Overview
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Electromagnetic relays are devices that connect and disconnect contact
points physically using an electromagnets.?
The magnetic attraction which activates the relay is generated by the magnetomotive
force, which is expressed by the product of coil turns and the current.
The operating time can be calculated from the magnetic attraction using
an equation of motion.
This note presents the use of a magnetic field analysis to evaluate the
operating time of a DC relay. |
Operating Time
Figure 1 shows the displacement versus time, Figure 2 shows the attraction
versus time, Figure 3 shows the current versus time and Figure 4 shows
the voltage versus time.
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 the less current flows. After the movable core contacts
with the stator core, the magnetic flux decreases, so the inductance becomes
smaller and the more current flows.
Even right after excitation stops at 0.005 seconds, the movable core still
contacts with the stator core because of the electromagnetic energy in
the coil through the diode. When the strength of the spring becomes stronger
than the attractive force, the movable core is released and returns to
its initial position. |
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Magnetic Flux Density Distribution
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Figure 5 shows the magnetic flux density distribution at each analysis
time; just before (0.0050 seconds) and just after (0.0052 seconds) excitation
stops and just after the iron core is released (0.0068 seconds). After
excitation stops, the magnetic flux density decreases gradually because
the current flows in the coil through the diode due to the effect of the
electromagnetic energy in the coil. |
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