Load and speed sensitive motor starting circuit and method

A circuit and method measures the voltage at the main motor winding (1) and detects the points in the electromagnetic wave cycle at which this voltage “crosses” zero. The method and circuit also measures the voltage at the auxiliary motor winding (2). The voltages measured in the main winding (1) and in the auxiliary winding (2) are compared by the circuit (13) as a means for starting and restarting the auxiliary winding (2). The circuit and method also detects the points in the electromagnetic wave cycle where the current in the auxiliary winding “crosses” zero and compares the phase of these current zero crossing points with a window pulse (11) that is generated when the main voltage crosses zero. When the zero current crossing points fall within the window pulse, the auxiliary winding (2) is up to proper operating speed and the auxiliary winding (2) is disconnected by the starting circuit. If the load on the main motor winding (1) increases or the main motor winding (1) speed decreases below a certain predetermined speed, the auxiliary winding (2) is switched back into the circuit to boost the speed of the main motor winding (1).

PRIORITY

This application is a 371 of PCT/US02/33750 and claims priority benefit dated Oct. 22, 2002.

FIELD OF THE INVENTION

The present invention relates generally to alternating current (AC) motors and to disconnect switches and circuits for use with AC motors. More specifically, the present invention relates to a circuit and method used with the start, or auxiliary, winding of an AC motor wherein the auxiliary winding is energized when starting the motor from rest and then disconnected at a given motor speed. The present invention also relates to such a circuit and method used with both split-phase and capacitor-start motors.

BACKGROUND OF THE INVENTION

It is well known that a single-phase AC motor produces an alternating magnetic field, one pulling first in one direction, then in the opposite direction as the polarity of the magnetic field changes. This is because the single-phase AC motor is energized by a single alternating current source. The major distinction between the different types of single-phase AC motors is how they go about starting the motor in a particular direction. Motor start is usually accomplished by some device or circuit that introduces a phase-shifted magnetic field on one side of the motor shaft, or rotor.

Split-phase motors achieve their starting capability by having two separate windings wound in the motor stator. The two windings are separated such that one winding is used only for starting. The starting, or auxiliary, winding is wound with a smaller wire size having higher electrical resistance than the main windings. Both windings are energized when the motor is started. The starting winding produces a field that appears to rotate. This rotation causes the motor to start. A centrifugal switch then disconnects the starting winding when the motor reaches a predetermined speed.

The winding and centrifugal switch arrangement of a capacitor-start motor is similar to that used in a split-phase motor. In the capacitor-start motor, a capacitor is used in series with the starting winding to produce a phase shift and the appearance of a rotating field. Here again, when the motor approaches a predetermined running speed, the starting switch opens thereby disconnecting the starting winding and the motor continues to run. One starting circuit for use with a motor of this type is disclosed in U.S. Pat. No. 4,622,506. The method and apparatus of the present invention is an improvement of that starting circuit.

Various types of switches, and controls therefor, are also well known in the electrical arts. This includes the mechanical switch and the centrifugal actuator mounted on the motor rotor, as alluded to previously. Mechanical switches of the centrifugal type are subject to problems such as limited life, fatigue, friction, vibration, mounting position, contact wear, among others. Also, the centrifugal switch includes a radial member that blocks axial airflow through the motor, which may impair cooling. This radial member also requires additional room in the motor housing, which may be objectionable in various applications.

In another known start winding disconnect system, Hall effect sensors or pick-up coils are used to detect motor speed to actuate a disconnect switch. This approach may be objectionable because of the requirement of adding an extra element such as a magnet on the motor shaft, and the pick-up coil to sense speed. These extra parts and the assembly required may be cost objectionable.

In another known disconnect system, a timer is started upon initial energization of the motor. When the timer times out, the disconnect switch is actuated to disconnect the auxiliary winding. This approach is not load or speed sensitive, but rather disconnects the auxiliary winding only after a preselected time regardless of motor speed and regardless of load. This approach is limited to dedicated applications where the load on the motor is known beforehand, and the delay time set accordingly. If the load on the motor is increased, the motor speed will not be up to the desired threshold at the noted cutout time. On the other hand, if the load on the motor is decreased, the motor will accelerate faster, and full voltage will be applied across the capacitor for a longer time than is desired, which in turn may damage the motor and/or the capacitor. Capacitor burn-out is a significant problem when reducing the loading of the motor in timed disconnect systems.

Another known approach is to sense current through the main winding and then actuate the disconnect switch at a designated condition. This requires a current sensor in series with the main winding and the start or auxiliary winding, which is objectionable to many manufacturers because of the cost of the extra components and the assembly cost of modifying the circuit and inserting such components in series in the circuit. This approach may also be objectionable due to the extra wattage and heat because current is still flowing through the sensor in the run mode after starting.

SUMMARY OF THE INVENTION

The present invention addresses and solves the above noted and other problems in a particularly simple and effective electronic control system for an auxiliary winding disconnect switch. The invention is load and speed sensitive, and is AC line voltage fluctuation insensitive. The invention eliminates the need for extra components on the motor shaft, around the shaft, or in series in the motor circuit. There is no need for physical modification of the motor components or the windings.

The present invention provides a new and useful circuit and method for measuring the voltage at the main motor winding and detecting the points in the electromagnetic wave cycle at which this voltage “crosses” zero. In other words, it detects the points at which the main motor winding voltage switches instantaneously from positive to negative and vice versa. The method and circuit also measures the voltage at the auxiliary motor winding. The voltages measured in the main winding and in the auxiliary winding are each compared by the circuit as a means for starting and restarting the auxiliary winding. The circuit and method also detects the points in the electromagnetic wave cycle where the current in the auxiliary winding “crosses” zero and compares the phase of these current zero crossing points with a window pulse that is generated when the main voltage crosses zero. When the zero current crossing points fall within the window pulse, this means that the auxiliary winding is up to proper operating speed and the auxiliary winding is disconnected by the starting circuit. If the load on the main motor winding increases thereby causing the motor rotor speed to decrease below a certain predetermined speed, the auxiliary winding will again be switched back into the circuit to boost the speed of the main motor winding.

As previously alluded to, the present invention provides an improvement over the circuit disclosed and claimed in U.S. Pat. No. 4,622,506. In the circuit of that embodiment, one was always measuring the voltage across the auxiliary winding. In the circuit of the present invention, one is always measuring the current zero crossing points in the auxiliary winding. More specifically, when the switch to the auxiliary winding is closed, the voltage of the auxiliary winding has no meaning because it contains no information as to motor speed, or RPM. This is why the auxiliary current is measured at this condition. When the switch is open, no current is passing through the auxiliary winding and there is no current information that is available. This is why the auxiliary winding voltage is measured at this condition, based on RPM information available.

Other aspects and advantages of the new and useful circuit and method will be apparent to those having skill in the art upon review of the attached drawings and the following detailed description.

DETAILED DESCRIPTION

Referring now to the drawings in detail wherein like numbered elements represent like elements throughout,FIG. 1shows a main winding1and auxiliary winding2of an AC motor that are each connected to an AC power source3. When the motor reaches a given threshold speed, a switch5that is connected in series with the auxiliary winding2is opened to disconnect the auxiliary winding2from the power source3. A current detection means6is also provided for detecting and measuring the current flow through the auxiliary winding2when it is energized. The current detection means6provides the circuit of the present invention with the capability of detecting the points in the sinusoidal AC current waveform at which the current of the auxiliary winding2“crosses” the zero point. This will also be called the “current zero crossing” of the auxiliary winding2. The significance of this will become more apparent later in this detailed description.

FIG. 2shows a control circuit, generally identified10, including main voltage detecting means7for sensing the magnitude of voltage across the main winding1, and auxiliary voltage detector means8for sensing the magnitude of voltage across the auxiliary winding2. A voltage comparator means13is provided which is responsive to the main and auxiliary voltage detectors7,8, respectively, and responds to a given relation between the magnitudes of the main and auxiliary winding voltages. A main voltage zero crossing means9is provided which senses the sinusoidal AC voltage waveform of the main winding voltage and the points at which the main winding voltage “crosses” zero or switches polarity. Window pulse means11responds to the main voltage zero crossing means9and generates a window pulse. The width of the window pulse that is generated is not a limitation of the present invention. An auxiliary winding current zero crossing means12is provided which senses the current flow through the auxiliary winding2and the points at which the auxiliary winding current also “crosses” zero or switches flow direction, as was mentioned previously. A phase comparison means14is provided which senses the auxiliary current zero crossing points in relation to the main voltage window pulse that has been generated. The voltage comparison means13generates a pulse shape15and the phase comparison means14generates a pulse shape16as well. Logic means17is provided to respond to the pulse shapes15,16to turn “off” or to turn “on” the switch5. This can be done, for example, by means of a triac driver18to disconnect or reconnect, respectively, the auxiliary winding2from the AC power source3. In effect, the triac driver18is switched on and off by negative voltage, which will become apparent later in this detailed description.

FIG. 3shows the detailed circuitry for the schematic shown inFIG. 2and like reference numerals are used to facilitate clarity. There are various portions of the detailed circuitry that correspond to the various functions illustrated inFIG. 2. For example, the final logic17portion of the schematic shown inFIG. 2corresponds to the NAND gate86shown inFIG. 3. The gate86is used with other circuit components to trigger the triac90and to turn the auxiliary winding2on and off. A pair of windings91,92are transformer windings that effectively provide current sensing means for the auxiliary winding2.

Again referring toFIG. 3, a power supply portion of the circuit includes a pair of transistors55,56that are connected to other components including a pair of zener diodes78,79, a resistor44and a capacitor67. The voltage across the main winding1is the same as the potential as across the resistor43, the capacitor69and the diode77. The main voltage sensing7portion of the circuit includes other resistors36,37,38, capacitor64, and a diode73. The auxiliary voltage sensing8portion of the circuit includes the resistors39,40,41,42, the capacitor65, the diode74and the transistor53. Voltage comparison13is accomplished by use of a comparator81. In actuality, the comparator81is one-fourth of a quad comparator chip, or other integrated circuit, which provides access to other comparators80,82,83. Specifically, the input pin6of the comparator81senses the main winding voltage7and the input pin7of the same comparator81senses the auxiliary winding voltage8. The output pin1of the comparator81feeds into the resistor45and the input pin6of the NAND gate85. Depending upon the input at the input pin5of the gate85, a pulse shape15is generated at the output pin4of the gate85. The significance of this will be discussed later in this detailed description.

Referring now to the main voltage zero crossing9and the auxiliary current zero crossing12portions of the circuit10, it will be seen that the voltage7across the main motor winding1is at the same potential as that across the resistors20,21of the circuit shown inFIG. 3. Also included in the main voltage zero crossing9portion of the circuit are two transistors50,51, a resistor22and a capacitor60. A delay portion of the circuit is also provided by the resistors23,24,25,26, the diode70, the transistor52and the capacitor61. As previously discussed, another comparator80is provided, the output pin2of which feeds into a pulse shape generator11portion of the circuit. The pulse shape generator11portion of the circuit includes the resistors27,46and the capacitor68.

The auxiliary winding current zero crossing12portion of the circuit includes the remaining pair of comparators82,83, a number of resistors28,29,30,31,32,33,34,35, a pair of capacitors62,63and a pair of diodes71,72. As shown, the output pins13,14of the comparators82,83, respectively, feed into one of the input pins2of a phase comparator gate84. The output pin3of the phase comparator gate84generates a pulse16on the “stop” side of the circuit by means of the resistor47, the capacitor66and the diode75. This output is fed into the input gate8of the final logic gate86. In this fashion, the pulse shapes15,16are used to turn the triac driver18“on” and “off” as required. The triac switching circuit is provided by virtue of the resistors48,49and the transistor57at the output pin10of the logic gate86. By use of this configuration, the triac driver18is switched on and off by negative voltage. SeeFIG. 2.

The initial starting of the motor on application of line voltage3is activated by means of the voltage comparison13. The voltage comparison13senses the low auxiliary winding voltage relative to the main winding voltage and initiates the triac turn on through logic17. Immediately after the first turn on, the auxiliary winding2is kept energized by the phase comparison14. The voltage comparison13cannot be used to maintain the starting condition because, after the first energization of the auxiliary winding2, the voltage on the auxiliary winding2is that of the line voltage source3, and does not represent the motor speed by means of induced voltage from the main winding1through the rotor. The maintenance of the auxiliary energization by means of winding current phase controls is explained in following paragraphs.

Referring now toFIG. 4, it will be demonstrated how the main voltage detecting means7in the circuit of the present invention views the magnitude of the main winding voltage, shown in the top graph as a generally sinusoidal waveform. The graph below it shows the points V1, V2, V3at which the main winding voltage crosses zero. That is, the points at which voltage polarity is instantaneously reversed. The next graph illustrates that, at each of these voltage zero crossing points V1, V2, V3, a window pulse P1, P2, P3, respectively, is generated by the window pulse means11. As the speed of the motor “ramps up” or increases, the auxiliary winding current zero crossing means12senses the current flow through the auxiliary winding2and the points I1, I2, I3at which the auxiliary winding current “crosses” zero or switches flow direction. Phase comparison means14is provided to sense the auxiliary winding current zero crossing points I1, I2, I3in relation to the main voltage window pulses P1, P2, P3. As the auxiliary current zero crossing point I3, for example, falls within the window pulse P3, the phase comparator14generates a pulse shape16to be received by the logic control17and turn off the triac driver18. If, because of load increase causing a reduction of motor speed, the auxiliary winding2needs to be reenergized, the voltage comparison means13is activated to the restart condition, which is the same as the initial motor start condition. The starting sequence is thereby reactivated to connect the auxiliary winding2. The sequence is that voltage comparison13is activated by the sensing of a low auxiliary winding voltage2relative to the main winding voltage1. This sensing is used for initial turn on the triac18after which the phase comparison means14is used to maintain the auxiliary winding2reenergization until the motor speed increases to the desird auxiliary winding denergization speed.

Referring now toFIGS. 1 and 5, it will be seen that function of the circuit is derived from the accessibility of RPM information of the motor. For example, when the switch5is open, the auxiliary voltage provides RPM information and there is no auxiliary current available. That is, the auxiliary winding voltage is really a function of the voltage across the main winding1and the motor RPM. The voltage across the auxiliary winding2has a direct relationship to the motor RPM. When the switch5is closed, the auxiliary voltage is the same as the line voltage and it contains no RPM information. On the other hand, the auxiliary current zero crossing does have RPM information. This is translated as shown in the uppermost figure showing the switch5“on” and “off” positions. As the switch5is initially closed, the speed of the motor “ramps up” to the point that the auxiliary winding2can be “cut out” of the circuit and the switch5opened up. During this time, auxiliary current zero crossing is determined by the circuit. After the switch5opens up, there is no current flowing through the auxiliary winding2and the voltage across the auxiliary winding2is a function of the voltage across the main winding1and the motor RPM. As the motor continues and RPM decreases due to load imposed on the motor, the switch5again needs to be closed to increase the motor torque to try and regain speed. This switch5closure “boosts” the motor so that the RPM increases to the point that the auxiliary winding1no longer needs to be energized. This continues throughout the operation cycle of the motor.

Accordingly, it will be apparent that there has been provided a new and useful circuit and method for measuring the voltage at the main motor winding and detecting points in the electromagnetic wave cycle where the main winding voltage “crosses” zero. The method and circuit also measures the voltage at the auxiliary motor winding. The voltages measured in the main winding and the auxiliary winding are each compared by the circuit as a means for starting and restarting the auxiliary winding. The circuit and method also detects the points in the electromagnetic wave cycle at which the current in the auxiliary winding “crosses” zero and compares the phase of these current zero crossing points with a window pulse that is generated when the main winding voltage crosses zero. When the zero current crossing point falls within the window pulse, the auxiliary winding is up to proper operating speed and the auxiliary winding is disconnected by the switching circuit. If the load on the motor increases causing the motor speed to decrease below a certain level, the auxiliary winding will again be switched back into the circuit to boost the speed of the main motor winding.

The scope of this invention is to include deenergization of the auxiliary winding based on current phase changes with motor speed of the main winding and/or the auxiliary winding current. With the auxiliary winding energized, both the main winding and auxiliary winding current phases change with motor speed during the motor starting or slowing. The disclosed circuit functions on the phase shift of the auxiliary winding relative to the line voltage phase. The main winding current phase shift with motor speed change is in the opposite direction as found for the auxiliary current. A change in logic and the circuit location of the current sensor from the auxiliary winding to the main winding could have been made to activate the deenergization of the auxiliary winding on the phase shift of the main winding current relative to the line voltage. Further, with two current sensors, the deactivation could have been activated by comparison of the phase of the auxiliary winding relative to the main winding. The circuit previously described in detail is an embodiment based on an attempt to minimize the cost of achieving the desired control. it is to be understood that the scope of the disclosure and appended claims are not limited to the specific embodiments described and depicted herein.