A rotor for an electric motor

Abstract: A rotor for an electric motor comprises a shaft, a commutator fitted to the shaft and connected to a winding wound around poles of an armature core, and a noise suppressor. The noise suppressor 40 has chip capacitors 44 connected to terminal portions 38 of commutator segments 32. The capacitors 44 have a common terminal 45 connected together to form a star connection via a ring formed from a conductive layer 42 on a board 41 or from resilient wire. The capacitors 44 may be soldered to the wire ring and to the commutator segments or may be held in recesses formed in the base of the commutator in direct contact with the commutator segments under resilient urgings of the ring to form a solderless connection. It is particularly suited to rotors using mechanical connection type commutators to form solder-free rotors.

Description: This invention relates to rotors for electric motors and in particular, to rotors for miniature permanent magnet direct current (PMDC) motors incorporating capacitors for suppression of electrical noise. Commutators are a major source of electrical noise in miniature PMDC motors. As the brushes transfer from one segment to another, sparks are produced which create electrical noise and increase wear of the commutator and brushes. By suppressing these sparks, the noise level can be reduced and the service life of the motor increased. Varistor or resistor rings may be fitted to the commutator, usually by soldering to tangs used for connecting the winding of the rotor to the commutator, for suppressing these sparks. See for example, GB 2202688. Alternatively, capacitors and/or chokes may be connected to the motor terminals, usually inside the motor end cap or on the brush mounting plate, to suppress the transmission of the noise to the motor's power supply. However, better noise suppression has been achieved by connecting capacitors directly to the winding of the rotor. The capacitors may have one lead connected together to form a star point and the other lead connected to a respective commutator tang, there being an equal number of capacitors and poles of the rotor. A known arrangement is shown in GB 2246913 in which the capacitors are electrolytic type capacitors disposed within the winding tunnels of the rotor with the star point connected at the opposite end of the rotor to the commutator. While this functions satisfactorily, it has certain drawbacks. It is not particularly suited to automated assembly and as the capacitors are of significant size, this technique cannot be used where the winding tunnels are relatively full of wire. JP-A-58022569 describes the use of chip-type capacitors for noise suppression mounted on a rotor to overcome the size problem. The chip-type capacitors are mounted in slots formed in a printed circuit board with conductive tracks formed on the board to enable the capacitors to be connected in star or delta. One terminal face of each capacitor is soldered to a respective commutator tang. One problem with having the capacitors mounted to a fixed plate such as a printed circuit board is that the gap between the commutator tangs and the terminal faces of the capacitors cannot be controlled. The size of the gap is important for making a good soldered connection. If the gap is too big, then too much solder is required to bridge the gap making it difficult to achieve a good electrical connection. If the gap is too small, there is difficulty in fitting the capacitor assembly to the commutator. However, there is a large tolerance on the length of the chips and the radial extent of the tangs after hot staking varies considerably. The present invention seeks to overcome these problems by providing a wound rotor with a chip capacitor noise suppression arrangement in which it is easy to connect the capacitor to the commutator segments. Accordingly, the present invention, in one aspect thereof, provides a rotor for an electric motor comprising: a shaft; an armature core having a plurality of poles fitted to the shaft; a rotor winding wound around the poles of the armature core; a commutator fitted to the shaft and having a base which supports a plurality of commutator segments, the commutator segments each having a contact portion for making sliding contact with a brush and a terminal portion for making electrical connection with the rotor winding; a capacitive noise suppression arrangement comprising a plurality of chip-type capacitors each having a first terminal face and a second terminal face, the capacitors extending radially with respect to the shaft and spaced circumferentially about the commutator; and a conductive member electrically contacting the first terminal faces of the capacitors, the first terminal faces thereby being electrically connected together to form a common connection, characterised in that the conductive member resiliently and radially urges the second terminal faces of the capacitors into electrical contact with respective commutator segments. Preferably, the capacitors may be soldered directly to the terminal portions of the commutator segments. Preferably, the capacitors are soldered to the conductive member. This allows for easier assembly and gives a reliable self supporting connection between the capacitors and the conductive member. A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic side view of a top half of a rotor showing a weldable commutator prior to fixing of a rotor winding; Figure 2 is an enlarged sectional side view of the commutator of Figure 1 after fixing of the rotor winding and fitting of a noise suppressor in accordance with the preferred embodiment of the present invention; and Figure 3 is an end view of the commutator assembly of Figure 2. In Figure 1, the top half of a rotor 20 is shown. The rotor 20 comprises a shaft 21 on which is fitted an armature core 22 comprising a plurality of stacked armature laminations. A rotor winding 23 is wound around the armature core to form wound armature poles. A commutator 30 is fitted to the shaft 21 adjacent the armature core 22. The commutator comprises a commutator base 31, a number of commutator segments 32 and a retaining ring 33. The commutator base is moulded from a suitable plastics material such as phenolic or nylon and has a segment support portion 34, a collar 35 and a spacer portion 36. The spacer portion 36 abuts the armature core 22 to axially space the commutator segments 32 from the armature core. The commutator segments 32 are retained on the commutator base by a retaining ring 33. The commutator segments have a sliding portion 37 for making sliding contact with carbon brushes of the motor and a terminal portion 38 to which connection portions 24 of the armature winding 23 are connected. In Figures 1 to 3, the terminal portion of the commutator is of the weldable type, having a U-shaped tang 39 which faces away from the armature core and around which a connection portion 24 of the winding 23 is looped during winding of the armature core 22, as shown in Figure 1. Once the rotor has been wound, the windings are electrically connected to the commutators by a hot forging process in which the tangs 39 are subjected to heat and pressure to collapse the tangs 39 onto the connection portion 24 and to remove insulation from that portion of the winding wire which is in contact with the tang. The tang 39 is seated on and supported by the collar during the forging operation. The tang 39 thus takes a form similar to that shown in Figure 2. The commutator has a number of segments corresponding to the number of poles of the rotor although only one is shown for clarity. The rotor construction so far described is well known in the art. In Figure 2, a noise suppression device is shown fitted around the tangs of the commutator. The noise suppression device 40 is more clearly shown in Figure 3. The noise suppression device 40 comprises a resilient wire ring 47 and a plurality of chip-type capacitors 44. Each capacitor 44 has two terminal faces 45 and 46 and is supported by the ring by being soldered at its outer terminal face 45 to the ring. The other terminal face 46 of the capacitors is soldered to a respective commutator tang. In the preferred embodiment of Figures 2 and 3, there are 3 capacitors, one for each commutator segment, corresponding to the number of poles of the rotor. As each capacitor is connected to the conductive ring 47, the ring forms a star point connection ring connecting one terminal 45 of each of the capacitors together and the capacitors are connected in parallel with the armature windings via the commutator segments. The resilient wire ring 47 has a split allowing it to be expanded. By soldering the capacitors to the wire ring 47 first, the resilience of the ring can be used to urge the capacitors into contact with the tangs to aid soldering or even avoid the need for soldering in certain arrangements. In some applications, where the capacitors are not soldered to the tangs, a small amount of flexible adhesive may be used to strengthen the retention of the capacitors under rapid speed changes. In Figure 2, the commutator base 31 has a small projection 36' formed at the end of the spacer portion which co-operates with a recess (not shown) in the armature core to correctly orientate the commutator on the shaft with respect to the armature core to ensure the desired commutation angle. Various modifications may be made to the embodiment described and it is desired to include all such modifications as fall within the scope of the invention as defined in the accompanying claims. For example, the wire ring may or may not be soldered to the capacitors and the capacitors may or may not be soldered to the commutator segments. While in the embodiment, the capacitors have been shown as being placed radially outward of the terminal portions of the commutator segments, if space permits, the capacitors could be mounted in direct contact with the sliding portions of the commutator segments or accommodated within recesses of the commutator base. While for clarity of description, a 3 pole rotor and commutator has been illustrated, rotors of other configurations may be used.

Claims: A rotor for an electric motor comprising: a shaft (21); an armature core (22) having a plurality of poles fitted to the shaft (21); a rotor winding (23) wound around the poles of the armature core; a commutator (30) fitted to the shaft (21) and having a base (31) which supports a plurality of commutator segments (32), the commutator segments (32) each having a contact portion (37) for making sliding contact with a brush and a terminal portion (38) for making electrical connection with the rotor winding (23); a capacitive noise suppression arrangement (40) comprising a plurality of chip-type capacitors (44) each having a first terminal face (45) and a second terminal face (46), the capacitors (44) extending radially with respect to the shaft (21) and spaced circumferentially about the commutator (30); and a conductive member (47) electrically contacting the first terminal faces (45) of the capacitors (44), the first terminal faces (45) thereby being electrically connected together to form a common connection, characterised in that the conductive member (47) resiliently and radially urges the second terminal faces (46) of the capacitors (44) into electrical contact with respective commutator segments (32). A rotor as defined in Claim 1, wherein the second terminal faces (46) of the capacitors (44) directly contact the terminal portions (38) of the commutator segments (32). A rotor as defined in Claim 1, wherein the second terminal faces (46) of the capacitors (44) are soldered directly to the terminal portions (38) of the commutator segments (32). A rotor as defined in Claim 3, wherein the commutator (30) has a plurality of tangs (39) formed on the terminal portions (38) for hot staking the rotor windings (23) and wherein the capacitors (44) are soldered to the tangs (39) after the tangs (39) have been hot staked. A rotor as defined in Claim 1 or Claim 2, wherein a flexible adhesive assists in retaining the capacitors (44) in contact with the commutator segments (32). A rotor as defined in any one of the preceding claims, wherein the conductive member (47) forms at least a part of a resilient ring. A rotor as defined in any one of the preceding claims, wherein the first terminal faces (45) of the capacitors (44) are soldered to the conductive member (47).

Inventors: HO SEUNG PUN, LAU JAMES CHING SIK, HO, SEUNG PUN, LAU, JAMES CHING SIK
Applicants: JOHNSON ELECTRIC SA, JOHNSON ELECTRIC S.A.
Application year: 1995
Main classification: H02K11
IPC codes: H01R39, H02K23, H02K13, H02K1, H02K11