Abstract:
A switch assembly including a switch (e.g., an electronic switch) and a controller connected to the switch to control the switch. The switch assembly can also include a power supply connectable to a power source and connected to the controller. The power supply is configured to receive power from the power source and controllably power the controller. The switch assembly can also include a generator and decision logic. The switch assembly can be used in an electric machine (e.g., a motor).

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to an electronic switch assembly and, more particularly, an electronic switch assembly that controls current through a circuit such as an auxiliary circuit (e.g., an auxiliary start circuit) of an electric machine.  
       BACKGROUND  
       [0002]     Single-phase induction motors of the split phase and capacitor start types typically have the start winding connected to the power source when starting the motor. Once started, however, it is common to remove the start winding, resulting in the motor being more efficient. One reason for the removal of the start winding and start capacitor (if present) is that the start winding and the start capacitor are not typically designed for continuous duty. That is, these components will fail if left permanently in the circuit. A common solution to this problem is connecting an electronic switch circuit in series with the start winding (and start capacitor) for controlling current through the start winding.  
         [0003]     The most common implementation of a start switch for the above motors is a centrifugal switch mounted on the shaft of the motor. The centrifugal switch senses the shaft speed of the motor and opens the start winding contacts at the appropriate speed. This speed is typically around 75% to 80% of the rated running speed of the motor.  
         [0004]     There are some problems associated with a motor including a centrifugal switch. Because the switch is opening an inductive load, a large spark occurs when the contacts open. This sparking pits the switch contacts and ultimately results in the switch failing. Another problem with the mechanical switch is that it must be adjusted in production to get an accurate switch-out speed. This is another step in the production process, which adds cost. Also, if adjustment difficulties arise, this step can slow production of the motor. Another frequently cited problem is that the switch must be mounted on the shaft of the motor and, thus, limits packaging options. The switch assembly adds length to the motor, which makes motor placement in tight quarters more challenging. A lesser problem is that the switch makes noise when it opens and closes. Some users may find the noise objectionable.  
       SUMMARY  
       [0005]     One alternative to a motor including a centrifugal start switch is a motor having an electronic start switch. In one embodiment, the invention provides a new and useful electronic switch assembly used to control the current through a circuit. As used herein, a circuit is a conductor or system of conductors through which an electric current can or is intended to flow. An example circuit is the start winding and start capacitor (referred to herein as an auxiliary circuit) of a single-phase induction motor of the capacitor start type. However, the electronic assembly is not limited to induction motors of the capacitor start type.  
         [0006]     In one construction of the electronic switch assembly, the assembly includes a power supply block, a switch control block, and a circuit control block. As used herein, a block is an assembly of circuits and/or components that function as a unit. The power supply block powers the electronic switch assembly. The switch control block includes an electronic switch and, generally speaking, opens (or closes) the switch based on a signal received from the circuit control block.  
         [0007]     In another embodiment, the invention provides an electric machine (e.g., a motor) having a winding (e.g., a start winding) controlled by the electronic switch assembly. Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is an electrical schematic of a motor including an electronic switch embodying the invention.  
         [0009]      FIG. 2  is a block diagram of a representative electronic switch assembly capable of being used in the circuit shown in  FIG. 1 .  
         [0010]      FIG. 3  is an electrical schematic of an exemplary power source capable of being used in the electronic switch assembly of  FIG. 2 .  
         [0011]      FIG. 4  is an electrical schematic of an exemplary switch control block and circuit control block capable of being used in the electronic switch assembly of  FIG. 3 .  
         [0012]      FIG. 5  is an electrical schematic of a portion of the electrical schematic shown in  FIG. 4  and, specifically, is an electrical schematic of a voltage sense circuit, a generator circuit, a NAND gate, and a switch driver.  
         [0013]      FIG. 6  is an electrical schematic of a portion of the electrical schematic shown in  FIG. 4  and, specifically, is an electrical schematic of a start-up set circuit, a timer circuit, a current sense circuit, and a latch circuit.  
         [0014]      FIG. 7  is a graph comparing a current in Amps through the auxiliary circuit of a single-phase, capacitor-start induction motor against time in milliseconds, and a percent speed of the motor against time in milliseconds. 
     
    
     DETAILED DESCRIPTION  
       [0015]     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The terms “connected,” “coupled,” and “mounted” and variations thereof herein are used broadly and, unless otherwise stated, encompass both direct and indirect connections, couplings, and mountings. In addition, the terms connected and coupled and variations thereof herein are not restricted to physical and mechanical connections or couplings.  
         [0016]      FIG. 1  schematically represents a single-phase, capacitor start induction motor  100 . The motor  100  includes a main winding  105 , a start winding  110 , a start capacitor  115 , and an electronic switch assembly  120 . Unless specified otherwise, the description below will refer to the motor  100 . However, the invention is not limited to the motor  100 . For example, the electronic switch assembly  120  described below can be used with a single-phase, split-phase induction motor; a capacitor-start, capacitor-run induction motor, and similar induction motors. It is also envisioned that the electronic switch assembly  120  (or aspects of the switch assembly  120 ) can be used with other motor types and other electric machines, where the electronic switch assembly  120  controls current through a circuit of the motor or machine. It is even envisioned that the electronic switch assembly  120  (or aspects of the switch assembly) can be used with any circuit, where the switch assembly  120  controls current through the circuit.  
         [0017]     With reference to  FIG. 1 , the main winding  105 , the start winding  110 , and the start capacitor  115  are conventional components of a capacitor-start, capacitor-run induction motor. It is envisioned that other components can be added to the motor  100 , and  FIG. 1  is meant only to be a representative induction motor capable of being used with the electronic switch assembly  120 .  
         [0018]      FIG. 2  shows a block diagram of one construction of the electronic switch assembly  120 . With reference to  FIG. 2 , the electronic switch assembly includes a power supply  200 , a switch control block  205 , and a circuit control clock  210 .  FIGS. 3 and 4  are detailed electric schematics showing one exemplary electronic switch assembly  120 .  
         [0019]     The power supply  200  receives power (e.g., 115 VAC or 230 VAC power) from a power source and provides a regulated (i.e., a constant or consistent) voltage. For the construction shown in  FIG. 2 , the power supply  200  is connected to the power line and provides a direct current (e.g., a −5 VDC) power.  
         [0020]      FIG. 3  is a detailed schematic showing one exemplary power supply  200  capable of being used with the electronic switch  120 . With reference to  FIG. 3 , the power supply  200  includes resistors R 1 , R 12 , and R 23 ; capacitor C 5 ; diode D 6 ; Zener diodes D 5  and D 9 ; and transistor Q 7 . During operation, when a positive half-cycle voltage is across the power supply  200 , diode D 6  blocks current through the power supply. When a negative half-cycle voltage is across the power supply  200 , diode D 6  conducts causing current to flow through resistor R 1 , thereby charging capacitor C 5 . Zener diode D 5  begins conducting when capacitor C 5  achieves a voltage determined by the Zener diode D 5 , thereby limiting the voltage across capacitor C 5 . Resistor R 5  dissipates the charge of capacitor C 5  when power is removed from the power supply  200 , allowing the electronic switch assembly  120  to reset.  
         [0021]     One feature of the circuit shown in  FIG. 3  is that the circuit prevents the electronic switch  120  from working should the motor  100  be hooked to the wrong supply voltage. To provide some background, motor manufactures frequently design motors for dual voltage operation (e.g., 115 or 230 VAC operation) to keep the number of different motor models produced to a minimum. A common mistake by technicians is to hook a 115 VAC configured motor to a 230 VAC power line. When power is applied to the motor, the electronic switch will perform as normal and the motor will start (if there were no voltage clamp circuit). When the switch circuit turns off the start winding, however, the triac will need to block a large voltage (e.g., 1200 V). The power supply clamp keeps the motor from starting and, thus, the triac is required to block a much relatively smaller voltage (e.g., 350 V). Because the motor did not start, the clamp circuit has the additional benefit of alerting the installer that something is wrong.  
         [0022]     Referring once again to  FIG. 3 , transistor Q 7 , resistor R 23 , and Zener diode D 9  form the power supply clamp circuit. More specifically, Zener diode D 9  has a set reverse breakdown voltage (e.g. 200 VDC) that results in the Zener diode conducting when the voltage applied to the power supply  200  is greater than the designed motor voltage (e.g., 130 VAC). When Zener diode D 9  conducts, transistor Q 7  switches on, thereby shorting the power supply. This circuit prevents the electronic switch assembly  120  from working should the motor be hooked to the wrong supply voltage by keeping the power supply  200  from powering the circuit.  
         [0023]     Referring again to  FIG. 2 , the electronic switch assembly  120  includes a switch control block  205 . The switch control block  205  includes a switch  215  connected in series with the circuit to be controlled. For the construction shown, the switch  215  is connected in series with the start winding  110  and the start capacitor  115 . The switch  215  can be any electronic switch that prevents/allows current through the switch  215  in response to a control signal. An example switch  215  is a triac. In one specific construction the electronic switch  215  is an “AC Switch” brand switch, Model No. ACST8-8C, produced by ST Microelectronics of France, which also provides a high voltage clamping device to the triac in the same package to give the triac better line transient immunity and ability to switch inductive loads. Unless specified otherwise, the switch  215  for the description below is a triac.  
         [0024]     Referring again to the construction shown in  FIG. 2 , the switch control block  205  includes a generator  220 , and NAND gate  225 . The generator  220  provides a signal to the NAND gate  225 , which compares the generated signal with a signal from the circuit control block  210  (described below). The result of the NAND gate  225  controls the switch  215 . Before proceeding further, it should be noted that, while the electronic switch shown is described with the NAND gate  230 , the circuit can be readily redesigned for other gate types.  
         [0025]     When the switch  215  is a triac, the generator  225  can be a pulse generator and the switch control  205  can also includes a voltage sense circuit  230 . Generally speaking, a triac is a bidirection gate controlled thyristor capable of conducting in either direction in response to a pulse. Therefore, the triac does not require a fixed control (or gate) voltage to allow current through the triac. Instead, the generator  220  can be a pulse generator that provides control pulses. To assist the pulse generator, the switch control block  205  includes the voltage sense circuit  230 . The voltage sense circuit  230 , generally, monitors the voltage applied to the switch  215  (i.e., the applied voltage to the auxiliary circuit) and generates pulses based on the applied voltage. For example, the voltage sense circuit  230  can monitor the voltage applied to the triac and generate pulses (also referred to as gating pulses) in relation to the inception of voltage after the zero crossings of the applied voltage. The pulses are applied to the NAND gate  225 . The NAND gate  225  decides whether a gating pulse should or should not be applied to the triac switch  215  based on the conditions of the circuit control block  215 , the result of which controls current through the triac  215 . It is envisioned that the voltage sense circuit  230  and the generator  220  can be designed differently for other types of gate logic and other types of switches (e.g., other types of electronic devices).  
         [0026]      FIG. 5  is a detailed schematic showing one exemplary switch control block including a triac Q 1 , a triac voltage sense circuit  530 , a pulse generator  520 , a NAND gate U 1 D, and a switch driver  570 . The triac voltage sense circuit  530  includes resistors R 10 , R 1 , R 18 , and R 19 ; diode D 3 ; Zener diode D 4 ; transistor Q 5 ; and NAND gate U 1 C. The pulse generator  520  includes capacitor C 1  and resistor R 3 . The output driver  570  includes resistors R 5 , R 7 , R 8 , R 16 , and R 17 ; and transistors Q 3  and Q 4 .  
         [0027]     One method to keep the cost of an electronic circuit as low as possible is to keep the current supplied by the power supply as low as possible. One way to help accomplish this in an electronic switch circuit is to use a triac as the switch  215 . A triac has the benefit of being a bidirectional gate controlled thyristor that only requires repetitive pulses to continuously conduct. Therefore, rather than providing a continuous signal to the triac (i.e., via the NAND gate  225 ), the voltage sense circuit  530  and generator circuitry  520  only need to generate short continuous pulses (e.g., 25 μs) where each pulse is generated each half cycle of the voltage applied to the triac switch Q 1 .  
         [0028]     With reference to  FIG. 5 , the voltage sense circuit  530  monitors the voltage across the triac (referred to as the triac voltage) and determines whether the absolute value of the triac voltage is greater than a threshold (e.g., 5V). When the absolute value of the triac voltage is greater than the threshold, a logic 0 is applied to pin  9  of the NAND gate U 1 C, thereby resulting in a logic 1 being applied to pulse generator  520 . The voltage at pin  8  begins charging capacitor C 1  and pulls pin  12  high at NAND gate U 1 D. A logic 1 is applied to pin  12  of U 1 D for the time constant of capacitor C 1  and resistor R 3 . Therefore, the result of the voltage sense circuit  530  and generator  520  circuitry is that pulses are provided to NAND gate U 1 D, the pulses are only generated when the triac voltage passes through zero voltage to the positive or negative threshold (i.e., are generated just after each zero crossing event), and the pulses are narrow relative to the AC cycle of the power source. The switch driver  570  drives the triac Q 1  based on the output of NAND gate U 1 D. While not necessary, the switch driver  570  is used because the triac Q 1  can float off of ground. The driver  570  prevents voltage from feeding back into NAND gates U 1 C and U 1 D if the triac Q 1  does float.  
         [0029]     A subtle feature of the circuit shown in  FIG. 5  relates to the line labeled  575  in  FIG. 5 . Line  575  locks out the voltage sense circuit  530  when the pulse is being applied to the gate of the triac Q 1 . This feature makes sure the full current pulse is applied to the triac Q 1  and, thus, prevents teasing the triac Q 1  ON. More specifically, as the current pulse is applied to the gate, the triac Q 1  will start conducting. The voltage across the main terminals of the triac Q 1  will go to near zero without line  575 . This can fool the voltage sensing circuit  530  into thinking the triac Q 1  is fully conducting, and the circuit terminates the current pulse to the gate. Line  575  prevents this by forcing the NAND gate U 1 C to provide a logic 1 result during the time constant of resistor R 3  and C 1 .  
         [0030]     Before proceeding further it should be noted that, in some constructions, the voltage sense circuit  230 , generator  220 , and NAND gate  225  are not required. That is, the circuit control block  210  (discussed below) can directly control the switch  215 .  
         [0031]     Referring again to  FIG. 2 , the electronic switch assembly  100  includes a circuit control block  210 . For the construction shown in  FIG. 2 , the control block  210  includes a latch  235 , a startup set circuit  240 , a current sense circuit  245 , an OR gate  250 , and a limit timer  255 . The latch  235 , which is shown as an SR latch, provides outputs to the switch control block  205  based on values received at the latch inputs, which are shown as inputs S and R. The outputs determine whether the switch  215  is on or off. Other latches and other arrangements for the SR latch can be used (e.g., if NAND gate  225  is replaced by an AND gate).  
         [0032]     The startup set circuit  240  sets the latch in the set condition while the motor power supply  200 , and consequently the electronic switch assembly, powers up. This ensures that the start winding  110  is energized for at least the duration of the set pulse, and that the current sense circuit  245  (discussed below) stabilizes before it is allowed to open switch  215 . An exemplary start-up circuit  640  is shown in  FIG. 6 . The startup set circuit  640  includes resistors R 4  and R 6 , capacitor C 2 , diode D 2 , Zener diode D 1 , and transistor Q 2 . The duration of the start-up period is set by how long it takes for capacitor C 2  to charge to a voltage greater than the reverse breakdown voltage of Zener diode D 1 .  
         [0033]     There are two ways that the latch  235  can be reset: A) either the magnitude of the current through switch  215  (i.e., through the controlled circuit) is greater than a threshold or a timer times out. For example, if the rotor of the motor was locked on startup, the magnitude of the start winding current would never increase and the start winding would remain connected until the thermal switch protecting the motor finally opens. With this high current flowing continuously in the motor start winding, the triac switch and current sensing resistor (discussed below) would get very hot and would likely fail. To keep circuit costs low, the limit timer is added to terminate the start winding current after a time period (e.g., 1 to 1.5 seconds), whether the motor is started or not. An exemplary timer circuit  655  is shown in  FIG. 6  as resistor R 9  and capacitor C 4 , where the period for the timer circuit  655  is determined by the RC time constant of resistor R 9  and capacitor C 4 . The timer changes the value of the signal (e.g., from a logic 0 to a logic 1) provided to the OR gate  250  ( FIG. 2 ) after the time period.  
         [0034]     Also provided to OR gate  250  is the result of the current sense circuit  245 . Referring again to  FIG. 2 , the current sense circuit  245  senses the current through the switch  215  and compares the sensed value to a threshold. The result of the OR gate is provided to the latch  235 , thereby controlling the latch  235 , the NAND gate  225 , and ultimately the switch  215 . More specifically, if either the current sense circuit  245  or the limit timer  255  generates a logic 1, the SR latch resets, thereby controlling the NAND gate  225  and the switch  210 . Before proceeding further, it should be noted that either the timer  255  or the current sense circuit  245  can be removed from the circuit control block  210 . Additionally, in other constructions, other sensors or circuits can be used in place of the current sense circuit  245  (e.g., a voltage sensor) and the current sense circuit  245  can sense other circuits (e.g., the main winding circuit) or components.  
         [0035]      FIG. 6  is a detailed schematic showing one exemplary circuit control block including set/reset latch circuit  635 , startup set circuit  640 , timer circuit  655 , and current sense circuit  645 . The set/reset latch circuit  635  includes NAND gates U 1 A and U 1 B. The current sense circuit  645  includes resistors R 2 , R 13 , R 14 , and R 15 ; capacitor C 6 ; diode D 7 ; and transistor Q 6 . For the current sense circuit, current flows from triac Q 1  ( FIG. 5 ) through resistor R 2  ( FIG. 6 ). This creates a voltage drop across resistor R 2 , which is used for sensing. Current from the negative half cycle of the applied power flows through diode D 7  and resistor R 13  to charge capacitor C 6 . The charging of capacitor C 6  relates to the voltage drop across resistor R 2 . When the voltage drop across resistor R 2  is greater than a threshold, switch Q 6  activates and pulls pin  5  of U 1 B low. This results in the reset of latch  635  and, then, latch  635  provides a logic 0 to NAND gate U 1 D, thereby deactivating triac Q 1 .  
         [0036]     One feature of the current sense circuit  645  is that the circuit  645  scales the switch-out point based on the initial start winding current. To provide some background, during low line conditions, the start winding current is lower and, during high line conditions, the start winding current is higher. This can potentially create a switch-out speed error. To compensate for this, the first two or three cycles of start winding current charges capacitor C 6  up to a value 0.7 volts (i.e., the diode forward drop) less than the peak voltage across the current sensing resistor R 2 . This sets the trip threshold value for the circuit. When the start winding current magnitude rapidly grows as the motor reaches operating speed, the voltage from base to emitter on transistor Q 6  becomes sufficient to turn transistor Q 6  ON. Therefore, the current sense circuit  245  scales the switch-out point to detect when the current of the auxiliary circuit flares (i.e., grows rapidly in magnitude).  
         [0037]     One feature of the electronic switch assembly shown in  FIG. 4  is that the assembly uses only three connections for connecting to the motor. Moreover, each connection is readily available. This reduces the complexity of adding the switch assembly shown in  FIG. 4 , and potentially reduces assembly time. However, for other constructions, more connections may be required.  
         [0038]     As stated earlier and best shown in  FIG. 1 , the electronic switch assembly  120  can control current through the start winding  110  and the start capacitor  115  of a single-phase, capacitor-start induction motor. In operation, as power is applied to the motor  100 , the power supply  200  charges and, when charged, the electronic switch assembly  120  energizes. As the voltage applied to the start winding  110  (and the electronic switch assembly  120 ) passes through zero, the voltage sense circuit  230  and generator  220  senses voltage on the switch  215  and generates pulses in relation to the inception of voltage after the zero crossings of the voltage. The pulses are provided to NAND gate  225 .  
         [0039]     The NAND gate  225  receives a control signal from latch  235 . Based on the control signal, the NAND gate  225  triggers (or “re-triggers”) the switch  215  into conduction. For the construction shown, when the NAND gate  225  receives a logic 1 from the latch  235 , the switch  215  conducts, and, when the NAND gate  225  receives a logic 0 from the latch  235 , the switch  215  prevents current through the auxiliary circuit.  
         [0040]     The startup set circuit  240  forces the switch  215 , via the latch  235  and NAND gate  225 , to conduct for a time interval after the power supply energizes the electronic switch assembly. The current sense circuit  245  monitors the magnitude of the current flowing through the switch assembly. When the magnitude is greater than a threshold, the current sense circuit  245  forces, via OR gate  250 , latch  235 , and NAND gate  225 , the switch  215  to prevent current flow through the auxiliary circuit (i.e., to “open” switch  215 ). Should the motor not come up to speed within a time interval, the timer  255  forces, via OR gate  250 , latch  235 , and NAND gate  225 , the switch  215  to prevent current flow through the auxiliary circuit. Preventing current flow through the auxiliary circuit prevents current flow through the start winding  110  and the start capacitor  115 .  
         [0041]     The electronic switch assembly  120  senses the magnitude of the auxiliary circuit current to determine the appropriate switch-out point for the auxiliary circuit.  FIG. 7  shows a representative auxiliary circuit current waveform  700 . It can be seen that as the rotor speeds up (waveform  705 ), the magnitude of the auxiliary circuit current stays relatively constant until the motor nears running speed. As the motor approaches running speed, the magnitude of the current grows rapidly because the start winding is no longer contributing to the output torque, but is rather fighting with the main winding. The electronic switch circuit  120  uses the flaring of the current to its benefit to deactivate the auxiliary circuit and, consequently, the start winding.  
         [0042]     Thus, the invention provides, among other things, a new and useful electronic switch assembly and motor having the electronic switch assembly. The embodiments described above and illustrated in the figures are presented by way of example only and are not intended as a limitation upon the concepts and principles of the invention. Various features and advantages of the invention are set forth in the following claims.