An electric current through a conductor will produce a magnetic field at right angles to the direction
of electron flow. If that conductor is wrapped into a coil shape, the magnetic field produced will
be oriented along the length of the coil. The greater the current, the greater the strength of the
magnetic field.
Inductors react against changes in current because of the energy stored in this magnetic field.
When we construct a transformer from two inductor coils around a common iron core, we use this field to transfer energy from one coil to the other. However, there are simpler and more direct uses
for electromagnetic fields than the applications we've seen with inductors and transformers. The
magnetic field produced by a coil of current-carrying wire can be used to exert a mechanical force
on any magnetic object, just as we can use a permanent magnet to attract magnetic objects, except
that this magnet (formed by the coil) can be turned on or off by switching the current on or off through the coil.
If we place a magnetic object near such a coil for the purpose of making that object move when
we energize the coil with electric current, we have what is called a solenoid. The movable magnetic
object is called an armature, and most armatures can be moved with either direct current (DC)
or alternating current (AC) energizing the coil. The polarity of the magnetic field is irrelevant for
the purpose of attracting an iron armature. Solenoids can be used to electrically open door latches,
open or shut valves, move robotic limbs, and even actuate electric switch mechanisms. However, if
a solenoid is used to actuate a set of switch contacts,we have a device so useful it deserves its own name( the relay).
Relays are extremely useful when we have a need to control a large amount of current and/or
voltage with a small electrical signal. The relay coil which produces the magnetic ¯eld may only
consume fractions of a watt of power, while the contacts closed or opened by that magnetic ¯eld
may be able to conduct hundreds of times that amount of power to a load. In e®ect, a relay acts as
a binary (on or off) amplifier.
One relay coil/armature assembly may be used to actuate more than one set of contacts. Those
contacts may be normally-open, normally-closed, or any combination of the two. As with switches,
the "normal" state of a relay's contacts is that state when the coil is de-energized, just as you would find the relay sitting on a shelf, not connected to any circuit.
Relay contacts may be open-air pads of metal alloy, mercury tubes, or even magnetic reeds,
just as with other types of switches. The choice of contacts in a relay depends on the same factors
which dictate contact choice in other types of switches. Open-air contacts are the best for high-
current applications, but their tendency to corrode and spark may cause problems in some industrial
environments. Mercury and reed contacts are spark-less and won't corrode, but they tend to be
limited in current-carrying capacity.
of electron flow. If that conductor is wrapped into a coil shape, the magnetic field produced will
be oriented along the length of the coil. The greater the current, the greater the strength of the
magnetic field.
Inductors react against changes in current because of the energy stored in this magnetic field.
When we construct a transformer from two inductor coils around a common iron core, we use this field to transfer energy from one coil to the other. However, there are simpler and more direct uses
for electromagnetic fields than the applications we've seen with inductors and transformers. The
magnetic field produced by a coil of current-carrying wire can be used to exert a mechanical force
on any magnetic object, just as we can use a permanent magnet to attract magnetic objects, except
that this magnet (formed by the coil) can be turned on or off by switching the current on or off through the coil.
If we place a magnetic object near such a coil for the purpose of making that object move when
we energize the coil with electric current, we have what is called a solenoid. The movable magnetic
object is called an armature, and most armatures can be moved with either direct current (DC)
or alternating current (AC) energizing the coil. The polarity of the magnetic field is irrelevant for
the purpose of attracting an iron armature. Solenoids can be used to electrically open door latches,
open or shut valves, move robotic limbs, and even actuate electric switch mechanisms. However, if
a solenoid is used to actuate a set of switch contacts,we have a device so useful it deserves its own name( the relay).
Relays are extremely useful when we have a need to control a large amount of current and/or
voltage with a small electrical signal. The relay coil which produces the magnetic ¯eld may only
consume fractions of a watt of power, while the contacts closed or opened by that magnetic ¯eld
may be able to conduct hundreds of times that amount of power to a load. In e®ect, a relay acts as
a binary (on or off) amplifier.
One relay coil/armature assembly may be used to actuate more than one set of contacts. Those
contacts may be normally-open, normally-closed, or any combination of the two. As with switches,
the "normal" state of a relay's contacts is that state when the coil is de-energized, just as you would find the relay sitting on a shelf, not connected to any circuit.
Relay contacts may be open-air pads of metal alloy, mercury tubes, or even magnetic reeds,
just as with other types of switches. The choice of contacts in a relay depends on the same factors
which dictate contact choice in other types of switches. Open-air contacts are the best for high-
current applications, but their tendency to corrode and spark may cause problems in some industrial
environments. Mercury and reed contacts are spark-less and won't corrode, but they tend to be
limited in current-carrying capacity.
No comments:
Post a Comment