40 November/December 2023 | E-Mobility Engineering Vector drives have similarities with scalar ones in that they drive the motor with sinusoidal current; however, vector drives provide smoother operation, quicker acceleration and superior torque control. These control systems often use field-oriented control using two orthogonal vectors whose magnitudes relate to the torque and magnetic flux within the motor. The control system must measure the position of the rotor in order to synchronise the system. This is often done by using sensors such as Hall effect sensors or a quadrature encoder interface. (Sensorless systems are also used in which the control system uses the back-EMF of the motor to determine rotor position.) The controller uses the Clarke and Park transform to calculate the magnitudes of the vectors, then uses these values as setpoints for the control loop. Motor drive measurements involve relatively high voltages. For example, the DC bus voltage in a 480 VAC threephase motor drive is typically around 680 VDC, while the common-mode voltages can also be relatively high. Field-oriented motor control algorithms maintain optimal performance for three-phase AC motors under all operating conditions. Real-time motor torque and power estimates optimise the power of the motor using a combination of customised field-oriented control algorithms and PWM switching to provide torque and system efficiency across the entire torque/speed spectrum. A dual-core microcontroller can allow additional code to run on the motor controller to act as a system controller, eliminating the need for costly additional controllers. This is particularly suitable for equipment such as materials handling trucks, mobile elevating work platforms, airport ground support and construction equipment. Wiring configurations Both the input and output of variablefrequency drives (VFDs) often use three phases. However, some VFDs used by automotive drive systems might be powered by single-phase AC or DC. In addition, three-phase systems can be wired and modelled in two configurations: star and delta. The wiring configuration determines the calculations used in power analysis, so it is important to understand and select the correct wiring configuration in order to get the expected results. These configurations apply to both the inputs and outputs of motor drives. Even though only two wattmeters are required to measure the total power in a three-wire system, there are advantages to using three. The threewattmeter configuration requires six oscilloscope channels: three voltages and three currents. This 3V3I configuration provides individual phase-to-neutral voltages and the power in each individual phase, which is not available in the twowattmeter configuration. For three-wire systems measured with 3V3I, the software includes a setting to convert line-to-line (L-L) to line-toneutral (L-N) voltages. Although there is no physical neutral in this system, it is possible to determine the instantaneous L-N voltages from the instantaneous lineto-line voltages. This point-by-point LL-LN conversion expresses all the voltages relative to a single reference and corrects the phase relationships between voltage and current for each phase. Turning on LL-LN conversion allows instantaneous power calculations by multiplying phase-toneutral voltages and phase currents. Three voltage channels and three current channels are required to measure the total power in a system that uses a neutral conductor between the line and the drive, or the drive and the motor. The voltages are all measured relative to the neutral. Phase-to-phase voltages can be accurately calculated from the phaseto-neutral voltage amplitudes and phases using vector mathematics. It is recommended to use built-in software in an oscilloscope that includes a three-phase autoset function, which automatically configures voltages and current sources based on the selected wiring configuration. It will optimally set up the vertical, horizontal, acquisition and trigger parameters on the oscilloscope, and can be done on all active power measurements. This greatly simplifies measurement set-up, especially for PWM waveforms on the output of the VFD, enabling faster and more accurate measurement. Focus | Motor control The Model HVi F5-R provides accurate speed and torque control of three-phase AC induction and permanent magnet AC (PMAC) motors (Courtesy of Curtis Instruments)
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