GB2256532A

GB2256532A - A core for an electrical choke

Info

Publication number: GB2256532A
Application number: GB9112353A
Authority: GB
Inventors: Trevor Bates
Original assignee: Delco Chassis Overseas Corp
Current assignee: Delco Chassis Overseas Corp
Priority date: 1991-06-08
Filing date: 1991-06-08
Publication date: 1992-12-09
Also published as: GB9112353D0

Abstract

An electrical choke (12) comprises first and second coils (14, 16) each wound around the coil carrying portion (22) of a respective core half (18, 20) of a core (15). The core halves (18, 20) are adapted to be connected together to form a closed magnetic circuit. The coils (14, 16) are wound on the core (15) in opposite directions so that the flux they produce from a d.c. current passing through them is cancelled out. <IMAGE>

Description

ELECTRICAL CHOKE The present invention relates to a choke for use in suppressing noise in an electrical circuit, and more particularly to a core for such a choke.

It is common to use small chokes in the form of a coil wound on a rod or tube of a ferrite or iron dust core to suppress noise caused by small, permanent magnet d.c. motors. Such noise can cause interference to radio reception.

It is also known for such chokes to have their coils wound in the form of a toroid on an annular core, which gives improved magnetic properties over a rod or tube-like core. One problem with using such a toroidal choke, however, is that saturation can occur if flux generated by the d.c.

current flowing through the coil exceeds a predetermined value. This is particularly the case when used with small d.c. motors, which can draw large currents.

One method of reducing this steady state flux in a toroidal choke is to have two coils, one connected to the positive terminal of, for example, the motor, and the other connected to the negative terminal, in such a manner that the flux generated by one coil flows in the opposite direction to that generated by the other coil, thereby cancelling out one another. In practice, total flux cancellation is never achieved due to manufacturing tolerances and the like, but the reduction in flux is normally sufficient to prevent saturation even at high d.c.

current levels in the coils.

A major problem with such toroidal chokes is that assembly is expensive due to the need to wind the coil or coils through the annular core. This is a time consuming process which requires special tools and which cannot be easily automated.

Another problem associated with small d.c motors is the heat generated by the choke due to the large currents flowing through its coil, and in some applications by the motor, or the environment in which the motor is made to work. This can be the case in particular on motors used with engine cooling fans, in which the temperature of the interference suppression chokes can exceed 1500C without cooling.

Such temperatures require the insulation of the coil to be of sufficient quality and thickness to cope with such conditions, and therefore to be more expensive. These temperatures also require the material of the core to be of high quality due to the possibility that the operating temperatures can exceed the Curie temperature of the core material, leading to a loss in magnetic properties.

The present invention seeks to provide an improved core and choke.

Accordingly, an aspect of the present invention provides a core for an electrical choke formed of at least two core portions and comprising first and second coil portions for receiving respective coils of a choke thereon, the coil portions being adapted to be disposed substantially adjacent one another in the core; and connecting means for connecting the coil portions together so as to form a substantially closed magnetic circuit.

Preferably, the core comprises two substantially identical core portions. This makes manufacture and assembly of the core easier since only one type of core part needs to be manufactured.

Advantageously, the connecting means comprises two arms each extending at an angle from a respective coil portion. Each arm may extend substantially perpendicularly from its respective coil portion. This enables the core to be designed such that, in use, an air gap is provided between the two coils to allow cooling of the coils by use of air flow in the environment in which the choke is made to work.

In an embodiment, each connector comprises a respective dovetail projection or recess. This can significantly facilitate assembly of the choke by enabling the coils to be slipped onto their respective coil portion without having to deform or bend the coil in the process.

According to another aspect of the present invention, there is provided a choke for use in suppressing noise in an electrical circuit comprising a core as herein defined and first and second coils fitted over a respective coil portion.

Tests have shown that the invention can provide a choke having an improved performance over existing chokes of the type having a coil wound on rods or tubes. A wider frequency bandwidth can be achieved, and the number of turns reduced as a result of the improved magnetic circuit.

Assembly costs can also be reduced because the choke is effectively two chokes assembled as one part.

The possibility of air cooling and reduced number of turns can result in lower operating temperatures, eliminating the need for high temperature wire insulation and more expensive core materials.

An embodiment of the present invention is described below, by way of illustration only, with reference to the accompanying drawings, in which: Figure 1 is a circuit diagram of a motor and interference suppression choke; Figure 2 is an elevational view of an embodiment of choke in unassembled form; Figure 3 is a plan view of the choke of Figure 2 disposed adjacent a brush plate of a motor; and Figure 4 is an elevational view of a second embodiment of core.

Referring to Figure 1, a motor 10 is connected to an interference suppression choke 12 formed of two choke halves and comprising first and second coils 14,16. The first coil 14 of the first choke half is connected to a positive supply terminal and to a first brush of the motor 10, while the second coil 16 of the second choke half, which is wound in the opposite direction to the first coil 14, is connected to a negative supply terminal and to a second brush of the motor 10. The choke 12 acts to suppress high frequency interference caused by noise from the motor 10, and is particularly useful for suppressing noise at the FM radio frequencies.

Figure 2 shows the components, in unassembled form, of an embodiment of choke 12 which can be used in the circuit of Figure 1. The choke 12 comprises substantially identical first and second coils 14,16 forming part of the first and second choke halves respectively. The coils 14,16 are wound so as to be substantially tubular in shape.

A core 15 is formed of first and second core halves 18,20, each associated with a respective choke half. Each core half 18,20 has a substantially cylindrical coil portion 22 and a first or second connecting member 24, respectively. In the embodiment shown, each connecting member 24 is in the form of an arm extending substantially perpendicularly from one end of the coil portion 22.

The coil portion 22 of each core half 18,20 is in the form of a rod having a diameter slightly larger than the inner diameter of the coils 14,16 so as to provide a tight fit of a coil on the coil portion 22.

A connector (not shown) is provided at the end of the connecting arm 24 of each core half 18,20 for connection with a cooperating connector (not shown) on the second end 28 of the coil portion 22 of the other core half.

The two core halves 18,20 could be connected together by any suitable means, and in some embodiments may not require connectors at the end of the arm 24 and second end 28 of the coil portion 22.

For example, a separate connecting device such as a clip or the like could be used, or the two core halves could simply be attached to one another by suitable adhesive, or other means.

In an alternative arrangement, the unit to which the choke is to be connected, for example the brush plate of a motor, could be designed to hold the two core portions 18,20 together by having, for example, an appropriately shaped recess therein.

To assemble the choke, each coil 14,16, which is previously formed by winding on a mandrel in conventional manner, is fitted over the coil portion 22 of its respective core half 18,20. Since the inner diameter of each coil 14,16 is slightly smaller than the diameter of the coil portion 22, it will fit tightly onto its core half 18,20 so as not to need any adhesive. However, if desired, or if a tight fit is not obtained, each coil 14,16 can be secured to its respective coil portion 22 by adhesive or by any other suitable means.

The coils 14,16 are fitted onto their respective coil portion 22 so as to be wound in opposite directions when the choke is in the assembled state, so that the fluxes they generate when a d.c. current is passed through them, in the manner indicated in Figure 1, substantially cancel out one another.

The two core halves 18,20 are then brought together in the manner shown by the dashed lines in Figure 2. As can be appreciated, the two core halves 18,20 become connected to one another such that the end of the connecting arm 24 of one core half 18,20 abuts the second end 28 of the coil portion 22 of the other core half 20,18, and if connectors are provided, the connectors become connected together.

The two core halves 18,20 can then be secured to one another, if connectors are not provided, in any of the ways described above. A closed magnetic loop, or toroid, is thus formed, with each coil 14,16 located on its respective coil portion 22.

In this embodiment, an air gap is provided between the two coils 14,16 when the choke 12 is assembled, the purpose of which is described below with reference to Figure 3.

In Figure 3, the choke 12 of Figure 2 is shown fitted to a brush plate 30 of a motor (not shown). First and second braided wires 32,34 connect an end of each coil 14,16 to a respective motor brush. The other end of each coil 14,16 is connected to its respective supply terminal (not shown) through an aperture in the brush plate 30.

In use, rotation of the motor causes air to flow through the gap between the two coils 14,16 of the choke 12 which cools the choke 12 to keep its operating temperature relatively low. The actual temperature at which the choke 12 operates in this arrangement, and in any other arrangement, can be determined by experiment, and thus suitable insulating material for the coils 14,16 and magnetic material for the core 15 can be found.

Figure 4 shows another embodiment of core formed also of two core halves 18,20, each having connectors formed therewith. The arm 24 of each core half 18,20 comprises a dovetailed recess 36 extending substantially perpendicularly to the longitudinal direction of the arm 24, while the second end 28 of each core half 18,20 comprises a dovetailed projection 38, also extending substantially perpendicularly to the arm 24.

The two core halves 18,20 can thus be connected to one another by sliding the dovetailed projection 38 of one core half 18,20 into the dovetailed recess 36 of the other core half 20,18, and vice versa. If desired, the connection can be secured by adhesive or by any other suitable means.

In an alternative arrangement, the core comprises a first core portion which includes the first and second coil portions 22 connected at a first end thereof by a cross-member so as to form a U-shaped portion comprising both coil portions 22, the second ends 28 of which can be connected together by a second core portion. The second core portion could be similar to one of the arms 24, with a dovetail recess 36 at either end cooperating with dovetailed projections 38 at the second ends of the two coil portions 22.

Alternatively, the first core portion may comprise one coil portion 22 and the two arms 24, each extending from a respective end of the coil portion 22. The second core portion 22 will then comprise the other coil portion 22, which can be fitted between the arms 24 of the first core portion.

Dovetail connectors could be provided to secure the connection.

In another arrangement, the core comprises two part-circular core portions adapted to be connected together to form an annular core, the coils being fitted to one or both of the core portions.

Claims

1. A core for an electrical choke formed of at least two core portions and comprising first and second coil portions for receiving respective coils of a choke thereon, the coil portions being adapted to be disposed substantially adjacent one another in the core; and connecting means for connecting the coil portions together so as to form a substantially closed magnetic circuit.

2. A core according to claim 1, comprising two substantially identical core portions.

3. A core according to claim 2, wherein the connecting means comprises two arms each extending at an angle from a respective coil portion.

4. A core according to claim 3, wherein each arm extends substantially perpendicularly from its respective coil portion.

5. A core according to claim 1, wherein the connecting means and the first coil portion form part of a first core portion, the connecting means comprising two arms, each extending from a respective end of the first coil portion; the second coil portion forming part of a second core portion and being connectable between the arms of the first core portion.

6. A core according to claim 1, wherein the first and second coil portions form part of a first core portion and are attached to one another by an arm extending between a first end of each coil portion; the connecting means forming part of a second core portion and comprising a member connectable between a second end of each coil portion.

7. A core according to any preceding claim, wherein the connecting means comprises a connector on one core portion cooperable with a connector on another core portion.

8. A core according to claim 7, wherein each connector comprises a respective dovetail projection or recess.

9. A core according to any preceding claim, wherein each coil portion is substantially of rod-like shape.

10. A core according to any preceding claim, wherein in use the core is adapted to provide an air gap between two coils fitted thereon.

11. A choke for use in suppressing noise in an electrical circuit comprising a core according to any preceding claim and first and second coils fitted over a respective coil portion.

12. A core substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

13. A choke substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.