Center-Tapped Input Filter for VSD Common-Mode Voltage
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Solution Overview
Problem
Variable speed drives (VSDs) with active converters generate high common mode voltages that cause premature bearing failures and insulation issues in HVAC&R applications, and there is a need for effective filtering to manage these voltages and maintain proper power factor control during voltage losses.
Innovation Solution
A circuit with a three-phase inductor and capacitor bank configuration is used to filter common mode and differential mode currents, providing a short circuit for high frequencies while passing fundamental frequencies, thereby reducing voltage stress on motor stators and preventing premature failures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an active converter is used in the VSD, then power factor correction and reduced input current harmonics are achieved, but common mode voltage increases causing bearing fluting and premature bearing failures
Solution Approach 1:
A common mode choke is introduced as an intermediary component between the active converter and the motor. This choke specifically targets and attenuates common mode voltage while allowing differential mode power transmission to continue, thus resolving the contradiction by filtering out the harmful common mode voltage generated by the active converter without affecting its power factor correction function
2Object-affected harmful factors
If conventional filtering is used, then some EMI is reduced, but it does not effectively address the high common mode voltages generated by active converters
Solution Approach 1:
The filtering approach is made local and specific by using a common mode choke that is dedicated solely to suppressing common mode voltages. Unlike conventional filters that attempt to handle all types of EMI, this choke is specifically designed and positioned to address the local problem of common mode voltage generation at the active converter output, providing targeted suppression where it is most needed
3Duration of action of moving object
If the VSD rides through extended voltage loss, then continuity of operation is maintained, but the d-q reference frame angle becomes incorrect requiring re-synchronization
Solution Approach 1:
The system performs preliminary action by maintaining the d-q reference frame angle information in memory during voltage interruptions. When power is restored, the system uses this previously stored angle information to quickly re-synchronize without requiring a complete re-lock sequence, thus maintaining measurement precision while enabling extended ride-through capability
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution effectively reduces common and differential mode currents and voltages, alleviating bearing and insulation failure issues, and enables proper power factor control and synchronization during voltage fluctuations.
Implementation Method 1
A three-phase input capacitor bank of three capacitors is connected in a wye configuration to the three center taps at one end, and to a common point at the opposite end. The three-phase input capacitor bank is configured to substantially provide a short circuit for frequencies above a predetermined fundamental frequency
Implementation Method 2
An input filter is connected to the VSD at the input to the converter stage for filtering a common mode component and a differential mode component induced by conducted electromagnetic interference or radio frequency interference present at the AC power source
Data Source
AI summary
Systems and methods for improved Variable Speed Drives having active inverters include an input filter for filtering common mode and differential mode currents. A three-phase inductor has three windings, each winding of the three-phase inductor having a center tap dividing each winding into a pair of inductor sections; and a three-phase input capacitor bank connected in a wye configuration to the three center taps at one end, and to a common point at the opposite end. The three-phase input capacitor bank provides a short circuit for frequencies above a predetermined fundamental frequency for shunting such frequencies through the three phase capacitor bank, while passing the predetermined fundamental frequency to an input AC power source.