PTC Heater Circuit for Cold-Start Impulse Current Limiting
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing electric transport refrigeration units face safety hazards due to high impulse currents generated during cold starts by PTC heaters, which can exceed the maximum allowable current, posing risks of scald and fire.
Innovation Solution
Incorporating a second protective resistor with a constant resistance value in series with the PTC heating resistor, connected in parallel with a temperature switch that enables or bypasses the protective resistor based on the surface temperature of the PTC resistor, determined by the operating voltage and maximum allowable current of the unit, to manage current flow effectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If a PTC heater is used to control surface temperature and avoid safety hazards, then the temperature can be kept below curie point temperature, but the impulse current during cold start may exceed the maximum current allowed by the transport refrigeration unit
Solution Approach 1:
A protective resistor is connected in series with the PTC heater before operation to limit the impulse current during cold start. This preliminary protective measure prevents excessive current from reaching the heater, and the protective resistor is subsequently bypassed through a temperature switch once the heater reaches operating temperature, allowing full power utilization.
Solution Approach 2:
A temperature switch acts as an intermediary component between the protective resistor and the power source. It monitors the heater's temperature and automatically switches to bypass the protective resistor when the heater reaches its curie point temperature, thereby eliminating the need for continuous current limitation while maintaining safety during startup.
2Power
If a protective resistor is added in series with the PTC heater to limit current, then the impulse current is controlled within maximum allowable limits, but the heating efficiency is reduced due to continuous current limitation
Solution Approach 1:
The circuit configuration dynamically changes based on temperature conditions. During cold start, the protective resistor remains in the circuit to limit current. Once the heater reaches curie point temperature, the temperature switch automatically bypasses the protective resistor, allowing the heater to operate at full power without current limitation, thus maintaining high heating efficiency during normal operation.
Solution Approach 2:
The protective resistor operates periodically - active during the cold start phase and inactive during normal operation. The temperature switch enables this periodic action by switching between two states: limiting current when temperature is low and bypassing the resistor when temperature reaches the curie point, optimizing both safety and efficiency at different operational stages.
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 configuration effectively prevents excessive current flow during cold starts, enhancing the safety and efficiency of the heater by ensuring the current does not exceed the maximum allowable value, thereby avoiding safety hazards and improving the overall working efficiency of the transport refrigeration unit.
Implementation Method 1
a first heating resistor, the first heating resistor being a thermistor
Implementation Method 2
a temperature switch connected in parallel with the second protective resistor, wherein the temperature switch enables or bypasses the second protective resistor according to surface temperature of the first heating resistor
Data Source
AI summary
The present application relates to a heater, the heater comprising: a first heating resistor, the first heating resistor being a thermistor; a second protective resistor connected in series with the first heating resistor, wherein the second protective resistor has a constant resistance value; and a temperature switch connected in parallel with the second protective resistor, wherein the temperature switch enables or bypasses the second protective resistor according to surface temperature of the first heating resistor. The present application also relates to a transport refrigeration unit, a refrigeration system for a transport unit, and a method of controlling a heater in a transport refrigeration unit TRU.