Overview
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Electromagnetic relays are devices that connect and disconnect contact
points physically using an electromagnet.
The magnetic attraction that activates the relay is generated by the magnetomotive
force, which is expressed by the product of coil turns and current.
The operating time can be calculated from the magnetic attraction at the
moveable core using an equation of motion.
This example presents the use of a magnetic field analysis to evaluate
the operating time of an electromagnetic relay driven by direct current
accounting for the eddy currents. |
Operating Time
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The displacement versus time is indicated in Fig. 1, the current versus
time is indicated in Fig. 2, and the attractive force versus time is indicated
in Fig. 3.
The current gradually increases just after the excitation starts, but the
current has difficulty increasing for around 2 milliseconds because of
the back electromotive force following the increased magnetic flux. Right
after the excitation stops at 5 milliseconds, the movable core still contacts
the stator core because of electromagnetic energy in the coil through the
diode. When the strength of the spring becomes stronger than the attractive
force at around 7 milliseconds, the movable core returns to its initial
position. |
Magnetic Flux Density Distribution
The magnetic flux density distribution just before the excitation stops,
just after the excitation stops, and just after the moveable core returns
to its initial position are indicated in Fig. 4.
After the excitation stops, the magnetic flux density decreases gradually
because the current still flows in the coil through the diode due to the
effect of the electromagnetic energy in the coil as indicated in Fig. 5. |
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