Electricity: motive power systems – Switched reluctance motor commutation control
Reexamination Certificate
1999-12-10
2001-04-24
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Switched reluctance motor commutation control
C388S805000
Reexamination Certificate
active
06222333
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to dynamo-electric machines and more particularly to controllers for DC brushless electric motors.
BACKGROUND OF THE INVENTION
It is conventional to use power MOSFET transistors to gate power to separate motor phase windings in order to minimize the number of transistors required and related drive circuitry, while still maintaining significant torque thereby minimizing cost. When a particular MOSFET transistor is gated on, current flows through the attached coil winding. When the same transistor is turned off, the energy field contained within the coil collapses creating a large voltage potential (V
flyback
) across the power transistor. This causes the transistor to go into an avalanche breakdown mode at its specified breakdown voltage (V
breakdown
). This effect limits the V
flyback
to V
breakdown
. The flyback energy is then dissipated between the coil and the transistor, creating a temperature rise in the power transistor proportional to I
flyback
×V
breakdown
.
For low power designs, this flyback temperature rise is tolerable, as the energy contained in the coil is relatively small. However, for higher power designs, the temperature rise is excessive and destroys the transistor. Also, the flyback energy is converted into heat instead of motion, so an efficiency loss is realized. Ideally, V
flyback
should be clamped with a diode so that the majority of the energy dissipates in the coil. However, after the flyback energy dissipates, and after the coil winding passes a new pole, the coil tries to generate an EMF voltage (V
emf
) of the same polarity as V
flyback
. If a clamping diode is used, V
emf
is also clamped creating a breaking effect thereby resulting in a major loss of energy.
SUMMARY OF THE INVENTION
An object of the invention is to provide a DC brushless motor controller which overcomes the prior art limitations noted above. Another object of the invention is the provision of such a controller which is a low cost, microprocessor-based controller which drives the motor with improved efficiency.
Briefly in accordance with the invention, SIDACs are used in series with diodes to clamp V
flyback
yet allow V
emf
to float to its natural level. The SIDACs are essentially TRIACs which trigger at a particular voltage higher than V
emf
and lower than V
breakdown
. When triggered, the SIDAC acts like a diode, clamping the flyback to a few volts. When not triggered, the SIDAC acts like an open, allowing V
emf
to float. A Hall Effect sensor is used in the preferred embodiment to sense the rotor position with a particular winding of the motor powered at the Hall Effect sensor trigger point. According to a feature of the invention, the phase timings are advanced or delayed to optimize performance and efficiency for a particular motor configuration of winding parameters and the torque and speed requirements. The motor is started with a relative phase advance of zero degrees and once the motor is running and stabilized, the control retards or advances the timing, ensuring the motor speed is within tolerance to optimize performance and efficiency. According to another feature of the invention, start-up time is limited to a first selected time period, e.g., two seconds, then, should the motor not start or if the signals are overly erratic, the control shuts down the motor for a second selected time period, e.g., sixteen seconds, allowing the motor and the control to cool down. According to a feature of the invention, two configuration resistors are provided for selecting from a matrix of a large number (e.g., 400) different combinations of target speeds, rotation direction and current limits. One such configuration resistor can be used during manufacture to select a range for the other such resistor which can be fixed internally or attached externally through a set of leads so that the motor configuration can be changed by an external control or relay.
REFERENCES:
patent: 5497094 (1996-03-01), George
patent: 5677605 (1997-10-01), Cambier et al.
patent: 5878189 (1999-03-01), Lankin et al.
patent: 5896487 (1999-04-01), Masten et al.
Garnett Ronald E.
Rowlette Mitchell R.
Baumann Russell E.
Duda Rina I.
Nappi Robert E.
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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