36 – Operating Time Analysis of an Electromagnetic Relay

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Application Note / Model Data

Overview

Electromagnetic relays are devices that use an electromagnet to physically connect and disconnect contact points. Magnetic flux is generated from the magnetomotive force, which is expressed as the product of the number of turns in the coil and the current that is applied to the coil. This flux produces an attraction force in the movable core, making the relay close.
To put it simply, the attractive force is determined from the area of the gap between the movable core and the stator core and the size of the magnetic flux density produced in said gap. With a relay whose movable core does not move linearly, however, it is hard to predict the magnetic flux density in the gap because it does not become parallel. The nonlinear magnetic properties of the iron core and yoke also affect the magnetic flux density in the gap. With a JMAG magnetic field analysis, it is possible to obtain the attraction force of the movable core while accounting for these factors.
This Application Note presents the use of the motion equation function to evaluate the operating time of an electromagnetic relay that uses a DC voltage drive.

Operating Time

Fig. 1 shows the displacement-time characteristic, fig. 2 shows the attractive force-time characteristic, fig. 3 shows the current-time characteristic, and fig. 4 shows the voltage-time characteristic. Just after excitation starts, the displacement of the movable core accelerates rapidly due to the attractive force. At this time, the magnetic flux from the movable core to the stator core changes substantially, so the inductance becomes larger and less current flows. The magnetic flux decreases after the movable core makes contact with the stator core, so the inductance becomes smaller and more current flows. Even right after the excitation stops at 0.005 s, the movable core is still in contact with the stator core because the electromagnetic energy stored in the coil flows through the diode. When the strength of the spring becomes stronger than the attractive force, the movable core returns to its initial position.

Magnetic Flux Density Distribution

Fig. 5 shows the magnetic flux density distribution at each analysis time: At 0.0050 s, just after the movable core makes contact with the stator core and just before the power source is cut off, at 0.0052 s, just after the power source is cut off, and at 0.0068 s, when the iron core begins to return to its initial position. After the excitation stops, the magnetic flux density distribution decreases gradually because the current still flows in the coil through the diode due to the electromagnetic energy stored in the coil.

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