Triac Control Circuit for Asynchronous Motor Starting
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 one motor winding terminal is not directly accessible.
Innovation Solution
A circuit that detects the voltage across a series connection of a resistive element and a triac, compares it to a threshold, and blocks triac reactivation when exceeded, using a resistive dividing bridge and zener diode for detection, and a MOS transistor or cathode-gate thyristor to control triac conduction, storing information to maintain blocking for at least two halfwaves.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Extent of automation
If a conventional triac control circuit with direct diode connection is used, then the motor can be started once, but the capacitor remains charged after thermostat shutdown preventing successive startings
Solution Approach 1:
The patent applies preliminary action by providing a discharge circuit that actively discharges the capacitor before the motor needs to restart. The discharge circuit includes a discharge triac and resistor that are activated when the thermostat shuts down, ensuring the capacitor is discharged in advance of the next potential start attempt, thus enabling reliable successive startings
Solution Approach 2:
The patent uses an intermediary approach by introducing a second triac (discharge triac) and associated discharge circuit as a mediator between the capacitor and ground. This intermediary discharge circuit allows controlled discharge of the capacitor without directly shorting it, enabling automatic reactivation while maintaining circuit safety and reliability
2Reliability
If a second resistive element with positive temperature coefficient is used to supply triac gate, then thermal protection is provided, but priming is delayed and electromagnetic noise is generated
Solution Approach 1:
The patent segments the thermal protection function by separating it from the priming function. The first triac handles priming immediately when voltage is applied, while the second triac with positive temperature coefficient resistive element handles thermal protection. This segmentation allows priming to occur without delay while thermal protection remains secured
Solution Approach 2:
The patent applies preliminary action by having the first triac activate immediately to prime the motor starting circuit, while the second triac is prepared in advance with the positive temperature coefficient resistive element to provide thermal protection. The preliminary activation of the first triac eliminates priming delay while the pre-positioned second triac ensures thermal security
3Device complexity
If the triac control circuit is simplified without discharge circuit, then device complexity is reduced, but the capacitor remains charged adversely affecting motor restarting
Solution Approach 1:
The patent merges the discharge function with the existing thermostat control mechanism. The discharge triac is controlled by the same thermostat that controls the motor, so when the thermostat shuts down, the discharge triac is automatically activated to discharge the capacitor. This merging approach adds minimal complexity while fully enabling motor restarting capability
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 automatic reactivation of the motor after power loss or thermal protection activation, reduces heat dissipation, and preserves the security of the resistive element, allowing for successive startings and maintaining thermal protection functionality.
Implementation Method 1
a resistive element 3 of positive temperature coefficient (PTC) having its resistance increasing along with temperature
Implementation Method 2
A disadvantage of the circuit of FIG. 1 is that it is used only once, on powering-on of the assembly. Due to the direct connection of diode D to terminal 1, it is no longer used when the thermostat turns off the motor, capacitor C remaining charged
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 which is at least capacitive, 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.