Jump to navigation Jump question: Q1. Figure 1 shows an AC-DC converter. Assume the capacitance C is large and its voltage is 100 V… search This article is about power in AC systems. For information on utility-supplied AC power, see Mains electricity.
The blinking of non-incandescent city lights is shown in this motion-blurred long exposure. The AC nature of the mains power is revealed by the dashed appearance of the traces of moving lights. Power in an electric circuit is the rate of flow of energy past a given point of the circuit. In alternating current circuits, energy storage elements such as inductors and capacitors may result in periodic reversals of the direction of energy flow.
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The portion of power due to stored energy, which returns to the source in each cycle, is known as reactive power. If the load is purely resistive, the two quantities reverse their polarity at the same time. If the load is purely reactive, then the voltage and current are 90 degrees out of phase. For two quarters of each cycle, the product of voltage and current is positive, but for the other two quarters, the product is negative, indicating that on average, exactly as much energy flows into the load as flows back out.
There is no net energy flow over each half cycle. Apparent power is the product of the rms values of voltage and current. Apparent power is taken into account when designing and operating power systems, because although the current associated with reactive power does no work at the load, it still must be supplied by the power source. Conductors, transformers and generators must be sized to carry the total current, not just the current that does useful work. Conventionally, capacitors are treated as if they generate reactive power and inductors as if they consume it. If a capacitor and an inductor are placed in parallel, then the currents flowing through the capacitor and the inductor tend to cancel rather than add. The complex power is the vector sum of active and reactive power.
The apparent power is the magnitude of the complex power. S is the complex power and the length of S is the apparent power. Reactive power does not do any work, so it is represented as the imaginary axis of the vector diagram. Active power does do work, so it is the real axis. Where V denotes voltage in phasor form, with the amplitude as rms, and I denotes current in phasor form, with the amplitude as rms. Also by convention, the complex conjugate of I is used. These are simplified diagrammatically by the power triangle.
The ratio of active power to apparent power in a circuit is called the power factor. For two systems transmitting the same amount of active power, the system with the lower power factor will have higher circulating currents due to energy that returns to the source from energy storage in the load. These higher currents produce higher losses and reduce overall transmission efficiency. 32 percent is reactive and has to be made up by the utility. Usually, utilities do not charge consumers for the reactive power losses as they do no real work for the consumer. This section does not cite any sources. In a direct current circuit, the power flowing to the load is proportional to the product of the current through the load and the potential drop across the load.
Energy flows in one direction from the source to the load. In AC power, the voltage and current both vary approximately sinusoidally. When there is inductance or capacitance in the circuit, the voltage and current waveforms do not line up perfectly. Energy stored in capacitive or inductive elements of the network give rise to reactive power flow.
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Reactive power flow strongly influences the voltage levels across the network. Voltage levels and reactive power flow must be carefully controlled to allow a power system to be operated within acceptable limits. Stored energy in the magnetic or electric field of a load device, such as a motor or capacitor, causes an offset between the current and the voltage waveforms. A capacitor is an AC device that stores energy in the form of an electric field. As current is driven through the capacitor, charge build-up causes an opposing voltage to develop across the capacitor. This voltage increases until some maximum dictated by the capacitor structure. Induction machines are some of the most common types of loads in the electric power system today.
These machines use inductors, or large coils of wire to store energy in the form of a magnetic field. When a voltage is initially placed across the coil, the inductor strongly resists this change in current and magnetic field, which causes a time delay for the current to reach its maximum value. Transmission connected generators are generally required to support reactive power flow. For example, on the United Kingdom transmission system generators are required by the Grid Code Requirements to supply their rated power between the limits of 0.
85 power factor lagging and 0. 90 power factor leading at the designated terminals. By making decisive switching actions in the early morning before the demand increases, the system gain can be maximized early on, helping to secure the system for the whole day. To balance the equation some pre-fault reactive generator use will be required. While active power and reactive power are well defined in any system, the definition of apparent power for unbalanced polyphase systems is considered to be one of the most controversial topics in power engineering. Originally, apparent power arose merely as a figure of merit. The 1920 committee found no consensus and the topic continued to dominate discussions.
In 1930 another committee formed and once again failed to resolve the question. Further resolution of this debate did not come until the late 1990s. A perfect resistor stores no energy, so current and voltage are in phase. For a perfect capacitor or inductor there is no net power transfer, so all power is reactive.
Where X is the reactance of the capacitor or inductor. This definition is useful because it applies to all waveforms, whether they are sinusoidal or not. This is particularly useful in power electronics, where nonsinusoidal waveforms are common. In general, we are interested in the active power averaged over a period of time, whether it is a low frequency line cycle or a high frequency power converter switching period. The simplest way to get that result is to take the integral of the instantaneous calculation over the desired period. This method of calculating the average power gives the active power regardless of harmonic content of the waveform. In practical applications, this would be done in the digital domain, where the calculation becomes trivial when compared to the use of rms and phase to determine active power.
Since an RMS value can be calculated for any waveform, apparent power can be calculated from this. For active power it would at first appear that we would have to calculate many product terms and average all of them. However, if we look at one of these product terms in more detail we come to a very interesting result. Therefore, the only product terms that have a nonzero average are those where the frequency of voltage and current match. Archived from the original on 2015-05-12. IEEE 100 : the authoritative dictionary of IEEE standards terms. Archived from the original on 5 April 2017.
Archived from the original on 2015-10-25. This extremely small Tube Tester measures tube characteristics in a pulsed mode. My current project in the form of a web-log. In contrast to most of these tube-lovers I have, at least up to now, little interest in tube amplifiers. I simply do not have the ears for it! I am however fascinated by the technology of these fragile and romantic devices and I love to read and write about their history.
It also applies to this project. Once the idea of a pulsed tube curve tracer was conceived, all the parts of the system seemed to fall into place as if they had been waiting to be put together. 1 Symbolic circuit diagram of the µTracer. For clarity the processor and other digital parts have been omitted. In a normal curve tracer these would be by far the most difficult and by far the heaviest parts of the tester. 1 A few snapshots from a very early publication describing a curve-tracer mechano-electrical curve tracer . The center picture shows an example a set of Ia-Va curves.
The right picture depicts the screen-grid current of the same tube under the identical measurement conditions. Philips Technical Review of 1938 . The left figure depicts the circuit diagram of the curve tracer. The anode voltage is derived from a 500 Hz high voltage generator, most likely a mechanical alternator. The stepping voltage for the control grid was generated by a rotating commutator.
2 A description of this apparatus designed to measure transmitting triodes appeared in the Philips Technical Review of 1939 . A year later in the same periodical an article is published describing a curve-tracer especially designed to characterize transmitter triodes . During normal operation of transmitter triodes, it is quite common that the control grid becomes positive resulting in a significant grid current. It is therefore important that these tubes can also be characterized in that biasing regime. 3 Click Here to view some more pictures of this amazing beast! After the war Philips emerged as the dominant tube manufacturer in Europe.
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The pentode patent in combination with its enormous production capacity all over Europe made its position as the foremost player in this field unchallenged. 4 As an example of what the curve-tracer of Fig. 3 was capable of these measurements on an ECL80 are shown. Left, the pentode section, middle, the pentode switched as tetrode and right, the triode section. The whole monster comprised some 200 tubes.
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Assuming an average life-time expectancy of 10. 1 depicts the test circuit that was used in the first experiments to ascertain whether a voltage in the range of 300-400 V can be achieved with a simple boost converter. The heart of the boost converter is formed by inductor L1, MOSFET T3, and diode D2. In the test circuit of Fig. 1 this pulse is generated by the differentiating network formed by C1 and R1, Diode D1 protects gate N1 against negative spikes caused by negative flanks of the square wave input signal.
T2 form a buffer to drive T3 with as steep as flanks as possible. The charging time of the output capacitor strongly depended on the input frequency. MOSFET be driven directly from the processor? In the original plan I had, the anode current was measured my measuring the cathode current via the voltage drop over a cathode resistance. This in reality measures the sum of the anode current and the screen current. The most left diagram in Fig. 2 depicts the pnp current mirror as I have intended it for this application.
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Note that T1 is switched as a diode, more precisely as the diode in the emitter-base junction. What is the maximum screen current that can be handled. The high-side voltage switch which pulses the high voltage to the anode and screen-grid of the tube is one of the key components of the circuit. It should be able the switch on the voltage quickly enough, and should have a low voltage drop.
Basically there are two options for this switch: a high voltage p-type MOSFET or a high voltage pnp transistor. I know that most people would choose for a MOSFET. They can easily handle the currents involved and can have a very low on-resistance. When the switch is pressed, they output of of N2 becomes high. The differentiating network C1, R3 in combination with N3 reduces the pulse length to ca. N7 buffer and invert this pulse.
The high-side switch itself is formed around T1 and T2. 4 First pulsed measurements on an EL84. I really could not resist trying the pulse circuit in combination with a tube. The first thing observed was that without C3 and C4 the tube immediately oscillated. 5 First pulsed measurements on an EL84.
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Having realized this, the left circuit of Fig. A much better solution is circuit B in Fig. Here T2 is used as a current source whose current is determined by the amplitude of the input pulse and the value of R1. For the control-grid bias it was assumed that a range of 0 to -20 V would be sufficient to cover most tubes.
This bias needs to be applied with respect to the cathode. Since the cathode current is measured by the voltage drop over a series resistance, some kind of circuitry was needed to correct for this voltage drop. A voltage drop over the cathode resistance was simulated with voltage source. An arbitrary voltage of 1 V was chosen.
Resistors of 12k1 and 47k were used because there were readily available. 9 shows the circuit that was used to test the circuit. The working of the circuit is simple. The BC548 npn is driven with the same 15-20 us pulse that is used for the high-voltage converters. I did have a whole bunch of BD138 transistors. Agreed,a power PMOS transistor would perform better in this circuit, but the performance is not that critical so that a simple BD138 performs good enough.
The voltage dividers reduce the high voltages so that they can be measured with the on-chip AD converter which has an input voltage range of 0-5V. They consist of a simple resistive voltage divider and a protection diode. The protection diode ensures that when, for any reason, the input voltage is too high, the voltage on the input of the AD converter is clamped to Vdd. Here Vin is the high voltage input, Vout the input voltage of the AD converter, and n the output value of the converter. To ensure the in the datasheet of the AD converter specified acquisition time, the impedance of the circuit connected to the AD converter should not be much higher than 2k. 1 One of the best parts of any project: bread board testing!
In this case the thyristor protection circuit is being tested. Some people are of the opinion that instead of actually building and testing a circuit, you could just as well simulate them! To a certain degree I agree with them. If you have good component models, simulation is a valuable and powerful tool. Most simulators and device models do not, or at best very poorly, include thermal effects. The same holds for junction breakdown behavior. 2 When all the difficult circuit parts have been tested on breadboard, everything comes together on perfboard.
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When you build up the circuit on a perfboard, do it step by step! The larger the circuit, the bigger the chance that you will make a mistake, and the more difficult it becomes to trace that mistake. In all projects that use a microcontroller, I almost always start at that end of the circuit. Once you have the mico-controller up and running, testing of the rest of the circuit becomes easier. The digital part, basically only the microcontroller and RS232 driver. The software for the microcontroller, which controls the analog electronics, performs the measurements and communicates with the PC.
However, at this moment the analog part of the circuit as I have it in mind is shown in Fig. I you have read the previous section, the circuit will look pretty familiar and straightforward. The digital part of the circuit could hardly by more straightforward. For maximum speed and accurate serial communication timing an external 20 MHz Xtal was used. MAX232 taking care of the necessary level translations. There really is not anything more to tell about it.
I decided to let the PC do the entire user interface so that the tasks for the microcontroller are rather limited. The microcontroller in the first place has to receive the settings from the PC. When the settings are received, it will charge the buffer capacitors in the boost converters and set the grid bias. When the anode, screen and grid voltages have reached their set-point, all boost converters are switched off to reduce noise, and the electronic switches are closed.