3.2 UPS Uninterruptible Power Supply Repair Cause Analysis
3.2.1 Power Supply Filter
this UPS is primarily to filter out high frequencies in the power output and reduce the distortion of the power waveform. There are two types of power cords:
—Common mode, which exists between the live wire and the ground wire, and between the neutral wire and the ground wire. Common mode exists simultaneously in the live wire and the neutral wire, and is equal in magnitude and phase.
—Differential mode, which exists between the live wire and the neutral wire. The differential mode exists simultaneously in the live wire and the neutral wire, with equal magnitude and opposite phase.
UPS ( Uninterruptible Power Supply) repair often involves both of the aforementioned conditions simultaneously in the power supply line . Therefore, a typical power filter is composed of a common-mode filter circuit (L1, L2, and Cy) and a differential-mode filter circuit (the difference between L1 and L2 and Cx). L1 and L2 are two independent coils with the same number of turns and the same winding direction wound on the same magnetic ring. When power frequency components pass through, the inductance is very small due to magnetic flux cancellation, allowing them to pass easily. When common-mode frequency components pass through, the inductance is very large due to magnetic flux addition, making them difficult to pass through and thus suppressed.
Common-mode inductors L1 and L2 are typically in the range of a few tenths to tens of mH. Common-mode capacitors Cy should generally be chosen with a larger value while keeping leakage current low. Differential-mode inductors are typically in the range of tens to hundreds of μH, and differential-mode capacitors Cx should be ceramic or polyester capacitors with sufficient voltage resistance. Commercially available power filters are generally designed for common-mode operation; to address differential-mode operation, two separate differential-mode suppression inductors should be added. For common-mode inductors, metallic magnetic materials or amorphous and microcrystalline magnetic materials are preferable. For differential-mode inductors, magnetic materials made from soft metallic powder that has been insulated, encapsulated, pressed, and annealed are preferable to open-faced ferrite materials.