Due to their excellent control capabilities and efficiency, frequency converters ( VFDs) are primarily used to drive electric motors. However, most traditional VFDs non-linearly draw current as part of their power conversion process, and the resulting distorted current waveform means that high-order harmonics are present. Harmonics can cause upstream electrical equipment to overheat, reduce power factor, decrease efficiency, and generate voltage harmonics, which may lead to the problems mentioned in IEEE 519.
Input harmonic filters are a common method for mitigating harmonics by capturing them to isolate the power supply from the nonlinear current consumption of the inverter. Adding such equipment to the system increases cost and space requirements and reduces efficiency. An alternative to filters is matrix drive technology, which can reduce harmonics without adding external equipment. The following six aspects highlight the potential benefits of using matrix drive technology.
Typically, the harmonics of a matrix drive are 5% lower than the total harmonic distortion (THD) at the inverter input. (Image source: Yaskawa)
1. Testing harmonics
Low input current harmonic performance is a key advantage of using matrix technology for power conversion. When operating a motor, current is drawn from lines with low harmonic levels. Typically, the harmonics of a matrix drive are less than 5% of the total harmonic distortion (THD) at the inverter input. Input filters mitigate harmonics generated by conventional inverters. The harmonics of a matrix drive throughout the entire load curve are consistently lower than those of an inverter with harmonic filters.
2. Testing efficiency
Harmonic filters require additional equipment and wiring to maintain low harmonics. As a result, they are less efficient across the entire load range compared to matrix drives, which ensure low input current harmonics, efficiency close to 98%, and maintain operating efficiency throughout the entire operating range.
3. Test reactive power
Besides lower efficiency, input harmonic filters require large capacitors to maintain low harmonic current performance. Large capacitors mean higher reactive current, which, circulating between the line and the filter, causes additional losses and reduces system efficiency. Utility companies typically charge based on kilovolt-amperes (kVA) of usage. Significant reactive power increases the kVA figure considerably, resulting in utility charges exceeding actual power (kW) demand.
A small input filter isolates the power supply from the inverter's AC-AC power output and converts the regenerated waveform back to a clean sine wave, saving potentially wasted energy. When driving a motor, the inverter draws low harmonic current from the power supply, eliminating the need for a harmonic filter.
The filter requires additional wiring for multiple devices that need to be installed inside a protective enclosure or a larger panel.
4. Regeneration
A bidirectional insulated-gate bipolar transistor (IGBT) switch connects AC input power to AC output power, thus automatically delivering regenerative energy to the circuit. When this happens, the voltage is typically redirected to other loads on the same power source, meaning less electricity is drawn from the power company, thus reducing utility consumption and utility costs.
Input harmonic filters can mitigate input current harmonics, but they cannot regenerate them. For regeneration using harmonic filters, additional components are needed to dissipate or regenerate energy generated by the application. Without them, undesirable drive failures may occur during motor operation.
5. Complexity
The matrix drive's power wiring and monitoring require only a simple three-wire input and three-wire output. Filters require additional wiring for multiple devices, which need to be installed inside a protective enclosure or a larger panel. In addition to the inverter wiring and panel required to drive the motors, filter-related wiring and equipment are also necessary.
6. No tripping, no fault operation
Input harmonic filter design requires a large amount of capacitance to maintain low harmonics. This large capacitance means higher reactive current and higher kilovolt-ampere (kVA) requirements, which reduces efficiency and increases operating costs. These filters also boost the voltage. When the inverter is not running the motor or is operating under light load conditions, the input filter circuit will boost the line voltage to increase the inverter's DC bus voltage, resulting in an overvoltage condition. This situation can lead to malfunctions or hinder operation.
The added equipment can overcome the effects of voltage rise. Contactors can isolate capacitors during idle and low-speed operation, solving the problems of high DC bus voltage and overvoltage tripping on the inverter, but may hinder low-harmonic operation under light loads. Matrix drives provide trip-free operation.
Filters are an effective method for managing inverter harmonics; however, filters increase costs related to equipment, installation space and time, and wiring. Matrix drive technology can eliminate low-harmonic currents, enable motor regeneration, and simplify installation, startup, and operation.
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