About the temperature protection and temperature measurement of the motor

Application of PTC thermistor 1. Delay start PTC thermistor It is known from the It characteristic curve of PTC thermistor that it takes a period of time for the PTC thermistor to reach the high resistance state after the voltage is applied. This delay characteristic is used For delayed start purpose. Application principle: When the motor starts, it must overcome its own inertia and at the same time overcome the reaction force of the load (for example, the reaction force of the refrigerant must be overcome when the refrigerator compressor is started), so the motor needs a larger current and torque when starting . When the rotation is normal, in order to save energy, the required torque will be greatly reduced. Add a set of auxiliary coils to the motor, it only works when it starts, and it will be disconnected when it is normal. Connect the PTC thermistor in series with the starting auxiliary coil. After starting, the PTC thermistor enters a high resistance state and cuts off the auxiliary coil. This effect can be achieved. 2. Overload protection PTC thermistor overload protection is a kind of PTC thermistor The protective element that automatically protects and recovers from abnormal temperature and abnormal current is commonly known as "self-resetting fuse" and "ten thousand fuse". It replaces traditional fuses and can be widely used for over-current and over-heat protection of motors, transformers, switching power supplies, electronic circuits, etc. The PTC thermistor for overload protection reduces the residual current value by limiting the consumption in the entire circuit through its resistance mutation. The traditional fuse cannot recover by itself after the circuit is blown, and the PTC thermistor for overload protection can be restored to the pre-protection state after the fault is removed, and the overcurrent and overheat protection function can be realized when the fault occurs again. Select the PTC thermistor for overload protection as the overcurrent and overheat protection element, first confirm the maximum normal operating current of the line (that is, the non-operating current of the PTC thermistor for overload protection) and the installation position of the PTC thermistor for overload protection (in normal operation) ) The maximum ambient temperature, followed by the protection current (that is, the operating current of the PTC thermistor for overload protection), the maximum operating voltage, and the rated zero power resistance. At the same time, the dimensions of the component should also be considered. Application principle: When the circuit is in the normal state, the current through the PTC thermistor for overload protection is less than the rated current, and the PTC thermistor for overload protection is in the normal state, and the resistance is small, which will not affect the normal operation of the protected circuit. When the circuit fails and the current greatly exceeds the rated current, the PTC thermistor for overload protection suddenly heats up and assumes a high resistance state, making the circuit in a relatively "off" state, thereby protecting the circuit from damage. When the fault is eliminated, the PTC thermistor used for overload protection will automatically return to a low resistance state, and the circuit will resume normal operation. 3. Overheating protection PTC thermistor The Curie temperature of the PTC thermistor sensor is from 40-300℃. On the RT characteristic curve of the sensitive resistance sensor, the section where the resistance value rises sharply after entering the jump zone can be used as temperature, liquid level, and flow sensing applications. According to the temperature-sensitive characteristics of PTC thermistors, it is designed to be used in overheating protection and temperature sensing applications, and is used in switching power supplies, electrical equipment (motors, transformers), and power devices (transistors). Features are small size and fast response time , Easy to install.

       The difference between PTC and KTY: Siemens uses KTY

       First of all, they are a kind of motor temperature protection device;

       PTC is a positive temperature coefficient resistor, that is, the resistance value rises with the rise of temperature;

       Another is that NTC is a variable resistor with a negative temperature coefficient. The resistance value decreases as the temperature rises, and it is not used for general motor protection. Kty has high precision, high reliability and strong stability. Mainly used in the field of temperature measurement. The kty is covered with a layer of silicon dioxide insulating material, a metal hole with a diameter of 20mm is opened on the insulating layer, and the entire bottom layer is metalized. The current distribution that is tapered from top to bottom is obtained through the arrangement of crystals, so it is named diffusion resistance. KTY has an actual online linear temperature coefficient in the entire temperature measurement range, thus ensuring high accuracy of temperature measurement.

       PT100 platinum thermal resistance is designed and manufactured by using the basic principle that the resistance value of platinum wire changes with the change of temperature. According to the resistance value R (℃) at 0℃, it is divided into 10 ohms (graduation number is Pt10). ) And 100 ohm (graduation number Pt100), etc., the temperature measurement range is -200 ~ 850 ℃. The temperature sensing element of the 10 ohm platinum thermal resistance is made of thicker platinum wire, and the temperature resistance is obviously excellent A platinum thermal resistance of 100 ohms, as long as it is used in the temperature zone above 650℃: 100 ohm platinum thermal resistance is mainly used in the temperature zone below 650℃, although it can also be used in the temperature zone above 650℃, but in the temperature zone above 650℃ Class A errors are not allowed. The resolution of a 100 ohm platinum thermal resistance is 10 times larger than that of a 10 ohm platinum thermal resistance. The requirements for the secondary instrument are correspondingly an order of magnitude. Therefore, the 100 ohm platinum thermal resistance should be used as much as possible to measure the temperature in the temperature range below 650℃. .