MOSFET Braking Switches for Elevator Dynamic Braking

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Solution Overview

Problem

Existing elevator systems employing dynamic braking require expensive relays to handle high currents, and existing designs using power resistors are inefficient and costly.

Innovation Solution

The use of MOSFET braking switches that couple motor windings in a star configuration, controlled by a controller to selectively short the windings based on sensed motor current or speed signals, providing dynamic braking without disabling the inverter's gate drives.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If switchover relays are used to connect motor leads in star configuration for dynamic braking, then dynamic braking function is achieved, but extremely high cost relays are required to handle the currents generated by the active DC source

Engineering Contradiction:

Improvedynamic braking functionVSAvoidcost of relays

Core Design Contradiction:

ReliabilityVSEase of manufacture

Solution Approach 1:

The patent replaces expensive, high-current rated relays with inexpensive MOSFET switches that can be rapidly switched. The MOSFETs handle the high current pulses during dynamic braking through their fast switching capability, eliminating the need for costly relay components while achieving the same braking function.

Inventive Principle:

Principle #27Cheap short-living objects (Disposable)

Solution Approach 2:

The patent substitutes mechanical relay switches with electronic MOSFET switches controlled by the inverter's gate drives. This electronic substitution allows for faster switching, lower cost, and better control while maintaining the dynamic braking function through the same star connection configuration of motor windings.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

2Reliability

If power resistors are used to short the DC power source for dynamic braking, then dynamic braking is achieved, but the system becomes inefficient and costly

Engineering Contradiction:

Improvedynamic braking capabilityVSAvoidsystem efficiency

Core Design Contradiction:

ReliabilityVSLoss of energy

Solution Approach 1:

Instead of dissipating the regenerative energy from the motor as heat through power resistors, the patent recovers this energy by feeding it back to the DC bus through the inverter. The motor acts as a generator during braking, and the inverter converts this generated energy back into electrical form that can be reused, transforming what would be wasted energy into a useful resource.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

Solution Approach 2:

The patent recovers the energy that would otherwise be discarded as heat in the power resistors. By using the inverter to feed back the regenerative braking energy to the DC bus, the system recovers and reuses this energy, improving overall system efficiency and eliminating the need for energy-dissipating resistors.

Inventive Principle:

Principle #34Discarding and recovering

3Reliability

If existing dynamic braking systems are used, then braking function is provided, but expensive components and reduced switch lifetime are required

Engineering Contradiction:

Improvebraking functionVSAvoidswitch lifetime

Core Design Contradiction:

ReliabilityVSDuration of action of stationary object

Solution Approach 1:

The patent uses periodic, pulsed switching of the MOSFETs controlled by the inverter's gate drives to achieve dynamic braking. This periodic action allows the switches to handle high currents in short pulses rather than continuously, significantly extending their operational lifetime while maintaining effective braking function.

Inventive Principle:

Principle #19Periodic action

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

This solution reduces costs by utilizing high-current capable MOSFET switches at a lower voltage, increases switch lifetime, and enhances system efficiency by enabling controlled braking only in maintenance mode, while maintaining the reliability of mechanical braking as a backup.

Implementation Method 1

Dynamic braking is a technique used to slow a motor through the use of back electromotive force (emf). Generally, dynamic braking operates by shorting terminals of a permanent magnetic machine, allowing the back emf to resist rotation of the rotor.

Methodology Applied

Scientific EffectBack electromotive force (emf): Electromagnetic Induction

Data Source

PatentEP2888190B1Elevator system using dynamic braking

Publication Date: 2019.01.02 OTIS ELEVATOR CO

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AI summary

An elevator system includes a motor having a plurality of motor windings; a plurality of braking switches coupled to the motor windings, the braking switches coupling the motor windings to a common electrical point; a sensor coupled to the motor, the sensor providing a sensed signal indicative of a parameter of the motor; and a controller providing a braking signal to the braking switches in response to the sensed signal to selectively control the braking switches to short the motor windings.