Electricity: motive power systems – Limitation of motor load – current – torque or force
Reexamination Certificate
2001-02-26
2003-01-21
Masih, Karen (Department: 2837)
Electricity: motive power systems
Limitation of motor load, current, torque or force
C318S466000, C318S803000, C388S800000, C388S806000
Reexamination Certificate
active
06509705
ABSTRACT:
The present invention relates to a power supply for DC motors, in particular for actuators for use in the adjustment of tables, beds and similar furniture, said power supply comprising a transformer having a primary side for connection to a mains voltage (AC) and a secondary side with a rectifier and a buffer capacitor having a terminal for at least a DC motor and typically also a terminal for a control.
Linear actuators as independent installation components are used e.g. in furniture, hospital and nursing equipment, machinery and within the manufacturing industry for performing various movements and positional adjustments.
For a good working position at tables or table-like arrangements it is important that tho level of the tabletop can be adapted to the individual person and his working position, sitting or standing. This has previously been accomplished in connection with more sophisticated tables and equipment where the costs of the height adjustment itself have been of inferior importance. However, the focussing in recent years on working environment has meant that also more ordinary working tables, such as desks, must be adjustable in height. As a consequence, the price of the height adjustment has become a decisive factor. This had led to a new product field, viz. lifting columns for tables, it being intended to meet the specific requirements that are associated with tables. These lifting columns are typically based on linear actuators as a drive assembly incorporated in the lifting column, which usually consists of two telescopic links. With a control panel, which may a switch in its simplest form, the user can adjust the level of the tabletop as desired.
For reasons of safety and costs, the electrical equipment of linear actuators is a low volt equipment. In addition to this being justifiable in terms of safety, it means lower component costs and simpler procedures of approval, which allows the prices of the products to be kept at an acceptably low level.
As a power supply for linear actuators use is typically made of a transformer with full-wave rectification and smoothing capacitor, which is a simple, reliable and inexpensive power supply, but still has the drawback that the motor speed decreases relatively drastically with an increased load on the actuator. In most situations it is seen as natural and to be expected that the speed of the structure is reduced in step with the load. For lifting columns for tables this relatively great difference in speed between an empty table and a heavily loaded table is so great that the users see this as a drawback. Concretely, the task is thus to minimize this difference in speed for lifting columns within the given framework. For competitive reasons, the solution must be neutral in terms of costs or essentially neutral in terms of costs.
Since it is desired to leave the basic structure of the actuators unchanged, it is practically just possible to consider the power supply and the control of the motors. As the manufacturing costs relative to the solutions applied must be substantially neutral, then e.g. frequency control and switch mode power supply are left out beforehand, as these are expensive solutions.
Accordingly, the object of the invention is to provide a power supply which, in terms of manufacture, is not more expensive or noticeably more expensive than those traditionally used for linear actuators, and which addresses the problem of declining performance without exceeding the permissible voltages for a low voltage structure.
With this as a starting point, it has been contemplated how the utilization of the available output of a given transformer can be optimized while observing the permissible voltages. In accordance with the Danish high power regulation, the voltage on the secondary side of the transformer may not exceed 60 volts, while the voltage in a low volt installation may generally not exceed 42.4 volts.
The invention provides a power supply of the type stated in the opening paragraph which is characterized in that it comprises voltage limiting means coupled in parallel with the motor and connected to switch means, so that the motor is connected on a first part of the sine half-waves, but is disconnected when the voltage determined for the voltage limiter is reached.
Hereby, minimal losses may be achieved in the power supply, while ensuring a good mean voltage as the voltage does not exceed an upper permissible value. The voltage is thus limited in the power supply at lower loads relative to the traditional power supply. When the load reaches a certain size, the output decreases like in the traditional power supply as the transformer cannot supply sufficient voltage. In case of a linear actuator, this means that the speed decreases considerably less at low loads than with a traditional power supply, while the decrease in speed at greater loads, where the transformer cannot supply sufficient voltage, is of the same size as with a traditional power supply. For example in case of lifting columns for tables, this is a significant improvement over the known structures. That the speed decreases relatively more on the last section toward maximum load is readily logical to the user, and is instrumental in making the user hesitate to overload the structure.
The voltage limiting means are expediently formed by a Zener diode, which is a reliable and relatively inexpensive component. A diode having a Zener voltage corresponding to the desired limitation of the maximum voltage is selected. Preferably, a diode having a Zener voltage of or about 39 volts is selected, which gives a suitable safety margin up to the permissible 42.4 volts.
The switch means are expediently a FET transistor having a gate arranged such that current is supplied to the motor when the gate is high, while current to it is interrupted when the gate is low. FET transistors are likewise a reliable and inexpensive component. The FET transistor may be controlled by a controller consisting of transistors and resistors. The circuit is realized such that the FET transistor may be coupled entirely ON and OFF in a simple manner, as it is arranged as a low-side FET transistor. To avoid power losses in the FET transistor, a transformer with an expedient frequency characteristic may be selected.
As a particular feature, FET transistors are protected against external transients (surge and burst pulses) by a single Zener diode. The same Zener diode serves as a snubbe limiter for the secondary side of the transformer. The diode ensures that the FET transistor is controlled to suppress these undesired spikes. The main energy from these undesired voltage spikes is even transferred to the buffer capacitor. In addition it is ensured that the FET transistor is not subjected to voltages above specification limits.
Within the given limits of the voltages the invention allows transformers with a higher power (higher voltage) to be used—in concrete cases 20%. This potential may be utilized for a higher speed. For transformers that can be switched from 230 volts AC to 115 volts AC, the extra power may be also be utilized such that the input of 115 volts AC may comprise 100 volts AC.
As another option, the actuator speed may be increased by lowering the motor impedance or increasing the spindle pitch. For reasons of costs, motors with permanent magnets are typically used in linear actuators. To protect the magnets against demagnetization because of increased start current, a soft start may be made—current limitation or PWM controlled start of the motor. Soft start may be realized by allowed the motor to idle for a period prior to stop, controlled by HW (hardware) or SW (software).
It has been found, however, that soft start of the motor may be provided in a simple and inexpensive manner with a measuring resistor arranged in the controller in the motor output after the buffer capacitor. During start, the measuring resistor will limit the voltage to the motor until its full EMF (electromotive force) has been reached.
Another way of utilizing the invention is to regulate to a lo
Bastholm Jeppe Christian
Kristiansen Kjeld
Dykema Gossett PLLC
Linak A/S
Masih Karen
LandOfFree
Power supply for DC motors does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Power supply for DC motors, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Power supply for DC motors will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3054578