Electrical machine, method of controlling an electrical machine, and system including an electrical machine

A method of controlling an electrical machine. The electrical machine includes a stator having a core and a plurality of windings, and a rotor disposed adjacent to the stator to interact with the stator. The method includes detecting a movement of the rotor, generating a three phase alternating current (AC) voltage signal by all phases of the electrical machine, monitoring for a transfer speed of the electrical machine, discontinuing the three phase AC voltage signal when the transfer speed is traversed, and switching to a back electromotive force (BEMF) control mode after discontinuing the three phase AC voltage signal.

FIELD OF THE INVENTION

The invention relates to an electrical machine and specifically a brushless, permanent magnet electrical machine. In particular, the invention relates to a system including a brushless direct current (BLDC) motor and a method for starting a BLDC motor.

BACKGROUND

BLDC motors (also known as electronically commutated or “ECM” motors) are becoming more prevalent in industries that typically did not use BLDC motors. For example, the need for increased efficiency in the heating and air conditioning market has led to the use of BLDC motors for powering the blower in heating, ventilation, and/or air conditioning systems (referred to herein as HVAC systems). An HVAC system is one example of an air-movement system. Other example air-movement systems include furnaces, heat pumps, blowers for gas-fired appliances (e.g., a gas water heater), etc.

Generally, BLDC motors are synchronous electric motors powered by direct-current (“DC”) electricity and have electronic commutation, rather than mechanical commutators and brushes. Further, BLDC motors include a rotor having a plurality of magnetic poles (e.g., a plurality of poles produced with permanent magnets) of alternating polarity disposed on a surface of a rotor core, and a stator that receives electrical power and produces a magnetic field in response thereto. The magnetic field of the stator interacts with a magnetic field of the rotor to cause movement of the rotor.

SUMMARY

BLDC motors use a means for determining the position of the rotor in order to commutate the motor. One method of commutating the motor is referred to as “sensorless” motor commutation. Sensorless motor commutation is often performed by sensing the back electromotive force (BEMF) produced by the motor. Typically, the BEMF signal produced in the stator windings is not large enough for sensorless motor commutation until the speed of the rotor reaches about ten percent of the rated motor speed. As a result, a means of starting the motor without using the BEMF signal may be necessary.

One method of starting a three-phase motor is described in U.S. Publication No. 2009/0160384, which is fully incorporated herein by reference. Typically, to start the BLDC motor, a controller aligns the rotor of the motor to a known position and then accelerates the rotor (e.g., by using the method described in U.S. patent application Ser. No. 12/398,675). Once the rotor reaches a sufficient speed, the rotor is allowed to coast for a short time (e.g., 20-200 ms) while the controller synchronizes the rotor to engage a normal running mode. During this startup process the air-movement system can generate ramp up noise. In particular, the power signal provided to the rotor can generate torsional torque ripple that excites system vibration modes and results in an audible noise for a short period during ramp up. Therefore, there is a need for an improved method for starting brushless electrical machines and BLDC motors, where the ramp up noise during the startup of the motor is avoided.

In one embodiment, the invention provides a method of controlling an electrical machine including a stator having a core and a plurality of windings, and a rotor disposed adjacent to the stator to interact with the stator. The method includes detecting a movement of the rotor, generating a three phase alternating current (AC) voltage signal by all phases of the electrical machine, monitoring for a transfer speed of the electrical machine, discontinuing the three phase AC voltage signal when the transfer speed is traversed, and switching to a back electromotive force (BEMF) control mode after discontinuing the three phase AC voltage signal.

In another embodiment, the invention provides an electrical machine including a stator having a core and a plurality of windings, a rotor disposed adjacent to the stator to interact with the stator, a memory, and a controller arranged to start the electrical machine. The controller is configured to detect a movement of the rotor, generate a three phase alternating current (AC) voltage signal by all phases of the electrical machine, monitor for a transfer speed of the electrical machine, discontinue the three phase AC voltage signal when the transfer speed is traversed, and switch to a back electromotive force (BEMF) control mode after the three phase AC voltage signal is discontinued.

In yet another embodiment, the invention provides an air-movement system including a system control board and a motor assembly including a stator having a core and a plurality of windings, and a rotor disposed adjacent to the stator to interact with the stator. The air-movement system further includes a drive circuit coupled to the motor assembly and having a controller and a memory, the controller being configured to start the motor assembly. Starting of the motor assembly includes detecting a movement of the rotor, generating a three phase alternating current (AC) voltage signal by all phases of the motor assembly, and discontinuing the three phase AC voltage signal when the motor assembly traverses a transfer speed.

DETAILED DESCRIPTION

FIG. 1illustrates an HVAC system2including a thermostat3, a system control board4, a motor assembly5, and input/output devices6. The thermostat3is coupled to the system control board4via a first communication line7A, and to one or more input/output devices6via a second communication line7B. Additionally, the thermostat3can be coupled directly to the motor assembly5. The system control board4is coupled to the motor assembly5via a third communication line7C, and to one or more input/output devices6via a forth communication line7D. The motor assembly5is coupled to one or more input/output devices6via a fifth communication line7E. As shown inFIG. 1, the communication lines7A-7E can represent a two-way system communication between the elements described above. Moreover, communication lines are schematic only, can include analog or digital communication, and can include wire or wireless communication.

Each one of the input/output devices6is also a schematic representation of input signals, output signals, and auxiliary devices operating in connection with the thermostat3, the system control board4, and the motor assembly5. Accordingly, more than one implementation of the construction of the HVAC system2is shown inFIG. 1. Moreover, other constructions of the HVAC system2can be possible by utilizing one, or a combination, of the primary devices (e.g. thermostat3, system control board4, and motor assembly5) and a number of input/output devices6. Additionally, it is envisioned that the primary devices discussed further below (e.g., the motor assembly5) can be used in other applications, either independently or simultaneously with respect to the operation of the HVAC system2.

In one construction, the thermostat3can include a set of ports (not shown) used to send output signals generated by the thermostat3. For example, the output signals generated by the thermostat3can include signals indicative of the status of the HVAC system2based on the input signals (e.g. ambient temperature and/or humidity levels) received by the thermostat3. The thermostat3can generate output signals, for example heating (W) and cooling (Y), such that the signals can be interpreted by receiving devices (e.g. the system control board4) as being “on” or “off.” For example, the thermostat can generate a signal W (i.e., a request for heating) through one of the output ports. The signal W can be interpreted by the system control board4, and as a result, the system control board4can generate a signal instructing or causing the motor assembly5to operate. In some constructions, the thermostat3is configured to generate signals indicative of requests of different levels of heating or cooling.

It is to be understood that the HVAC system2illustrated inFIG. 1represents only one exemplary construction of an air-movement system, and thus other constructions are possible. Therefore, the operation of the HVAC system2can be implemented in other air-movement systems that include BLDC motors. For example, similar air-movement systems can include furnaces, heat pumps, blowers for gas-fired appliances (e.g., a gas water heater), etc. Further, the HVAC system2(or any other air-movement system) can operate the motor assembly5without the input from a thermostat3. In these constructions, the system control board4of the air-movement system2can generate a signal instructing the motor assembly5to operate based on an input from other external devices or based on a request from an internal module of the system2.

In one construction, the system control board4can relay signals generated by the thermostat3to the motor assembly5. More specifically, the system control board4processes the signals from the thermostat3and generates instructions for operating the motor assembly5. The system control board4can also be operable to communicate with other input/output devices6, such as humidity control systems, gas burner controls, gas ignition systems, other motors, safety systems, service systems, and combustion blowers. Accordingly, the system control board4can generate instructions for the motor assembly5based on signals received from the thermostat3, as well as signals received from alternative devices coupled to the system control board4, such as safety systems, ambient sensors, gas ignition systems, and other HVAC system components.

In some constructions of the HVAC system2, the system control board4communicates with the motor assembly5utilizing at least one serial port. More specifically, the system control board4and the motor assembly5can be coupled via a serial cable. In some cases, the system control board4can generate and send instructions to the motor assembly5, as well as receive diagnostics from the motor assembly5via the same serial port. In other cases, the motor assembly5and the system control board4can send and receive other information besides instructions and diagnostics utilizing the serial ports based on an operational mode of the system control board4.

In one construction, the motor assembly5includes a permanent magnet, brushless direct current (BLDC) motor.FIGS. 2-3illustrate portions of an exemplary BLDC motor. However, the invention is not limited to the motor disclosed inFIGS. 2-3; other BLDC motors or electrically commutated motors (ECMs) can incorporate the invention. Although the BLDC motor is described in relation to an air-movement system (e.g., HVAC system), it is understood that the described BLDC motor incorporating the invention can be implemented in other systems and used in different industries.

FIG. 2is a partial exploded view of the stator and rotor of an electrical machine (e.g., motor) according to one construction of the motor assembly5. ForFIG. 2, the electrical machine is a motor10having a rotor15and a stator20. The rotor15is coupled to a shaft17and held by one or more bearings. In general, the stator20receives electrical power, and produces a magnetic field in response thereto. The magnetic field of the stator20interacts with a magnetic field of the rotor15to produce mechanical power with the shaft17.

The rotor15includes a plurality of magnetic poles25of alternating polarity exhibited on a surface of a rotor core30. The rotor core30includes laminations (e.g., magnetic steel laminations), and/or solid material (e.g., a solid magnetic steel core), and/or compressed powdered material (e.g., compressed powder of magnetic steel). One construction of the rotor15includes a sheet of permanent magnet (e.g., hard magnetic) material disposed on the rotor core30. Another construction of the rotor15can include a plurality of strips of permanent magnet material attached (e.g., with adhesive) around the core30. The permanent magnet material can be magnetized by a magnetizer to provide a plurality of alternating magnetic poles. Additionally, the number of magnetic strips can be different than the number of rotor magnetic poles. Yet another construction of the rotor15contains blocks of permanent magnet material placed inside the rotor core30.

It is to be understood that the description of the invention is not limited to a particular mechanical construction, geometry, or position of the rotor15. For example,FIG. 3shows the rotor15located inside and separated by a radial air gap from the stator20. In another construction of the motor10, the rotor15can be positioned radially exterior to the stator20(i.e., the machine is an external- or outer-rotor machine).

One method to reduce cogging and ripple torque, which may arise in some BLDC motors, is skewing the magnetization of the magnetic poles25with respect to the stator20. Alternatively, stator teeth of the stator20can be skewed with respect to the rotor magnetization. As shown inFIGS. 1 and 2, the “magnetization” of the rotor15refers to the line pattern31along the length of the rotor15delineating alternating magnetic poles25on the rotor core30.

With reference toFIGS. 2 and 3, the stator20includes a stator core105having a plurality of stator teeth110, stator windings112, and a back iron portion115. In one construction, the stator core105includes a stack of magnetic steel laminations or sheets. In other constructions, the stator core105is formed from a solid block of magnetic material, such as compacted powder of magnetic steel. The stator windings112can include electrical conductors placed in slots120(i.e., the space between adjacent stator teeth110and receives stator windings112) and around the plurality of teeth110. Other constructions and types of the stator core105and stator windings112known to those skilled in the art can be used and are not limiting on the invention.

In some constructions of the motor10, electrical current flows through the stator windings112and produces a magnetic field that interacts with the magnetization of the rotor15to provide torque to the rotor15and shaft17. The electrical current can be an (m) phase alternating current (AC), where (m) is an integer greater than or equal to two. The electrical current can have various types of waveforms (e.g., square wave, quasi-sine wave, etc). The stator windings112receive electrical current from an electrical drive circuit.

In the construction shown inFIG. 3, the rotor15is produced by fixing three arc shaped magnets26on the rotor core30. Other rotor designs and constructions are also possible. A magnetizer is used to produce on the rotor15a number of alternating magnetic poles that interact with the stator20.

FIG. 4illustrates a drive circuit125that receives AC power from a power source130and drives the motor10in response to an input135. The AC power is provided to a filter140and a rectifier145that filter and rectify the AC power, resulting in a bus voltage VDC. The bus voltage VDC is provided to an inverter150and to a voltage divider155. The voltage divider155reduces the bus voltage VDC to a value capable of being acquired by a controller160(at a terminal162). The controller160includes a processor165and a memory170.

Generally speaking, the processor165reads, interprets, and executes instructions stored in the memory170to control the drive circuit125. The controller160, which may be in the form of a microcontroller, can include other components such as a power supply, an analog-to-digital converter, filters, etc. The controller160issues drive signals at terminals175and180to control the inverter150. The inverter150includes power electronic switches (e.g., MOSFETs, IGBTs) to vary the flow of current to the motor10. For example, the inverter150can be in the form of a bridge circuit. A sense resistor185is used to generate a voltage having a relation to a bus current of the inverter150. The voltage of the sensor resistor185is provided to the controller160at a terminal187. Other methods of sensing current can be used to sense the bus current. The controller160can receive values associated with phase currents and phase voltages provided by the inverter150.

The drive circuit125also includes a BEMF voltage divider190and variable gain amplifiers195A,195B, and195C. The BEMF voltage divider190and variable gain amplifiers195A,195B, and195C provide voltage values to the controller160at terminals200A,200B, and200C, respectively. The voltage values provided to the controller160by the variable gain amplifiers195A,195B, and195C have a relation to the BEMF of each phase voltage.

During operation of the HVAC system2, the motor controller160can start and control the motor10by providing drive signals to the inverter150based on inputs received at the controller160. The controller160can receive input signals from the input interface135or a serial port interface. In some constructions, the input interface135can be configured to receive input signals from one or more voltage sensors, current sensors, and auxiliary systems. Voltage sensors and current sensors can be used to measure voltages and currents, respectively, in the motor10or other devices operating in cooperation with the motor10. Thus, the voltage sensors and current sensors can be coupled or placed within the motor10, or alternatively, these sensors can be placed at a remote location. Moreover, the drive circuit125can be coupled or placed within the motor10, or alternatively in close proximity to the motor10. Signals generated by auxiliary inputs can be received at the input interface135and can include signals from safety systems or other input/output devices6as schematically illustrated inFIG. 1.

The starting procedure of the motor10is stored as software instructions in the memory170. The processor165reads the instructions from the memory170, interprets the instructions, and executes the interpreted instruction resulting in the operation of the motor10as described below. Other circuit components (e.g., an ASIC) can be used in place of the processor165and the memory170to control the motor10.

In some constructions of the HVAC system2, the thermostat3, system control board4, and motor assembly5are configured to start and operate the HVAC system2utilizing a set of specific startup methods. For example, the thermostat3can generate signals indicative of temperature requirements which can turn on the motor assembly5and consequently the HVAC system2. The signals generated by the thermostat3can be sent to the system control board4. In some cases, the system control board4can be used to simultaneously control the HVAC system2and other input/output devices6such as auxiliary systems or safety devices (e.g. smoke detection systems, alarm systems, ambient humidity control). The system control board4can process the signals generated by the thermostat3to generate instructions for the motor assembly5. The motor assembly5can receive the instructions from the system control board4, utilizing a serial port interface (not shown), to drive a blower or a fan (not shown) producing generally a constant air flow, for example.

As explained above, the power signal provided to the rotor15generates torsional torque ripple that excites system vibration modes and results in an audible noise for a short period during ramp up. Thus, in order to avoid the audible noise during the ramp up of the BLDC motor10, the memory170stores software instructions for the processor165. The processor executes the instructions and the controller160starts the motor10by using a routine or a method that eliminates that ramp up noise.

In particular, the processor165starts and ramps up the motor10to a predetermined switch over speed with a three phase AC (sinusoidal) voltage signal instead of the three step method that can be used to commutate the motor10. In a typical startup, the processor165generally runs two phases at a time and the third phase is used to receive feedback about the position of the rotor15. The proposed startup technique uses an open loop vector-like drive and all three phases of the motor are simultaneously generating three phase sinusoidal voltage. The preprogrammed magnitude and frequency of the power signal during startup commutates the motor10from zero revolutions per minute (“RPM”) to a predetermined switch over speed (e.g., 300 RPM).

During this startup process, the rotational speed is not monitored and, therefore, there is no feedback about the position of the rotor15or the BEMF produced by the motor. When the motor10reaches the predetermined switch over speed, the AC signal is turned off (i.e., it is disconnected by the processor165) and the processor165switches to a BEMF control mode (i.e., a closed loop control, receiving feedback). In the BEMF control mode, the drive circuit125can estimate the rotor position through sensorless control. One of the advantages of the described startup method is that the process provides a technique for avoiding a startup noise without changing the existing hardware of the drive circuit125and the motor10.

FIG. 5illustrates a flow chart describing one possible method for starting the brushless permanent magnet electrical motor10utilizing the electrical drive circuit125. The process illustrated by the flow chart can be started automatically or manually (at block300). A pre-charge power stage takes place at block305. The pre-charge power stage can include charging energy storing devises (e.g., capacitors) that are used to develop voltages required for switching the power electronic switches (e.g., IGBTs) on. Various methods for charging the capacitors can be used.

Occasionally, the rotor15is in motion when the method for starting the motor10is initiated. The controller160monitors the BEMF to detect movement of the rotor15(at block310). The variable gain amplifiers195are switched to a high gain mode to detect possible low BEMF signals produced by the motor10. Low BEMF signals are generally indicative of significantly slow motion of the rotor15. The controller160usually determines the rotational speed of the rotor15by measuring the time between BEMF crossings. For example, if the time between BEMF crossings increases, it is determined that the rotor15is slowing down. The speed of the rotor15may be classified as one of various states. For example, states determined by the speed of the rotor15can include a no moving state, a slow moving state, or a fast moving state. If the speed of the rotor15falls under the slow moving state, the rotor15is stopped by shorting phases A, B, and/or C (at block315).

The controller160classifies the speed of the rotor15under the no moving state when there is relatively no rotation of the rotor15. In such case, the controller160starts and ramps up the motor10to a predetermined switch over speed with a three phase AC voltage signal (at block320). As discussed above, this is open-loop, preprogrammed magnitude and ramp to get the motor10from zero RPM to a switch over or transfer speed. For quiet operation of the motor10, it is best to produce a sine wave current during ram up of the motor (at block320). Because the motor BEMF waveform is not purely sinusoidal a sine wave current may only be produced by a modified sine wave voltage. During this motor control, the motor current is not measured. Therefore, to produce more sine wave current, the open-loop voltage waveform can be intentionally distorted during320.

At block322, the controller160checks if the motor10has reached the switch over speed. If the motor has not reached the switch over speed, the controller160continuous to ramp up the motor10with three phase AC voltage signal (at block320). The controller160proceeds to a coast state (at block325) when the controller160determines that the motor10has reached the switch over speed. At that time, the controller160turns off the inverter150as the rotor15is allowed to coast (at block325). The controller then switches to a BEMF control mode. Monitoring the BEMF allows the controller160to determine a period in relation to the rotational speed of the rotor15(at block330).

With reference to block305, the controller160proceeds directly to determine the period (at block330) when the rotational speed of the rotor15is classified under a fast moving state (at block305). In some cases when the controller160determines that the rotational speed of the rotor15is classified under the fast moving state (at block305), the rotor15may also be rotating in the reverse direction.

The controller160allows the rotor15to rotate in a forward direction after turning on the inverter150, and monitors the BEMF for a predetermined amount of time (at block335). The controller160determines if the rotational speed of the rotor15is above a threshold value after the predetermined amount of time. The threshold value in relation to rotational speed of the rotor15may vary based on factors such as the size of the motor10or the load coupled to the motor10. When the speed of the rotor15is above the threshold value, the controller160returns to the hardware initialization procedure (at block305). Alternatively, when the speed of the rotor15is below the threshold value, the controller proceeds to a run mode (at block340).