With the development and advancement of power electronics technology, modern variable frequency speed control systems employ multi-segment PWM (explained at the end of the article) technology. This technology utilizes the switching on and off of IGBTs (explained at the end of the article) to convert DC voltage into a pulse train of a specific shape, and achieves voltage regulation and frequency conversion by controlling the pulse width and pulse train period. Although modern variable frequency speed control systems have made the input current essentially sinusoidal, thus reducing the pollution of power supply quality by high harmonic currents, and have achieved a staged, approximately sinusoidal output voltage waveform, protecting the insulation of high-voltage motor windings.
However, based on the actual operation of its variable frequency speed control system, the application of variable frequency speed control devices to the design of modern AC motors still has its own particularities and requirements, which are issues that designers must carefully study and consider.
New variable frequency speed control systems have special requirements for AC motor design.
(1) Although the output power supply voltage waveform of the variable frequency speed control device is basically similar to a sine wave, it still contains high-order harmonic components, which causes the output torque of the motor shaft to fluctuate and generate pulsating torque. This will change the torsional resonance frequency of the shaft system. When the frequency of this torque fluctuation is close to or coincides with the inherent rotational frequency of the mechanical system of the shaft system, it will lead to the sudden occurrence of resonance, which will inevitably increase the vibration torque many times over. In severe cases, it can cause serious damage to the motor and the driven mechanical equipment.
Practice has shown that the pulsating torque caused by higher harmonic components will also increase the oscillation resistance of the motor's ventilation and cooling fans and blades; the design must also consider increasing the structural strength and damping effect of the cage bars and end rings of the rotor damping winding accordingly.
(2) During the variable frequency speed regulation process, designers must carefully consider the fatigue strain and impact transient torque that the variable frequency speed regulation control system will inevitably generate on the overall structure of the motor. In particular, it is necessary to take more effective technical measures for the main insulation of the high-voltage, high-power motor windings and the binding and fastening of the winding coil ends. Moreover, from the perspective of the development of motor design and manufacturing processes, more effectively eliminating the magnetic field resonance frequency at the winding coil ends is still an important technical issue that should be studied in depth in modern motor product design and process design.
(3) Frequent frequency conversion speed regulation process causes the motor to generate frequent changes in impact current, which will inevitably cause thermal stress and mechanical stress fatigue of rotating parts and components such as the cage bars and end rings of the damping winding, i.e. alternating thermal stress and mechanical stress fatigue damage.
(4) The variable frequency speed control device is the power supply for the AC motor variable frequency speed control system. Although very effective technical measures have been taken to greatly improve the output voltage waveform, the high-order harmonic components contained in the variable frequency speed control device will still increase the shaft voltage. Although it may not cause obvious harm, it should still be given great attention by the designer.
(5) If the motor is running at a low speed of 20%-30% of the rated speed controlled by the variable frequency speed control device, and the lubrication system of the sliding bearing is designed, the thickness of the lubricating oil film layer will be significantly reduced. The oil film between the shaft and the bearing bush will inevitably be destroyed, which will inevitably lead to metal-to-metal contact and grinding, and will also cause abnormal temperature rise of the sliding bearing.
(6) When designing an AC motor using a variable frequency speed control device, it is essential to carefully exclude dangerous rotational speeds that could cause resonance from the speed control range.
(7) For AC motors using variable frequency speed control devices, the calculation of their ventilation and cooling systems must be based on low speed, and their ventilation and cooling capacity must be verified and calculated.
(8) In the design of AC motors using variable frequency speed control devices, the insulation structure design must be enhanced accordingly to prevent the instantaneous generation of ultra-high frequency overvoltage impacts by power semiconductor devices, which may cause damage to the inter-turn insulation of the motor winding coils, thus shortening the insulation life of the motor or causing serious inter-turn insulation breakdown, leading to an inter-turn short circuit accident.
(9) When designing an AC motor with a variable frequency speed control device, it is necessary to prevent low-speed surge during the lowest speed operation in order to maintain stable operation at the lowest speed.
(10) The design of the variable frequency speed control system must ensure that the power supply system is not polluted, that is, the outflow of harmonic components to the power supply source must be strictly controlled.
Explanation of PWM and IGBT
PWM is a method of digitally encoding analog signal levels. Using a high-resolution counter, the duty cycle of a square wave is modulated to encode the level of a specific analog signal. The PWM signal is still digital because at any given moment, the full-amplitude DC supply is either fully present or completely absent. A voltage or current source is applied to the analog load as a repeating sequence of on/off pulses. On is when DC supply is applied to the load, and off is when the supply is disconnected. Any analog value can be encoded using PWM, provided the bandwidth is sufficient.
An IGBT (Insulated Gate Bipolar Transistor) is a composite, fully controllable, voltage-driven power semiconductor device composed of a BJT (Bipolar Junction Transistor) and a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) . It combines the advantages of both MOSFETs (high input impedance) and GTRs (low on-state voltage drop). GTRs have a low saturation voltage and high current density, but require a large drive current; MOSFETs have very low drive power and fast switching speed, but a large on-state voltage drop and low current density. IGBTs combine the advantages of both devices, offering low drive power and a low saturation voltage. They are ideally suited for applications in DC power conversion systems of 600V and above, such as AC motors, frequency converters, switching power supplies, lighting circuits, and traction drives.
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