Triac Control Circuit for Asynchronous Motor Starting
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Solution Overview
Problem
Conventional circuits for controlling triacs in asynchronous motors are limited in their ability to enable successive startings without excessive heat dissipation and are not compatible with thermal protection mechanisms, especially when the auxiliary winding is disconnected after the motor has started, and they do not allow automatic restarting due to charged capacitors and inaccessible terminals.
Innovation Solution
A circuit that detects the voltage across a series connection of a resistive or capacitive element and a triac, compares it with a threshold, and blocks the triac's reactivation when the threshold is exceeded, using a resistive dividing bridge, zener diodes, and transistors to store the blocking information for at least two halfwaves, ensuring efficient control and compatibility with thermal protection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Extent of automation
If a conventional triac control circuit is used with a charged capacitor, then the motor cannot be restarted after thermal protection activation, but this prevents automatic restarting functionality
Solution Approach 1:
The patent extracts the capacitor discharge function from the triac control circuit by introducing a separate discharge circuit with a discharge resistor connected in parallel with the capacitor. This allows the capacitor to be discharged independently of the triac control, enabling the triac to be retriggered even when the capacitor is charged, thus restoring automatic restarting capability while preserving thermal protection functionality.
Solution Approach 2:
The patent introduces a discharge resistor as an intermediary element that provides an alternative discharge path for the capacitor. This intermediary component allows the capacitor to discharge through a dedicated path rather than through the triac control circuit, resolving the conflict between automatic restarting and thermal protection functionality.
2Loss of energy
If the auxiliary winding is disconnected after motor starting, then power dissipation is reduced, but the circuit cannot support successive startings without excessive heat dissipation in the resistive element
Solution Approach 1:
The patent applies preliminary action by discharging the capacitor before each triac triggering event through a dedicated discharge circuit. This preliminary discharge ensures that the capacitor does not retain charge that would cause excessive voltage across the resistive element during successive startings, thereby reducing power dissipation while maintaining the ability to restart the motor multiple times.
Solution Approach 2:
The patent implements periodic action by continuously discharging the capacitor through the discharge resistor during intervals between motor operations. This periodic discharge maintains the capacitor in a low-energy state, preventing excessive heat dissipation in the resistive element during successive startings while allowing the system to reset and prepare for the next operation cycle.
3Adaptability or versatility
If a resistive element with positive temperature coefficient is used for security, then the auxiliary winding can be disconnected after starting, but the circuit becomes complex and cannot handle inaccessible terminals
Solution Approach 1:
The patent extracts the capacitor discharge function from the main triac control circuit and places it in a separate, independent discharge circuit. This separation simplifies the main control circuit while maintaining compatibility with thermal protection mechanisms and accommodating inaccessible terminals, as the discharge circuit can be configured independently of the motor terminal accessibility.
Solution Approach 2:
The patent creates a universal solution by designing a discharge circuit that can operate independently of the specific motor configuration or terminal accessibility. The discharge circuit serves multiple functions: it discharges the capacitor for automatic restarting, works with thermal protection mechanisms, and accommodates various terminal configurations including inaccessible terminals, thereby reducing overall circuit complexity.
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 allows for automatic reactivation of the motor, preserves the security of the resistive element, and maintains the operation of thermal protection, enabling successive startings without excessive heat dissipation and ensuring the triac remains blocked until the threshold is reset.
Implementation Method 1
a resistive element 3 of positive temperature coefficient (PTC) having its resistance increasing along with temperature
Implementation Method 2
auxiliary winding Ls in series with element 3 and triac T are connected in parallel with main winding Lm. The gate of triac T is connected to the junction point of a resistor R and a capacitor C
Implementation Method 3
The starting time (conduction of winding Ls) is set by the time constant brought by resistor R and capacitor C
Implementation Method 4
A resistive dividing bridge formed of two resistors R1 and R2 in series between terminal 53 and ground M
Implementation Method 5
a zener diode having its threshold voltage setting the triggering of the blocking circuit
Data Source
AI summary
A method and a circuit for controlling a triac intended to be series-connected with a resistive element of positive temperature coefficient or a capacitive element, and a winding for starting an asynchronous motor, for supply by an A.C. voltage, the present invention including the steps of: detecting a voltage representative of the voltage across the series connection of the element and of the triac; comparing this detected voltage with respect to a threshold; and blocking a turning back on of the triac when the threshold has been exceeded.