Motor Drive Earth Fault Detection via Charging Circuit
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Solution Overview
Problem
Existing motor drive systems face challenges in detecting and preventing earth faults during installation, which can lead to excessive currents and component damage, particularly due to the high cost of high-current contactors required to prevent fault currents.
Innovation Solution
A motor drive system that includes a rectifier bridge, a converter bridge with an intermediate DC circuit, a controlled main relay, a charging circuit with a current limiting component, and a voltage sensor connected to a fault evaluation circuit, allowing for earth fault detection without the need for high-current switches between the converter bridge and the elevator motor, using a charging circuit that bypasses the rectifier bridge and directly connects to the intermediate DC circuit via diodes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If high-current contactors are provided between the converter bridge and the elevator motor to prevent fault currents, then component protection is improved, but device cost and complexity increase significantly
Solution Approach 1:
A charging circuit with a charging switch is introduced as an intermediary component between the mains and the intermediate DC circuit. This charging switch acts as a mediator that controls the charging process and enables earth fault detection, replacing the need for expensive high-current contactors while maintaining component protection
Solution Approach 2:
The charging circuit is activated before the main relay is closed to charge the intermediate DC circuit. This preliminary action allows the system to detect earth faults during the charging phase, preventing fault currents from damaging components when the main relay is subsequently closed
2Reliability
If high-current contactors are provided between the converter bridge and the elevator motor to prevent fault currents, then earth fault detection reliability is improved, but device cost increases
Solution Approach 1:
The charging switch is implemented using inexpensive semiconductor components (such as IGBTs or MOSFETs) rather than expensive high-current contactors. These electronic switches are sufficient for the temporary charging operation and earth fault detection, significantly reducing device cost while maintaining detection reliability
Solution Approach 2:
The mechanical high-current contactors are replaced with an electronic charging circuit controlled by semiconductor switches. This substitution eliminates the need for expensive mechanical switching devices while achieving the same earth fault detection function through electronic control
3Reliability
If a motor relay is used to open the motor side circuit for earth fault protection, then component protection is improved, but the connection between converter bridge and motor is interrupted
Solution Approach 1:
The protection function is segmented into two independent parts: the charging circuit with charging switch for earth fault detection, and the main relay for power connection. This segmentation allows the charging circuit to perform protection functions without requiring the motor relay to interrupt the permanent connection between the converter bridge and the motor
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
This solution effectively detects earth faults and prevents component damage without the need for expensive high-current switches, ensuring a direct and uninterrupted connection between the converter bridge and the elevator motor, while using low-cost electronic components to manage current flow and voltage monitoring.
Implementation Method 1
a voltage sensor connected between the positive and the negative branch of the intermediate DC circuit, which voltage sensor is connected to a fault evaluation circuit
Implementation Method 2
The charging circuit comprises at least one charging switch which connected in series with at least one current limiting component
Implementation Method 3
between a positive and a negative branch of the intermediate DC circuit at least one capacitor and/or battery is connected
Implementation Method 4
the charging circuit bypasses the rectifier bridge and is directly connected with the intermediate DC circuit, of course via corresponding diodes or other voltage rectifying components
Data Source
Figure 1
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Figure 4
AI summary
The invention relates to a motor drive (10) comprising a rectifier bridge (12) for connection with mains, a converter bridge (26) configured to be connected with an elevator motor and an intermediate DC circuit (28) in-between, whereby between a positive and a negative branch (30, 32) of the intermediate DC circuit (28) at least one capacitor (34) and/or battery is connected, whereby at the mains side of the rectifier bridge (12) a controlled main relay (14) with contacts (16a, 16b, 16c) is arranged, which are configured to connect or disconnect the rectifier bridge (12) with the corresponding mains phase (L1, L2, L3), characterized in that the motor drive (10) comprises a charging circuit (24) being connected between the mains and the rectifier bridge (12) and/or intermediate DC circuit (28), which charging circuit comprises at least one charging switch (20) connected in series with at least one current limiting component (22, 42), and which motor drive (10) comprises a voltage sensor (38) connected between the positive and the negative branch of the intermediate DC circuit (28), which voltage sensor (38) is connected to a fault evaluation circuit (40) comprising at least one reference value and a comparator, which evaluation circuit (40) is configured compare the actual sensor (38) signal of the voltage sensor (38) to the reference value and to operate the main relay (14) dependent on the comparison result.