Electricity: motive power systems – Switched reluctance motor commutation control
Reexamination Certificate
2000-09-19
2003-04-08
Ro, Bentsu (Department: 2837)
Electricity: motive power systems
Switched reluctance motor commutation control
Reexamination Certificate
active
06545438
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to cooling modules such as those incorporating brushless DC fans or thermoelectric cooling devices, and particularly relates to cooling modules of a type suitable for use in personal computers and similar electronic equipment.
2. Description of the Related Art
Brushless DC fans have found wide use in cooling electronic equipment such as personal computers. Such fans incorporate a brushless DC motor (BLDC motor) which includes a stationary armature. At least two winding stages are configured around an armature core to provide a predetermined number of poles, and permanent magnets are mounted on the rotor of the fan. Frequently such fans incorporate two winding stages which are arranged to generate four magnetic poles in the armature, although greater numbers are possible. A traditional two-terminal BLDC fan motor usually incorporates circuitry to electronically commutate the winding stages to operate the motor at its full speed for the particular power supply voltage applied. Such circuitry frequently incorporates a rotor sensor to determine when the rotor has rotated sufficiently for the next winding stage to be energized. BLDC motors are also frequently known as electronically commutated motors.
BLDC fans have significant advantages over other kinds of fans. There is no rotating commutator or brush assembly as with traditional DC motors. Consequently, much fewer dust particles are generated which, if shed, may contaminate the equipment. There are fewer parts to wear out, and there is much less ignition noise than with a brush assembly. Furthermore, the magnetic coils of a BLDC motor are mounted on a rigid frame which improves the thermal dissipation of the motor and provides for greater structural integrity of the motor. BLDC fan motors are also much more electrically quiet than other kinds of motors previously used.
Despite all of these advantages, BLDC motors typically still wear out faster than the electronic systems they are included to protect. Mainly, such wear-out is caused by a failure of the bearings supporting the rotor assembly. Also, any fan generates acoustic noise which may be ergonomically displeasing to an operator of equipment incorporating such a fan. Likewise, as with all fans, the fan may become blocked by foreign matter, or filters included in the air flow path generated by the fan may become increasingly clogged with particulate matter which results in a higher strain on the motor driving the fan, and ultimately, an inability of the fan to provide the air flow for which the system was designed.
To reduce the detrimental aspects of using BLDC fans, fan-speed management has proven useful to drive such fans only when needed and only at a speed sufficiently useful to cool the system within which the fan is incorporated. Such fan-speed management results in less acoustic noise, either because the fan is totally off at certain times or is throttled to a much lower speed for a typical operation than its maximum speed which is only driven in a worst-case cooling demand. By throttling back the power of a BLDC fan, significant power dissipation is also saved, which is particularly important in battery-powered systems.
There are a variety of ways that BLDC motors, and particularly BLDC fans, have been controlled in attempt to provide fan-speed management. In its simplest implementation, such fans may be turned fully on or fully off. In a system incorporating such an on/off technique, the fan is turned on typically when a measured temperature somewhere in the system exceeds a first threshold limit, and the fan is turned off when that same temperature is below a second threshold limit (usually lower than the first threshold limit to provide hysteresis). This technique is effective when the fan is unnecessary for most operating conditions and may, therefore, be left off except in the most unusual circumstances. Nonetheless, such control has its drawbacks. For example, when the fan is on, it usually runs at full speed, in which case the acoustic noise is at its worst. Furthermore, if the temperature of the system hovers near the threshold temperatures, a displeasing frequent cycling of the fan may occur as the temperature is driven below the second threshold and allowed to rise above the first threshold with frequent cycling of the power to the fan.
In many systems, a temperature proportional cooling method has been devised to variably increase the speed of a cooling fan as a function of the heat generation or the temperature of the system to be protected. One method of controlling the speed of a brushless DC motor is to regulate the voltage powering the motor to a lesser voltage than the nominal operating voltage that the motor is normally specified to require. Referring now to
FIG. 1
, a standard brushless BLDC motor
40
is shown having one power terminal tied to ground, and the other power terminal connected to a triple darlington transistor circuit (i.e., transistors
41
,
42
and
43
) to a 12-volt positive power supply voltage. A variable control voltage, indicated in the figure as a variable voltage
44
, is applied to the control terminal of transistor
41
and the current is amplified by transistor
42
, and yet again, by transistor
43
, to provide on the power terminal
45
of the BLDC motor
40
a substantially fixed voltage which is a function of the control voltage
44
. Raising the control voltage
44
similarly raises the voltage applied to terminal
45
, which is usually about 1.5 volts below the control voltage
44
. The current drawn through BLDC motor
40
is generally proportional to the voltage applied across the motor. For example, if the current drawn by BLDC motor
40
at 12 volts is 300 milliamps, the current drawn through the BLDC motor
40
will usually be approximately 150 milliamps if terminal
45
is instead controlled to a 6-volt level. The linear voltage regulator circuit shown in
FIG. 1
has several advantages, not the least of which it is extremely easy to design. It provides for a fan speed decrease which is almost linear with decreasing voltage. However, the power dissipation in the voltage regulator itself is substantial. At low fan speeds, the power dissipation of the regulator circuit may actually exceed the power consumed by the fan. This suggests large transistors may be necessary, and external heat sinks may need to be provided to dissipate the heat generated by such a large amount of current flowing through a circuit with a large voltage drop across the same circuit. Generating all of this additional heat seems counter-productive to the purpose of having the fan incorporated within the system in the first place. Consequently, such a linear voltage regulator fan-speed control has found decreasing use in recent years. Nonetheless, such a circuit does extend fan life if the system is designed to rarely require the fan to operate at full speed. Likewise, controlling the fan in this fashion may reduce acoustic noise typically generated by the system.
Another method of controlling the speed of a brushless DC motor involves chopping the power to intermittently apply full power to the motor at certain times, and applying no power to the motor at other times. This is usually accomplished with a relatively low-frequency (30-250 Hz) pulse-width-modulated (PWM) signal. One terminal of the BLDC motor may be connected to one power supply, and the other terminal may be connected to ground through a power switching device which is driven appropriately by the PWM signal.
Referring now to
FIG. 2A
, the brushless DC motor
40
discussed previously is here shown with one terminal connected to a +12 volt source of voltage and a second terminal
54
coupled through an MOS transistor
50
to ground. The gate of transistor
50
is generated by a pulse-width-modulated signal generator
51
, which is responding to a control signal for controlling the speed of the BLDC motor
40
. By using the pulse-width-modulated signal coupled to the gate of driver transis
LJM Products, Inc.
Ro Bentsu
Thompson & Knight L.L.P.
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