Motor Control Circuit

1. Field of the Invention

The present invention relates to controls for controlling the speed of AC electrical motors.

2. Description of the Prior Art

One common type of speed control for AC electric motors involves the use of a gated bilateral switch such as a triac which is connected in series with the motor. The triac may be triggered or gated at different phase angles of each half-cycle of the input AC line voltage in order to vary the voltage applied to the motor and therefore the motor speed.

In many applications, it is desirable to be able to control the speed of an electric motor over a wide range of speeds. One such application is in fan control systems for controlling temperature in a commercial confined stock growing operation for raising livestock such as hogs. In typical operations, the animal waste drops through the floor and into a pit below the floor. Constant ventilation must be maintained at a low speed so that the methane gas generated by the animal waste is dissipated, and does not endanger the livestock. As temperature increases in the building, fan speed should increase to increase ventilation and help reduce the temperature. In this type of system, therefore, the electric motor driving the fan must operate at a constant, very low speed under most conditions, but must be capable of much higher speeds depending upon the temperature.

One problem which has been encountered with prior art system phase angle firing motor controls is that there has been a tendency of the electric motors (particularly permanent split capacitor or shaded pole single-phase motors) to stall at low speed as the temperature increases. This is clearly undesirable, since it limits severely the range of voltages and speeds over which the motor may operate. In systems of the type described above, operation over a wide range of speeds without stalling is a necessity.

Another problem with the prior art motor control circuits has been encountered when the temperature sensor was remotely located with respect to the remainder of the circuit. There has been an increasing tendency of the motor to stall, and this tendency appears to be affected by the length of the leads running from the temperature sensor to the remainder of the control circuit. In many applications, it is highly desirable to position the temperature sensor in a remote location with respect to the motor control circuit and the motor. Limitations on sensor lead length severely limit the usefulness of the motor control circuit.

The present invention is a motor control circuit which controls speed of the motor over wide ranges without stalling. The control circuit includes a gated bilateral switch means such as a triac connected in series with the motor, an integrating circuit means, and a bilateral switch means such as a diac which is connected to the integrating circuit means and a gate electrode of the gated bilateral switch means. The phase angle at which the gated bilateral switch means receives a signal at its gate electrode is dependent upon the charging rate of the integrating circuit means.

The motor control circuit also includes a condition responsive circuit, such as a bridge circuit which provides an unbalance signal which is a function of a sensed condition, such as temperature. Light emitting means is connected to the bridge circuit, and provides a light output which is a function of the signal imbalance. A charging circuit including photosensing means senses light emitted by the light emitting means and supplies a charging current to the integrating circuit which is a function of the light received. The additional charging current supplied by the charging circuit causes the integrating circuit means to charge at a faster rate, thereby changing the phase angle at which the gated bilateral switch mens is fired, and increasing the speed of the motor.

The FIGURE shows an electrical schematic diagram of the motor control circuit of the present invention.

The motor control circuit of the present invention, as shown in the FIGURE, controls the speed of a single phase permanent split capacitor or shaded pole electric motor 10. Although in the FIGURE motor 10 is shown as a single electric motor, in successful embodiments of the present invention a plurality of electrical motors have been connected in parallel and have been controlled simultaneously by the motor control circuit.

First and second input terminals 12 and 14 of the motor control circuit receive the AC line voltage, which in one preferred embodiment is 230 volt single-phase AC power. The circuit also includes first and second main conductors 16 and 18 between which the components of the motor control circuit, and motor 10 are connected. Fuse 20 is connected between first terminal 12 and first conductor 16 to provide protection for the electronic components of the circuit. Second conductor 18 is connected directly to second terminal 14. A bilateral gated switch, such as triac 22, has its main current carrying electrodes 23a and 23b connected in series with motor 10 between first and second conductors 16 and 18. Triac 22 also has a gate electrode 24 which must receive a gate signal in order to trigger triac 22 into a conductive state and permit current to flow through the series circuit including triac 22 and motor 10. Once triggered, triac 22 continues to conduct until the voltage across electrodes 23a and 23b reaches zero, which of course occurs at the end of each half-cycle of the AC line voltage.

The triggering circuit for triac 22 includes a bilateral semiconductor switch such as diac 26 and an integrating circuit which includes capacitors 28 and 30, resistor 32, and potentiometer 34. During each half-cycle, the integrating circuit charges until the voltage across diac 26 triggers diac 26, and thereby applies a gate signal to gate electrode 24 of triac 22. Selection of the minimum motor speed is made by adjusting potentiometer 34, which varies the rate of charging of the integrating circuit.

At minimum speed conditions, capacitors 28 and 30 are charged through potentiometer 34 and windings of motor 10 during each half-cycle of the AC line voltage until the voltage across diac 26 is sufficient to fire diac 26. The firing of diac 26 applies a trigger pulse to gate electrode 24 of triac 22, thereby turning on triac 22. In the embodiment shown in the FIGURE, capacitors 28 and 30 and resistor 32 are used as part of the integrating circuit to ensure a sufficiently long trigger pulse to gate electrode 24. This is particularly important at very low speeds so that triac 22 is properly triggered.

Capacitor 36 and resistor 38 are connected across triac 22 and form a commutation circuit for turn-off of triac 22. Varistor 40 is also connected across triac 22. The purpose of varistor 40 is to eliminate transients. For example, if transients appear in the AC line voltage, varistor 40 prevents those transients from adversely affecting triac 22. Conversely, if the switching of triac 22 could otherwise cause transients on the line which may adversely affect other electrical equipment, these transients are eliminated by varistor 40.

The circuit also includes a voltage sensing circuit which is connected across the terminals of motor 10. This circuit includes DC voltmeter 42, diode 44, and resistor 46. Diode 44 rectifies the voltage across motor 10 so that voltmeter 42 may be an extremely low cost DC voltmeter, rather than a more expensive AC voltmeter. The purpose of the voltage sensing circuit is to provide a visual indication to the operator of the voltage which appears across motor 10, thereby permitting the operator to select the proper set point for the minimum motor speed.

The motor control circuit shown in the FIGURE provides automatic control of the speed of motor 10 as a function of a sensed condition, such as temperature. Thermistor 48 is a negative temperature coefficient temperature sensor and is connected in a sensing bridge 50 which includes resistors 52 and 54, potentiometer 56, and resistors 58, 60 and 62. Resistor 64 and diode 66 connect terminal 70 of bridge 50 with first conductor 16. Similarly, resistor 72 and diode 74 connect terminal 76 of bridge 50 with second conductor 18. Resistors 52 and 54 and thermistor 48 are connected in a first leg of the bridge circuit between terminal 70 and terminal 78. Potentiometer 56 and resistor 58 form a second leg of the bridge between terminal 70 and terminal 80. Resistor 60 forms a third leg of the bridge between terminal 78 and terminal 76, and resistor 62 forms a fourth leg of the bridge between terminal 80 and terminal 76.

Bridge 50 is normally balanced so that at a predetermined minimum temperature, the potential difference between terminals 78 and 80 is zero. In this state, the bridge is balanced. As temperature increases, the resistance of thermistor 48 decreases, and a potential difference is created between terminals 78 and 80. The amount of unbalance and therefore the amount of potential difference is a function of the temperature sensed by thermistor 48.

Connected between terminals 78 and 80 is a light emitting circuit including resistor 82, light emitting diode 84, and protective diode 86. When the potential difference between terminals 78 and 80 reaches a threshold value, light emitting diode 84 turns on and emits light. As the temperature increases, the light from light emitting diode 84 increases. The motor control circuit of the present invention also includes an additional charging circuit which includes resistors 88 and 90, photosensors 92 and 94, and switch 96. Resistors 88 and 90 form a voltage divider between first and second conductors 16 and 18. Photosensors 92 and 94 are photoconductive sensors which receive light emitted by light emitting diode 84. The conductivity of photosensors 92 and 94, therefore, is controlled by the light emitted by light emitting diode 84. As the conductivity of sensors 92 and 94 increases, and when switch 96 is closed for automatic operation, additional charging current is permitted to flow which increases the rate of charging of the integrating circuit. This decreases the time required for the charge on capacitors 28 and 30 to reach a threshold value which triggers diac 26 and triac 22. In other words, with increasing temperature, charging current to the integrating circuit is increased, thereby causing earlier and earlier firing of triac 22 during each half-cycle. This phase angle change in triggering triac 22 results in greater voltage across the motor 10, and thereby in a higher motor speed.

Initially, the operator sets the minimum speed of the motor 10 by varying potentiometer 34 to produce the charging rate of the integrating circuit which results in the desired minimum speed. This operation is done with switch 96 open, so that the automatic temperature control portion of the circuit does not affect the minimum speed setting. Switch 96 is then closed to connect the additional charging circuit to the integrating circuit and permit automatic operation.

A temperature increase results in an increase in the unbalance of bridge 50, which is sensed by light emitting diode 84, which in turn supplies light signals to photosensors 92 and 94. Depending upon the amoung of light sensed by light sensors 92 and 94, the additional charging current to the integrating circuit is varied. An increase in temperature results in increased charging current, thereby advancing the phase angle at which triac 22 is triggered during each half cycle. This in turn results in an increase in motor speed with increase in temperature.

One important advantage of the motor control circuit of the present invention is that it does not suffer from stalling of motor 10 as thermistor 48 senses higher and higher temperatures. This has been a problem with previous phase angle firing motor control circuits. In one preferred embodiment, in which motor 10 was a 230 volt motor, the motor control circuit of the present invention permitted motor speed control over essentially the entire range of possible motor speeds. In other words, motor 10 could be operated without stalling over a range from about zero to about ninety-eight percent full voltage across motor 10.

Another significant advantage of the present invention is that it is insensitive to the length of sensor lead wires 98 and 100, which connect sensor 48 to bridge 50. In previous motor controls, the length of lead wires affected the operation of the motor control circuit. In some cases, the lead wires could not exceed about six feet without causing stalling of the electric motor. With the present invention, wires 98 and 100 may be as long as five hundred feet without any effect upon the operation of the motor. This is because gate electrode 24 of triac 22 is isolated from thermistor 48. The control signals which vary the phase angle firing of triac 22 are supplied through light emitting diode 84 and photosensors 92 and 94. The length of lead wires 98 and 100 do not affect the firing point of triac 22.

Another important advantage of the present invention is that photosensors 92 and 94 are not directly in series with either triac 22 or motor 10. For that reason, sensors 92 and 94 may have an extremely high dark resistance (e.g. 15 to 50 Mohm) without creating motor stalling problems.

Table 1 is a list of component values of the various components of the circuit shown in FIG. 1. In several cases, alternative sources of the components are listed.

In conclusion, the present invention is a motor speed control which provides variable speed control over essentially the entire operating range without stalling. The present invention also permits the temperature sensor to be positioned remote from the remainder of the circuit, since the sensor lead lengths do not adversely affect operation of the motor.

Although the present invention has been described with reference to a preferred embodiment, workers skilled in the art will recognize that changes made be made in form and detail without departing from the spirit and scope of the invention.