Motor Control Relay Arc Suppression via Current Reduction

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Solution Overview

Problem

In air-conditioning devices, the use of relays to control the number of motors driven can lead to arc occurrence and stress on the relay when the current is turned off, potentially causing breakdown.

Innovation Solution

A motor control device with an inverter circuit, current detectors, and a controller that manages the relay's state by reducing motor currents and rotation speeds to minimize stress on the relay, including a mechanism to switch the relay off only when currents are low and to switch it on only when necessary, preventing excessive current flow.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Adaptability or versatility

If a relay is used to switch motors on and off, then the number of driven motors can be changed, but arc occurs on the relay contact and stress is applied to the relay, potentially causing breakdown

Engineering Contradiction:

Improvenumber of driven motorsVSAvoidrelay durability

Core Design Contradiction:

Adaptability or versatilityVSReliability

Solution Approach 1:

The control device reduces the current flowing through the motor before turning off the relay. This preliminary action of current reduction prevents high current from being present when the relay contact opens, thereby eliminating arc occurrence and protecting the relay from stress and potential breakdown.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The system changes the electrical parameter (current magnitude) before performing the switching operation. By dynamically adjusting the current level to a safe threshold before relay disconnection, the patent modifies the operating conditions to prevent harmful arcs while maintaining the ability to switch motors on and off.

Inventive Principle:

Principle #35Parameter changes

2Speed

If relay switching is performed while current flows through the motor, then motor operation can be changed quickly, but excessive current causes arc and stress on the relay

Engineering Contradiction:

Improveresponse speed of motor switchingVSAvoidarc and stress on relay

Core Design Contradiction:

SpeedVSObject-affected harmful factors

Solution Approach 1:

Before the relay switching operation is executed, the control device performs a preliminary current reduction phase. This ensures that when the relay contact opens, the current is already at a safe level, eliminating arcs and stress while maintaining quick response capability.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The system provides beforehand protection by reducing current to a safe level before the relay switching event. This cushioning action prepares the electrical conditions in advance to prevent harmful effects during the switching operation.

Inventive Principle:

Principle #11Beforehand cushioning (Prior cushioning)

Data Source

PatentUS11424698B2Motor control device and air-conditioning device

Publication Date: 2022.08.23 MITSUBISHI ELECTRIC CORP

AI summary

A motor control device includes: an inverter circuit configured to convert a direct-current voltage into an alternating-current voltage, and to supply the alternating-current voltage to a plurality of motors connected in parallel; current detectors configured to detect motor currents flowing through the respective motors; a relay provided between at least one of the plurality of motors and the inverter circuit; and a controller configured to change the number of motors to be driven. When one of two or more motors under driving is stopped, the controller lowers a current instruction value to the two or more motors under driving. When a monitor current that is the motor current detected by the current detector of the motor connected to the relay is decreased, the controller switches the relay from an on state to an off state.