VFD Controllers, Applications and Schemes This article discusses the concept of a Variable Frequency Drive (VFD) in many of the most important aspects in choosing the right VFD for applications. An important factor is the type of control required for a particular application because it is not uncommon to mistake the type of control that does not provide the required performance. gibbonsgroup.co.uk) Also, the expected effects on kinetic applications when VFDs are used in conjunction with grounding requirements are discussed. VFD is often associated with low voltage applications; MV implementations are mostly ignored, so the main differences are highlighted. Finally, VFD diagrams are introduced to shed some light on common graphics that can be encountered. Table of Contents: Introduction to Control Types of VFDs VFD: V/f Control (Speed ​​Control at Constant Torque) Phasor (Vector) Control Direct Torque Control Impact from the inverter on the insulation system Rise time Cable length Frequency switching High frequency withstand currents VFD Grounding of LV and MV Applications Schemes of VFDs 1. Introduction to VFDs The simplest way to control the speed of a motor is to run it at full speed, but many applications require a variable speed. Several technologies were used before the advent of the variable frequency drive (VFD) to control speed as follows: control valves, dampers, and vanes; eddy current clutches. Fossil fuel beam engines have variable pitch hydraulic coupling and DC solid state control. Initially, DC motors were used as VFD motors because they could easily achieve the required speed and torque without the need for sophisticated electronics. Desire to simulate the excellent performance of a DC motor, such as rapid response to torque and speed accuracy, while using powerful, inexpensive and maintenance-free AC motors. Figure 1 shows the block diagram of a typical three-phase VFD controller. – Block diagram of a typical Variable Frequency Drive (VFD) Figure 1 – Block diagram of a typical VFD The function of each VFD block is as follows: A full-wave rectifier converter converts the applied alternating current into a DC vector. The link connects the output of the rectifier to the input of the inverter. The DC bus acts as a filter to smooth out uneven and undulating outputs to ensure that the rectified output is as similar as possible to pure DC. Inverter The inverter takes the filtered DC current from the DC vector and converts it into a pulsed DC waveform. By controlling the inverter’s output, the pulsating DC waveform can simulate the AC waveform at different frequencies. Such as SCRs and transistors. The participating control circuits equitably coordinate the switching of power devices, usually through a control panel that dictates the firing of power components in the correct sequence. A built-in microprocessor is used for all internal logic and resolution requirements. The number of converter pulses determines the number of switching operations that are used during one base period to convert AC to DC. The primary transformer for a three-phase system is the B6 topology (see Figure 2). The B6 switch uses 6 relays during one prime period. This results in a toggle every 60 degrees. Table 1 – Transformer pulses execution Number of transformer pulses Execution 6 – Impulse base on 1 circuits B6 12 – Impulse base on 2 circuits B6 18 – Impulse base on 3 circuits B6 24 – Impulse base on 4 circuits B6 Table 1 lists common transformer pulse numbers . In addition, the number of secondary terminals of an isolation transformer (if any) is proportional to the number of pulse motor according to the following formula: xmfr # Secondary x 6 = Converter # pulses For example, a drive with one secondary would have 6- pulse transformer while A secondary 4 motor results in a 24 pulse transducer. Figure 3 shows a 12-pulse transformer with two B6 transformers and two secondary transformers. This article/technical guide requires a premium membership. You can choose a Plus, Pro, or Enterprise membership plan on an annual basis. Subscribe and enjoy the study of specialized technical articles, online video courses, electrical engineering guides and research. With an EEP Premium Membership, you get an additional core that enhances your knowledge and expertise in the low, medium and high voltage engineering fields. Check the benefits of each plan and choose the membership plan that best suits you or your organization. Get 15% off the Pro plan, use discount code: UX226 Login »Learn More»
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