### Force and resistance

Solenoid Force is impacted by ampere turns. Ampere turns are a function of the current that passes through the coils and the number of turns of copper wire wrapped around the bobbin. The solenoid’s size determines the amount of copper wire that can be used, so the main determinant of Force is the amount of current.

Lower resistance leads to greater current flow. In other words, thick copper wire increases Force. Normally, higher or lower resistance in solenoids of the same size can be compared to judge their Force. ### Temperature rise and resistance

If resistance is low and current flow is large, the Force will increase, but the solenoid’s internal temperature will quickly rise.

There is a risk of fire or explosion if the temperature rise exceeds the solenoid’s limit. Therefore, a solenoid must be selected on the basis of not just its Force but also its temperature rise.

When current passes through continuously, or when a solenoid is used in a high-temperature environment, its Force will decrease. In this case, choose a solenoid with a small temperature rise and large resistance. ### Temperature rise reduces pulling force

As described above, when solenoid temperature rises, Force decreases. The table below shows the rate of increase and decrease in Force according to temperature when the standard temperature is 20℃. Please refer to the table to choose a Force with a safety margin. The amount of heat generated in the coil also causes a temperature rise in external parts. On the other hand, external parts can be a surrounding heat sink and thereby can be used to control the temperature rise. They also have a point of equilibrium where, once their temperature rises to a certain point, they do not get any hotter.

The solenoid’s insulation grade will vary depending on this equilibrium point.
The table that follows shows the insulation grades of ordinary solenoids. ### Measuring the temperature rise

Measuring the temperature rise of the solenoid simply by measuring the solenoid body itself may lead to differences from the solenoid’s actual temperature, due to the impact of surrounding equipment when the solenoid is mounted in an actual mechanical environment, and also due to changes in ambient temperature and varying time periods when electric power is flowing.

In this case, the temperature rise can be calculated by measuring the solenoid’s resistance after the solenoid has been actually mounted and the electric power has been turned ON. (Resistance method)

The following formula is used to obtain the temperature rise 