Electromagnetic actuator equipped with two return springs

Details Electromagnetic actuator equipped with two return springs Electromagnetic actuator equipped with two return springs

2000-07-26

2001-07-24

Donovan, Lincoln (Department: 2832)

Electricity: magnetically operated switches, magnets, and electr

Magnets and electromagnets

With magneto-mechanical motive device

C335S251000, C335S255000, C335S258000

Reexamination Certificate

active

06265957

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to an electromagnetic actuator, in particular for a trip device of an electrical switchgear apparatus.
FIG. 7
represents a known actuator of the state of the technique. This actuator
110
comprises a fixed magnetic circuit
112
, made of ferromagnetic material, formed by a shell closed at one of its ends on a fixed core
122
. A movable assembly
114
is designed to slide parallel to a fixed geometrical axis and comprises a mobile core
116
and a rod
118
associated to the mobile core and passing axially through an opening of the fixed core
122
. A spiral-wound compression spring
140
biases the movable assembly
114
to a rest position.
A coiled winding with two fixed coils
130
,
132
is fitted inside the shell and surrounds the mobile core
16
. This coiled winding is designed to generate a magnetic control flux in the magnetic circuit so as to move the movable assembly towards the fixed core against the action of the spring
140
to an active position.
Such a device is conventionally used in shunt releases (MX) and as closing electromagnet (XF) of a circuit breaker. In case of actuation of the electromagnet, an inrush current flowing in the two coils
130
,
132
causes movement of the mobile core
116
, and consequently of the rod
118
, which then protrudes outwards thus enabling either opening of the associated circuit breaker in the case of a shunt release (MX) or closing of the circuit breaker in the case of a closing electromagnet (XF). It is therefore the electromagnetic energy supplied by the coils
130
,
132
during the inrush phase which causes actuation of the circuit breaker. In other words, the rod
118
must be able to perform the mechanical work necessary for movement of the latch to which it is associated, this work corresponding to the energy supplied by the coils
130
,
132
in the inrush phase. The inrush phase is followed by a holding phase during which only one of the two coils
130
,
132
is supplied. A minimum axial air-gap is maintained by fitting a spacer
141
between the mobile core and the fixed core. When the voltage is lower than a dropout threshold, the current flow in the coil winding is interrupted and the mobile core
116
is separated from the fixed core by the action of the spring
140
. As switching to this position does not have any action on the circuit breaker, the power of the spring is relatively indifferent in this phase. The spacer
141
prevents the mobile core
116
from remaining “stuck” to the fixed core
122
due to the remanence effect of the magnetic circuit when the power supply to the coil is interrupted.
In a device of this kind, the dimensioning of the different elements, in particular of the spring and the minimum air-gap in the active position, is difficult. The potential energy of the contracted spring, which has to return the movable assembly to the rest position on its own, must be great enough to overcome the remanent magnetic energy. The presence of the air-gap enables the sticking effect to be limited but induces a risk of nuisance unsticking, i.e. of an involuntary return to the rest position, in particular in response to a mechanical shock on the rod or a large vibration of the movable assembly. If it is chosen to reduce the air-gap, the potential energy of the return spring then has to be increased accordingly, so that the inrush energy necessary to move the movable assembly to the active position is also increased.
OBJECT OF THE INVENTION
The object of the invention is to overcome these shortcomings and to provide a high-sensitivity electromagnetic actuator, of reduced volume and with a low inrush and holding energy, which in addition has a low sensitivity to mechanical shocks and vibrations. According to the invention, this object is achieved by an electromagnetic actuator comprising:
a fixed magnetic circuit made of ferromagnetic material comprising:
a shell and
a fixed core situated at one end of the shell and connected thereto,
a movable assembly designed to slide along a fixed geometric axis between a rest position and an active position and designed to produce a mechanical work when moving from its rest position to its active position, the movable assembly comprising:
a mobile core whose axial air-gap with the fixed core is reduced when the movable assembly moves from its rest position to its active position, the axial air-gap between the mobile core and the fixed core being zero in the active position,
an actuating means associated to the mobile core,
a first return spring biasing the movable assembly to its rest position,
an excitation circuit comprising at least one fixed control coil designed to generate a magnetic control flux in the magnetic circuit, which flux oppose s the action of the first spring, the excitation circuit being designed to switch from an inrush mode in which it delivers a high power sufficient to move the movable assembly from its rest position to its active position, to a holding mode in which it delivers a lower power sufficient to hold the movable assembly in the active position,
a second spring with a greater stiffness than that of the first spring, designed to return the movable assembly flexibly to its rest position,
a first stop,
a second stop, mobile and designed to operate in conjunction at least with the second spring and with the first stop, in such a way that, in a first part of the axial travel of the movable assembly from its rest position to its active position, the second stop is not in contact with the first stop and the action of the first spring is preponderant, and that in the remaining travel up to the active position, the second stop is immobilized with respect to the first stop and the action of the second spring is preponderant.
During the first phase of activation, the effect of the spring with lesser stiffness is preponderant, so that the movable assembly is subjected to a large acceleration. At the end of the first phase, the kinetic energy stored by the movable assembly is great. In addition the axial air-gap is reduced, so that during the second phase of activation contraction of the second spring is possible. The zero air-gap between the mobile core and the fixed core contributes to decreasing the supply energy of the coil necessary to hold the actuator in the active position and ensures a better resistance to mechanical shocks and vibrations. At the moment the movable assembly returns to the rest position, the increase of the magnetic remanence effect resulting from the absence of an air-gap is compensated by the second spring.
According to a preferred embodiment, the first spring is arranged between the fixed core and the movable stop, and the second spring is arranged between the movable stop and the movable assembly, so that in the first part of the travel, the two springs cooperate in series, and that in the second part of the travel, only the second spring continues to work. If k
1
is the stiffness of the first spring and k
2
that of the second spring, the stiffness of the system in the first phase is k
1
k
2
/(k
1
+k
2
), a value which will be all the more close to k
1
the greater k
2
is compared with k
1
. During the second phase, the stiffness of the system is equal to k
2
. This series fitting is particularly advantageous when the radial dimensions of the actuator and the diameter of the coil are sought to be reduced as a priority.
According to another embodiment, the first spring is arranged between the fixed core and the movable assembly whereas the second spring is arranged between the fixed core and the second stop, so that in the first part of the travel the first spring is working alone, and that in the second part of the travel the two springs are cooperating in parallel. The stiffness in the first phase is then equal to k
1
and the stiffness in the second phase is equal to k
1
+k
2
, a value all the more close to k
2
the greater k
2
is compared with k
1
. This arrangement, which in practice requires a greater radial dimension, and therefore bulkier coil

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