Several methods of VFD (Variable-frequency Drive) load test
I. Introduction
With the rapid development of VFD (Variable-frequency Drive) technology in China and the rapid rise of VFD (Variable-frequency Drive) manufacturers, large VFD (Variable-frequency Drive) applications and manufacturers are in urgent need of VFD (Variable-frequency Drive) ) performance test, optimize the loading device. How to choose an effective testing unit has become a topic worth studying.
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2. Introduction to the principle of slip motor
Since the following content mostly uses electromagnetic speed-adjustable asynchronous motors, commonly known as slip motors, it is necessary to give a brief introduction to the principle of slip motors. The electromagnetic speed-adjustable asynchronous motor is composed of three parts: an ordinary squirrel-cage asynchronous motor, an electromagnetic slip clutch and an electrical control device. The asynchronous motor is used as a prime mover. When it rotates, it drives the armature of the clutch to rotate together. The electrical control device is a device that provides the excitation current of the excitation coil of the slip clutch.
Figure 1 is a schematic diagram of the structure of the electromagnetic slip clutch, including three parts: armature, magnetic pole and excitation coil. The armature is a cylindrical structure made of cast steel, which is connected with the shaft of the squirrel-cage asynchronous motor, commonly known as the active part; the magnetic pole is made into a claw-shaped structure, mounted on the load shaft, commonly known as the driven part. There is no mechanical connection between the active part and the driven part. When the excitation coil passes through the current to generate a magnetic field, the claw structure forms many pairs of magnetic poles. At this time, if the armature is dragged and rotated by the squirrel-cage asynchronous motor, it will cut the magnetic field and interact to generate torque, so the magnetic poles of the driven part will rotate with the armature of the active part, and the speed of the former is lower than that of the latter. Because only when there is relative motion between the armature and the magnetic field, the armature can cut the lines of magnetic force. There is no essential difference between the principle that the magnetic poles rotate with the armature and the principle that the rotor of an ordinary asynchronous motor moves with the rotating magnetic field of the stator winding. The difference is that the rotating magnetic field of the asynchronous motor is generated by the three-phase alternating current in the stator winding, and the electromagnetic slip clutch The magnetic field is generated by the DC current in the excitation coil, and it acts as a rotating magnetic field due to the rotation of the armature.
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Figure 1 Schematic diagram of the basic structure of the electromagnetic slip clutch
When running stably, the load torque is equal to the electromagnetic torque of the clutch. When the load is constant, the magnitude of the excitation current determines the speed of the driven part. The larger the excitation current, the higher the speed; on the contrary, the smaller the excitation current, the lower the speed. According to this characteristic, the speed and torque of the driven part can be adjusted very conveniently by using the electric control circuit.
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3. Stall method of a single slip motor
This method is to directly use a single slip motor to output the spindle of the slip motor (as shown in Figure 1), and connect it mechanically to the machine base. At this time, the speed of the output spindle is always zero. By loading the DC voltage on the excitation coil to adjust the magnitude of the excitation current and output torque, which is used to adjust the size of the load, as shown in Figure 2.
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Figure 2 Schematic diagram of single slip motor stall method
This method requires the user to prepare a 0~60~90V/2~8A (maximum) DC adjustable voltage source. If there is no suitable power supply, it can be realized by using a voltage regulator plus a rectifier filter circuit, as shown in Figure 3. In addition, since the speed controller is usually attached when the slip motor is purchased, it can be realized by canceling the voltage closed-loop control part of the original slip motor speed controller and changing it into a single-phase SCR voltage regulation circuit. However, the disadvantage of this method is that the voltage output is nonlinear. In the initial stage, the output voltage changes slowly and the load is slow. When the output voltage is high, the output voltage changes quickly and the load adjustment is difficult.
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Figure 3 DC excitation voltage generation circuit - voltage regulation and rectification circuit diagram
This method has the advantages of simplicity and low cost, and is suitable for medium and high-speed loading test occasions of small and medium power VFD (Variable-frequency Drive). Since the fast loading and unloading cannot be realized through the excitation, the dynamic performance test and the performance test of the power generation state cannot be realized. In addition, due to the low speed of the slip motor, the relative operating speed of the slip motor is low, so low-speed loading cannot be realized.
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4. Two asynchronous motors dragging each other through slip motors
This method uses a slip motor to be coaxially connected to another asynchronous motor, and the two motors can be driven by two VFDs (Variable-frequency Drive), as shown in Figure 4.
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Figure 4 Schematic diagram of two asynchronous motors dragging through a slip motor
This method can adjust the size of the load by applying a DC voltage to the excitation coil, or adjust the size of the load by adjusting the relative speed of the two motors. That is to say, the loading of reverse electric operation and the loading of same-phase power generation operation can be realized. Due to the existence of relative speed, compared with the above three single-slip motor solutions, zero-speed or low-speed loading can be achieved. The disadvantage is that the loading of the slip motor is realized by electromagnetic induction and slip, the loading response speed is slow, and fast loading cannot be achieved, so it cannot meet the high-precision and fast performance test.
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5. Two AC motors pair towing method
This method uses two asynchronous motors of the same power to be connected coaxially, and the two motors are respectively driven by two VFDs (Variable-frequency Drive), as shown in Figure 5. One of the motors is driven by a test VFD (Variable-frequency Drive), and the other is driven by a closed-loop vector control VFD (Variable-frequency Drive) with precise torque control functions, such as Emerson's TD3000 series products. By changing the magnitude and direction of the torque, it can be used as the load of the motor under test, and the performance of the VFD (Variable-frequency Drive) can be verified.
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Figure 5 Two AC motors towing method
This method can realize the loading of reverse electromotive operation, and can also realize the loading of in-phase power generation operation. Due to the closed-loop torque control, it can realize zero-speed, low-speed and high-speed high-torque and high-precision loading. Due to the mechanical hard connection of the motor, the torque response of the asynchronous motor is faster than that of the slip motor, and the loading response speed is faster, which can meet the test requirements of most occasions, but it is not enough for high-precision and fast performance tests. satisfy.
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6. Towing method of AC and DC units
This method adopts a coaxial connection between a DC motor and another asynchronous motor, as shown in Figure 6. Among them, the asynchronous AC motor is driven by the tested VFD (Variable-frequency Drive), and the DC motor is driven by a DC speed controller that can operate in four quadrants. Through precise torque control, the DC motor can change the magnitude and direction of the test torque, and the load of the motor under test can be changed arbitrarily, and the performance of the VFD (Variable-frequency Drive) test can be verified.
This method can realize the loading of reverse electromotive operation, and can also realize the loading of in-phase power generation operation. Due to the closed-loop torque control of the DC motor, it can realize zero-speed, low-speed and high-speed high-torque and high-precision loading. Due to the mechanical hard connection of the motor, the torque response of the DC motor is fast, and the load response