Electricity: motive power systems – Switched reluctance motor commutation control
Reexamination Certificate
2002-10-28
2004-05-25
Duda, Rina (Department: 2837)
Electricity: motive power systems
Switched reluctance motor commutation control
C318S132000, C318S434000, C318S459000, C318S500000, C388S928000, C388S928100, C388S923000
Reexamination Certificate
active
06741049
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to DC motors, and more particularly to low voltage start-up of brushless DC motors.
As the density and operating speeds of complex integrated circuits increases, the power dissipated thereby needs to be dispersed as heat, typically with the use of fans. Such fans are typically driven from the DC power supply connected to the power dissipating circuit such as to ensure the fan runs when the circuit is powered. DC fan motors such as those used to cool high density or high power integrated circuits, e.g., in a modem PC, are normally designed to operate with the same supply voltages as are used for the integrated circuits themselves for simplicity and cost effectiveness.
DC fan motors typically include brush-type permanent magnet motors and brushless motors. As is well known, brush-type motors typically include an armature, having windings, attached to a rotor. Brushes press against a commutator attached to the armature. As the armature turns, the brushes come into contact with different segments of the commutator and change the current path through the winding. The interaction between the magnetic field created in the armature and the permanent magnetic field in the stationary part of the motor results in rotation of the armature. Operation of a brushless motor is similar except that the permanent magnets are coupled to the rotor instead of the stationary part, and the windings are on the stationary part instead of the rotor. The winding phases of a brushless motor are switched on and off electronically by means of a control circuit. Hall effect sensors are typically used to detect the (rotational) position of the rotor, which is used by the control circuit as feedback to control the timed switching of the windings.
FIG. 1
illustrates an example of a brushless DC motor
10
including permanent magnets
15
coupled to a rotor
20
. Sensors
25
detect the rotational position of magnets
15
. Windings
30
, provided in an armature
35
, are controlled by a drive controller (not shown).
One trend in integrated circuit design is to reduce the supply voltage and thus reduce the power dissipation as much as possible. However, the integrated circuits and therefore the system cannot be operated without additional cooling since the temperature rise caused by the power density can exceed the safe operating limits of the silicon. The performance of the fan is thus essential for the safe and reliable operation of the system. However, as these voltages drop to low levels, e.g., below 3.5 volts, the control of the fan motors becomes increasingly difficult. Analogue circuitry, which is often needed to handle and process signals from sensor devices, such as Hall effect sensors, used to detect the rotation of the magnet in the DC motor, may not operate effectively or accurately as the supply voltage drops below about 3.5 volts.
Specialized integrated circuits have been developed, such as the Melexis US79 series of fan drivers, that can handle all the functions required to ensure reliable fan operation. However, the analogue content of these integrated circuits, essential for their correct operation to specification, sets a lower limit to the operating voltage of about 3.5 volts. Below this figure the performance of analogue circuitry cannot be reliably predicted.
To maintain operation at lower voltages other techniques are typically used. For example, back Electro Motive Force, EMF, generated by the inductive windings of the DC motor when the current through the windings is turned off during normal commutation is typically used to boost the supply voltage available to the analogue circuitry. Digital circuitry can be configured and arranged to operate satisfactorily at voltages down to about 1.5 volts. However, when the motor is stationary, commutation is not taking place so there is no back EMF generated to supply to the analogue circuitry. The smooth and satisfactory performance of the DC motor under these conditions cannot be ensured and indeed the motor may not start if the analogue circuitry cannot resolve the situation sufficiently to enable the selection of, and drive to, the correct winding of the motor.
Accordingly, it is desirable to provide systems, methods and circuitry to generate sufficient voltage to ensure analogue circuitry of a DC motor performs sufficiently predictably to ensure satisfactory start up of the DC motor.
BRIEF SUMMARY OF THE INVENTION
The present invention provides systems, methods and circuit arrangements for ensuring proper start-up of brushless DC motors including components operating at low voltages compatible with modem IC design voltages.
According to the invention, a local oscillator and logic circuit pulses the open winding of a brushless DC motor at start up and the back EMF is used to generate a voltage to boost the voltage available to the control circuit for optimizing performance when starting with low supply voltage. As the rotor of a motor rotates and the windings are commutated by the drive electronics there is generated in each winding a voltage caused by the collapse of the current and the inherent inductance of the winding. These voltages exceed the normal operating voltage of the motor. The energy in these voltages is used to generate a regulated power feed to the analogue circuitry of the control circuit at a suitable voltage level.
During steady state conditions, when the fan is running, the commutation of the windings is continual and there is ample energy available to power the analogue electronics, and, if required, the associated digital electronics as well. At start up, however, when the motor is stationary, there is no commutation and thus no additional voltage pulses from which to generate a supply for the analogue circuitry. Accordingly, additional circuitry is included to drive one of the motor windings with short voltage pulses such as to create inductive voltages that can be used to create the desired regulated power feed for the analogue circuitry. This feed, once established, enables the analogue circuitry to accurately determine the state and position of the rotor and cause the correct winding to be driven. The motor will then start and usual steady state conditions will become established.
According to an aspect of the present invention, a circuit arrangement for driving at low voltage a brushless dc motor having a rotor and at least two windings for driving one or more permanent magnets on the rotor is provided. The arrangement typically includes drive circuits configured to provide drive signals to the windings, one or more sensors arranged to determine the rotational position of the rotor, and an analogue processing circuit configured to process signals received from the one or more sensors so as to provide a feedback signal. The circuit arrangement also typically includes a regulation circuit configured to extract energy from inductive voltages produced by the windings and to generate a voltage power source for the processing circuit, an oscillator circuit configured to provide a pulse signal, and a control circuit configured to receive the feedback signal and the pulse signal. The control circuit also configured to control the drive circuits such that in a first mode of operation, when the rotor is turning, the drive circuits are selectively enabled based on the feedback signal, and in a second mode of operation, when the rotor is not initially turning, the drive circuits are pulsed based on the pulse signal so as to generate inductive voltages in the windings.
According to another aspect of the present invention, a system for driving at low voltage a brushless dc motor having a rotor and at least two windings is provided. The system typically includes driving means for driving the individual windings on the dc motor so as to rotate the rotor, sensor means for determining the rotational position of the rotor, and analogue interface circuitry for interfacing to the sensor circuits and providing a feedback signal based on signals received from the sensor me
Kawaji Hideki Ted
White Mark R.
Duda Rina
Gray Gerald T.
Melexis NV Miroelectronics Integrated Systems
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