As we all know, Solenoid valve has wide application in our daily life, such as Construction and agricultural machinery, municipal technology, Forest technology and etc. As the important component of a solenoid valve, hydraulic solenoid coil operated solenoid valve by creating magnetic potential and magnetic flux. In the working process of the solenoid coil, there is a noticeable problem that is the temperature rise of the solenoid coil. Temperature rise is the difference between coil temperature and environment temperature. Too large temperature rise not only affect the performance of valve like responding speed but also decrease the insulated grade of the solenoid coil. What’s more, temperature rise of solenoid coil lead to the breakdown or short circuit between the adjacent turns, even destroy coils.
The temperature rise of solenoid coil is relative to many factors, including the property of magnetic materials, size of magnetic circuit gap, size of shading coil, deformation characteristics of spring, immersion paint quality. The maximum permissible temperature rise is decided by insulation class of coil. Generally, a temperature rise value is less than the maximum permissible temperature rise value. The environment temperature of the solenoid valve is controlled by the maximum permissible temperature of coil’s insulation class and temperature rise value of solenoid coil. General solenoid valve coil use B-class insulation, if environment temperature is no more than 60℃, the solenoid coil temperature rise is less than 70 ℃.
Temperature-rise test of coil
To enhance the service life of hydraulic solenoid coil, temperature rise test is usually used in the processing of solenoid valve. In the coil temperature rise test, the selection of coils ‘heat transfer coefficient always has experiential and uncertainty characters. It is easy to design solenoid valve with too big or too small structure, which has a bad influence in cooling coil or causes waste. In a word, it is vital that accurately calculate the temperature rise of the coil in advance. Based on the rich experience in manufacturing hydraulic solenoid coil, Kaidi makes this analysis on how to accurately calculate the temperature rise of the coil.
Temperature rise and heat dissipation of solenoid coil
When a hydraulic solenoid is powered, if the heat produced by hydraulic solenoid coils resistance loss can’t be dissipated, and coil temperature rise continually. The solenoid coil is linked to the matter around it, the difference between coil temperature and environmental temperature is bigger, heat dissipation is better. Heat dissipation methods of coil include heat conduction, convection, and heat radiation.
As for the Heat dissipation, according to the heat transfer theory, heat occurs due to the conductibility is proportional to the one power of temperature rise; heat occurs due to the natural convection is proportional to the 0.25 power temperature rise; heat occurs due to the heat radiation is proportional to the four power of temperature rise. Heat is dissipated by the air convection takes the main part of the total heat. In consideration of complicated connection between heat dissipation and temperature rise, in the real calculation, put three heat dissipation methods combine together and think with comprehensive heat transfer coefficient Kt. In the coil’s temperature range of use, heat dissipation of coil is proportional to its temperature rise, as below:
Ps = Pcd + Pdl +Pfs = KtθS
Ps is the total radiating power; Pcd, Pdl and Pfs respectively represent radiating power through heat conduction, convection, and heat radiation; θ represents temperature rise of solenoid coil; S represents heat radiating area of the coil. Above formula is Newton’s law of radiation, it has the advantage of simple form and easy calculation. Kt is related to many other elements like temperature, fluid property, flow rate and the shape or size of the cooling surface.
- Steady temperature rise θs formula
ρθ: resistance coefficient of solenoid coil conductor
f: space factor of winding
b: width of coil window
N: turns per coil
I: coil current
- on-load factor of coil:
T= 1 – evtr
- The temperature rise of coil equals to the product of steady temperature rise and on-load factor.
Heat transfer coefficient empirical formula of coil
By above analysis, we can know that Heat transfer coefficient Kt is relative to a lot of elements. The test is the best way to get accurate Kt value. Through the heat test of the coil, the main factors affecting the heat transfer coefficient of the coil are temperature rise θ and heat radiating area S. Based on the test data, the heat transfer coefficient empirical formula of coil is Kt =f(θS).