Electricity: motive power systems – Synchronous motor systems – Braking
Reexamination Certificate
2001-06-28
2003-05-20
Dang, Khanh (Department: 2837)
Electricity: motive power systems
Synchronous motor systems
Braking
C318S254100, C310S06800R
Reexamination Certificate
active
06566839
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the operation of variable speed electric motors, and more particularly to a switched capacitor resonance circuit for increasing the available speed of a variable speed electric motor without increasing the voltage supplied to the motor, or the current supplied to the motor circuit.
2. Description of the Prior Art
Variable speed electric motors, particularly stepper motors, are employed in a wide variety of applications where precise movements are desired. A typical application of a variable speed motor is in a closed circuit television (CCTV) system where a camera unit is mounted on a movable base. Movement is imparted to the base using one or more variable speed motors which cause the camera to scan, pan and/or tilt.
Because electric motors are inherently inductive, at high speeds the supply of voltage must be great enough to drive the AC current through the motor windings. A typical stepper (or DC brushless) motor will have a given inductance based on the number of motor windings, with more windings providing additional inductance. Additional windings are preferred because they allow for more strength, or torque, in the motor. Thus, a typical strong electric motor will have a higher inductance than a weaker electric motor for equal amounts of current.
As an electric motor is operated, its speed (RPM or frequency) may be increased according to the needs of the application. The usable speed of an electric motor is determined by available torque which is directly proportional to current driven through the motor. The current in the motor is limited by resistance of the windings, the inductance of the windings, and the back EMF of the motor. As the frequency of the motor increases, so also does the reactance in the motor circuit, the reactance being a function of the inductance (i.e. number of windings) of the motor itself. At a given high frequency, the continually increasing reactance begins to affect the ability of current to flow through the motor circuit. As the frequency increases beyond this point, the supply voltage is unable to support the necessary current, or torque, desired in the circuit such that the motor speed begins to drop off. Eventually, a point is reached where the circuit is unable to accelerate the motor further. At such a high speed sufficient current cannot be forced into the motor and torque will fall to unusable levels.
One solution to this problem is to increase the supply of voltage provided to the circuit; however, in many systems this is not feasible or increased voltage is simply not available. Another solution is to change to a less inductive, higher current motor (i.e. a motor with fewer windings). A less inductive motor will have lower back EMF and lower inductance; however, it will not be as strong, and will require a higher drive current to reach the same torque values as a more inductive motor. Unfortunately, the necessary higher current for such a motor may not be feasible or available to the motor circuit.
It is therefore desirable to provide a way to increase the torque in a high inductance electric motor where limited voltage or current is available to the motor circuit (i.e. increasing motor torque without increasing the voltage supplied to the motor circuit, and without using a weaker motor which would increase the overall current required by the motor circuit).
SUMMARY OF THE INVENTION
The present invention provides a circuit that is useful for increasing the torque of electric stepper (and brushless DC) motors, allowing such motors to be operated at higher speeds and at higher torque without raising the supply voltage. The present invention also allows for a wider dynamic range of speed and torque to be realized. This is accomplished by connecting one or more capacitors in series with each motor winding, and selectively activating the capacitors at higher speeds where the frequency of the motor resonates with the capacitor(s). This allows higher currents to flow through the windings resulting in higher torque. Since the motor winding inductance is known, the capacitor value (or values) can be calculated at the resonant frequency (or frequencies—speeds) at which the motor will be running.
The driving circuitry must have the capability of switching the capacitor(s) in and out of the motor circuit. This is required because at zero speed when the motor is stopped, a DC current must be present in the winding to hold torque. For this condition, the capacitor cannot be connected to the motor winding (cannot be in series). The same holds true for lower speeds, since the capacitor(s) will have a very high reactance at low speeds and will actually hinder acceleration of the motor, resulting in very low winding current and torque. Because of these conditions, the resonant frequency of the motor/capacitor(s) and the point(s) where the capacitor(s) is/are switched into the circuit must be chosen carefully.
The following illustrative discussion is based on switching a single capacitor into the motor circuit; however, the present invention includes the potential use of multiple capacitors, used one after another and at different frequencies, to further extend the motor torque.
At the start of operation the capacitor switching device is closed such that the capacitor is out of the circuit, and the speed is zero. The switching device may be a mechanical relay, optical relay, or other solid state switch. If the switch is open at startup, the motor will not start because the capacitor will not pass DC current required at startup. The motor speed (frequency) then increases as the motor accelerates. Eventually, the motor will accelerate to a frequency A (the drop-off frequency) where the reactance from the motor itself begins to prevent the necessary current from reaching the motor. Current delivered to the motor drops off as it is accelerated beyond drop-off frequency A until it reaches frequency C where no further acceleration occurs (drop-out frequency).
A capacitor value is chosen which resonates with frequency B (the resonant frequency). Resonant frequency B is deliberately selected to be higher than drop-off frequency A, but lower than cut-off frequency C. There is a range of frequencies on either side of resonant frequency B (both above and below frequency B) which approach the resonance of frequency B. Selection of resonant frequency B establishes a point somewhere between drop-off frequency A and resonant frequency B where the dropping frequency of the motor crosses into the range of frequencies approaching resonant frequency B. It is at this cross over point that the capacitor is switched on in the circuit. Because the capacitor resonates with the motor at resonant frequency B, it reinforces the rotation of the motor within a range of frequencies on either side of resonant frequency B without the need for additional voltage. This allows the motor to accelerate to and slightly beyond resonant frequency B, therefore increasing current and torque, without increasing the voltage.
The value for the capacitor is selected based on the available current and voltage, and the number of windings in the motor, and is established so that it has a resonant frequency B at a point that is above the drop-off frequency A of the motor, but not so high that it is beyond the cut-off frequency C, preferably closer to frequency C than to frequency A. When the lower end of the frequency range of resonant frequency B is reached, this is detected by microprocessor circuitry, and the capacitor is switched into the circuit. Additional capacitors may also be included in the circuit above resonant frequency B, establishing higher resonant frequencies D, E, and so on until the back EMF of the motor prevents additional acceleration. The next sequential capacitor is switched into the motor circuit at the same time that the previous capacitor is switched out of the circuit so that only one capacitor is activated at a time. As motor speed is reduced, the capacitors are switched back
DaSilva Daniel Robert
Pretzer John
Waehner Glenn
Dang Khanh
Miller Mark D.
Pelco
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