GB2287133A - Reducing vibration/noise in a three brush DC motor - Google Patents

Abstract

An electric motor (12) for a windscreen wiper assembly, comprising angularly-offset low and high speed brushes (4, 6), and a common brush (5) with capacitive means acting between the low speed brush (4) and the, or each, high speed brush (6). The capacitive means is preferably an electrolytic capacitor (20) having its negative side connected to the, or each, high speed brush (6). Alternatively, capacitors (10) are arranged across the rotor windings. <IMAGE>

Description

REDUCED NOISE ELECTRIC MOTOR DESCRIPTION This invention relates to an electric motor having reduced mechanical noise characteristics and is especially, but not exclusively, related to such a motor for use with windscreen wiper assemblies.

In known DC electric motors for windscreen wiper assemblies, three brushes can be used, a first for low speed operation of the motor and a second for high speed operation of the motor, with the third acting as a common brush for either the low speed or high speed brush, depending upon whether the motor is being operated at low or high speed. Generally, the low speed brush is diametrically opposed to the common brush, whilst the high speed brush is off-set at an angle thereto.

Due to the inherent generative characteristics of such motors, a standing DC voltage of between, say, 5 volts and 8 volts is generated across the low and high speed brushes. Also, during low or high speed operation of the wiper motor, the low or high speed brush short circuits successive armature coils connected between adjacent pairs of commutator segments, due to the low or high speed brush bridging and making effective ohmic contact between successive adjacent pairs of commutator segments as the associated armature rotates. As a consequence, current pulses flow through the successively short circuited armature coils to induce voltage pulses peaking up to 80 to 100 volts across those brushes and superimposed upon the standing DC voltage thereacross.

Typically, when the wiper motor is operating at low speed, the standing DC voltage across the low and high speed brushes is about 5 volts and when the motor is operating at high speed, the standing DC voltage across the two brushes is in the region of about 8 volts.

Although the superimposed voltage pulses are of only approximately 1 microsecond duration, they generate mechanical forces at a rate of, say, twelve per armature revolution, which vibrate and excite the mechanical structure of the wiper motor, thereby resulting in audible noise. When the wiper motor is operating at high speed, the audible noise is typically in the region of 50dB(A) to 60dB(A). The vibrational response of the wiper motor structure is enhanced when such twelfth order excitation, or multiples thereof, coincides with the inherent mechanical resonant frequencies of the various components of the motor structure. Two such resonant frequencies predominate in the design of such a wiper motor, the first being associated with the so-called Mend frame assembly component of the motor and the second being associated with the armature assembly, as discussed above. In order to reduce the noise level in such a motor, either the inherent mechanical resonant frequency responses of the motor have to be reduced or the electro-mechanically generated forces derived from the voltage pulses generated by successive short circuiting of adjacent armature coils by, say, the high speed brush bridging successive pairs of adjacent commutator segments, have to be reduced, or both.

To date, the first type of mechanical resonance has been reduced by suitably designing the physical structure of the motor and/or mounting the motor on an associated vehicle using vibration damping techniques.

Other techniques have been employed to reduce the second type of electro-mechanically generated resonances discussed above but with very little success, if any.

It is an object of the present invention to eliminate, or at least substantially reduce, the disadvantages associated with known electric motors which experience the second type of electromechanically generated vibrations discussed above.

Accordingly, a first aspect of the present invention resides in an electric motor comprising low and high speed brushes arranged to operate the motor at respective low and high speeds thereof, another brush common to each of the low and high speed brushes, with the or each high speed brush being angularly off-set with respect to the low speed brush, and capacitive means associated or associable with at least one winding of the motor armature.

Accordingly, a second aspect of the present invention resides in an electric motor comprising low and high speed brushes arranged to operate the motor at respective low and high speeds thereof, another brush common to each of the low and high speed brushes, with the or each high speed brush being angularly off-set with respect to the low speed brush, and capacitive means connected across at least one, but preferably each, winding of the motor armature.

A third aspect of the invention provides an electric motor comprising low and high speed brushes arranged to operate the motor at respective low and high speeds thereof, another brush common to each of the low and high speed brushes, with the or each high speed brush being angularly off-set with respect to the low speed brush, and capacitive means connected between the low speed brush and the or each high speed brush.

Preferably, the low speed and common brushes are generally diametrically opposed to each other with respect to the armature of the motor.

During low or high speed operation of the inventive motor as defined above in accordance with the first, second or third aspect of the invention, the short circuiting of successive coils of the rotating motor armature by the low or high speed brush, when the motor is operating at low or high speed, generates the previously-mentioned successive voltage pulses between the low and high speed brushes, resulting in mechanical vibration of the structure of the motor and, as a consequence, the generation of audible noise.

Thus, and in accordance with the invention, it has been found that by associating capacitive means with at least one of the windings of the armature, for example, by connecting such means across each winding or between the low and high speed brushes, such audible noise can be reduced, preferably, by up to SdB(A).

This is particularly advantageous when the motor is operating at high speed, in which case, the capacitive means connected between the low and high speed brushes preferably has a capacitance of at least 470if, with the negative side of the capacitive means preferably being connected to the high speed brush.

However, it has been found that audible noise can also be reduced using capacitive means of lesser capacitance, as will be described in more detail hereinbelow in relation to the preferred embodiments of the invention. Such capacitive means is preferably an electrolytic capacitor having its negative electrode connected to the high speed brush.

In a preferred embodiment of inventive two speed electric motor, the motor is arranged to be used as a wiper assembly motor.

Preferred embodiments of two speed DC electric motor for use with windscreen wiper assemblies in accordance with the invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 is a diagrammatic circuit arrangement of a conventional prior art DC electric motor used with a windscreen wiper assembly; Figure 2 is a diagrammatic circuit arrangement of one embodiment of DC electric motor in accordance with the second aspect of the invention, for use with a windscreen wiper assembly; Figure 3 is a diagrammatic circuit arrangement of a second embodiment of DC electric motor in accordance with the third aspect of the invention, for use with a windscreen wiper assembly; Figure 4 is a graph of the DC voltage generated between high and low speed brushes of the prior art electric wiper motor of Figure 1; Figure 5 is a part of the graph shown in Figure 4 but on an enlarged scale; Figures 6 to 11 are respective graphs of the AC voltage between high and low speed brushes of the second embodiment of inventive electric motor shown in Figure 3, using capacitors of increasing capacitance; Figures 12 to 14 are respective graphs of the audible noise level versus operating voltage for the second embodiment of inventive electric motor shown in Figure 3, with and without different capacitors of the same capacitance; and Figures 15 to 18 are respective graphs of the audible noise level (dB(A)) versus operating voltage for the inventive electric motor as shown in Figure 9, with and without four different capacitors of the same capacitance.

Referring firstly to Figure 1 of the drawings, a DC electric motor, indicated generally at 1, comprises twelve spaced armature commutator segments 2 and twelve associated series windings 3. A first socalled "low speed" brush 4 for low speed operation of the motor 1, is diametrically opposed to another, socalled "common brush" 5. Off-set at an angle of, say, about 700, to the low speed brush 4 is a second, socalled "high speed" brush 6 for high speed operation of the motor 1.

The motor 1 can be operated at low or high speed by actuation of a switch indicated generally at 7 and connectable to the positive side of a voltage power supply. The common brush 5 is connected to the other, negative side of that supply.

With the switch 7 suitably set for low or high speed operation of the motor 1, a DC voltage of about 5 volts or 8 volts is generated between the low and high speed brushes 4, 6 as a result of the inherent generating characteristics of the motor 1.

During low or high speed operation of the motor 1, the low and/or high speed brushes 4, 6 bridge the gaps 8 between successive pairs of commutator segments 2, thereby short circuiting successive windings 3 of the rotating motor armature. As a consequence, current pulses are generated within successive windings 3, resulting in voltage pulses which can peak up to 100 volts in certain circumstances. These voltage pulses or peaks generate, in turn, mechanical input forces to the physical structure of the motor 1, resulting in the generation of audible noise, particularly when the frequency of the voltage pulses or peaks coincides with the resonant frequencies of certain components of the motor structure, for example, the armature assembly itself and/or an associated end frame assembly for the motor.

A typical graph of the DC voltage generated between the low and high speed brushes 4, 6 for high speed operation of the motor 1 is shown in Figure 4 over a 10 millisecond period, whilst the graph shown in Figure 5 represents part of the graph shown in Figure 4 but on an expanded log scale.

As can be seen from those two graphs, the DC voltage comprises a generally constant standing DC voltage A of, say, 7 to 8 volts, upon which are superimposed the voltage pulses or peaks B. As discussed above, it is these voltage pulses or peaks B which result ultimately in the generation of audible noise within the structure of the motor 1 which, in this particular case, has twelve commutators 2 and twelve associated armature windings 3 generating mechanical input forces at a rate of twelve per revolution of the armature. Thus, the resonant response of the motor structure is enhanced when twelfth order excitation, or multiples thereof, coincides with the resonant frequencies of certain components of the motor, for example, the armature assembly and/or the end frame mounting assembly.

The standing DC voltage generated between the low and high speed brushes 4, 6 during low speed operation of the motor 1 is typically between 5 and 6 volts, whilst that generated during high speed operation of the motor is typically 7 to 8 volts.

For high speed operation of the motor 1 with, say, a 20 volt power supply across the common and high speed brushes 5, 6, an effective 2 volts (rms) ripple is superimposed upon that standing DC voltage, whilst a 14 volt supply will provide slightly lower values for both the standing DC and ripple voltages.

The resultant audible noise generated within the motor structure can be as high as 50 to 60dB(A).

In order to reduce that noise level, the second aspect of the invention resides in connecting capacitive means, for example, capacitors, across respective armature windings, an embodiment thereof being shown in Figure 2. Here, a DC electric motor 11 for a two speed wiper assembly is substantially the same as the prior art motor 1 described above with respect to Figure 1, except that a capacitor 10 is connected across each armature winding 3. Other suitable capacitive means may be used in addition or as an alternative to the capacitors 10.

Such an inventive arrangement tends to reduce the voltage pulses or peaks B superimposed upon the standing DC voltage A generated across the low and high speed brushes 4, 6 during corresponding operation of the motor 11.

The construction of the motor 11 is such that it is comparatively difficult physically to incorporate the capacitors 10 or other capacitive means within the armature of the motor and, for this reason, an alternative form of electric wiper motor has been designed in accordance with the third aspect of the invention.

A preferred form of such an inventive motor is shown in Figure 3 at 12, its basic construction being substantially identical to that of the prior art motor 1 of Figure 1 and the inventive embodiment of motor 11 of Figure 2, except that the capacitive means is connected between the low and high speed brushes 4, 6, preferably in the form of a capacitor 20 It has been found that, particularly for high speed operation of the motor 12, a capacitor of comparatively high capacitance, for example, at least 470if, is preferred. Also, the capacitor 20 is preferably electrolytic.

Figure 6 is a graph of the AC voltage generated between the high and low speed brushes 4, 6 of the motor 12 during high speed operation thereof, employing a capacitor 20 of 4.7if capacitance. It can be seen from this graph that the voltage peaks B' are less than the voltage pulses or peaks B (Figures 4 and 5) generated by the prior art motor of Figure 1.

A capacitor 20 of 22F capacitance reduces the voltage peaks B' further, as shown in the graph of Figure 7, whilst a 47iF capacitor 20, as shown in Figure 8, reduces the voltage peaks B' yet further, as well as providing some further smoothing of the AC voltage generated between the high and low speed brushes 4, 6.

Figures 9 and 10 are respective graphs of the AC voltage generated between high and low speed brushes 4, 6 employing capacitors 20 of 100ZF and 220pF, resulting in even further reduction of the voltage peaks B' and smoothing thereof.

When a 470F capacitor 20 is used in the inventive wiper motor 12 of Figure 3, the voltage peaks are reduced to substantially zero, as shown in Figure 11. Thus, for high speed operation of the inventive motor 12, audible noise can be reduced by 2 to 5dB(A).

Figures 12 to 18 are respective graphs illustrating the reduction in noise level (db(A)) using no capacitor and one of 470F capacitance for high speed operation of the motor 20, versus the input voltage therefor.

For a low tolerance motor such noise level reduction can be as high as lOdB(A) but for high tolerance motors, such reduction is usually in the region of 2 to 5dB(A), typically 2 to 3dB(A), as high tolerance motors tend to be quieter than low tolerance motors anyway.

Tests have shown that the audible noise level of these types of motor tends to be less erratic when capacitive means, such as the capacitor 20, is employed.

Preferably, a 470at electrolytic capacitor rated at 10 volts DC can be used and it has been found that there is no detrimental effect upon that type of capacitor if the following operating conditions are taken into consideration: a) the voltage rating must be at least as great as that of the highest ripple voltage plus the standing DC voltage; b) the ripple current must not exceed that recommended for the capacitor, which would reduce its life expectancy or, alternatively, a de rating should be employed; c) the temperature rating of the capacitor must not be exceeded at any time, even during stall of the motor; and d) the capacitor 20 should preferably not be connected inside the motor structure, as failure of the capacitor could cause an explosion, in which case, the capacitor is preferably mounted on an associated circuit board exterior of the motor structure.

It is to be appreciated that, although the embodiments of wiper motor described above are for two speed operation, motors having three speed operational characteristics may be embraced by the basic inventive concept, in which case, a corresponding number of additional brushes will be involved.

It is to be appreciated also that the inventive concept further embraces multi-speed electric motors used for purposes other than wiper assemblies.

Claims (12)

1. An electric motor comprising low and high speed brushes arranged to operate the motor at respective low and high speeds thereof, another brush common to each of the low and high speed brushes, with the or each high speed brush being angularly off-set with respect to the low speed brush, and capacitive means connected between the low speed brush and the or each high speed brush.

2. An electric motor according to claim 1, wherein said capacitive means has a capacitance of at least 470my.

3. An electric motor according to claim 1 or 2, wherein said capacitive means is an electrolytic capacitor.

4. An electric motor according to claim 3, wherein the negative side of the electrolytic capacitor is connected to the or each high speed brush.

5. An electric motor comprising low and high speed brushes arranged to operate the motor at respective low and high speeds thereof, another brush common to each of the low and high speed brushes, with the or each high speed brush being angularly off-set with respect to the low speed brush, and capacitive means associated or associable with at least one winding of the motor armature.

6. An electric motor comprising low and high speed brushes arranged to operate the motor at respective low and high speeds thereof, another brush common to each of the low and high speed brushes, with the or each high speed brush being angularly off-set with respect to the low speed brush, and capacitive means connected across at least one, but preferably each, winding of the motor armature.

7. An electric motor according to claim 5 or 6, wherein said capacitive means is a capacitor having a capacitance of at least 470of.

8. An electric motor according to claim 5, 6 or 7, wherein said capacitive means is an electrolytic capacitor.

9. An electric motor according to any preceding claim arranged to be used as a wiper assembly motor.

10. An electric motor according to any preceding claim, wherein the motor is a two speed electric motor.

11. A two speed DC electric motor according to claim 1, substantially as hereinbefore described with reference to the accompanying drawings.

12. A windscreen wiper assembly incorporating an electric motor according to any preceding claim.