Patent Publication Number: US-9843279-B2

Title: Phase current detection system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 14/035,465 filed Sep. 24, 2013, now U.S. Pat. No. 9,240,751. This application is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     The present invention relates to systems and methods for controlling the operation of an electric motor based on monitored phase currents. 
     SUMMARY 
     In one embodiment, the invention provides a motor control system that includes an inverter and a plurality of current sensors each positioned in-line between the inverter and a phase coil of the motor. Each current sensor measures the current provided to each phase coil of the motor and provides a signal indicative of each phase current to a controller. In some embodiments, the currents sensors are provided as one or more current sense integrated circuits. A protection circuit protects the current sense integrated circuit from ground bounce by coupling a diode and an opposite facing Zener diode in series between the power supply pin and the ground pin of the integrated circuit. 
     In another embodiment, the invention provides a motor control system comprising a three-phase inverter. The three-phase inverter includes a first high-side switch and a first low-side switch arranged in series between a power supply and ground. A first node between the first high-side switch and the first low-side switch is couplable to a first phase coil of an electronically commutated motor. A second node between a second high-side switch and a second low-side switch is couplable to a second phase coil of the motor and a third node between the third high-side switch and the third low-side switch is couplable to a third phase coil of the motor. A first, second, and third current sensor are coupled between the motor and the first node, second node, and third node, respectively. A controller is configured to receive a signal from each current sensor and control the inverter to control the motor based on the measured phase currents. 
     In yet another embodiment, the invention provides a phase current monitoring system for an electronically commutated motor. The phase current monitoring system includes a first current sense integrated circuit coupled between a first node and a first phase coil of the motor. The first node is between a first phase high-side switch and a first phase low-side switch. A protection circuit protects the integrated circuit from ground bounce and includes a first diode coupled between the power supply pin of the current sense integrated circuit and the ground pin of the integrated circuit. The power supply pin is couplable to a power supply voltage and the ground pin is couplable to ground. The first diode is oriented to allow current flow from the power supply to the ground. A first Zener diode is also coupled between the power supply pin and the ground pin in series with the first diode. The first Zener diode is oriented such that any voltage rise on the power supply pin above a normal operating voltage is clamped during ground bounce. A controller is configured to receive a sensed first phase current from the first current sense integrated circuit and to controllably operate the first phase high-side switch based at least in part on the sensed first phase current to control the operation of the electronically commutated motor. 
     Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a motor control and current sense circuit according to one embodiment. 
         FIG. 2A  is a schematic diagram of a motor control and current sense circuit according to a second embodiment. 
         FIG. 2B  is a schematic diagram of an IC protection circuit for the circuit of  FIG. 2A . 
     
    
    
     DETAILED DESCRIPTION 
     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. 
     Electrically commutated motors are controlled using a drive circuit such as illustrated in  FIG. 1 . The drive circuit  100  of  FIG. 1  provides three-phase power to commutate the motor  101 . The operating parameters of the drive circuit  100  are controllably varied to adjust speed and torque of the motor as necessary. The inverter includes a plurality of controllable switches including high-side switches  103 ,  105 , and  107  and low-side switches  109 ,  111 , and  113 . Each phase coil of the motor  101  is connected to the inverter between a high-side switch and a low-side switch. The switches are controllably opened and closed to selectively provide power from a power source  103  to each coil of the electric motor  101  and to, thereby, control the operation of the motor  101 . 
     A series of three current sense resistors  115 ,  117 , and  119  are coupled between each low-side switch  109 ,  111 , and  113  and ground. A controller  121  is coupled between each low-side switch and each current sense resistor. Through this coupling, the controller  121  monitors the output current from the inverter on each line of the three-phases of the motor  101  and calculates various parameter values to maintain target speed and torque settings. The controller  121  controllably opens and closes each of the high-side switches  103 ,  105 , and  107  and low-side switches  109 ,  111 , and  113  to control the operation of the motor  101 . Furthermore, if the monitored output current from the inverter indicates an error, the controller  121  will shut down the motor or reduce the power to provide for safe operation. 
     However, the construction illustrated in  FIG. 1  is only capable of detecting current flowing through the three phases of the motor  101  and returning through the low-side switches  109 ,  111 , and  113 . Current that does not circulate into the low-side switches will not be sensed by the system of  FIG. 1 . For example, if there is a ground fault in the motor winding, the current will flow into ground and does not return through the low-side switches. The system of  FIG. 1  is not able to detect such a ground fault. Furthermore, the heat generated by the current sense resistors  115 ,  117 , and  119  results in power loss and increased ambient temperatures—particularly in high-power motor drives. 
       FIG. 2A  illustrates another motor control and current sense circuit  200  for use with an electronic motor  201 . Like the system of  FIG. 1 , the system  200  includes a three-phase inverter that includes a series of three high-side switches  203 ,  205 , and  207  and three low-side switches  209 ,  211 , and  213 . Each phase coil of the motor  201  is coupled to the inverter between a high-side switch and a low-side switch. The switches are controllably opened and closed by a controller (not pictured) to control the flow of current from a power source (V+) through a high-side switch (e.g.,  203 ) through a phase coil of the motor  201  and back through a low-side switch (e.g.,  211 ) to ground. 
     However, unlike the system of  FIG. 1 , system  200  does not include any current sense resistors connected between the low-side switches and ground. Instead, a current sense integrated circuit  215  is coupled between the inverter and each phase coil of the motor  201 . The current sense IC  215  can include one or more of a variety of current sense ICs including, for example, the ACS709 current sense circuit manufactured by Allegro Microsystems. The current sense IC  215  measures the current of the phase coil of the motor  201  from the inverter and provides an output to a controller (not pictured). The current sense IC  215  in this example uses a Hall Effect sensor to measure the current and processes the information to generate an analog voltage corresponding to the phase current. As described, above the controller uses the measured current on each phase to control the operation of the high-side and low-side switches and to, thereby, control the operation of the electric motor  201 . Furthermore, because the current sense IC is positioned in-line between the motor  201  and the inverter, it is able to also detect fault conditions such as ground fault conditions in the motor. 
     In the example of  FIG. 2A , the system  200  includes three separate current sense ICs  215 ,  217 , and  219  that each provide an in-line measurement of current provided to one of the motor phase coils from the inverter. However, in some alternative constructions, a single current sense IC chip can be used that receives power from each of the three high-side switches and measures the current in-line to each of the three phase coils of the motor. Furthermore, although the examples describes herein include three-phase motor control, other constructions can utilize the in-line current measurements systems and techniques with more or fewer than three phase coils in the motor  201 . 
     The three-phase inverter motor control systems described above in reference to both  FIG. 1  and  FIG. 2A  use a pulse-width modulated (PWM) control to generate sine waves with positive and negative half cycles like an AC waveform from a DC bus. As a result, the in-line IC based current sense circuit of  FIG. 2A  could potentially experience a “ground bounce” phenomenon on the secondary side of the current sense IC. A “ground bounce” occurs when the ground of the supply voltage VCC of the current sense IC dips below the ground potential causing the VCC to rise above 5 Vdc. A ground bounce can cause catastrophic failure of the IC. 
       FIG. 2B  illustrates an IC protection circuit that protects the current sense IC  215  from damage due to the ground bounce phenomenon. The power supply pin (VCC) of the current sense IC  215  is tied to a reference voltage (+5V). It is also coupled to the ground terminal (GND) through both (1) a capacitor C 13  and (2) a combination of a standard diode and a Zener diode coupled in opposing directions (D 3 ). As a result, the ground is prevented from becoming negative. 
     The VIOUT pin provides a voltage representative of the measured current on the phase coil of the motor. The VIOUT pin is coupled to the controller (U_CURRENT_OUT) through a resistor R 1  and an inductor L 5 . The node between the resistor R 1  and the inductor L 5  is coupled to ground through both a resistor R 10  and a capacitor C 12  arranged in parallel. The output (U_CURRENT_OUT) is also coupled to ground through another capacitor C 16 . 
     The open-circuit voltage pin (VOC) is coupled to a node between a first resistor R 11  and a second resistor R 9  arranged in series between the reference voltage (+5V) and ground. The FILTER output pin is coupled to ground through a capacitor C 7  and the VZCR output pin is coupled to ground through another capacitor C 14 . 
     The FAULT output pin of the current sense IC  215  provides a flag to the controller (U-FAULT) indicating a fault condition in the circuit or the motor. The FAULT output pin is tied to a reference voltage (+5V) through a resistor R 3 . A RESET output from the controller can reset the current sense IC  215  by selectively opening or closing a controllable switch Q 1 . When the switch Q 1  is open, the reference voltage (+5V) is applied to the FAULT_EN pin of the current sense IC  215  through a resistor R 6 . However, when the switch Q 1  is closed, the reference voltage is shorted to ground through the switch Q 1  and the FAULT_EN pin is brought low. In some constructions, the FAULT output pin is active low when a fault condition occurs and remains low until the controller acknowledges the faults and resets the fault condition. 
     Thus, the invention provides, among other things, a phase current detection and motor control system including a plurality of phase current sensors positioned to detect current flowing into the phase coil of the motor. The current sensors in some constructions are embodied in an integrated circuit that is protected from ground bounce by a protection circuit. Various features and advantages of the invention are set forth in the following claims.