Note: Descriptions are shown in the official language in which they were submitted.
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_LECTROMAGNETIC ACTUATOR ARRANGEMENT__________________________________
The present invention relates to electromagnetic
a_~uator arrangements and relates particularly but no-t
exclusively to residual current circuit breakers.
It is necessary for residual current circuit
breakers in particular to trip very quickly, e.g. in
less than 20 milliseconds. In order to meet this
requirement, it has been necessary to provide a very
powerful biasing spring which trips the actuator when
the actua-tor is de-energised. A correspondingly
powerful and bulky electromagnet has been required to
overcome the spring bias in the energised condition.
An object of the present invention is to provide an
electromagnetic actuator arrangement which is faster-
acting and/or requires a less powerful electromagnet
than comparable conventional actuator arrangements.
According to the present invention, an
electromagnetic actuator arrangement comprises an
electromagnetic actuator, including at least one
operating member of magnetically soft material which
cooperates with biasing means and is magnetised when
said electromagnetic actuator is energised and means for
reversing the magnetising force acting on said operating
em'3-er when the said electromagnetic actuator is
de-energised.
The invention is applicable particularly but not
exclusively to circuit breakers and relays, which
incorporate -two such operating members, namely a fixed
yoke and a movable armature. By reversing the magnetising
force on de-energising an electromagnet actuator of this
type, the residual magnetic flux in the armature is
opposed by the reversed magnetic field and the normal
tenden_y of the armature to stick to the yoke immediately
after the winding has been de-energised is avoided.
In fact during the brief period in which the
residual magnetic flux in -the armature maintains its
original polarity, the armature is repelled by the
reversed magnetic field arising from the reversed
magnetising force and thus the biasing force is briefly
augmented at -the instant of de-energising the winding.
Accordingly -the invention is also applicable -for example
to solenoids and other devices which incorporate only a
single "opera-ting member" of magnetically soft
material.
In typical preferred embodiments, particularly of
relay and circuit-breaker arrangements, the armature
will be removed from the magnetic field of the winding
by the biasing means by the time that the residual
magnetic flux in the armature has been reversed by the
reversal of magnetising force on de-energising the
actuator. The resulting force of attraction acting on
the armature will accordingly be very weak in comparison
with the opposing biasing force and the tripping time
will not be adversely affected.
A particularly convenient way of limiting the
reverse magnetisation is to reverse the magnetising
force by means of a capacitor (which may for example by
connected in parallel with the winding) which is
arranged to discharge through the winding on de-
energisation of the actuator. Preferably the discharge
is oscillatory so that the residual magnetisation of the
armature is thereby reduced to a very low level.
In a preferred embodiment the winding of the
electromagnetic actuator is energised from a current
force (preferably a substantial~ c~n~tant current
source) and a gate-controlled semi-conductor switching
device is arranged both to bypass current from said
current source in response to a control signal applied
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to the ga-te of said device on de-energising said
electromagnetic actuator, and to conduct the current
which is disch~rged by said capacitor through said
winding.
A preferred embodiment of the invention is
described below by way of example only with reference to
the accompanying drawing, Figure 1, which is a ci~cuit
diagram of a residual current circuit breaker in
accordance with the invention.
The circuit shown in Figure 1 comprises live (L)
and neutral (N) mains input terminals which are
connected via a two-pole swi-tch SWl to mains output
terminals, which may in turn be connected to an
electrical appliance (not shownj. Switch SWl is
controlled by an electromagnetic actuator comprising a
movable armature ARl and a yoke Yl which is provided
with a winding Llo Armature ARl is mechanically linked
to switch SWl and the latter is biased open by a tension
spring SP which acts on the armature. Armature ARl and
yoke Yl are composed of magnetically soft iron and
contact one another when switch SWl is closed.
In use, switch SWl is closed manually and energised
by a rectified constant current source comprising a
series-connected diode Dl, resistors R3, R4 and R5 and
R6 and capacitor C6 (which are connected between the
mains conductors) and resistor R2 (which is connected
between -the junction of R3 and C6 and one end of winding
Ll).
The other end of winding Ll is connected to pin 6
of an RA3783 integrated circuit ICl ad thence via pin 4
thereof to the free terminal of smoothing capacitor C6.
Accordingly winding Ll is energised a~.d maintains switch
SWl ON.
A ferrite-core transformer Tl is coupled to the
35 mains conductors and its output is fed to pins 2 and 3
of ICl.
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The potential difference across pins 2 and 3 is
monltored by ICl and in the event that it exceeds a
predetermined threshold value (as a result of an
inbalance in the forward and return currents in the
mains conductors due to a potentially dangereous leakage
of current to earth) an output voltage is generated at
pin 5. ~ series-corrected discharge capacitator Cl and
thyris-tor SCRl are corrected in parallel with winding Ll
and the ~ate of SCRl is connected to pin S. The cathode
of thyristor SCRl is also connected to the negative
terminal of capaci-tor C6 so that thyristor SCRl bypasses
the output current of the curren-t source as well as
providing a discharge path for capacitor Cl through
winding Ll. The current from R2 is sufficient to hold
SCRl ON whilst capacitor Cl is discharging.
Accordingly, when an earth leakage current is
detected, a voltage appears at terminal 5 of ICl which
fires thyristor SCRl, thereby bypassing the forward
current from resistor R2 of the current source and
allowing capacitor Cl to send a brief reverse discharge
curren-t through winding Ll which generates a reverse
magnetising force which opposes the residual flux in
~rm2ture ARl and yoke Yl. Accordingly, armature ARl is
immedia-tely repelled from yoke ~1 and is rapidly removed
from the yoke by biasing spring SP. By this time, the
current in winding Ll has fallen to zero. Thus switch
SWl breaks the mains circuit very rapidly.
I~hen safe conditions have been restored, the
circuit may be re-set by depressing switch SWl
manually.
A pair of test contacts TC are connected in series
with a resistor R7 between the neutral output terminal
and a live input terminal of the mains conductors to
enable a residual current to be generated artificially
in order to test the circuit.
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The sensitivity of the circuit can be adjusted by
altering the value oF a resis-tor R8 which is connected
across the winding of Tl or the value of resistor Rl
which is connected between terminals 1 and 7 of ICl. It
is by-passed by a noise suppression capacitor C4 and an
additional noise-suppression capacitor C3 is connected
between pins 1 and 8 of ICl. Resistor R8 is by-passed
by a noise-suppression capacitor C5 and a noise-
suppression capacitor C2 is also connected between -the
gate and cathode of thyristor SCRl.
It should be noted that the circuit au-tomatically
switches switch SWl OFF in the event of loss oE mains
supply.
