GB2080056A - Control Circuit for Reducing AC Hum in Split Phase Electric Motors - Google Patents

Abstract

The annoying, audible hum which is usually encountered during low speed operation of a permanent split capacitor motor is substantially eliminated by a control circuit (10) which is connected in series with one of the motor's windings (W1, W2). The circuit comprises an NPN transistor (21) having its emitter- collector circuit connected in series with the motor capacitor (C1) and auxiliary winding (W2). The transistor collector terminal is connected, during motor operation, to one side of an AC power supply and through a fixed resistor (R3) to the transistor and through a potentiometer (R1) and a fixed resistance (R2) to the emitter. By adjusting the potentiometer to vary the resistance in the emitter-base circuit the current flow in the emitter- collector circuit can be controlled to vary the speed of the motor. <IMAGE>

Description

SPECIFICATION Control Circuit for Reducing AC Hum In Split Phase Electric Motors This invention relates to an improved control circuit for controlling the speed of split phase electric motors, and more particularly to a circuit which permits operations of permanent split phase capacitor motors at low speeds without any objectionable AC hum.

There are numerous appliances in the modernday household which are operated by small, split phase electric motors of the fractional horsepower variety. Many such items (for example, refrigerator, exhaust fans, ceiling fans, etc.) are within earshot of the members of the household, when in use, and it is therefore most desirable that their associated motors operate as quietly as possible.

One of the principal problems heretofore encountered in connection with electric motors of the type described is that, when used with electronic speed controls, and particularly at lower speeds, they tend to develop an objectionable hum, which can be very distressing to any one within hearing distance of the motor.

Efforts have been made in the past to minimize signal distortion inherent in electronic speed controls and consequent undesirable humming during low speed operation of motors of the type described, but such efforts have not proved to be successful.

According to this invention there is provided an improved motor control circuit which comprises a transistor having its emitter-collector circuit connected in series with one of the motor windings, means connecting its collector to its base and to an A.C. power supply, and a variable resistor connecting its base to its emitter and adjustable to control current flow in the emittercollector circuit.

Preferably said means connecting said collector to said power supply and to said base comprises a second resistor connected at one end to said collector and at its opposite end to said base.

Conveniently a third resistor is connected in series with the variable resistor between said emitter and base.

Suitably said second resistor has an ohmic value substantially greater than that of said third resistor.

Ideally said transistor is NPN transistor or its complimentary equivalent.

Other features of the invention will be apparent hereinafter from the specifications and from the recital of the appended claims, particularly when read in conjuction with the accompanying drawings, in which: Figure 1 is a wiring diagram illustrating the windings of a conventional permanent split phase capacitor motor, together with a control circuit made according to one embodiment of this invention, and shown connected in series with the auxiliary winding of the motor to permit operation thereof at variable low speeds; and Figure 2 is a wiring diagram illustrating this control in greater detail.

Referring now to the drawing by numerals of reference and first to Figure 1 , W1 and W2 represent the main and auxiliary windings, respectively, of a conventional permanent split capacitor motor, which is of the type that is frequently employed to drive household ceiling fans, exhaust fans, etc. Opposite ends of the main winding W1 are connected by lines L1 and L2 to one side of a conventional OnOff switch 51, the opposite side of which is connected to an AC power supply, which may be of the usual 120 volt, 60 Hertz household variety.

The auxiliary winding W2 is connected between lines L1 and L2 in parallel with the main winding W1, and in series with a capacitor C1 and a control circuit 10, by means of a line L3 which connects one end of winding W2 to line L2, by a line L4 which connects the opposite end of winding W2 to one side of the capacitor Cl, by a line L5 connecting the opposite side of the capacitor C1 to the control circuit 10, and by a line L6 which connects circuit 10 to Li. As a consequence, when switch S1 is closed, the control circuit 10, the capacitor C1 and auxiliary winding W2 are connected in series across the AC power supply. Also at this time, of course, the winding W1 will also be connected directly across the power supply.

Referring now to Figure 2, the control circuit 10 comprises an NPN power transistor 21, which has its emitter connected directly to line L5, it collector connected directly to line L6, and its base connected by a line 22 to the movable contact P of a 100 ohm potentiometer (POT) having a variable resistance Ri. The fixed or zero end of the pot resistance R1 is connected by a line L8 to one end of a fixed resistor R2, the opposite end of which is connected to line L5, thereby placing resistor R2 in the emitter-base circuit of the transistor 21. Line 22 is also connected through another fixed resistor R3 and a line L9 to line L6, so that resistor R3 is in the collector-base circuit of the transistor.By way of example, the resistor R2, may have a value of approximately twenty-seven ohms, while resistor R3 has a much larger value, for example, in the vicinity of fifteen thousand ohms.

In operation, and assuming that the switch S1 is closed, the pot contact P can be adjusted manually to control the value of R1, and consequently the forward biasing voltage that is developed across the emitter-base circuit of the transistor 21. In turn, this permits manual control of the current flow through the emitter-collector circuit. For example, assuming that the adjustable pot contact P is moved toward the left in Figure 2 to reduce the value of the resistance R1 placed in the circuit, then the forward biasing voltage between the base and the emitter will be reduced, thereby reducing the current flow through the emitter-collector circuit. At this stage the transistor 21 functions as a high resistance in parallel with R2 and R3 between lines L5 and L6.

This, in turn, reduces current flow through winding 'at2.

On the other hand, as the movable pot contact P is moved toward the right Figure 2, so as to increase the value of resistance Tri, the voltage drop between the base and the emitter will increase, and thereby effectively will increase the current flow in the emitter-collector circuit. This permits the transistor 21 to function more nearly as a short circuit and allows most of the current flow to take place through the emitter-collector circuit of the transistor, rather than through the high resistance circuit represented by R3, Tri, and R2.

At intermediate settings of the pot contact P the transistor 21 acts as a variable resistance between the emitter and collector, either increasing or decreasing the forward biasing voltage between the emitter and the base, depending upon the position of the movable contact P of the pot. The feedback resistor R3 and the biasing resistor R2 tend to stabilize the operation of the control circuit.

In practice it has been found that this control circuit provides relatively simple and inexpensive means for controlling the intermediate speed ranges of the motor represented by the windings W1 and W2. For low speed operation of the associated motor, the pot is adjusted so that the resistance R1 is almost negiigible, thus switching the transistor 21 almost to its blocking mode. This causes a decrease in current through W2 which causes a decrease in motor torque.On the other hand, as the pot is adjusted to increase the resistance of Rl,thereby causing a corresponding increase in the current flow in the emittercollector circuit of the transistor 21, the current flow through the auxiliary winding W2 increases which causes an increase in motor torque and thereby causing a progressive speeding up of the motor, In all such operations, however, the control circuit 10 tends to minimize, if not substantially eliminate, the audible hum which heretofore tended to accompany the intermediate and low speed operations of permanent split capacitor motors of the type described.

The transistor 21 may be of the type sold, for example, by Sylvania GTE as type ECG-280, while the adjustable potentiometer can be of the wire wound variety rated at one hundred ohms, 2 to 4 watt, and sold for example by Clarostat under the catalog No. A43-100. Moreover, while in the embodiment disclosed above it has been suggestted that an NPN power transistor be employed, it will be apparent to one skilled in the art that an equivalent solid state element, or the compliment of the solid state element could be utilized in its place. Likewise, although specific values have been given for resistors R2 and R3, it will be appreciated that these values also can be modified within reason without departing from this invention. Also, the circuit 10 or an equivalent thereof obviously could be used to control current flow in the other winding (W1), if desired.

Claims (10)

Claims

1. A control circuit for a permanent split capacitor motor of the type having a main winding, an auxiliary winding, and a capacitor connected in series with said auxiliary winding, comprising a transistor having its emittercollector circuit connected in series with one of said windings, means for connecting the collector of said transistor to one side of an AC power supply, and to the base of said transistor, and a first, variable resistor connected between the base and emitter of said transistor, and adjustable to control the current flow through the emittercollector circuit of said transistor during operation of said motor

2. A control circuit as defined in claim 1, wherein said connecting means includes a second resistor connected at one end to the collector of said transistor, and at its opposite end to the base of the transistor.

3. A control circuit as defined in claim 2, including a third, fixed resistor connected in series with said first resistor between the emitter and base of said transistor.

4. A control circuit as defined in claim 3, wherein the ohmic value of said second resistor is substantially greater than that of said third resistor.

5. A control circuit as defined in any one of claims 2 to 4, wherein said transistor is an NPN power transistor and said resistors are connected in series across the emitter and collector of said transistor.

6. A control circuit as defined in claim 1, wherein said first resistor is one of a plurality of resistors connected in a series circuit with each other and in parallel with said emitter-collector circuit of said transistor.

7. A control circuit as defined in claim 6, wherein a second one of said resistors is connected between the collector and base of said transistor and has an ohmic value substantially greater than that of said variable resistor.

8. A control circuit as defined in either claim 6 or 7, wherein said transistor is selected from the group consisting of an NPN transistor and its complimentary equivalent.

9. A control circuit substantially as described herein with reference to, and as shown in, the accompanying drawing.

10. Any novel feature or combination of features disclosed herein.