PWM Converter Switching for Overcurrent Protection
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Solution Overview
Problem
Existing motor driving devices with PWM converters face challenges in reducing the capacity of the PWM converter and power storage unit while protecting elements from overcurrents, especially when the DC voltage in the DC link drops below the input voltage peak value, leading to potential breakage of diodes and the need for increased capacitance and cost.
Innovation Solution
A motor driving device that includes a PWM converter, an inverter, a power storage unit, a switch, and a command unit that controls the connection and disconnection between the AC power supply and the PWM converter to limit input current and prevent overcurrents, using a power storage unit like a capacitor or flywheel to manage energy storage and conversion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the capacity of the PWM converter is reduced to lower cost, then the device complexity and cost are reduced, but the converter cannot handle high input currents when DC voltage drops below input voltage peak, leading to reliability issues
Solution Approach 1:
The patent introduces a switch as an intermediary component between the AC power supply and the PWM converter. This switch is controlled by a control unit to disconnect the PWM converter from the AC power supply when the DC voltage drops below the input voltage peak, preventing harmful reverse currents from flowing through the diodes. This allows the use of a smaller, lower-cost PWM converter while maintaining system reliability through the protective intermediary mechanism.
2Reliability
If the capacitance of the power storage unit is increased to prevent overcurrents, then the reliability is improved, but the device complexity and cost increase
Solution Approach 1:
The patent uses a switch as an intermediary protective device that disconnects the AC power supply from the PWM converter when voltage conditions are unfavorable. This active protection mechanism eliminates the need for oversized power storage capacitors, allowing the use of smaller, lower-cost capacitance while maintaining reliable operation and preventing overcurrent damage to converter elements.
3Use of energy by moving object
If the DC voltage in the DC link drops below the input voltage peak value, then the power storage device can supply energy during motor acceleration, but reverse currents may flow through the diodes causing potential breakage
Solution Approach 1:
The switch acts as a protective intermediary that monitors the DC voltage level and disconnects the AC power supply from the PWM converter when the DC voltage drops below the input voltage peak. This prevents reverse currents from flowing through the diodes during motor acceleration when the power storage device supplies energy, eliminating the harmful effect while preserving the beneficial energy supply function.
Solution Approach 2:
The control unit continuously monitors the DC voltage level in the DC link and provides feedback control to the switch. When the DC voltage drops below the input voltage peak, the control unit activates the switch to disconnect the AC power supply, preventing reverse current flow. This feedback mechanism enables the system to safely utilize the power storage device for energy supply during motor acceleration without exposing the diodes to damaging reverse currents.
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 reduces the capacity of the PWM converter and power storage unit, preventing overcurrents and allowing for safe operation even when the DC voltage is below the input voltage peak, enabling efficient motor driving without alarms and allowing for smaller capacitance if a capacitor is used.
Implementation Method 1
A capacitor is provided in the DC link, which connects the DC side of the converter and the DC side of the inverter. The capacitor has a function as a smoothing capacitor for suppressing the ripple components of the DC output of the converter, and a function as a power storage device that may store DC power.
Implementation Method 2
The PWM converter is constituted of a bridge circuit having semiconductor switching elements and diodes connected in reverse parallel therewith, and performs power conversion between AC power on the AC power supply side and DC power in the DC link side with switching operations of the semiconductor switching elements therein being PWM-controlled.
Implementation Method 3
an inverter (reverse converter) that is connected with a DC link, which is the DC side of the rectifier, and that performs power conversion between DC power in the DC link and AC power as driving power for or regenerative power from the motor
Data Source
AI summary
A motor driving device includes, a PWM converter that performs power conversion between AC power and DC power in a DC link, an inverter that converts the DC power in the DC link to AC power for a motor and that converts the AC power from the motor to DC power for returning to the DC link, a power storage unit that stores the DC power, a switch that connects or disconnects between an AC power supply and the PWM converter in response to a command, and a command unit that continues outputting a connection command to the switch while a DC voltage in the DC link is boosted up to a prescribed voltage by the DC power, and that initiates outputting a disconnection command to the switch after the DC voltage in the DC link reaches the prescribed voltage and before the inverter initiates powering operation.