1. Introduction With the development of frequency conversion speed regulation technology, frequency converter speed regulation has become the mainstream of AC speed regulation, and has been widely used in chemical fiber, textile, steel, machinery, papermaking and other industries. Since the general-purpose inverter generally adopts V/f control, that is, variable voltage variable frequency (VVVF) mode speed regulation, therefore, the inverter correctly sets its voltage-to-frequency ratio before use, which is essential for ensuring the normal operation of the frequency converter. The voltage-to-frequency ratio of the inverter is determined by the ratio of the reference voltage of the inverter to the reference frequency, that is, the reference voltage/reference frequency=voltage-to-frequency ratio. The setting of the reference voltage and the reference frequency parameter is not only related to the rated voltage and rated frequency of the motor (the voltage-to-frequency ratio of the motor is the ratio of the rated voltage of the motor to the rated frequency), but also the mechanical characteristics of the load must be considered. For the general asynchronous motor in general speed application, the reference voltage and the reference frequency are set according to the factory value (reference voltage 380V, reference frequency 50Hz), that is, the use requirements are met. However, for the more special motors used in some industries, the parameters of the reference voltage and the reference frequency must be reset according to the actual situation. Since these two parameters are not described in detail in the instruction manual of the inverter and related books, it is not easy for many users to set this parameter correctly. To this end, this paper combines the basic control method of frequency conversion speed regulation and the relationship between the mechanical characteristics of the load and the reference voltage and reference frequency parameters, and gives examples to explain the setting method of the reference voltage and the reference frequency parameters in detail.
2. The relationship between the basic control mode of the frequency conversion speed regulation and the reference voltage and the reference frequency There are two situations when the motor uses the frequency converter to adjust the speed - the fundamental frequency (reference frequency) is below the speed regulation and the fundamental frequency is above the speed regulation. An important factor that must be considered is to keep the main flux of the motor as constant as possible. If the magnetic flux is weak (the voltage is too low), the motor core cannot be fully utilized, the electromagnetic torque becomes small, and the load capacity decreases. If the magnetic flux is strong (voltage is too high), the motor is in an overexcited state, and the motor is severely heated due to excessive excitation current. According to the motor principle, the effective value of the electromotive force per phase of the stator of the three-phase asynchronous motor is: E1=4.44f1N1Φm where: E1--the effective value of the electromotive force induced by the air gap flux per phase of the stator, V; f1--stator frequency, Hz; N1——the effective number of turns per phase of the stator; Φm-per magnetic flux per pole can be seen from the equation, the value of Φm is determined by E1/f1, but since E1 is difficult to directly control, it can be ignored when the electromotive force is high. The stator leakage impedance drops and is replaced by the stator phase voltage U1. Then to ensure that Φm does not change, as long as U1/f1 is always a certain value. This is the basic situation of adjusting the fundamental speed below the fundamental frequency, and it is the constant voltage frequency ratio (constant flux) control mode, which belongs to constant torque speed regulation. The reference frequency is the highest frequency of the constant torque speed regulation zone. The voltage corresponding to the reference frequency is the reference voltage, which is the highest voltage of the constant torque speed regulation zone. When the speed is below the fundamental frequency, the voltage will change with the frequency. , but the ratio of the two does not change. When the speed is above the fundamental frequency, the frequency can be adjusted from the fundamental frequency up to the upper limit frequency value, but since the motor stator cannot exceed the rated voltage of the motor, the voltage no longer changes with the frequency, and the reference voltage value remains unchanged. The magnetic flux must be weakened with increasing frequency, the torque is reduced correspondingly, and the power remains basically unchanged, which belongs to the constant power speed regulation zone. It can be seen that the reference frequency is the lowest frequency of the constant power speed regulation zone, which is the turning point of the constant torque speed regulation zone and the constant power speed regulation zone, and the reference voltage value does not change with the frequency variation in the entire constant power speed regulation zone.
3. The mechanical characteristics of the load and the reference voltage and the setting of the reference frequency are reasonably used. It is necessary to understand the mechanical characteristics of the driven load. According to different purposes, the load can be basically divided into three categories: constant torque load, constant power load and square torque load. The torque required for constant torque load is basically not affected by the speed change (T= fixed value). For this type of load, the whole working area of ​​the inverter is preferably running below the fundamental frequency. At this time, the output characteristics of the inverter are just right. Can meet the requirements of the load. The higher the speed of the constant power load, the smaller the required torque (T × N = fixed value). For a constant power load, if the operating frequency of the motor is above the fundamental frequency, the required mechanical characteristics will be The output characteristics of the inverter match. As for the square torque load, the torque required is proportional to the square of the speed (T/N2 = fixed value), and the motor should be operated below the fundamental frequency. It should be noted that the operating frequency of the square torque load must never exceed the power frequency (unless the inverter capacity is one level larger). Otherwise the inverter and motor will be heavily overloaded.
4, setting example Example 1: A chemical fiber spinning metering pump motor model is FTY-550-6, namely 550W, 6-pole three-phase permanent magnet synchronous motor. The nameplate parameters are as follows: Operating voltage: 62.5 - 125 - 475V. Operating frequency: 25 - 50 - 190HZ, motor power: 275 - 550 - 2090W, speed: 500 - 1000 - 3800R / min, current: 4A. The working range is wider. The nameplate parameters are different from the general asynchronous motor. The value on the left is the lower limit of the normal operation of the motor (no loss of step), the value on the right is the maximum value when the motor is working normally, and the middle value is the rated value (50HZ). The motor has a voltage-to-frequency ratio of 125V/50HZ=2.5, and uses a three-inch SAMCO-I 1HF1.5K inverter. If only according to the motor parameter setting, the rated voltage and rated frequency of the motor are both the reference voltage and the reference frequency of the inverter. The reference voltage (code is CD005) is set to 125V, and the reference frequency (CD006) is 50HZ (factory value). No change, this setting, when the motor works below the fundamental frequency, the motor drives the metering pump without any problem, but the metering pump belongs to the constant torque load. If the metering pump requires a higher speed (such as 90HZ), then the frequency can be Adjusted to 90HZ, but the motor working voltage is still 125V, the actual pressure ratio is 125/90HZ=1.39, as shown in Figure 2a, the electromagnetic torque becomes smaller, unable to provide the required torque of the load, so that the metering pump can not work normally. The correct setting should be: CD005=475V, CD006=90HZ, where the reference voltage is set to 475V, but since the inverter does not have a boost function, its actual output voltage is determined by the maximum value of the input voltage, so this setting It is only effective for increasing the slope of the V/F pattern, and it does not really reach 475V. Therefore, it can also be set as follows: CD005=380V, CD006=152HZ, the frequency-to-frequency ratio of the inverter is still 380V/152HZ=2.5, and the whole working section of the motor is in the constant torque speed regulation range, which satisfies the requirements of load characteristics. .
Example 2: A three-phase asynchronous speed-regulating motor for textiles, rated power 60W, rated voltage 110V, rated frequency 50HZ, speed range 40-110HZ, rated current 0. 34A, 4 poles, so the voltage-to-frequency ratio of the motor is 110V/50HZ=2.2. The driven load is a constant power characteristic. The drive inverter was originally prepared to use Fuji FRN1.5G11S-4CX (driving six motors), but the reference voltage of the inverter (Fuji inverter rated voltage) can only be adjusted to 320V at the lowest. According to the voltage-to-frequency ratio of the motor, the motor must be operated. At 50HZ, the working voltage is 110V, and the motor can work normally. However, the load operating speed adjustment range is wide. If the operation is required at 110HZ, then the motor voltage will reach 242V, which is more than twice the rated voltage. The result can be imagined. If the motor operating voltage is set to 110V at 110HZ, the rated voltage is 320V (the lowest value), and the reference frequency is 320HZ. When the motor is running at 110HZ, the voltage is exactly the rated voltage of the motor. However, the frequency-to-frequency ratio of the inverter is 320V/320HZ=1. Therefore, when the motor runs at 40HZ, its voltage is only 40V. Obviously, there is not enough power to drive the load. Therefore, this type of Fuji inverter can not meet the requirements of use. Switch to Sancha SAMCO-I IHF1.5K inverter, set the reference voltage CD005=110V, the reference frequency CD006=50HZ, so that the motor keeps the voltage value at 110V from 50-110HZ speed regulation, the motor works at constant power The speed zone is consistent with the mechanical characteristics of the load and will no longer exceed the rated voltage or power of the motor.
5. Conclusion This article elaborates on the method of setting the reference voltage and reference frequency of the inverter according to the mechanical characteristics of the load and the motor parameters. It can be seen that the correct setting of the reference voltage and reference frequency of the inverter can fully utilize The performance of the inverter can meet the requirements of the load on the inverter. If the setting is not correct, the inverter and the motor will not even work. Therefore, correctly setting the voltage-to-frequency ratio of the inverter is the key to using the inverter.
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