ELECTRIC MOTOR ARMATURE CURRENT CONTROL CIRCUIT

Abstract: An electric motor armature current control circuit includes an electric motor (16) with an armature in series with a protection device (13) comprising a pair of polymer PTC elements (17,18) connected in electrical series with the motor armature windings and in electrical parallel and thermal contact with each other. The first polymer PTC element (17), a temperature compensating element, has a very low cold resistance and a trip temperature higher than the maximum expected ambient temperature to which it is to be exposed in motor operation and generates heat in motor operation to trip to a much higher resistance and become self-regulating at that trip temperature. The second polymer PTC element (18) is maintained at least at the trip temperature of the first through thermal contact and has a lower resistance at that temperature than the cold resistance of the first. Its trip temperature is higher so as to be reached only with excessive armature current. Due to the temperature compensation element, the protection device limits excessive armature winding current after a period of time independent of ambient temperature.

Description: This invention relates to the limitation of excessive armature current in a wound armature electric motor and particularly to such limitation by polymer PTC (positive temperature coefficient) elements placed in series with the motor armature to carry the armature current. The material of such polymer PTC elements has a resistance below a trip temperature sufficiently low as to not significantly affect the armature current, the resistance rising sharply and greatly above the trip temperature. Polymer PTC materials, which may have a cold resistance on the order of 0.5 ohm or less and a very sharp resistance change at the trip temperature, are more suitable than other PTC materials such as barium titanate (as, for example, shown in US patent no. 3886401) for use directly in series with a motor armature, since they have a comparatively low voltage drop and power dissipation. The material is heated directly by the armature current therethrough and trips when the heat generated by the armature current causes the temperature thereof to increase to the trip temperature. However, since the trip temperature is essentially constant, the trip time for an excessive armature current depends on the starting temperature of the PTC material. Such starting temperature is generally the ambient temperature, which may vary greatly in some environments such as motor vehicles. The trip time following the onset of an excessive armature current must be short enough at low ambient temperatures to allow an increase in resistance in time to protect the motor. However, the trip time at high ambient temperatures must be sufficiently long to prevent nuisance tripping on short transient overcurrents that are not damaging to the motor. The problem is illustrated in Figure 3, which shows the variation of resistance over time for a single typical prior art polymer PTC element carrying an excessive motor armature current. It can be seen that the trip time is highly variable inversely with ambient temperature. It is desirable to reduce this variability. US-A-3,673,538 discloses an armature current control circuit using a pair of PTC elements as in the pre-amble of claim 1. The object of the present invention is to provide an electric motor armature current control circuit having a protection device which limits armature current at excessive temperature, in which the variation in trip time is reduced. To this end, an electric motor armature current control circuit in accordance with the present invention is characterised over US-A-3,673,538 by the features specified in the characterising portion of claim 1. The invention is an electric motor armature current control circuit comprising an electric motor having a field and an armature with armature windings, an electric power supply, and a protection device connected in series with the motor armature windings and electric power supply, whereby an armature winding current also flows through the protection device. The protection device comprises a pair of polymer PTC elements connected in electrical series with the motor armature windings and in electrical parallel and thermal contact with each other. The first polymer PTC element has a resistance effective to generate heat with armature winding current flow therethrough and a first trip temperature higher than the maximum expected ambient temperature to which the first polymer PTC element is to be exposed in motor operation. Above the first trip temperature its resistance increases greatly from a low resistance essentially negligible in its effect on the armature winding current to a high resistance. The heat generated by normal armature current is effective to raise the temperature of the first polymer PTC element above the first trip temperature; and the resulting high resistance is effective to reduce electric current therethrough sufficiently as to make the first polymer PTC element self regulating at the first trip temperature. The second polymer PTC element is maintained by the first through thermal contact at least at the first trip temperature. The second polymer PTC element has a resistance effective to generate heat with armature winding current flow therethrough and a second trip temperature sufficiently higher than the first to require excessive armature winding current to raise the temperature thereof to the second trip temperature. Above the second trip temperature the resistance of the second polymer PTC element increases greatly from a low resistance lower than that of the first polymer PTC element and essentially negligible in its effect on the armature winding current to a high resistance protectively limiting the armature winding current. The first polymer PTC element also has, at the second trip temperature, a high resistance protectively limiting the armature winding current. Therefore, the protection device limits excessive armature winding current after a period of time independent of ambient temperature. The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a combined perspective view and circuit diagram of a preferred embodiment of this invention, which view shows a motor armature current control circuit with a protection device and the structure of the protection device; Figure 2 is a curve of electrical resistance vs. time for the embodiment of Figure 1; and Figure 3 is a curve of electrical resistance vs. time for a prior art arrangement of a single polymer PTC element showing a high variability of trip time inversely with ambient temperature. Referring to Figure 1, a source of DC electric power is represented as battery 10, which has one terminal grounded and another at supply voltage B+ connected through a switch 11 to a terminal 12 of a protection device 13. Protection device 13 has another terminal 15 connected in series with the armature of a (DC permanent magnet) motor 16 to ground. Thus, when switch 11 is closed, armature current flows from battery 10 through protection device 13 in series with the armature of motor 16. Protection device 13 comprises two polymer PTC elements 17 and 18, each having a different trip temperature between low and high resistances. The low resistance of each (which are not necessarily the same) is a value essentially negligible to affect the armature current of motor 16, such as 0.5 ohm or less. This resistance is almost constant up to the trip temperature, above which it increases greatly to a value such as fifty ohms or more, which is effective to limit the armature current of motor 16 for its protection. Polymer PTC elements 17 and 18 are both in electrical and thermal contact with an extension 20 of terminal 15. Polymer PTC element 17 is in electrical contact with an extension 21 of terminal 12; and polymer PTC element 18 is in electrical contact with an electrically conducting member 22 connected to terminal 12. Extension 21 and electrically conducting member 22 extend from terminal 12 in a Y configuration parallel to each other and to extension 20 of terminal 15, with extension 20 extending thereinto from the open end of the Y. Extension 20 is sandwiched between polymer PTC elements 17 and 18, with the combination thereof sandwiched between extension 21 and electrically conducting member 22. Thus, polymer PTC elements 17 and 18 are connected electrically in parallel with each other and in series with terminals 12 and 15. In addition, polymer PTC elements 17 and 18 are in thermal contact, over a wide area through extension 20, so that heat from one raises the temperature of the other. Polymer PTC element 17, the ambient temperature compensating element, has a cold resistance below a trip temperature which is effective with armature current flowing therethrough to generate heat to raise its temperature to the trip temperature, at which the resistance greatly increases to a higher resistance effective to render the element self regulating at the trip temperature. The trip temperature is set just above the maximum expected ambient temperature to which protection device 13 is to be exposed. When switch 11 is first closed, a current flows through polymer PTC element 17 to heat it quickly to its trip temperature, which may be, for example, 85 degrees C. The heat generated by polymer PTC element 17 also raises the temperature of polymer PTC element 18, since the two are in thermal contact. When polymer PTC element 17 trips, its resistance increases greatly, which reduces its power dissipation. From this point on, it operates as a self regulating temperature maintenance element for polymer PTC element 18. Polymer PTC element 18 also has a resistance which generates heat; but it trips at a temperature higher than does polymer PTC element 17 by an amount sufficient to require excessive armature current to raise the temperature thereof to its trip temperature. A convenient trip temperature for polymer PTC element 18 is 135 degrees C. Its resistance below its trip temperature is lower than that of polymer PTC element 17 and is essentially negligible in its effect on the armature current of motor 16; but, when it trips, its resistance increases greatly to a level sufficient to protectively limit the armature current of motor 16. After polymer PTC element 17 warms up to its trip temperature of 85 degrees C, it maintains element 18 at this temperature until a condition develops which causes an excessive current flow through motor 16 and protection device 13. Most of this current flow will be through polymer PTC element 18, since the resistance of polymer PTC element 17, greater than that of polymer PTC element 18 when cold, has increased still further. Since polymer PTC element 18 essentially always starts from the same temperature, regardless of ambient temperature, it always takes approximately the same time to trip; and this time is controllable through the design parameters of protection device 13. The excessive current may be a fault current such as results from a short circuit, but it does not have to be. Some motors are designed for short term, intermittent use with an armature current that does no damage in such use but may burn the motor out in longer term, continuous operation. In the latter case, the excessive current is present as soon as the switch is closed; and the fault condition is operation of the motor continuously for too long a period. Protection device 13 will protect against either type of fault. The effect of the temperature compensating polymer PTC element can be seen by comparing Figure 2, resistance vs. time with the element (Figure 3, resistance vs. time) without it. The time and resistance scales of the Figures are not necessarily identical: the Figures indicate the general behavior of the protection devices. In Figure 3, the time from initial excessive current to trip temperature is shown for three ambient temperatures: +85C, +24C and -24C. The trip time may be identified as the portion of the curve where the resistance increases greatly, which portion is labeled with the ambient temperature. The time is seen to vary greatly, inversely with ambient temperature. Figure 2 shows the times for the same ambient temperatures for the present invention. Due to the fact that the temperature compensating polymer PTC element causes the protection device to start from approximately the same temperature regardless of ambient temperature, the time difference is greatly reduced.

Claims: An electric motor armature current control circuit comprising an electric motor (16) having a field and an armature with armature windings; an electric power supply (10); and a protection device (13) comprising a first PTC element (17) having a first trip temperature higher than the maximum expected ambient temperature to which the first PTC element is to be exposed in motor operation, above the first trip temperature the resistance increasing greatly from a low resistance to a high resistance, and a second PTC element (18) connected in electrical parallel with the first PTC element and in thermal contact therewith for being maintained at least at the first trip temperature, the second PTC element having a second trip temperature sufficiently higher than the first PTC element and above the second trip temperature the second PTC element resistance increasing greatly from a low resistance lower than that of the first PTC element to a high resistance; characterised in that the protection device (13) is connected in series with the motor armature windings and electric power supply, whereby any armature winding current also flows through the protection device, the first and second PTC elements (17,18) being of the polymer type and being connected in electrical series with the motor armature windings, the first PTC element having a resistance effective to generate heat with armature winding current flow therethrough, the heat generated by normal armature current being effective to raise the temperature of the first PTC element above the first trip temperature and the resulting high resistance being effective to reduce electric current therethrough sufficiently as to make the first PTC element self regulating at the first trip temperature; the second PTC element having a resistance effective to generate heat with armature winding current flow therethrough and requiring an excessive armature winding current to raise the temperature of the second PTC element to the second trip temperature, the low resistance of the second PTC element being essentially negligible in its effect on the armature winding current, and the high resistance of the second PTC element protectively limiting the armature winding current, the first PTC element also having, at the second trip temperature, a high resistance protectively limiting the armature winding current, whereby the protection device (13) limits excessive armature winding current after a period of time independent of ambient temperature. An electric motor armature current control circuit as claimed in claim 1, in which the first and second trip temperatures are substantially 85 and 135 degrees C, respectively. An electric motor armature current control circuit as claimed in claim 1 or claim 2, in which the protection device (13) has a pair of terminals (12,15) for connection in series with the motor armature windings, one (15) of the terminals having an extension (20) extending between the first and second polymer PTC elements (17,18) for electrical connection to one side of each and thermal connection to both, and the other (12) of the terminals having parallel extensions (21,22) in a Y configuration to the other side of each polymer PTC element with the extension (20) of the one (15) of the terminals extending parallel between the parallel extensions of the other (12) of the terminals and the polymer PTC elements so as to connect the first and second polymer PTC elements in electrical parallel between the terminals.

Inventors: THORNTON, DENNIS P.
Applicants: GENERAL MOTORS CORPORATION
Application year: 1988
Main classification: H02H5
IPC codes: H01C1, H02P29, H01C7, H02H7, H02H9