Electricity: motive power systems – Switched reluctance motor commutation control
Reexamination Certificate
2001-11-20
2003-10-14
Ro, Bentsu (Department: 2837)
Electricity: motive power systems
Switched reluctance motor commutation control
C318S459000, C318S500000, C318S599000, C318S132000, C318S434000, C388S928100, C388S804000, C388S806000, C388S811000, C388S822000
Reexamination Certificate
active
06633145
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to motor driving and control circuitry, and is more specifically related to an improved circuit and method for back electromotive force (BEMF) detection in a brushless motor.
2. Discussion of Related Art
Three-phase brushless DC motors have many uses, among which include both high speed and low speed applications. Conventional high speed applications include spindle motors for computer hard disk drivers, digital video disk (DVD) drivers, CD players, tape-drives for video recorders, and blowers for vacuum cleaners. Motor for high speed applications typically operate in a range from a few thousand rotations per minute (rpm's) to 20,000 rpm's, for example. Low speed applications include motors for farm and construction equipment, HVAC compressors, fuel pumps and the like. Motor for low speed applications typically operate in a range from less than a few hundred rpm's to a few thousand rpm's, for example. Compared to DC motors employing brushes, brushless DC motors enjoy reduced noise generation and improved reliability because no brushes need to be replaced due to wear.
FIG. 1
shows a cross-section of a typical brushless, DC motor
10
. The motor
10
includes a permanent magnet rotor
12
and a stator
14
having a number of windings (A, B, C shown in FIG.
2
). The windings are each formed in a plurality of slots
18
. The motor
10
illustrated has the rotor
12
housed within the stator
14
. The stator
14
may also be housed within the rotor
12
. The rotor
12
is permanently magnetized, and turns to align its own magnetic flux with one generated by the windings.
Power to the motor
10
is often provided in a pulse width modulation (PWM) mode. The PWM mode is a nonlinear mode of power supply in which the power is switched on and off at a very high frequency in comparison to the angular velocity of the rotor. For example, typical switching frequencies may be in the range of 20 kHz. In a typical on-off cycle lasting about 50 &mgr;s, there may be 40 &mgr;s of “on” time followed by 10 &mgr;s of “off” time. Given the short duration of off times, current still flows through the motor windings so there is virtually no measurable slow down in the angular velocity of the rotor
12
during these periods. Accordingly, PWM mode provides a significant power savings advantage over modes in which power is continuously supplied.
In order to operate the motor
10
, the flux existing in the stator
14
is controlled to be slightly in advance of the rotor
12
thereby continually pulling the rotor forward. Alternatively, the flux in the stator
14
may be controlled to be just behind the rotor
12
, in which case the polarity is set such as to repel the rotor
12
, thereby aiding rotation. Therefore, to optimize the efficiency of the motor
10
, it is advantageous to monitor the position of the rotor
12
so that the flux in the stator
14
may be appropriately controlled and switched from one stage to the next. If the rotor
12
movement and the flux rotation should ever get out of synchronization, the rotor
12
may become less efficient, start to jitter or stop turning.
A conventional motor can be represented in circuit form as having three coils A, B, and C connected in a “Wye” or “Y” configuration, as shown by reference numeral
20
in
FIG. 2
, although a larger number of stator coils are often employed with multiple rotor poles. Typically, in such applications, eight-pole motors are used having twelve stator windings and four N-S magnetic sets on the rotor, resulting in four electrical cycles per revolution of the rotor. The stator coils, however, can be analyzed in terms of three “Y” connected coils, connected in three sets of four coils, each physically separated by 90 degrees.
In operation, coils A, B and C are energized with a PWM drive signal that causes electromagnetic fields to develop about the coils. The resulting attraction/repulsion between the electromagnetic fields of the coils A, B, and C and the magnetic fields created by the magnets in the motor causes the rotor assembly of the motor to rotate.
The coils are energized in sequences to produce a current path through two coils of the “Y”, with the third coil left floating (or in tristate), hereinafter floating coil FC. The sequences are arranged so that as the current paths are changed, or commutated, one of the coils of the current path is switched to float, and the previously floating coil is switched into the current path. The sequences are defined such that when the floating coil is switched into the current path, the direction of the current in the coil that was included in the prior current path is not changed. In this manner, six commutation sequences, or phases, are defined for each electrical cycle in a three phase motor, as shown in Table A.
TABLE A
Phase
Current Flows From:
Current Flows To:
Floating Coil
1
A
B
C
2
A
C
B
3
B
C
A
4
B
A
C
5
C
A
B
6
C
B
A
When the motor is on, the rotation of the rotor induces a BEMF voltage in each of the windings of the motor. Such BEMF is represented by the Bemf voltage sources in FIG.
2
. With respect to whichever phase is currently floating, the BEMF in that phase is monitored to determine when to advance in the communication sequence. More particularly, the BEMF in the floating coil is monitored to determine when it crosses zero at which point the position of the rotor is assumed to be known. The point at which the BEMF crosses zero is referred to as the “zero-crossing”. Each time a zero-crossing is detected, the motor advances in its commutation sequence by 30 electrical degrees.
A conventional technique to measure the BEMF is to measure, during a floating period, the voltage at a coil tap (nodes Va, Vb, and Vc. in
FIG. 2
) for the floating coil. The measured voltage at the coil tap is presumed to be the BEMF. Accordingly, the coil tap voltage for the floating coil is monitored to detect zero-crossings at which times the commutation sequence is advanced. However, unless the center tap voltage V
CT
is zero, this BEMF calculation is not fully accurate.
Known methods of detecting BEFM include comparing the floating phase coil tap voltage with the center tap voltage, or a virtual center tap voltage configured by a resistor network. During the PWM-on and PWM-off states, the center tap voltage V
CT
is significantly deviated from zero. This generates high common mode noise. To offset the center tap voltage V
CT
for zero-crossing detection, voltage divider and filter circuits have been used. However, such voltage divider and filter circuits reduce the sensitivity of the circuits and delay zero-crossing detection.
SUMMARY OF THE INVENTION
A system and method of advancing the commutation sequence of a brushless DC motor is provided. The system and method advantageously monitors for zero crossing detections during PWM-off states. Because a PWM signal typically oscillates at a frequency significantly greater than the frequency at which the commutation sequence advances, zero-crossings which may happen to begin during a PWM-on state are still detectable during the PWM-off state with minimal delay. For example, the frequency of the PWM signal may be in the range of 20 kHz-100 kHz while the frequency at which the commutation sequence advances is typically on the order of 100 Hz. Accordingly, timely advancement of the commutation sequence is minimally impacted if the zero-crossing begins to occur during a PWM-on state. Further, as zero-crossing detection is accomplished during PWM-off states, the filter circuits previously used to offset the center tap voltage for zero-crossing detection are no longer needed, thereby avoiding reduced circuit sensitivity and delays in zero-crossing detection.
It has been observed that, especially in low speed and/or low voltage applications, variations in the center tap voltage V
CT
from zero during PWM-off states may have an adverse effect on zero-crossing detection. Variations in the center tap voltage V
CT
from a zero often occur during PWM-off state
Haughton Kwan A.
Hopkins Thomas L.
Nolan Dennis C.
Shao Jianwen
Duda Rina I.
Jorgenson Lisa K.
Ro Bentsu
Santarelli Bryan A.
STMicroelectronics Inc.
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