DC Bus Clamp Circuit for ASD Overvoltage Protection

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Solution Overview

Problem

Adjustable speed drives (ASDs) for AC electric motors face challenges in protecting against overvoltage conditions caused by high frequency electrical transients and voltage surges, which can lead to component failure and reduced motor lifespan, especially in systems with high capacitance or multiple motors sharing a common DC bus.

Innovation Solution

A protective circuit with a damping element consisting of a resistor and capacitor connected in parallel, and diodes that act as switches to engage only during overvoltage conditions, reducing the power ratings and component count, and can be mounted internally or externally to the ASD.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If snubber circuits are designed to handle continuous operation with significant voltages and currents, then protection reliability is improved, but component power ratings and costs increase

Engineering Contradiction:

Improveprotection reliabilityVSAvoidcomponent power ratings

Core Design Contradiction:

ReliabilityVSPower

Solution Approach 1:

The circuit dynamically engages protection only during overvoltage transients through diode switching, rather than continuously handling full power. The damping element connects to the DC bus only when voltage exceeds the diode forward voltage plus zener voltage, automatically disengaging during normal operation. This dynamic behavior reduces required power ratings while maintaining protection reliability.

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The protective circuit operates periodically during transient overvoltage events rather than continuously. The diodes act as automatic switches that engage the damping element only during voltage spikes, allowing the circuit to handle significant voltages and currents only when needed, thereby reducing average power requirements and component ratings.

Inventive Principle:

Principle #19Periodic action

2Adaptability or versatility

If multiple motors operate from a common DC bus, then system versatility is improved, but vulnerability to voltage spikes and overvoltage conditions increases

Engineering Contradiction:

Improvesystem versatilityVSAvoidvoltage spike vulnerability

Core Design Contradiction:

Adaptability or versatilityVSObject-affected harmful factors

Solution Approach 1:

The protective circuit provides individual protection for each motor-inverter connection point on the common DC bus. Each inverter can be equipped with its own damping element and diodes, creating segmented protection zones. This allows voltage spikes from one motor to be contained and dissipated locally without propagating to other motors sharing the common bus, thereby maintaining system versatility while reducing vulnerability.

Inventive Principle:

Principle #1Segmentation

3Reliability

If detection and shutdown is used for overvoltage protection, then component reliability is improved, but system productivity decreases due to downtime

Engineering Contradiction:

Improvecomponent reliabilityVSAvoidsystem productivity

Core Design Contradiction:

ReliabilityVSProductivity

Solution Approach 1:

The damping element is pre-positioned in the circuit but remains inactive during normal operation. When an overvoltage transient occurs, the diodes automatically engage the damping element to absorb and dissipate the voltage spike before it can damage components. This beforehand cushioning approach protects components without requiring detection or shutdown, maintaining both reliability and productivity.

Inventive Principle:

Principle #11Beforehand cushioning (Prior cushioning)

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 effectively reduces electrical transients and power requirements, providing continuous protection with lower component ratings and costs, while allowing for flexible mounting options to suit various system capacitances and configurations.

Implementation Method 1

at least one damping element having a resistor (36) and a damping capacitor (38) connected in parallel with the resistor (36)

Methodology Applied

Scientific EffectDamping: Damping

Implementation Method 2

a damping element having a resistor (36) and a damping capacitor (38) connected in parallel with the resistor (36)

Methodology Applied

Scientific EffectJoule Heating: Joule Heating

Implementation Method 3

A first diode (40) is connected in series with the damping element between the common point (24) and the positive DC bus rail (20). A second diode (42) is connected in series with the damping element between the negative DC bus rail (22) and the common point (24)

Methodology Applied

Scientific EffectDiode conduction: Diode

Data Source

PatentUS8233258B2DC bus clamp circuit to prevent over voltage failure of adjustable speed drives

Publication Date: 2012.07.31 ROCKWELL AUTOMATION TECH INC

AI summary

The present invention relates to a protective circuit to provide over voltage protection for an ASD. The protective circuit provides the benefits of fewer components with lower power ratings than existing protective circuits. The protective circuit may be incorporated directly in the ASD for continuous protection or mounted externally and connected to the ASD under operating conditions that require the circuit. This flexibility for mounting the protective circuit allows the capacitor of the protective circuit to be sized either in relation to capacitive elements on the DC bus within the ASD or according to external capacitance observed at the output of the ASD. In addition, the circuit is only operative during an overvoltage condition allowing for power ratings lower than would be required for continuous operation.