Electricity: motive power systems – Induction motor systems – Primary circuit control
Reexamination Certificate
1999-12-24
2001-03-27
Masih, Karen (Department: 2837)
Electricity: motive power systems
Induction motor systems
Primary circuit control
C318S254100, C318S434000, C318S132000
Reexamination Certificate
active
06208112
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method as well as to a device for controlling a voltage/frequency converter controlled single or polyphase electric motor.
DESCRIPTION OF RELATED ART
Such a method may for example be applied to single-phase or polyphase permanent magnet motors but also to asynchronous motors. Polyphase permanent magnet motors are for example formed three-phase and comprise on the rotor side permanent magnets and on the stator side windings. In order to set the rotor into rotation in the for example three-phase stator winding a rotating rotary field must be produced which for example may be effected with the help of an inverse rectifier. With this one differentiates between block commutation and sine commutation.
With block commutation a constant intermediate circuit voltage in dependence on the rotor position with a changing polarity is connected to the respective phase windings. By way of this into the phase windings block-shaped currents are impressed against the trapezoid-shaped motor induction voltage. The motor rotational speed is determined by the exit voltage of the converter whose mean value may be changed by way of pulse width modulation.
With a sine-commutated motor against the sinusoidal motor induction voltage there is impressed a sinusoidal current into the phase windings. In contrast to block commutation the windings are continuously flown through by current. The pulse-width modulated inverse rectifier varies the width of the individual pulses such that as a base oscillation there arises a sinusoidal voltage.
Independently of the type of the commutation one constantly strives to bring the rotating rotary field in the stator to correspond to the magnetic field of the rotor, since then the motor then runs particularly smoothly and with a uniform torque. This may for example be effected in that the phase shifting between the rotary field produced by the voltage impingement and the intrinsic induction of the motor is evaluated and the motor drive-control is correspondingly corrected. This error position between the rotary field rotating in the stator and the rotary field produced in the rotor by permanent magnets or the induction produced by way of this may be evaluated sensorically, for example via Hall sensors attached in the motor near to the stator. Such an acquisition via rotor position sensors is constructionally very expensive and furthermore yet requires expensive evaluation electronics, which however with sine-commutated motors at present may not be avoided, since the voltage induced within the motor phase windings on account of the continuous sinusoidal voltage impingement is practically no longer measurable.
Although with block-commutated motors the intrinsic induction may also be acquired in that this may be measured in the level current-free phase, these block-commutated motors however have the disadvantage that these on account of their hard drive-controlling as a rule have a considerably worse running manner than the sine-commutated motors, in particular a more erratic running and higher running noise. The later is particularly disadvantageous with the application in combination with heating circulatory pumps since the acoustic oscillations of the drive may propagate almost undampened via the heating tube system.
It is to be understood that with the application of such a method for controlling an asynchronous motor the phase position of the intrinsic induction is not brought to correspond with that of the phase winding current but in a predetermined ratio. Otherwise a control with asynchronous motors is usually not effected. It is however known for optimizing the efficiency with a constant rotational speed to minimize the motor current in that the voltage and frequency are changed within predetermined limits until the power consumption of the motor is minimized.
SUMMARY OF THE INVENTION
It is the object of the present invention therefore to provide a smoother-running single or polyphase electric motor, in particular a permanent magnet motor in which the running quietness is increased by control of the phase deviation between the electrical and the magnetic field or of a predetermined ratio of these. With this the design cost and the measurement cost is to be as small as possible.
The part of the object with regard to method is achieved by the features specified in claim
1
. Claim
11
defines the construction of the invention with regard to the device.
The invention thus envisages the measurement of the intrinsic induction of the motor, in particular the voltage induced in at least one motor phase winding, and bringing it to correspond with the winding current of the same motor phase winding or in a predetermined ratio with this, and specifically by way of the fact that according to phase position by way of the control the frequency and/or voltage supplied to the motor in the next control interval is reduced or increased until the phase positions of the intrinsic induction and of the associated phase winding current correspond or essentially correspond or have the predetermined ratio. Then and only then is there to be expected a particularly smooth and low-noise running of the motor, since then the rotating rotary field of the stator winding with respect to the rotating magnetic field of the rotor are at the desired and directed relationship. Usefully with this the control is effected such that the frequency increase or frequency reduction and/or the voltage increase or voltage reduction is fixed in dependence on the measured phase difference for the next control interval, for example proportionally or according to other suitable control curves.
The method according to the invention may for example be applied to a permanent magnet motor as a commutation method. It may also be applied independently thus additionally with the application on another commutation method in order to optimize the running of the motor. With an asynchronous motor the method according to the invention may preferably be applied for optimizing the power and efficiency. The particular advantage of this method in comparison to the usual, previously described method for optimizing the efficiency lies in the fact that with the method according to the invention continuously one always intervenes in a correcting manner, whereas with the known method quasi by way of inputting various settings a good operating point is empirically evaluated.
The result of the control is more exact the more is measured and the more is readjusted. For this reason it is advantageous to evaluate the residual induction (back electromotive force) by way of suitable voltage or current measurement in each motor phase winding in order thus to be able to control also in each motor phase winding.
In order to be able to measure the intrinsic induction, thus the voltage induced in the motor in a simple manner and without complicated electronic evaluation means, it is useful to separate the motor phase windings in which the intrinsic induction is to be measured, from the supply network, so that the measurement is not disturbed by the applied supply voltage. A separation from the network may where appropriate be done away with when one measures only during a comparatively short time interval in which the supply voltage is zero.
It is particularly useful when the measurement of the induced voltage is triggered by the detection of the zero crossing of the motor current in the corresponding motor phase winding, which means that the respective motor phase winding directly after detection of the zero crossing of the associated phase winding current is switched off, i.e. is separated from the supply network and then in a comparatively short time interval the intrinsic induction of this motor phase winding is measured, whereupon this motor phase winding in turn is connected to the supply network. It has been shown that for the measurement of the intrinsic induction only a fraction of a period is necessary so that the separation from the supply network in practice cannot be noticed. The d
Jensen Niels Due
Vadstrup Pierre
Grundfos a/s
Jacox Meckstroth & Jenkins
Masih Karen
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