GB2389242A

GB2389242A - Varistor assembly for motor noise suppression

Info

Publication number: GB2389242A
Application number: GB0212471A
Authority: GB
Inventors: Lucux Law
Original assignee: Johnson Electric SA
Current assignee: Johnson Electric SA
Priority date: 2002-05-30
Filing date: 2002-05-30
Publication date: 2003-12-03
Also published as: GB0212471D0

Abstract

To overcome the problems associated with conventional annular ceramic varistor a noise suppression element 18 comprises an annular printed circuit board (pcb) 23, conductive pads 24 and chip varistors 25 soldered thereto by surface mounted re-flow techniques, the element being fitted to the commutator by soldering to the segment terminal flags 26. Alternatively single clip varistor 25 may be resiliently mounted by fingers30 associated with brush arms 29, fig 3, or motor terminals 22, fig 4. Chokes and capacitors may also be present.

Description

f Electric Motor This invention relates to electric motors and in

particular, to a miniature PMDC motor fitted with a varistor for noise suppression.

Miniature electric motors may be used in applications where the electrical noise produced by the motor must be kept below a predetermined level. This is commonly achieved by the use of chokes, capacitors, varistors, or combinations thereof.

However, as each component significantly increases the final cost of the motor, it is 10 desirable to reduce the number of noise suppression components to a minimum.

Ceramic varistors have excellent noise suppression properties, especially when mounted directly onto the commutator of the motor to suppress the electrical noise at the source. However, the ceramic ring varistors are expensive, difficult to mount and 15 require carefill handling due to their brittle nature. The varistors are also configured specifically for a particular type of motor, i e. a 5 pole motor requires a varistor with 5 terminals, while a 3 pole motor requires a varistor with only 3 terminals. Also, the size of the hole in the varistor must match the physical size of the commutator, too large and the solder connections are difficult with a greater possibility of 20 misalignment affecting the balance of the rotor.

As an alternative, it is known to mount capacitors on the rotor and connected either directly or via a PCB to the commutator segments. The capacitors may be leaded with the body of the capacitor located in the winding tunnels of the rotor or chip type 25 capacitors soldered on to the PCB However, there are applications where a varistor provides a better solution and thus there is a need for an improved method of mounting one or more varistors in an electric motor.

Accordingly, the present invention provides a miniature electric motor having a permanent magnet stator; a wound rotor comprising a shaft, a commutator, a rotor core, rotor windings wound around the rotor core and connected to the commutator; at least two brush assemblies associated with the commutator for transferring electric 35 power to the rotor windings; and a noise suppression circuit; wherein the noise suppression circuit includes at least one chip type varistor Preferred and/or optional features are defined in the dependent claims.

( Three preferred embodiments will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a partial sectional view of a PMDC motor constructed in accordance with a first embodiment of the present invention, Figure 2 is a view of a PCB as used in the motor of Figure 1; 10 Figure 3 is an internal plan view of an end cap for a PMDC motor according to a second embodiment of the present invention; and Figure 4 is a view similar to Figure 3 of an end cap according to a third embodiment of the present invention Referring to Figure 1, the PMDC motor lO has a deep drawn metal housing It supporting two permanent magnets 12 forming a stator and a wound rotor 13 The wound rotor 13 comprises a shaft 14, a rotor core 1 5, rotor windings 16 wound around the core 15 and a commutator 17. Fitted to the commutator is a noise suppression 20 element 18. The motor to also has an end cap 20 which closes off one end of the housing I l and supports brushes 21 (only one shown) and motor terminals 22. Here the motor terminals 22 are male spade type connectors.

The noise suppression element 18 is shown more clearly in Figure 2 The element 25 comprises an annular PCB 23 having three conductive pads 24 and three chip type varistors 25. As there are three varistors 25, this embodiment is for use on a 3-pole armature Each varistor 25 is soldered across two of the pads 24 as shown in Figure 2 This soldering is simple to achieve and in mass production, could be done by a surface mounting reflow technique.

When fitted to the commutator' the conductive pads 24 are soldered directly to flags 26 on terminals of the commutator (shown in phantom in Figure 2) or directly to the commutator terminals similar to the current practice for annular ceramic varistors In this way, the varistors are easily connected to the commutator with a higher degree of 35 reliability as the reflow soldering process can be highly automated with minimal impact on the varistors and the connection between the PCB and the commutator reduces the heat and pressure applied directly to the varistor which is a significant cause of premature failure of the annular ceramic varistor

( Referring now to the embodiment shown in Figure 3 where like numbers refer to like parts, the end cap 20 has brushes 21 connected to the motor terminals 22 In this embodiment, the motor terminals 22 are female type terminals arranged to accept 5 male spade connectors. A bearing 27 is shown located centrally of the end cap for rotatably supporting the motor shaft The brushes 21 comprise brush bodies 28 of a graphite material fitted to beryllium copper brush arms 29 for resilient urging of the brush bodies 28 into contact with the commutator The brush arms 29 also make resilient electric contact with the motor terminals 22 Near the base of the brush arms 10 29 where they are secured to the end cap (either directly or by way of a separate brush holder (not shown)), a finger 30 extends towards the opposing brush arm. Each finger 30 resiliently contacts a respective terminal end of a chip type varistor 25 located in a compartment 31 on the inner surface ofthe end cap 20 between the brush arms 29.

15 This arrangement provides a solder free reliable, easy to assemble method of incorporating a varistor into a motor circuit with minimal parts. If the varistor is not required, it can be left out without any change to the motor or end cap.

The alternative embodiment of Figure 4 is very similar to the embodiment of Figure 3 20 with the exception that the varistor chip 25 is located between the motor terminals 22 instead of between the brushes 21 or brush holders. Here, a finger 30 is shown extending from each motor terminal 22 across a recess or channel 31 towards the opposite finger. The fingers 30 resiliently engage the varistor chip 25 to electrically connect the varistor 25 across the motor terminals 22.

In this embodiment, the motor terminals 22 are connected to the brushes 21 by way of chokes 32. The chokes 32 are part of the noise suppression circuitry and while two chokes are shown, depending on requirements only one may be necessary. Also, capacitors, not shown, could be incorporated into the noise suppression circuitry in 30 known configurations in addition to the varistor chip The embodiments described above are given by way of example only and various modifications will be apparent to persons skilled in the art without departing from the scope of the invention as defined in the appended claims.

One such variation would be to omit the finger on the brush arms or motor terminals and use a separate spring or two, either U-shaped or coiled to resiliently connect the varistor to the brush arms, brush holders or motor terminals Alternatively, only one

finger could be used which resiliently presses the chip into contact with the opposite brush brush holder or terminal, as the case may be.

Also, the chip varistor could be soldered directly between the brush holders or motor 5 terminals While this works very well and provides security of attachment and retention of the varistor, it is not solder free which may be a requirement in certain applications

Claims

( Claims

I A miniature electric motor has a permanent magnet stator; 5 a wound rotor comprising a shaft, a commutator, a rotor core, rotor windings wound around the rotor core and connected to the commutator, at least two brush assemblies associated with the commutator for transferring electric power to the rotor windings; and a noise suppression circuit, O wherein the noise suppression circuit includes at least one chip type varistor.
2. A motor according to claim 1, wherein the rotor has n poles and the noise suppression circuit has n chip type varistors.

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3. A motor according to claim 2, wherein the commutator has n segments and each varistor is electrically connected across a respective pair of adjacent segments.
4. A motor according to claim 3, wherein the varistors are mounted on a printed circuit board which is connected to the commutator segments
5 A motor according to claim 4, wherein the printed circuit board has n conductive pads electrically connected to the n commutator segments, respectively, with the n varistors connected across respective pairs of adjacent pads 25
6. A motor according to claim 5, wherein each commutator segment has a terminal connecting the commutator to the rotor windings and wherein the printed circuit board is annular and physically located about the commutator in the region of the terminals.

30
7. A motor according to claim 6, wherein the printed circuit board is directly soldered to the terminals of the commutator segments
8. A motor according to any one of claims 4 to 7, wherein the varistors are soldered to the printed circuit board
9. A motor according to claim 1, wherein the noise suppression circuit has a single chip type varistor.

(
10 A motor according to claim 9, wherein the varistor is electrically connected across the brushes.
11 A motor according to claim 9, wherein 5 the motor has a metallic housing containing the permanent magnet stator; and an end cap closing one end of the housing and supporting two motor terminals; wherein the two blush assemblies are supported by the end cap in association with the motor terminals respectively, and 10 the single chip type varistor is electrically connected between the brushes and held on the end cap.
12. A motor according to claim 11, wherein the varistor is held in the end cap in a recess in an inner surface of the end cap.
13. A motor according to claim 11 or 12, wherein the varistor is connected to the brush assemblies by way of a conductive resilient connector
14 A motor according to claim 13, wherein the brush assemblies have resilient 20 conductive brush arms and the connector comprises a finger formed on one brush arm and making resilient contact with a terminal end of the varistor while urging an opposite terminal end of the varistor into contact with the other brush arm.
15 A motor according to claim 13, wherein the brush assemblies have resilient 25 conductive brush arms and the connector comprises fingers formed integral with the brush arms which resiliently contact terminal ends of the varistor
16. A motor according to claim 15, wherein the fingers hold the varistor to the end cap.
17. A motor according to claim 11 or 12, wherein the varistor is electrically connected across the brushes by resilient fingers formed on the motor terminals
18 A motor according to claim 11 or 12' wherein the varistor is held to the end 35 cap by solder connections.

(
19 A motor according to any one of the preceding claims, wherein at least one of the brushes is connected to a respective one of the motor terminals by a further component of the noise suppression circuit.

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20. A motor-according to claim 19, wherein the further component is a choke.
21. A motor substantially as hereinbefore described, with reference to the accompanying drawings