Electricity: motive power systems – Generator-fed motor systems having generator control – Alternating-current-motor system
Reexamination Certificate
1995-12-07
2001-03-06
Martin, David (Department: 2837)
Electricity: motive power systems
Generator-fed motor systems having generator control
Alternating-current-motor system
C363S170000, C363S175000
Reexamination Certificate
active
06198238
ABSTRACT:
BACKGROUND
1. Field of Invention
The present invention is related to the field of electric power production and consumption. The present invention is further related to field of power electronics and power conversion systems involving AC frequency change.
2. Background—Prior Art
Methods for the interconversion of electrical power at a given input voltage, current, and frequency, to electrical power at a different output voltage, current, and frequency are well known in the art. The simplest device for such interconversion is the common transformer, which may be used to trade voltage for current with little loss of power to inefficiency. A transformer is not capable of altering the frequency of the power delivered to the load, and is not capable of functioning with DC power input.
More complex systems for power interconversion are capable of changing frequency and dealing with DC power input. The earliest such devices were motor/generator pairs and synchronous converters. These devices converted the input electrical power into an intermediate mechanical form, and then converted the mechanical power back into electricity. An AC induction motor, for example would act as the prime mover for a DC generator, thus allowing AC mains power to be converted to DC power. Similar devices may be used for frequency conversion.
Advances in semiconductor technology have made the field of power electronics viable for applications over a wide range of power levels. Power electronics is the application of electronic switching devices such as transistors to problems of power interconversion. Efficiency is of paramount importance in power electronic applications, in contrast to signal level electronics, in which fidelity of signal reproduction is of greater import.
The basic element found in power electronics applications is the switching element. An ideal switching element is either on, meaning that it has zero resistance, or off, meaning that it passes zero current. The transition between on and off in the ideal element is instantaneous. An ideal switching element would dissipate no power, either passing current without loss, or preventing the flow of current entirely. Again, this may be contrasted to signal electronics applications wherein active devices are operated in the linear mode, and therefore must dissipate power. Currently available semiconductor devices for the switching element approach ideal capabilities very well; a single transistor, which because of a slight imperfection must dissipate 10 watts of power as heat, may be capable of controlling several hundred watts of power delivered to a load.
Power dissipation in the switching elements may be divided up into several processes. Conduction loss is loss that occurs when the device is on. Conduction loss is similar to resistance loss; when current flows through a bulk material there is a slight voltage drop, which is accompanied by heat generation. Drive loss is the electrical power required to control the switching element. In something like a bipolar transistor, drive loss may be considerable. Switching losses are losses that result from current being carried during the transitional state between on and off or off and on. During this transitional state, the switch appears to be a high resistance, but considerable current may be carried. Often, switching losses, in particular, turn-off losses, account for the majority of the total losses of an operating device. Simply operating devices more slowly can substantially increase efficiency.
The simplest of the power electronics power converters to understand is the pulse width modulated DC controller, or chopper. This device is simply a switch element placed in series with a load and connected to a source of DC power. The switch element is switched on and off at a rapid rate. By varying the duty cycle of the switching element, i.e., by changing the ratio between ‘on’ state and ‘off’ state, the power delivered to the load may be varied. The variable duty cycle switching element acts as a variable resistance, without dissipating power in the fashion of an actual resistor. Though use of suitable filtering components such as inductors and capacitors, the pulsing nature of the DC power delivered to the load may be eliminated, and a smooth variable DC voltage may be delivered to the load. Additionally, such filter components allow one to trade current for voltage, and allow for output voltages greater than the input voltage. The latter device is known as a switch mode power supply or DC to DC converter. For many power applications, such as resistance heaters or lighting applications, pulsing DC is acceptable and the controller is simple in the extreme.
Of similar complexity is the SCR controller. The
S
ilicon
C
ontrolled
R
ectifier, or SCR, is one of the oldest power electronic components, the semiconductor analog of the thryatron gas discharge tube. SCR devices are available with current capacities in the thousands of amperes and voltage ratings in the thousands of volts, meaning that a single device can switch many megawatts of power. The primary difficulty involved in the use of the SCR is that it is not self-commutating. Once an SCR is turned on by the application of a control pulse, it does not turn off. In order to commutate an SCR, the current flow must be stopped externally. Once current has ceased to flow, the SCR will ‘turn-off’, and will prevent current flow until the application of the next control pulse.
SCR devices may be used to great advantage when controlling AC current flows, wherein the flow of current necessarily stops and reverses twice with each AC cycle. The natural cessation of current may be used to commutate the SCR switch.
An SCR device may be used to control the power delivered to a load as follows: the SCR is connected in series with the load to a source of AC power. It is controlled by a device which is capable of triggering the SCR at a variable time after the beginning of the AC cycle; such devices are commonly called ‘cosine firing circuits’. If the SCR is triggered at the beginning of the AC cycle, then the SCR conducts for the entire half-cycle until the current flow reverses. The full power of the first half-cycle is delivered to the load. If the SCR is triggered near the end of the first half-cycle, then very little power is delivered to the load. The addition of a second SCR allows the second (negative going) half-cycle of the AC waveform to be similarly used. Thus through the use of two SCR devices and a simple delay circuit, control of the AC power delivered to a load is achieved. Packaged devices operating on this principal are used in home lighting dimmers and small motor controls.
Both the DC chopper and the AC SCR based controller may be used to produce an AC power output. Such output may be of use in supplying power to a conventional power distribution system, or for the operation of a motor requiring AC power input. Roughly, the methods described above are used, but the control circuitry is additionally used to vary the output in the same fashion that the desired AC output varies. In order to produce the desired output with reasonable fidelity, the pulse rate of the switching device must be of a higher frequency than the desired output frequency. A device which converts DC input power into an AC output in this fashion is commonly called an inverter.
Well known in the art is a device which consists of a rectification section for the production of DC power from input AC power, combined with a three phase inverter system for the production of AC. This AC power is then used to operate an AC motor. The benefit of a so called DC link converter is that the output AC power is of arbitrary and variable frequency and voltage, thus providing substantial control of motor operations. Such devices allow for control of motor synchronous speed, and with appropriate feedback devices can control the motor based upon speed, torque, acceleration, or other factors.
Also well known in the art is the cycloconverter. This is an SCR based converter for producing a polyp
Borealis Technical Limited
Martin David
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