Inverter Frequency Control via Current Detection

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Solution Overview

Problem

Existing inverter devices take a long time to adjust frequency corresponding to load changes, especially when in an over-torque state, due to reliance on induction motor speed for frequency determination.

Innovation Solution

An inverter device comprising a converter circuit, inverter circuit, current detecting unit, calculating unit, and pulse-signal output unit that calculates and adjusts output frequency based on detected output current, allowing for frequency adjustment corresponding to load magnitude without being affected by load changes.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Speed

If frequency is determined based on induction motor speed in over-torque state, then motor control is achieved, but adjustment time to reach target frequency increases significantly

Engineering Contradiction:

Improvefrequency adjustment speedVSAvoidadjustment time

Core Design Contradiction:

SpeedVSLoss of time

Solution Approach 1:

Instead of determining frequency from motor speed (conventional approach), the patent inverts the relationship by detecting output current and determining frequency based on current magnitude. This inversion allows the system to proactively set frequency according to load requirements rather than reactively responding to motor speed changes, thereby reducing adjustment time significantly.

Inventive Principle:

Principle #13The other way round (Inversion)

Solution Approach 2:

The system performs preliminary detection of output current to anticipate load requirements before the motor actually reaches the desired speed. By detecting current magnitude early in the acceleration process and pre-calculating the appropriate frequency, the system prepares the frequency command in advance, eliminating the delay that would occur if frequency were only adjusted after speed changes were detected.

Inventive Principle:

Principle #10Preliminary action

2Reliability

If frequency adjustment is delayed due to load changes, then motor protection is compromised, but rapid adjustment may cause instability

Engineering Contradiction:

Improvemotor protection reliabilityVSAvoidfrequency adjustment stability

Core Design Contradiction:

ReliabilityVSStability of the object's composition

Solution Approach 1:

The system continuously monitors output current magnitude and uses this feedback to dynamically adjust frequency. The feedback loop compares detected current with predetermined thresholds and automatically adjusts frequency accordingly, providing both rapid response to load changes and stable operation through continuous monitoring and adjustment. This ensures motor protection while maintaining system stability.

Inventive Principle:

Principle #23Feedback

Solution Approach 2:

The system changes the control parameter from speed-based frequency determination to current-based frequency determination. By monitoring output current magnitude and adjusting frequency based on current thresholds, the system achieves rapid response to load changes while maintaining stability through controlled parameter transitions rather than abrupt changes.

Inventive Principle:

Principle #35Parameter changes

3Device complexity

If frequency is determined by motor speed, then simple control logic is maintained, but response to load magnitude changes is slow

Engineering Contradiction:

Improvecontrol logic complexityVSAvoidfrequency adjustment efficiency

Core Design Contradiction:

Device complexityVSProductivity

Solution Approach 1:

The patent replaces the mechanical speed-sensing approach with an electrical current-detection approach. Instead of using speed sensors and mechanical feedback, the system uses electrical current detection to determine load magnitude and adjust frequency accordingly. This substitution maintains control logic simplicity while dramatically improving response efficiency to load changes.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

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

Enables rapid adjustment of frequency to match load magnitude, preventing motor overload and ensuring stable operation by suspending or resuming acceleration based on torque current limits.

Implementation Method 1

a converter circuit unit to convert an alternating-current voltage to a direct-current voltage

Methodology Applied

Scientific EffectElectromagnetic rectification: Electromagnetic Induction

Implementation Method 2

an inverter circuit unit to convert a direct-current voltage obtained by conversion by the converter circuit unit to an alternating-current voltage

Methodology Applied

Scientific EffectPulse width modulation:

Data Source

PatentUS10079556B2Inverter device

Publication Date: 2018.09.18 MITSUBISHI ELECTRIC CORP

AI summary

An inverter includes: a converter circuitry to convert an alternating-current voltage to a direct-current voltage; an inverter circuitry to convert a direct-current voltage obtained by conversion by the converter circuitry to an alternating-current voltage and to supply the alternating-current voltage obtained by conversion to a load; a current detector to detect an output current of the inverter circuitry; a calculator to calculate an output frequency based on the output current detected by the current detector, and a pulse-signal outputter to output a pulse signal based on the output frequency calculated by the calculator. The inverter circuitry converts the direct-current voltage obtained by conversion by the converter circuitry to the alternating-current voltage based on the pulse signal output from the pulse-signal outputter, so as to obtain a frequency corresponding to the magnitude of the load.