter. AC--Direct--AC inverter video: 1. http://you.video.sina.com.cn/b/15567438-1400105074.html ； Principle of frequency converter 2. http://v.ku6.com/show/9f8POxyoJbMiAgsK.html Installation and wiring of frequency converter 3. http://v.ku6.com/show/MIBKbDujlpTohGLk.html Parameter setting two http://v.ku6.com/show/-Cw66a5iqt7oDL-u.html Parameter setting three http://v.ku6.com/show/u6WPSXcixf6hWguM.html Parameter setting seven 4, 5, http://v.ku6.com/show/a3He3UagCnD9Orz6.html Application example of frequency converter 3. Frequency converter wiring diagram 3.1 Main circuit wiring (1) Three-phase AC input terminals (r, s, t) of the frequency converter. The power input terminals are connected to the three-phase AC power supply through a circuit breaker for line protection or a circuit breaker with leakage protection. There is no need to consider the connection phase sequence. Special attention should be paid here that the three-phase AC power supply must not be directly connected to the output terminals of the inverter, otherwise it will cause damage to the internal components of the inverter. (2) The three-phase AC output terminals (U, V, W) of the inverter should be connected to the motor in the correct phase sequence. If the motor direction is wrong, just exchange any two phases of (u, v, w). This can also be achieved by setting the inverter parameters. It should be noted that the output terminal cannot be connected to phase capacitors and surge absorbers. (3) DC reactor connection terminals P+, P) The DC reactor connection terminals are connected to the DC reactor used to improve the power factor. There is a short-circuit conductor connected to the terminal. When using the DC reactor, the short-circuit conductor must be removed first. Notice: When the DC reactor is not used, this conductor does not need to be removed. (4) Braking unit connection terminals P and B) Generally, low-power inverters (0.75~15kw) have built-in braking resistors, while the braking resistors above 18.5kw must be external. (5) DC power input terminals The DC input terminals of the external braking unit are the positive and negative poles of the DC bus respectively. (6) Ground terminal (pe) The inverter will generate leakage current. The greater the carrier frequency, the greater the leakage current. The leakage current of the entire frequency converter is greater than 3.5ma. The size of the leakage current is determined by the usage conditions. To ensure safety, the frequency converter and motor must be grounded. 3.2 Notes (1) The grounding resistance should be less than 10ω. The diameter requirements of the grounding cable should be determined according to the power of the inverter. ； (2) Do not share the ground wire with welding machines and other power equipment ； (3) If the power supply line is shared with ground zero, it is best to consider laying a separate ground wire. ； (4) If multiple inverters are grounded, they should be connected to the earth respectively. Do not allow the ground wire to form a loop. 3.3 Control loop terminal wiring (1) Since the overcurrent of the low-voltage inverter control loop cable is generally very small, the size and specifications of the control loop cable can be standardized. In order to avoid malfunctions caused by interference, the control loop connection line should use twisted shielded wire. (2) Laying of control lines and main circuit cables. Lay the control lines of the inverter separately from the main circuit cables or other power cables, and try to keep them at least 100mm away from the main circuit. ； Try not to lay them parallel to the main circuit cables, and do not cross them with the main circuits. If they must cross, they should be vertically crossed. (3) Cable shielding The shielding of the inverter cable can be grounded metal pipe or shielded cable. One end of the shielding layer is connected to the common terminal (com) of the inverter control circuit, but do not connect it to the ground terminal (e) of the inverter. The other end of the shielding layer is left floating. (4) Switching control line The switching control line of the inverter is not allowed to use shielded wires, but two wires of the same signal must be twisted together, and the twisting distance of the twisted wires should be as small as possible. And connect the shielding layer to the ground terminal e of the frequency converter. The longest signal line cable must not exceed 50m. (5) Grounding of the control loop ●The wires of the weak voltage and current loop should be grounded at one point. The grounding wire should not be used as a circuit for transmitting signals. ； ●The grounding of the wire is done on the inverter side, using a dedicated grounding terminal and not sharing it with other grounding terminals. 3.4 The peripheral circuit of the main circuit is at the input circuit, that is, the power end, and should usually be connected to the circuit breaker QS (commonly known as the automatic air switch or air switch) and the contactor KM. VF is the code name of the frequency converter. The function of the circuit breaker is to turn off QS to isolate VF from power supply L1-L3 when installing, inspecting, or repairing the inverter. In addition, QS has automatic tripping protection functions such as short circuit. Contactor is also called AC contactor. Its function in the circuit is to facilitate operation. When the inverter fails, it can quickly cut off the power supply of the inverter. In most cases, the output end of the frequency converter should be directly connected to the motor. When wiring the output end, the following matters should be paid attention to:: (1) Contactors are not allowed to be connected between VF and M, so as to prevent the motor from starting directly at a certain frequency and causing overcurrent. (2) There is no need to connect a thermal relay between VF and M, because VF has a thermal protection function. (3) If one frequency converter is used to start multiple motors, each motor can be connected in series with a thermal relay. (4) Capacitors are not allowed to be connected to the output end of the frequency converter, capacitor filtering is not allowed, and capacitive single-phase motors are not allowed to be connected. The main circuit diagram of the frequency converter uses the frequency converter 1VF to control the blowers M1 and M2 respectively. 1KM11 and 1KM21 are the AC contactors used by the frequency converter to switch the M1 and M2 blowers respectively. The frequency converter can only start, stop and adjust the speed of one of the blowers at the same time. 4. Frequency converter insulation: Conduct insulation test on the main circuit before installation (1) The insulation test of the main circuit of the frequency converter generally uses a 5000v megger. (2) Disconnect all the terminals of the inverter's main circuit, control circuit and external circuits. (3) Use wires to connect the common terminal COM (some are marked as CM) of the frequency converter to the main terminals R, S, T, U, V, W, P, P+, B, etc.) firmly. Note that the exposed part of the wire must not come into contact with the shell or parts. (4) The other end of the megohmmeter is firmly connected to the ground terminal PE of the frequency converter. (5) Shake the megohmmeter at a speed of 120r/min. If the measured value is greater than 5 megohms, the insulation is normal. Conduct an insulation test on the control circuit before installation (1) Disconnect all the connecting wires between the inverter control circuit terminals and the external circuit. (2) Use a 500v megohmmeter to connect the control terminals to the COM terminal one by one. When the measurement is greater than 1 megohm, the insulation is normal. 5. Frequency converter ventilation The frequency converter has power loss during operation and is converted into heat energy, causing its own temperature to rise. Roughly speaking, for every 1 kva of inverter capacity, the power loss is about 40w to 50w. Therefore, when installing the inverter, we must consider the heat dissipation of the inverter and how to fully dissipate the heat generated by the inverter during operation, so we must pay attention to the installation method. (1) Wall-mounted installation The shell design of the inverter is relatively strong. Generally, it is allowed to be installed directly on the wall, which is called wall-mounted. In order to ensure good ventilation, all frequency converters must be installed vertically. The distance between the frequency converter and surrounding objects should meet the following conditions: greater than 100mm on both sides and 150mm above and below. In order to prevent debris from falling into the air outlet of the frequency converter and blocking the air duct, it is best to install a baffle above the air outlet of the frequency converter. (2) Cabinet installation method: When there is too much dust on site, relatively high humidity, or there are many peripheral accessories of the inverter and need to be installed together with the inverter, cabinet installation can be used. Cabinet-type installation of frequency converters is currently the best installation method, because it can effectively shield radiation interference, and can also prevent dust, moisture, and light. Precautions for cabinet installation: ●When a single frequency converter adopts the in-cabinet cooling method, an exhaust cooling fan should be installed on the top of the frequency converter cabinet and try to be installed directly above the frequency converter (this facilitates air circulation) ； ●Multiple frequency converters should be installed side by side as much as possible. If vertical installation is necessary, a partition should be installed between the two frequency converters. Since the cooling fan is a vulnerable item, the inverter fan is mostly controlled by a temperature switch. When the temperature inside the inverter is greater than the set temperature, the cooling fan will run. ； Once the temperature inside the inverter is lower than the temperature set by the temperature switch, the cooling fan will stop running. 6. Lightning protection of the frequency converter The lightning protection measures of the frequency converter device are another important peripheral measure to ensure the safe operation of the frequency converter. This issue is particularly important in areas with active lightning or active seasons. Today's inverter products are generally equipped with lightning absorption networks, which are mainly used to prevent damage to the inverter due to instantaneous lightning intrusion. However, in actual work, especially when the power line is introduced overhead, the lightning absorption network of the frequency converter alone cannot meet the requirements, and a special lightning arrester for the frequency converter needs to be installed. Specific measures include: (1) A special lightning arrester for frequency conversion can be installed at the power incoming line (optional) ； (2) Or embed a steel pipe 20m away from the frequency converter for special grounding protection according to the specifications. ； (3) If the power supply is introduced by cable, a lightning protection system in the control room should be installed to prevent lightning from entering and damaging the equipment. Practice has shown that the above methods can basically effectively prevent lightning strikes. 7. Precautions: 1. Things to note when setting parameters: Before using the inverter, set the inverter output voltage to 380v and the base frequency to 50hz ； For driving pumps and fans, the maximum frequency and upper frequency of the load are set to 50hz, and the lower limit frequency is set to 15-20hz. ； The acceleration and deceleration time should be determined based on the motor capacity and load inertia. The 11kw inverter can be set smaller (within 10s) ； Inverters above 11kw can be set longer (15-60s). 2. Things to note during wiring: Do not connect the power cord to the output terminal of the inverter ； Do not mistake the "N" terminal on the inverter output terminal strip for the power supply neutral terminal. ； The control circuit wiring should be as far away from the main circuit wiring as possible. 3. Grounding precautions: The smaller the grounding resistance of the PE grounding terminal of the frequency converter, the better. The cross-sectional area of ​​the grounding wire should be no less than 2mm2, and the length should be controlled within 20m. The grounding of the frequency converter must be separated from the grounding of the power equipment and cannot be grounded together. The shielding layer of the signal input line should be connected to PE, and the other end must not be connected to the ground, otherwise it will cause signal changes and fluctuations, causing the system to oscillate. 4. Precautions during operation: To run and stop the inverter, do not turn on and off the inverter power supply. Instead, use the run and stop terminals on the inverter control terminals, or use the run and stop buttons on the operation keyboard. Starting test: Slowly increase the operating frequency from 0hz and observe whether the drag system can start and at what frequency. If it is difficult to start, the starting frequency should be increased. Start the experiment: If the starting current is too large and trips, the speed-up time should be appropriately extended. If the starting current is too high at a certain speed, you should try to solve it by changing the starting method. Shutdown test: Check whether overvoltage or overcurrent causes tripping during shutdown. If so, extend the deceleration time appropriately. When the output frequency is 0hz, check whether the drag system has crawling phenomenon. If so, DC braking should be appropriately strengthened.

Go back upstairs: When our 1250Kw asynchronous motor (drives the acid-making fan) is started with variable frequency, the initial setting frequency is 20Hz according to the frequency converter manufacturer's requirements. What impact will it have on the components of the fan? Thanks!