DC electromagnets are classified into two types: rotary and direct-acting. Rotary electromagnets are more commonly used based on their structural type.
Direct-acting DC Electromagnets
The structural characteristic of direct-acting DC electromagnets is their relatively long armature stroke. Their structure is mostly solenoid, with the armature moving linearly within the coil cavity. The leakage magnetic field pattern and magnetic field pattern at the working air gap differ from other types of electromagnets. This difference is reflected in their performance; their electromagnetic attraction includes solenoid force, generated by leakage flux. Due to their simple structure, solenoid electromagnets are widely used as electromechanical conversion elements in pneumatic servos, especially pulse-width modulation (PWM) servos.
Rotary DC Electromagnet
Rotary DC electromagnets, also known as valve-type electromagnets or snap-action electromagnets, are mostly used in DC relays. Their structural characteristics are as follows:
The armature rotates around a corner: The advantage of this structure is that the frictional torque is small and it is not easily damaged. After appropriate heat treatment of the friction surface, the wear will be very small. Electromagnets with this structure have a long service life, and the number of rotations can reach thousands of times.
A pole cap is located at the end of the iron core column: The iron core column of the rotary electromagnet is relatively thin and long, and the leakage flux cannot generate attraction force to do work, resulting in a very flat attraction force characteristic curve for the electromagnet. To obtain a relatively flat attraction force characteristic curve, the electromagnetic attraction force under the rated stroke must be relatively increased.
The core column has a pole cap at its end: The core column of a rotating electromagnet is relatively thin and long, with leakage flux. This leakage flux cannot generate attraction force to do work, resulting in a very flat attraction force characteristic curve. To obtain a flatter attraction force characteristic curve, the electromagnetic attraction force at the rated stroke must be relatively increased.
A non-magnetic shim is placed at the end face of the core column: Magnetic shims are usually stamped from thin aluminum or copper sheets. Their function is to weaken residual magnetism to prevent the armature coil from sticking to the core after de-energization, ensuring reliable armature release.