Electricity: motive power systems – Switched reluctance motor commutation control
Reexamination Certificate
1998-02-10
2002-05-07
Fletcher, Marlon T. (Department: 2837)
Electricity: motive power systems
Switched reluctance motor commutation control
C318S434000, C318S132000
Reexamination Certificate
active
06384554
ABSTRACT:
The invention pertains to a drive circuit for brushless dc motors.
Drive circuits of this kind are known from DE-OS 35 37 403 A1. These circuits exhibit a number of properties that are advantageous for the operation of a commutatorless dc motor.
Sample functions include:
low loss speed control of the motor,
temperature governed speed control of the motor,
signal output in case of excess temperature,
reduction of acoustic and EMI radiation.
One particular advantage of these circuits consists in the fact that they can be fully integrated onto a relatively small silicon module (chip). There, these circuits on their allocated silicon module, represent a very high functional density. It is therefore difficult and complicated to implement additional module functions of this type since signal functions and performance control functions will have to be present side-by-side on the same silicon chip.
For greater power or higher speed motors, however, more extensive speed control functions and control functions are required.
In addition, it is an advantage to achieve a yet higher efficiency for the motor and associated drive circuit.
Therefore, it is the task of the invention to describe an electronic circuit that will ensure an improved overall efficiency for the drive circuit and motor by means of a wider speed range and to include an expanded level of function.
The invention is suitable both for single phase motors with permanent magnetic auxiliary torque according to DE-OS 23 46 380, and also for multiphase motors or commutatorless motors without permanent magnets (reluctance motors).
The invention is based on the fact that for purposes of additional energy savings through increasing the overall efficiency of the motor and drive circuit, the following activities are be combined:
First, according to this invention it is an advantage specifically to shift in time the rising and/or falling edges of motor current pulses by means of a suitable circuit, and, of course, especially as a function of the speed of the motor. This will achieve both reliable start-up behavior and also good efficiency for the motor.
For all motors in the range of high and maximum speed, it is an advantage to shift forward the moment of connection of a motor current pulse in order to provide the necessary current maximum in a timely manner due to the finite rate of rise of a motor current pulse. Depending on the motor and speed range, shifts of the commutation timing by several ten degrees (electric) is an advantage.
In the case of motors with permanent magnetic auxiliary torque, it is particularly useful to shift the turn-off edge of a motor current pulse forward in time within the range of minimum speed (increase of the extinction angle), where the stated extinction angle can amount to as much as about 90° (electric).
After start-up of the motor (ramp phase) within an average speed range, normally no essential shift in the moments of commutation is needed, whether for the ignition angle at the beginning of current flow or for the extinction angle at the end of a motor current pulse.
In this range, therefore, we can succeed, even without specific measures, in bringing the time rise of a stator current pulse into approximate coincidence with the shape of the induced counter-EMF of an (unconnected) stator coil, in a generally known manner.
Secondly, the first activity according to this invention, is combined with a circulated (clocked) current flow to the motor (i.e., usually the stator), and, of course, with a clock frequency preferably outside of the range of human hearing.
This measure has the advantage that by means of differing individual pulse widths in the clocked method of current flow, the effective motor current can be preset and adjusted in a simple manner without any large losses in performance due to this adjustment process, as is already generally known. In addition, in a similarly known manner, the advantage results that the effective maximum rate of rise of the motor current can be increased through simultaneous increase in the supply voltage.
It is understood that the supply voltage cannot be increased indefinitely, so that also the effective maximum rate of rise of the motor current remains limited to a finite value.
For high motor speeds it is therefore an advantage to effect a preshifting of the injection timing, even in the case of circuited or clocked motor current.
In this case, one decisive parameter, in addition to the motor speed, is the electric time constant of the motor coils: the ratio of inductivity to resistance, L/R.
In addition, according to the invention it is an advantage to modify the high frequency timing of the motor current during one commutation phase in the direction of smaller pulse duty factors, and, of course, toward the end of one commutation phase. It has proven advantageous to reduce the percentage of the pulse duty factor in two steps each of about 5% after passage of 50% and 75%, respectively, of one commutation phase. This will also avoid additional expense as is necessary in a continuous reduction of the pulse duty factor during one current flow phase, and, furthermore, the latter method has the same or better power reduction effect.
In a third aspect of the invention actions are illustrated that will ensure the provision of differing ignition or extinction angles and thus cause a preignition or advanced extinction of a motor current pulse.
Proceeding from a standard ignition with an ignition angle of zero, it is initially not possible in the range of high and maximum speed of the motor to implement in advance any commutations without a knowledge of future moments of commutation, as this is actually necessary for preignition (according to definition).
According to this invention, in order to solve this problem, the signal output for a commutation is undertaken by means of a galvanomagnetic sensor. However, in this case the sensor is intentionally placed at a location, e.g., between stator and rotor, that will cause a forwardly shifted signal output of, e.g., 5° (electric), as compared to normal signal output. By means of a delay feature in the drive circuit, a delay in these commutation signals will also be possible. Thus it is possible to generate advance ignition angles up to a specified angular value. A delay feature that can delay the commutation signals in a variable manner is a particular advantage, so that the actual start of a commutation process can take on any value between the stated value and later phase angles or moments.
At greater speeds that can necessitate a more prominent forward shift of the ignition angle, the problem again arises of the absence of knowledge of the phase position of the rotor at the desired moment of ignition, at which the commutation process of the stator current is to begin. In addition, the problem exists that the optimum moment of commutation is being shifted constantly, i.e., depending on the speed of the rotor, it consists of another phase position or speed setting of the rotor.
However, according to the invention, in this case, a solution is possible according to the following logic:
The additionally necessary phase preshifting of the moment of ignition occurs practically only at high speeds. At these speeds, the rotational motion due to the mechanical inertia of the rotor, i.e., of the stored mechanical energy, is determined practically for several future rotations of the rotor.
Accordingly, it is possible, with a knowledge of the current rotational speed of the rotor and of the last moment of signal output of the rotational position sensor, to calculate a moment that corresponds to a desired or necessary, future moment of ignition at the current rotational speed (angular velocity) of the rotor.
In this case, according to the invention, a generally known device will be used that determines the rotational velocity of the rotor from the progress of the last determined sensor signals.
In this case, the following mathematic relation will be used:
Z=K
(
n
)+(
K
(
n
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K
(
n−
1)*(1+&phgr;(&ohgr;)/
Karwath Arno
Moini Mojtaba
Wünsch Eberhard
Duda Rina I.
Fitch Even Tabin & Flannery
Fletcher Marlon T.
Papst Licensing GmbH
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