PTC Sensor Circuit for Motor Thermal Protection
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Solution Overview
Problem
Current protective devices for electric motors, such as those used in refrigeration compressor motors, fail to reliably detect and prevent dangerous temperature overloads due to the delay in responding to rapid temperature rises, especially at high current densities, leading to potential motor damage.
Innovation Solution
A protective device with a sensor circuit comprising at least one first PTC sensor and a second PTC sensor with a higher nominal response temperature, along with a tripping device that evaluates the second-order derivative of the sensor signal to detect changes in curvature, allowing for earlier detection of critical temperature situations, and includes a fixed resistor to generate turning points in the characteristic curve for enhanced monitoring.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If a single PTC sensor is used to monitor winding temperature, then the sensor is small and can be installed in the motor winding, but the sensor cannot follow rapid temperature rises due to thermal contact resistance between the sensor and the winding
Solution Approach 1:
The patent divides the temperature monitoring function into multiple PTC sensors with different nominal response temperatures (NAT1, NAT2, NAT3) arranged in series. Each sensor monitors a different temperature range, allowing the system to detect rapid temperature rises at multiple threshold levels simultaneously, overcoming the limitation of a single sensor's thermal response delay.
Solution Approach 2:
The patent uses multiple PTC sensors with predetermined nominal response temperatures to detect temperature rises before they reach dangerous levels. The sensors are pre-configured to trigger at specific temperature thresholds (e.g., 120°C, 140°C, 160°C), enabling early warning and preventive action before the winding temperature becomes critically high.
2Measurement precision
If PTC sensors are connected in series to form a sensor circuit, then the total resistance provides temperature information, but the characteristic curve lacks turning points making it difficult to detect critical temperature situations early
Solution Approach 1:
The patent changes the electrical parameters of the sensor circuit by connecting PTC sensors with different nominal response temperatures in series. This creates a composite resistance characteristic curve with multiple turning points corresponding to each sensor's NAT. The evaluation device detects these turning points to identify critical temperature situations early, reducing detection delay.
3Reliability
If the motor is switched off only when the recorded temperature reaches the nominal response temperature, then the tripping device operates based on clear threshold detection, but the actual winding temperature is already well above the specified threshold value causing motor damage
Solution Approach 1:
The patent employs multiple PTC sensors with progressively higher nominal response temperatures to provide early warning before the winding reaches dangerous temperatures. The first sensor (NAT1 = 120°C) provides early detection, allowing the motor to be switched off before the actual winding temperature exceeds safe limits, preventing motor damage while maintaining reliable operation.
4Loss of time
If multiple PTC sensors with different nominal response temperatures are used, then early detection of critical temperature situations is enabled, but the device complexity increases
Solution Approach 1:
The patent combines multiple PTC sensors with different nominal response temperatures into a single series-connected sensor circuit. The total resistance of the circuit reflects the temperature state at multiple thresholds simultaneously. The evaluation device processes this combined signal to detect turning points corresponding to each sensor's NAT, enabling early temperature detection while maintaining a relatively simple circuit structure.
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 enables early detection and prevention of temperature overloads, effectively protecting the motor from rapid temperature rises and extending its service life by providing dynamic and static protection criteria, allowing for increased utilization and reduced material usage.
Implementation Method 1
PTC thermistors typically have a characteristic as shown in 1 is shown. From room temperature to a few C° below the nominal switch-off temperature NAT, the characteristic curve is flat and then increases exponentially.
Implementation Method 2
a sensor circuit (2) with at least one first PTC sensor (PTC1), at least one second PTC sensor (PTC2) whose nominal response temperature (NAT2) is greater than the nominal response temperature (NAT1) of the first PTC sensor
Data Source
Figure 1
Figure 2
Figure 3
AI summary
The protection device has a sensor circuit with a positive temperature coefficient sensor, and another positive temperature coefficient sensor whose rated operating temperature is bigger than the rated operating temperature of the former positive temperature coefficient sensor. A loosening tool is provided with an evaluation device for evaluating the sensor signal emitted on the basis of the total resistance of the sensor circuit and for generating an output signal for the loosening tool depending on the sensor signal.