Motor Drive Unit Mode Switching to Reduce Relay Overcurrent and Wear
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Solution Overview
Problem
Existing motor drive units for refrigeration cycle apparatuses face challenges in preventing overcurrents through make/break contacts of relays and minimizing the number of relay operations, which affects the relay's lifetime.
Innovation Solution
A motor drive unit that includes a controller to switch between open-windings mode and star-connection mode based on motor current values, preventing overcurrents by immediately switching to open-windings mode when the motor current exceeds a threshold, and reducing relay operations by maintaining the open-windings mode during high-rotational-speed operations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the motor current exceeds a threshold during star-connection mode, the system switches to open-windings mode to prevent overcurrent, but this frequent switching increases relay operations and reduces relay lifetime
Solution Approach 1:
The patent replaces the mechanical relay with a solid-state switching mechanism implemented through control logic in the controller. The controller directly controls the power switches (IGBTs/MOSFETs) to transition between star-connection and open-windings modes without requiring mechanical make/break contacts. This substitution eliminates the mechanical wear and contact degradation issues that limited relay lifetime, while maintaining the overcurrent protection function through electronic current monitoring and switching control.
2Reliability
If the system frequently switches between star-connection mode and open-windings mode to maintain reliable operation, then overcurrent protection is improved, but the number of relay operations increases and relay lifetime decreases
Solution Approach 1:
The patent eliminates the mechanical relay by implementing switching control through solid-state power electronic devices (IGBTs and MOSFETs) that are already present in the inverter circuitry. The controller manages the switching between connection modes by controlling these semiconductor switches, replacing the need for mechanical make/break contacts. This approach maintains reliable overcurrent protection while eliminating the wear-related lifetime limitations of mechanical relays.
Solution Approach 2:
The control system uses the existing current detection capability and power switching infrastructure of the motor drive unit to perform the mode switching function. The controller monitors motor current and automatically transitions between star-connection and open-windings modes using the same power switches that drive the motor, making the system self-sufficient without requiring additional mechanical switching components.
3Reliability
If a mechanical relay is used to switch between star-connection and open-windings modes, then the system can operate reliably within current limits, but the relay has a limited lifetime corresponding to the number of operations
Solution Approach 1:
The patent replaces the mechanical relay with solid-state power switches (IGBTs and MOSFETs) that are integral to the inverter circuit. These semiconductor devices can switch between star-connection and open-windings modes without mechanical wear, enabling unlimited operational cycles. The solid-state switching mechanism provides the same reliability function while eliminating the operational lifetime limitation inherent in mechanical contact systems.
Data Source
AI summary
According to one embodiment, a motor drive unit includes a motor including a plurality of phase windings in a mutually unconnected state, a first inverter which controls application of electric power to one ends of the phase windings, a second inverter which controls application of electric power to the other ends of the phase windings, relays each of which includes a make break contact connected between the other ends of the phase windings, and a controller which switches, according to a value of a current flowing through the motor, between an open-windings mode and a star-connection mode.