Electricity: motive power systems – Switched reluctance motor commutation control
Reexamination Certificate
2002-03-13
2004-05-04
Leykin, Rita (Department: 2837)
Electricity: motive power systems
Switched reluctance motor commutation control
C318S132000, C318S434000, C318S257000, C318S258000
Reexamination Certificate
active
06731082
ABSTRACT:
RELATED APPLICATIONS
Not applicable.
FEDERALLY SPONSORED RESEARCH
Not applicable.
Microfiche APPENDIX
Not applicable.
BACKGROUND OF THE INVENTION
This invention relates to an electronically commutated brushless D.C. motor and the pulse width modulated (PWM) control signal generating circuitry used to provide control signals for this motor.
Brushless D.C. motors include a rotating permanent magnet rotor, a stator carrying field coils, and a drive circuit for sequentially exciting the field coils with digital pulses, thereby creating electronic commutation. Electronically commutated motors eliminate or reduce the disadvantages inherent in motors with mechanical structures for a commutator. Specifically, RFI (radio frequency interference) losses and EMI (electro-magnetic induction) losses are reduced or eliminated. Brush and armature maintenance is eliminated, and power consumption attributed to armature-brush arcing is also eliminated.
Typically, an electronic timing drive circuitry incorporating active electronic components, i.e., transistors, FETs (field effect transistors), MOSFETs (metal oxide semiconductor field effect transistors) has been used to provide PWM drive pulses. The PWM drive signal circuit is connected directly to a power supply. Drive pulse generation has been synchronized with rotor position by the incorporation of monitoring or feedback circuitry, including the use of optical position sensors and/or magnet position sensors, such as Hall effect devices.
Many such DC motors are used in electronic applications that may utilize batteries as the energy source. Examples of applications of such battery-powered fans are laptop computers, and telecommunication equipment. More and more, desktop computing devices and non-portable telecommunication equipment are being backed-up by arrays of batteries. Therefore, this equipment must be designed to operate on battery power.
When batteries are used to power such equipment, it is desirable that this equipment be very efficient, so that there is a minimum of energy consumption. This has become an important design requirement for cooling fans that are used in such battery-powered equipment. It has become important that these fans, and the motors that power them, be designed for efficiency and for minimum energy consumption. It is also desirable that fan motors used for cooling telecommunications equipment provide maximized airflow to pressure characteristics, with a minimum of noise and DC power consumption.
In systems situations, such as telecommunications systems, batteries are used in the system and charged while the system and the battery driven equipment is in operation. Sometimes, the batteries are charged to a higher voltage than the nominal value required for the equipment. Examples of this are the lead-acid batteries used in telephone centers. In the presence of power shortages, or blackouts, these batteries may discharge to a voltage value below the rated voltage for the system. As an example, battery banks rated at a nominal working voltage of 48 volts, may be permitted to swing between 40-56 volts. Under severe load conditions, additional batteries may be added to a bank (which increases its operating voltage). Under such conditions, a nominal bank output voltage may be permitted to swing between 60-72 volts.
If a DC fan motor rated at 48 volts were installed in the system where the battery bank operated between 60-72 volts, the motor would consume excessive power. It would also produce and excessive amount of noise. Motor controller electronics and coil driver power transistor switching circuits may be subjected to overheating and burnout. Even when the battery bank operates about its nominal voltage value, there may be a considerable voltage swing to which a motor is subjected. It is therefore desirable to provide a DC fan motor that will maintain a constant speed and operate under minimum power consumption under power supply voltage variations.
While the variations recited above were directed to battery powered systems, voltage variations can occur even with regulated power supplies operating from ac current. Moreover, power surges are not unheard of with these ac powered-dc supplies. Such power surges manifest themselves as voltage variations. Therefore, a constant speed motor circuit is also desirable in ac powered dc supply systems.
PWM control pulses are of fixed height and frequency and are of variable pulse width. As the pulse width varies so does the power delivered to the motor coils. As the power varies, so does motor speed. Erratic motor speeds increase power consumption, can create more heat, and create electrical noise.
Excessive supply voltages and variations in supply voltage, whether originating with batteries or not, will result in changes in the PWM signals produced. These changes have occurred in prior PWM controller circuits when erratic voltage levels, drifting voltage levels, and voltage spikes have been incurred. It is desirable that the effects of these changes be avoided.
In prior circuits, the PWM motor drive is used to switch on and off the incoming energy source in order to control the current (or voltage) supplied to a load (motor coil). Generally higher frequency pulses have in the past been utilized in the generation of PWM control pulses. The frequency used to carry the pulses of variable width (PWM) has been higher than the frequency of the waveform that needed to be controlled.
The PWM technique has been widely used to control the power supply voltage applied to a DC fan motor in order to control fan speed. Chinomi, et al. (U.S. Pat. No. 6,256,181 B1) disclose a pulse width modulated (PWM) motor drive circuit. Chinomi changes the drive pulse rate by controlling (changing) drive pulse width. Erdman, et al. (U.S. Pat. No. 6,271,638 B1) use a capacitor-coupled bridge circuit power supply to further reduce power consumption. Erdman further uses a Hall sensor control of the pulse generator to limit current usage. Erdman uses a stall sensor to deactivate the field coil driver pulse circuitry in the presence of a stall and fault condition.
Horiuchi, et al. (U.S. Pat. No. 5,969,445), have used a brushless motor, incorporating a sensor magnet for sensing rotor position. This sensor magnet provides a feedback signal indicative of each rotor pole position and thereby the rotational speed of the brushless motor. The Horiuchi drive circuit, which creates electronic commutation, incorporates FETs, and obtains power from a power supply containing an AC-to-DC converter. The Horiuchi sensed rotor position signal is used to optimize the drive pulse effects as a function of pulse generation timing (i.e. leading and/or falling edges) verses rotor magnet position.
Schmider, et al. (U.S. patent application Pub. 2001/000 4194), use bi-stable multivibrator circuits for implementing the electronic commutation of the motor's field magnets. One or more comparator circuits control the switching state of the multivibrator. The Schmider comparator circuit is controlled by a voltage induced in a field coil which has just been deactivated. The need for a separate rotor position sensor is eliminated.
Hall effect devices (Hall generators) have been substitute for the Horiuchi-type magnet sensor (U.S. Pat. No. 6,211,635 B1, Kambe, et al.). Kambe uses a single Hall generator in his motor to determine rotor position and to generate synchronization signals for the drive pulse circuit operation.
In the use of PWM digital drive pulses, a current spike or over voltage can occur at the time of or shortly after each coil current transition time. Shunt or snubber circuits have been used to protect the drive circuitry from these spikes. These devices limit over voltage or over current transients, during or after the operation of switching current (leading edge or falling edge of a pulse) in a field coil. Snubber circuits are shown in Markaran, et al. (U.S. patent application Pub. 2001/0000293 A1). Both Markaran's field coil switch and his snubber circuit switch are implemented by MOSFETs. When Markaran's fi
Leykin Rita
Paul & Paul
Pelko Electric (HK) Ltd.
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