1. The application background

Reactors and transformers are often used in photovoltaic inverter systems, which ensure the safe, stable and efficient operation of photovoltaic power generation systems and power grids by limiting current, improving voltage fluctuations, regulating voltage levels and other functions.

In particular, high-power reactors and transformers above 500W will produce large heat during the working process, and overheating may lead 

to equipment damage and even fire. Therefore, in order to ensure the safe and reliable operation of the equipment, the temperature switch is a small and efficient solution that can be used to monitor the temperature and take corresponding protective measures.

2. The protection principle

The temperature switch is a mechanical passive component, mainly set in the control loop. When the circuit failure or current exceeds the rated value, the circuit abnormal heating causes the temperature to rise, the temperature switch through the internal heat sensitive element bimetallic induction of external temperature changes, once the temperature exceeds the set threshold, the double deformation, disconnect the contact, cut off the circuit, thus playing a whole machine protection.

When the heat sensitive element acts, the contact is disconnected,  and the contact resistance changes instantaneously from within 50ma to close to  infinity, thus giving the control loop signal, IC or MCU after receiving the signal through the MOS tube or other executive elements and then turn off the main loop current, thus playing a whole machine protection role.

When the ambient temperature is reduced to the reset temperature of the temperature switch, the heat sensitive element returns to the initial state, the contact is re-closed, and the resistance value signal within 50ma of the circuit is given, and the actuator operates according to the program setting.

3. the installation position

In the application of photovoltaic inverter, the inductive devices such as DC reactor, sine wave filter reactor and grid-connected transformer used are the key heating components of the product. When designing the overheating protection function of the product, the temperature switch is often used to monitor the heating temperature of the reactor and transformer coil, and the installation method is external or embedded.

External installation: The temperature switch should be close to the surface of the enameled wire, and tied or bonded firmly with high temperature resistant materials. 

• Advantages: easy to maintain, easy to observe, easy to install;

• Disadvantages: appearance impact, affected by the external environment, protection performance is slightly inferior.

Embedded installation: can timely induction coil temperature change. It is necessary to consider the influence of the installation position and the embedded volume on the winding, and choose the appropriate temperature switch model and installation position according to the design of the reactor and transformer products.

• Advantages: beautiful appearance, good protection performance, strong protection;   

• Disadvantages: difficult maintenance, high installation cost, difficult fault detection.

4. selection factors

• Rated current and voltage: In the photovoltaic inverter application, the temperature switch is often used to control the loop, and the current passed is the signal current of the MA level. Our temperature switch can meet the application requirements.

• Protection temperature: often depends on the insulation level of the reactor or transformer, and can also be selected according to the design and installation position of different temperatures, usually choose more temperature points: 90/100/110/125/140/150τ .

• Contact switch form: divided into normally closed, normally open two, normally closed for the initial state of contact closure, to the protection temperature contact disconnect. Normally open is just the opposite. Choose different switch forms according to the design.

• Size and installation method: According to the product design of the reactor and transformer, combined with the size and installation space of the temperature switch to choose the appropriate installation method (external/embedded) to ensure that the installation is convenient and does not affect the normal operation of the equipment.

• Shell epoxy encapsulation products: the insulation layer of the temperature switch will be a little sharp after heat shrinkage, and the enamelled wire with a thin paint layer will have the risk of cutting if it is forced to assemble,  causing short circuit or insufficient insulation voltage.  It is recommended to use the shell epoxy-enclosed temperature switch, which is round and smooth and will not cause harm to the enamelled wire. Our company ST01, ST06, BW-B, BW-E series have epoxy encapsulation products.