Feature: Components
Line Reactors for VFD
Line reactors are commonly found in motor drives, automotive, industrial automation and power quality applications. Since they are installed at the input side of the VFD, they are also known as “input reactors”. Teir primary role is to protect the drive from incoming line voltage distortion, such as spikes and surges. Switching transients can
exceed 1000V, so the inductive characteristics of the line reactor restrict these sudden surges and eliminate nuisance safety tripping. Also, by limiting the inrush current to the rectifier in the VFD, the line reactor can minimise harmonic distortions.
Figure 2: Line reactors for VFD
Dv/Dt reactors Common applications for the Dv/Dt reactors include servo drives, closed loop vector drives, motor drives with short motor cables, robotics and industrial automation. Tey are installed at the output side of the VFD, before the motor, hence their other name of “load reactors”. Te term “Dv/Dt reactor” is used because they provide efficient reduction of high output voltage Dv/Dt from IGBT motor drives. Te high design inductance of
Figure 3: Dv/Dt reactors
the Dv/Dt reactor will smoothen the rough voltage spikes and reduce distortions as the voltage is applied to the motor. Tese type reactors effectively protect the motor coil insulation from premature ageing and destruction.
Harmonic filters Tere are two types of harmonic filters: passive and active. Passive harmonic filters use inductors, capacitors and power resistors to filter noise on the VFD. Active harmonic filters
can work with many variable speed drives and actively reduce noise by constantly monitoring electricity and injecting currents to mitigate harmonics. Tey are designed to pass current at the fundamental frequency but block current at higher
frequencies. Since these filters provide excellent harmonic mitigation, they help systems meet the IEEE 519 standard.
Dv/Dt filters Dv/Dt filters are L-C combination devices, installed at the output side of the VFD, before the motor. Teir primary role is to protect the motor by regulating the drive output voltage, and prevent spikes from reaching the motor. Among their primary functionalities are: • Limit the peak voltage and protect the motor insulation windings from premature wear and tear;
• Minimise interference propagation to neighbouring equipment and lines;
Figure 5: Dv/Dt filter
• Teir output current ripple is lower, but the voltage is still pulse- width modulated (PWM);
• Tey protect the motor a lot longer than load reactors.
Sine wave filters for VFD Sine wave filters are L-C type devices installed at the output side of the VFD, before the motor. Teir high inductance and capacitance and sinusoidal output help optimise the output signal. Tese devices protect the motor by regulating the PWM wave distortions from the drive, to resemble an AC sine wave. Tey minimise motor drive losses by reducing high frequency leakage- and bearing-currents and eliminating the pulse reflections in the motor cable. Table 1 summarises the features of various reactors and filters.
Figure 6: Sine wave filters
Figure 4: Harmonic filters for VFD
Table 1: Summary of features for various reactors and filters for VFD and motor applications
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