Electricity: motive power systems – Limitation of motor load – current – torque or force
Reexamination Certificate
2002-03-13
2003-12-16
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Limitation of motor load, current, torque or force
C318S254100, C318S132000, C318S244000, C318S246000, C388S907200, C361S099000, C361S091500, C363S056020, C363S058000, C363S132000, C363S136000
Reexamination Certificate
active
06664750
ABSTRACT:
RELATED APPLICATIONS
Not applicable.
FEDERALLY SPONSORED RESEARCH
Not applicable.
MICROFICHE APPENDIX
Not applicable.
BACKGROUND OF THE INVENTION
The present invention relates to a coil driver, bridge circuit for a direct current motor. Specifically, it relates to bridge circuits for providing the excitation current to the field coils in an electronically commutated D.C. motor.
An electronically commutated, brushless, D.C. motor can have a rotatable permanent magnet rotor, a stator that carries field coils, coil driver circuits, and coil drive control timing circuitry for controlling the operating state of each coil driver circuit for thereby implementing the sequential excitation of the motor field coils. A brushless D.C. motor eliminates or reduces the disadvantages inherent in the mechanical structure of a mechanically commutated motor, i.e., a motor with a commutator and brushes. Specifically, RFI (radio frequency interference) losses and EMI (electromagnetic induction) losses are reduced or eliminated. Also, the brush maintenance factors and the armature maintenance factors are eliminated. Moreover, the power consumption, attributed to armature-brush arcing, is also eliminated.
Typically, a motor electronic timing circuit, which incorporates active electronic components, has been used to provide and/or control the excitation of each motor coil, by controlling the operating state of each, respective, coil driver circuit. In the motor, each coil driver circuit is connected directly between a power supply and each motor coil. The operating state of the coil driver circuit is controlled by control pulses generated by the electronic timing circuit. The electronic timing of the generation of control pulses to each coil driver circuit can be synchronized with rotor position, wherein monitoring and/or feedback circuitry is incorporated, including the use of optical position sensors or magnet position sensors. The power supply and the control pulse generating, electronic timing circuit are sensitive to reverse voltage and voltage spike conditions which can be generated in a coil portion of the circuit network, under electrical fault conditions, rotor stall conditions, and other effects. Each coil driver circuit is a bridge circuit connecting the control portion of the network to the load (coil) portion of the network. It is desirable for bridge coil driver circuits to incorporate reverse voltage and voltage spike protection, thereby isolating the sensitive power supply circuitry and sensitive electronic control circuitry.
Pulse width modulation (PWM) is used in the electronic timing circuit to control each bridge coil driver circuit. The width of these pulses affects the instantaneous torque developed by the coil excitation in the motor, and the pulse rate affects the motor speed. The duty cycle of the generated drive pulses (pulse width) is controlled electronically.
The sensitive circuitry used to generate the PMW control pulses cannot be subjected to over voltage and over current spikes caused by motor faults and stalls, nor can it be subjected to spikes often generated when the excitation drive current to a field coil is stopped. Such circuit conditions can burnout this delicate pulse drive circuitry. Moreover, such sensitive circuitry cannot normally carry (pass through) the power needed to excite of the motor field coils, without again burning out. Bridge circuits, whose operation is controlled by the PWM output from the electronic timing circuit, can be used to power the motor field coils directly.
The use of turn off, turn on, “snubber circuits” as a part of a coil bridge circuit, to protect power electronics from overvoltage or overcurrent conditions, is not new. These snubber circuits bridge the connection between the electronic timing circuit and the motor (phase) field coils that are excited. Snubber circuits are devices that limit such over voltage and over current transients that can occur during or immediately after the deactivation of a field coil. These snubber circuits have taken one of two forms, dissipative snubbers and regenerative snubbers.
Dissipative snubbers are circuits which shunt such overvoltage and overcurrent transients to ground. The most common configuration for a dissipative snubber is a Resistor-Capacitor-Diode circuit (RCD) connected between one end of a field coil and ground, with the field coil excitation “switch” connected in parallel with the RCD snubber. This device has proven to be somewhat unsatisfactory in unipolar brushless D.C. motors where low voltage, high current is used. In a unipolar brushless D.C. motor, the current in the coil windings flows in one direction through the coils from a D.C. source to ground.
RCD snubbers have been used in low side drive or boost converter type motor drives. With high side drive configurations, these elements are replaced by diodes connected to the positive voltage. A high side drive is where the excitation switch is connected between the voltage supply (power supply) and the load (field coil) being excited. Some high power, low voltage type switching devices have been used in such snubber. Examples are MOSFETs (metal oxide semiconductor field effect transistors). Reverse protection diodes are used to protect motor (phase) field coils from shorting out. Other diodes provide reverse protection to MOSFET containing circuitry because of the MOSFET circuit source to drain diodes that become forward biased during a reverse polarity condition.
Regenerative snubbers return the energy stored in snubber circuit elements back to the positive voltage supply. Many of these have used transformers, or active elements such as D.C. power supplies (batteries). These prior regenerative snubbers have been used to maintain a constant voltage across the switches in order to prevent a braking of the motor through back electromotive force (EMF); and have also been used to return excess energy stored in the motor field coils back to the positive voltage supply. The elimination of the motor braking action reduces the previously induced voltage spikes due to braking EMF.
Markaran (U.S. patent application Ser. No. 2001/0000293 A1) shows and actively controlled regenerative snubber (power output circuit) for a unipolar brushless D.C. motor. Markaran's circuitry has an output bridge snubber circuit connected to each motor phase (field) coil. The Markaran bridge circuit includes the series connection of a coil and a capacitor in parallel with a motor field coil, and a cross-connected diode, from the grounded side of the coil to the intermediate point of this series LC circuit. The Markaran bridge circuit maintains a constant voltage across its switches (which are each implemented by transistor devices) to prevent avalanching (i.e., breaking down of the transistor switch) which would result in the braking of the motor through conduction of motor back EMF. Markaran's concern is minimizing switching losses. Markaran operates his bridge switches at a constant frequency “f
c
”.
Upon opening the Markaran field coil excitation switch S
1
, the residual energy in the Markaran field coil L begins to discharge through the snubber diode D
1
and into the capacitor C connected to ground. This limits the voltage drop across the coil activation switch S
1
once it is opened. Once the voltage on the Markaran capacitor C reaches a pre-selected threshold voltage, his second switch S
2
is closed and the capacitor C begins to discharge through the inductor L
2
. Markaran's stated purpose for the inductor L
2
is to slow the discharge of the capacitor C and to ensure that the capacitor is discharged in a sinusoidal fashion for harmonic minimization of the discharge current. The value of the snubber diode generally locks in the effective voltage range of the circuit, which is then limited to that value.
Alvaro, et al. (U.S. Pat. No. 6,239,565 B1) show an output bridge circuit for exciting two phase (field) coils W under the control of signals from a driver circuit DC. Alvaro activates a coil excitation switch SW to excite a desired
Martin Edgardo San
Nappi Robert E.
Paul & Paul
Pelko Electric (HK) Ltd.
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