GB1594334A - Thermal protection for electric motors - Google Patents

Description

(54) THERMAL PROTECTION FOR ELECTRIC MOTORS (71) We, OTTER CONTROLS LIMITED, a British Company of Otters 'Ole, Buxton, Derbyshire, SK17 6LA, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following state ment :- This invention relates to thermally sensitive current controlling devices for use in protecting electric motors from an overheating condition.

A common cause of damage to electric motors arises when an armature or field winding short circuits, causing excess current to flow and consequent overheating of the motor which may damage the motor beyond repair. Nowadays there are many small electric motors employed in motor cars for various ancillary functions such as automatic window raising and lowering, seat adjustment, and headlamp wipers. Such motors are normally coupled via high ratio gearing and although they may only be required to operate intermittently for short periods of time, they may have to develop a considerable power in these periods.

Thus there is a risk of an electrical fault or overloading causing overheating of the motor.

Such motors have previously been protected by, amongst other arrangements, a thermally sensitive switch similar to that described in British Patent Specification 903,807 mounted on the exterior of the motor and connected in the motor circuit so as to break the circuit when a thermally sensitive switch actuator detects an overheating condition. Whilst such protector has the advantage of cheapness, it does have a disadvantage in that the sensitivity to an overheating condition is not very great because of the poor heat flow path between the motor and the actuator. Further, the actuator must contend with changes in ambient temperature of from--30"C to + 70"C, which may affect the temperature of the exterior of the motor and reduce the sensitivity of the actuator to an overheating condition of the motor.

The present invention is based on the concept of mounting a thermally sensitive electric switch or current limiting device within the motor in good thermal connection with a brush holder of the motor, the brush being one of the sources of heat in an overheating condition.

Accordingly, the present invention provides a combined brush holder and thermal protector for an electric motor, comprising a metal part for holding a brush and arranged to make a good thermal and electrical contact with the brush, and a thermally sensitive current controlling device for limiting or interrupting the current flowing to the metal part when an overheating condition arises in operation of the motor, said current controlling device including a thermally responsive control element mounted in direct thermal connection with said metal part.

The metal part preferably defines at least part of a sleeve for holding the brush. The metal part may define the entire sleeve, or it may define a portion of U-shaped section, a further member forming the remainder of the wall of the sleeve.

The combination preferably comprises two metal parts, one part for holding the brush as stated above and the other part forming a motor current input terminal for connection to a power supply, the current controlling device being arranged to control the flow of current between the two parts.

Said current controlling device may comprise a thermally responsive element in the form of a PTC (positive temperature coefficient) resistor the resistance of which sharply increases at a certain temperature. Such resistor may take the form of a slab sandwiched between said two metal parts.

Alternatively said current controlling device may comprise a switch having a stationary and a movable contact and including a thermally responsive bimetallic snapacting actuator, preferably as described and claimed in our Patent No. 1,542,252, carrying the movable contact of the switch. The actuator may be welded or brazed or otherwise mounted with a good thermal connection to the metal part for holding the brush. The stationary contact of such switch is mounted on the metal part in which the said power supply terminal is formed. The two metal parts are secured together with an intervening layer of insulation.

Some preferred embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, wherein: Figure 1 is a plan view of part of an electric motor incorporating a combination according to the invention; Figure 2 is a plan view of a thermally sensitive switch actuator employed in the combination of Figure 1; Figure 3 is a sectional view along the line III-III of Figure 1; Figure 4 is a plan view of a metal plate for forming into a part incorporating a terminal of the combination of Figure 1; Figure 5 is a plan view of a metal plate for forming into a part incorporating the brush holder of the combination of Figure 1; Figure 6 is a perspective and Figure 7 an exploded view of a second embodiment of the invention as applied to a somewhat larger motor; and Figure 8 is a perspective and Figure 9 an exploded view of a further embodiment of the invention employing a PTC resistor as the thermally responsive element.

Referring the drawings there is shown a plastics body member 2 of an electric motor.

The body member 2 has a central bearing 4 for receiving an axle of the rotor of the motor.

On diametrically opposed sides of the bearing 4 are mounted brushes, one of which is shown at 6, for engaging the rotor. Brushes 6 are located in sleeves 8 constituting brush holders and are biassed towards the rotor by springs 10 mounted on spigots 12 and passing through slots 14 (Fig. 3) in the sleeves 8. Raised stops 16 for the springs 10 may be formed integrally with the sleeves 8 as shown on the right hand side of Figure 1 or separately as shown on the left hand side.

The brush holder assembly shown on the right hand side of Figure 1 is formed of two metal parts, the lower one being generally in the form of a plate having a depending terminal 18 and the upper one being generally in the form of a plate having an upwardlyextending portion of inverted U-shaped section defining in part the sleeve 8. These parts are held together in face-to-face contact on body member 2 by means of a rivet 20 thus providing continuous conductive contact between the terminal 18 and the right hand brush.

The left hand brush holder assembly is further illustrated with reference to Figures 2 to 5. It comprises an upper part 22 formed by bending up a plate as shown in Figure 5 and a lower part 24 formed by bending up a plate as shown in Figure 4. Upper part 22 includes a portion of inverted U-shaped section defining in part sleeve 8 and an upwardly extending tab 26. Lower part 24 includes a downwardly extending terminal 28 and an upwardly extending tab 30 having a contact 32 fixed thereto.

Mounted to the tab 26 is a thermally responsive bimetallic snap-acting actuator 34 shown in detail in Figure 2. This actuator is as described and claimed in British Patent Specification No. 1,542,252 and is of dished configuration and has a central tongue 36, the free end of the tongue being welded to tab 26. At the periphery of the actuator opposite the free end of the tongue is mounted an electrical contact 38 for engagement with fixed contact 32. Actuator 34 and contacts 32, 38 together form a thermally sensitive electric switch electrically connected in series between terminal 28 and brush holder 8.

Parts 22 and 24 are secured together with an intervening layer of insulation 40 therebetween. Insulation 40 is of nylon reinforced plastics and has burred-over projections 42 extending through apertures in parts 22 and 24. As shown in Fig. 3, sleeve 8 is defined by parts 22, and insulation 40.

The left hand brush holder assembly is secured to the body member 2 by means of a rivet 44. It is to be noted that aperture 46 in lower part 24 is larger than aperture 48 in upper part 22, to prevent the rivet 44 from contacting the lower part 24.

Actuator 34 is in good thermal contact with the brush holder 8 by virtue of thermal conduction along the part 22 via mounting tab 26. Thus should the electric motor be overloaded or a fault arise, for example, in the armature windings causing overheating of the motor, the brush holder 8 will act as a source of heat. In fact, the brush holder 8 will receive much of the energy dissipated at the point of engagement of the carbon brush and the armature. This heat will be conducted along upper part 22 and will rapidly heat actuator 34. There will also be, by virtue of excess electrical current flowing into the motor, self-heating of the actuator 34 caused by current flowing therethrough.

As soon as actuator 34 is heated to its operating temperature it snap-acts to an oppositely dished configuration in which movable contact 38 disengages from fixed contact 32 thereby to interrupt the flow of current to the motor and thus protect the motor from further damage.

Figures 6 and 7 show another embodiment in which the terminals are provided at bolts 50. One brush holder assembly again comprises upper and lower parts 22 and 24 separated by insulation 40, the upper part 22 including a sleeve defining the brush holder 8. The actuator 34 is welded to the upper part 22 so as to be in good thermally conductive connection with the brush holder 8 and the contact 38 is arranged to touch contact 32 fixed to the lower part 24. Insulation tubes 52 are disposed in aligned apertures in the parts 24 and 40 for insulating rivets 54 used to hold the assembly together.

This embodiment operates in the same manner as that previously described but affords greater protection for the bimetallic actuator.

Figures 8 and 9 show an arrangement employing a PTC resistor 56 as the thermally responsive element. A spring washer 58 is provided to apply proper pressure to the resistor 56, and insulation tubes 52 define the spacing between parts 22 and 24. By placing the spring washer 58 below rather than above the resistor 56, the flat base to the sleeve 8 is maintained and the resistor 56 is kept in good thermal contact with the upper part 22.

It may be seen that the PTC resistor 56 is in series between the brush holder 8 and its associated terminal and so is effective to limit the current supplied to the motor when its temperature rises above the point at which its resistance sharply increases.

WHAT WE CLAIM IS: 1. A combined brush holder and thermal protector for an electric motor, comprising a metal part for holding a brush and arranged to make good thermal and electrical contact with the brush, and a thermally sensitive current controlling device for limiting or interrupting the current flowing to the metal part when an overheating condition arises in operation of the motor, said current controlling device including a thermally responsive control element mounted in direct thermal connection with said metal part.

2. A combination as claimed in claim 1 wherein said metal part defines at least part of a sleeve for holding the brush.

3. A combination as claimed in claim 2 comprising said metal part together with a further metal part providing a current input terminal for connection to a power supply, the current controlling device being arranged to control the flow of current between the two parts.

4. A combination as claimed in any preceding claim wherein said thermally responsive element comprises a positive temperature coefficient resistor the resistance of which sharply increases at a certain temperature.

5. A combination as claimed in claim 4 as dependent on claim 3 wherein said resistor comprises a slab disposed between said two metal parts.

6. A combination as claimed in claim 1, 2 or 3 wherein said device comprises a switch having a stationary contact and a movable contact, the switch including a thermally responsive bimetallic snap-acting actuator carrying the movable contact of the switch.

7. A combination as claimed in claim 6 as dependent on claim 3 wherein said actuator is mounted in good thermal connection with said first mentioned metal part, the stationary contact of the switch is mounted on said further metal part, and a layer of electrical insulation is disposed between said metal parts.

8. A combined brush holder and thermal protector substantially as shown and described with reference to Figures 1 to 5 of the accompanying drawings.

9. A combined brush holder and thermal protector substantially as shown and described with reference to Figures 6 and 7.

of the accompanying drawings.

10. A combined brush holder and thermal protector substantially as shown and described with reference to Figures 8 and 9 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. upper part 22 so as to be in good thermally conductive connection with the brush holder 8 and the contact 38 is arranged to touch contact 32 fixed to the lower part 24. Insulation tubes 52 are disposed in aligned apertures in the parts 24 and 40 for insulating rivets 54 used to hold the assembly together. This embodiment operates in the same manner as that previously described but affords greater protection for the bimetallic actuator. Figures 8 and 9 show an arrangement employing a PTC resistor 56 as the thermally responsive element. A spring washer 58 is provided to apply proper pressure to the resistor 56, and insulation tubes 52 define the spacing between parts 22 and 24. By placing the spring washer 58 below rather than above the resistor 56, the flat base to the sleeve 8 is maintained and the resistor 56 is kept in good thermal contact with the upper part 22. It may be seen that the PTC resistor 56 is in series between the brush holder 8 and its associated terminal and so is effective to limit the current supplied to the motor when its temperature rises above the point at which its resistance sharply increases. WHAT WE CLAIM IS:

1. A combined brush holder and thermal protector for an electric motor, comprising a metal part for holding a brush and arranged to make good thermal and electrical contact with the brush, and a thermally sensitive current controlling device for limiting or interrupting the current flowing to the metal part when an overheating condition arises in operation of the motor, said current controlling device including a thermally responsive control element mounted in direct thermal connection with said metal part.

2. A combination as claimed in claim 1 wherein said metal part defines at least part of a sleeve for holding the brush.

3. A combination as claimed in claim 2 comprising said metal part together with a further metal part providing a current input terminal for connection to a power supply, the current controlling device being arranged to control the flow of current between the two parts.

4. A combination as claimed in any preceding claim wherein said thermally responsive element comprises a positive temperature coefficient resistor the resistance of which sharply increases at a certain temperature.

5. A combination as claimed in claim 4 as dependent on claim 3 wherein said resistor comprises a slab disposed between said two metal parts.

6. A combination as claimed in claim 1, 2 or 3 wherein said device comprises a switch having a stationary contact and a movable contact, the switch including a thermally responsive bimetallic snap-acting actuator carrying the movable contact of the switch.

7. A combination as claimed in claim 6 as dependent on claim 3 wherein said actuator is mounted in good thermal connection with said first mentioned metal part, the stationary contact of the switch is mounted on said further metal part, and a layer of electrical insulation is disposed between said metal parts.

8. A combined brush holder and thermal protector substantially as shown and described with reference to Figures 1 to 5 of the accompanying drawings.

9. A combined brush holder and thermal protector substantially as shown and described with reference to Figures 6 and 7.

of the accompanying drawings.

10. A combined brush holder and thermal protector substantially as shown and described with reference to Figures 8 and 9 of the accompanying drawings.