Electricity: motive power systems – Switched reluctance motor commutation control
Reexamination Certificate
1999-10-20
2001-05-29
Dang, Khanh (Department: 2837)
Electricity: motive power systems
Switched reluctance motor commutation control
C318S700000, C318S721000
Reexamination Certificate
active
06239563
ABSTRACT:
The invention relates to control of the start-up and operation of a single-phase synchronous motor with a permanent-magnet rotor, taking into account disturbance cases and the maximum permissible demagnetization current, in particular for driving the circulation pump in dishwashers and the compressor in refrigerators.
PRIOR ART
Purely passive single-phase synchronous motors which are used, for example, for driving small pumps and are operated without any closed-loop control are in widespread use. Such synchronous motors have an extremely simple construction, and thus a low cost outlay. As a result of the difficulty in start-up owing to the mass inertia of the parts to be moved, these motors are limited to a rating of about 30 W. They are thus not suitable for circulation pumps in dishwashers, unless additional measures are taken.
With such single-phase synchronous motors with permanent-magnet excitation, reliable start-up is in principle possible only if special measures, such as airgap forming or additional magnets or the like, are taken to ensure that the flux vector of the rotor magnet in the magnetic rest position is not parallel to the flux vector of the stator.
The magnetic rest position of the rotor is in this case the position of the rotor at which its magnetic field, without any additional current flow, has the lowest energy content and at which it remains stationary in the absence of friction influences.
One measure in order to increase the rating of such a motor above 30 W may be, for example, a special coupling between the motor and pump, in which case the pump is also of special design, although start-up takes place passively without additional electronic aids.
The performance range of such arrangements of synchronous motors is also limited, in particular by the irregular torque profile in the start-up range.
Furthermore, their efficiency is heavily dependent on the applied voltage, as a result of which the synchronous motor for a known load must also be designed for the worst-case operating voltage range.
Patent Application DE 195 33 344, which has not yet been published, describes an apparatus for controlling a single-phase synchronous motor, in which the single-phase synchronous motor includes at least one stator winding which is connected in series with the AC voltage source, a sensor for measuring the magnetic field of the stator, preferably a Hall sensor, a sensor for measuring the current, a sensor for measuring the supply voltage, an electronic switch, preferably one which changes to the switched-off state (for example a triac) at the current zero-crossing, and an electronic circuit which logically links the signals from the sensors and controls the switch appropriately.
With this apparatus, during the starting process, means for phase-gating control of the supply voltage (electronic switch and electronic circuit) in accordance with the polarity of the magnetic-field sensor and the required rotation direction are enabled and the electronic switch is switched after the end of a delay time, so that a driving torque is then produced in the required rotation direction.
During run-up, time intervals for enabling the means for phase-gating control are defined from the magnitude and the gradient of the signal of the magnetic-field sensor, and the AC voltage is switched on after the end of a delay time.
During synchronous running, time intervals which occur in a corresponding manner, likewise periodically, are defined, in which the means for phase-gating control are enabled, and the AC voltage is switched on after the end of a delay time.
One disadvantage arises in the starting phase from the fact that such a single-phase synchronous motor has a good and a poor starting direction in the magnetic rest positions.
The good starting direction is in this case that in which the flux vector of the rotor magnet (see
FIG. 1
) points in the opposite direction to the flux vector of the stator. Its speed can then rise virtually over 180°, and it can overcome the dead point at which the flux vectors of the rotor and stator are parallel. In the other direction, the angle to the dead point is only a few degrees, and the probability of overcoming the dead point is correspondingly poor.
In the case of this apparatus, a confident assumption is now made that the rotor starts to carry out an oscillating movement, particularly in the poor starting direction, which is increased by resonance effects and, at a specific amplitude, allows the rotor to rotate. This method may require a current which causes the permanent magnet to be at least partially demagnetized. This is particularly true if there are friction effects which prevent the desired resonance effect due to excessive damping, or if poorly matched rotor and load rotation masses reduce this effect.
The run-up is also prevented, in particular, if the rotor sticks to some extent in the bearings, which cannot be excluded after a lengthy shutdown time. An additional disadvantage results from the characteristics of Hall sensors. These have a large offset voltage and high temperature drift, which make it possible to detect the position of the magnetic rest position unambiguously only with major complexity, which is not feasible here for price reasons.
The process of running such a motor up to synchronous running is governed by the fact that no driving current pulses are available over relatively long time intervals, due to the discrepancy between the power supply frequency and the rotor rotation speed. Thus, in order to provide the required mean drive torque, the driving current pulses which are present must be greater. If one of these current pulses now occurs in such a manner that the flux vectors of the rotor and stator are essentially parallel and opposite, the rotor magnet can be at least partially demagnetized, if the current pulse and thus the magnetic field are of sufficient intensity.
A disadvantage of the said apparatus is that it has no device to limit the current in these areas of risk.
A further disadvantage during run-up and synchronous operation results, in particular, from the fact that the means for phase-gating control are enabled first of all, and the means are then switched on after a delay time. During this delay time, the rotor continues to rotate and it is possible for the current which builds up to produce a braking effect, or for the rotor to rotate into a region in which there is a risk of demagnetization.
This may be the case not only at rotation speeds greater than the synchronous rotation speed, but also if the angle at the time when the means for phase-gating control are enabled is not ideal.
A disadvantage of the said apparatus also results from the fact that periodically occurring time intervals are defined for enabling the means for phase-gating control. Time intervals imply the use of timing elements, for example timers, which, however, are no longer in synchronism with the rotor rotation during acceleration or deceleration of the motor, and thus lead to triggering errors which can cause braking of the motor or, as is even worse, can increase the risk of demagnetization.
A further disadvantage results from the use of a triac, particularly if the moment of inertia of the rotor and the connected pump is small and the motor is thus highly dynamic, as a result of which over-speeding is possible. In this case, at rotation speeds greater than the synchronous rotation speed, there is a very high risk (arising from a current pulse which can no longer be switched off if a triac is used) of the rotor ending up, due to its fast rotation, in an angle range in which the magnetic field resulting from the current pulse can lead to demagnetization of the rotor.
A further disadvantage results from the use of cheap Hall sensors. Owing to the wide scatter in their characteristics, the determination of the rotor magnetic field in mass-produced motors is likewise subject to wide scatter. This leads to errors, for example when measuring amplitude values for defining times at which the electronic switch is intended to be tr
Dang Khanh
Kinberg Robert
Venable
LandOfFree
Electronic starting and operating control system for a... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Electronic starting and operating control system for a..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Electronic starting and operating control system for a... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2551693