Electricity: electrothermally or thermally actuated switches – Thermally actuated switches – With bimetallic element
Patent
1996-10-07
1998-12-29
Picard, Leo P.
Electricity: electrothermally or thermally actuated switches
Thermally actuated switches
With bimetallic element
337102, 337380, H01H 3752
Patent
active
058545840
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention concerns a device for protecting a fractional-horsepower motor from overheating. The motor is intended to accommodate a high heat load. The motor has a stator of permanently magnetic material and a rotor with an electric winding. The motor is employed for rapidly turning automotive accessories on and off. Its power supply accommodates a make-or-break switch. A bimetal component opens the switch, preferably suddenly and as suddenly as a snap-in disk for example, once a certain temperature has been attained. The bimetal component is mounted on a support that has one or more parts. The switch, the bimetal component, and its support are electrically connected serially. There is a specific ratio between the electric impedance and heat capacity of bimetal component and those of its support, and they are extensively in surface-to-surface contact.
Many small motors of this type are employed for various accessory functions in motor vehicles--for automatically raising and lowering windows, adjusting seats, sliding roofs, or folding tops and for operating pneumatic and hydraulic pumps and antenna-drive mechanisms, for example. The motors are direct-current, permanent-magnet components and have power ratings of 12 or 24 volts. They are usually connected downstream to a transmission with a high ratio. Although they generally operate only briefly, the motors must accomplish a lot during those brief periods. They can easily overload during normal operation when the component being driven is obstructed in its assigned path or when the controls are applied improperly or excessively. The motor, especially its rotor winding and brushes or brush holders, can overheat.
The fractional-horsepower motors employed for such and similar purposes must also be small and light in weight. The requisite outputs can accordingly only be achieved at loads ranging mainly from a maximum level to short-circuit or breakdown levels. This is particularly true of what are called brief-operation motors. Other factors critical to the operation of such motors in vehicles are the wide fluctuations in temperature and battery voltage. The temperatures can range from -40.degree. to +80.degree. C., and the potentials from 9 to 15 or 18 to 30 V. Also significant in the event of obstruction is the extent to which the motor's carbon brushes cover the commutator segments. In a typical window opening-and-closing motor with a 10-groove armature, a coverage of 2 segments will yield a total impedance of 0.479 .OMEGA., and a coverage of 4 segments a total impedance of 0.385 .OMEGA.. The initial obstruction currents will, even at a constant power of 13 V and an ambient temperature of +25.degree. C. for instance, be 27.1 A for 2 segments and 33.7 A for 4 segments. To some extent bridged windings at a coverage of 4 segments will lead, due to the low armature impedance and to the low mass of the circuit, to higher temperature-variation rates and accordingly shorter time constants.
The obstruction currents that will then occur over an operating-temperature range of -40.degree. to +80.degree. C., at a power of 13 V, at the different commutator coverages, and at impedance variations over the copper-winding temperature gradients are 36.7 to 45.6 A at an ambient temperature of -40.degree. C. and 22.7 to 28.5 A at an ambient temperature of +80.degree. C.
SUMMARY OF THE INVENTION
The device in accordance with the present invention will accordingly need to satisfy the demands that will now be described in order to cope with obstruction subject to the aforesaid conditions.
At an ambient temperature of +25.degree. C. and at an initial obstruction current of 27.1 to 33.7 A at 13 V, the device will have to switch the motor off in more than 4 and less than 10 seconds.
It will need to discontinue an initial obstruction current of 18.3 to 23.4 A at 9 V in less than 30 seconds, within, that is, the motor's time constant in terms of the particular operating situation, before the permissible threshold temperatures of the winding or
REFERENCES:
patent: 4532488 (1985-07-01), Blackburn
Fogiel Max
Gandhi Jayprakash N.
Picard Leo P.
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