A grid-leak detector is a combination diode rectifier and audio amplifier used as a detector in vacuum tube A.M. radio receivers. In the circuit, the grid of the detector -- usually a low-mu or medium-mu triode -- is connected to the secondary of the final R.F. or I.F. transformer through a capacitor (100 µµF to 330 µµF -- 250 µµF being typical). This capacitor eliminates R.F. in the output circuit. (An R.F. choke in the plate circuit may also be used to eliminate any transient R.F. in the output circuit.) The grid is negatively biased through a grid leak resistor (1 to 5 megohms -- 2.2 megohms being typical). This resistor may be parallel connected with the grid capacitor, or it may be connected directly to ground. As D.C. current flows through the grid leak resistor, the control grid functions like the plate of a diode, which causes rectifying action to occur. Frequency variations in voltage across the bias resistor are amplified through the tube as in a normal audio amplifier.
A grid-leak detector has considerably greater sensitivity than a diode. The sensitivity is further increased by using a tetrode or pentode with a sharp cut-off control grid instead of a triode. The operation is equivalent to that of the triode circuit except controlled feedback is applied and controlled by adjustment of the screen-grid voltage.
All grid-leak detectors require a plate by-pass capacitor connected to ground to regulate plate current. For triodes, this capacitor should have a value of 0.001 µF to 0.002 µF. For tetrodes and pentodes, this capacitor should have a value of 250 µµF to 500 µµF. Tetrodes and pentodes grid-leak detectors also require a screen grid by-pass capacitor of at least 0.47 µF.
The heyday for grid-leak detectors was the 1920s when battery-operated, multi-dial T.R.F. radios using low-mu triodes with directly heated cathodes was the norm. When indirectly heated cathodes and A.C. powered receivers were introduced in 1927, most manufacturers switched to plate detectors, and later to diode detectors.
Although the regenerative grid-leak detector was one of the more sensitive detectors of its day, its many disadvantages limited it for use only in the simplest receivers. However, this did not stop some manufacturers from using regenerative detectors in their radios. Many of the cathedral radios and other table models made by Philco during the early 1930s used a regenerative detector fed by a superheterodyne converter tube. This allowed the detector to double as a sort of I.F. amplifier, thus saving money by eliminating the need for another tube.
2009年1月22日星期四
Grid bias
Grid bias is a DC voltage applied to electron tubes (or valves in British English) with three electrodes or more, such as triodes. The control grid (usually the first grid) of these devices is used to control the electron flow from the heated cathode to the positively charged anode. Bias point in small-signal applications is set to minimize distortion and achieve sufficiently low power draw. In high-power applications, biasing is typically set for maximum available output power or voltage, with a secondary target of either low distortion or high efficiency.
In a typical voltage amplifier, including power stages of most audio power amplifiers, DC bias voltage is negative relative to cathode potential. Instant grid voltage (sum of DC bias and AC input signal) should never rise above cathode potential to prevent grid-to-cathode currents that overload preceding amplifier stages and may cause severe even-order distortion. High transconductance tubes develop significant grid currents even with small negative bias; in these cases, maximum instant voltage ceiling is lowered to -1.0..-0.5 Volt.
High efficiency Class B+ push-pull amplifiers operate at higher bias points (near zero or even positive values). These designs take care of grid currents through the use of cathode followers or interstage transformers easing current load on the driver stages, and deep negative feedback to minimize distortion.
High power transmitter tubes (oscillators and modulators) are frequently positively biased to maximize radio frequency output. Distortion is minimized by using band-pass filter loads tuned to the desired radio frequency.
Bias voltage is obtained through:
An external voltage source (fixed bias) - a battery or a dedicated DC power supply. When the cathode potential is raised above ground (as in cascode circuits), bias voltage is obtained by tapping into main (positive) plate power supply.
Automatic bias - using a cathode resistor to raise cathode potential above grid (tied to ground) and stabilize plate current;
Grid leak bias - diverting DC grid current through a high value grid resistor.
In a typical voltage amplifier, including power stages of most audio power amplifiers, DC bias voltage is negative relative to cathode potential. Instant grid voltage (sum of DC bias and AC input signal) should never rise above cathode potential to prevent grid-to-cathode currents that overload preceding amplifier stages and may cause severe even-order distortion. High transconductance tubes develop significant grid currents even with small negative bias; in these cases, maximum instant voltage ceiling is lowered to -1.0..-0.5 Volt.
High efficiency Class B+ push-pull amplifiers operate at higher bias points (near zero or even positive values). These designs take care of grid currents through the use of cathode followers or interstage transformers easing current load on the driver stages, and deep negative feedback to minimize distortion.
High power transmitter tubes (oscillators and modulators) are frequently positively biased to maximize radio frequency output. Distortion is minimized by using band-pass filter loads tuned to the desired radio frequency.
Bias voltage is obtained through:
An external voltage source (fixed bias) - a battery or a dedicated DC power supply. When the cathode potential is raised above ground (as in cascode circuits), bias voltage is obtained by tapping into main (positive) plate power supply.
Automatic bias - using a cathode resistor to raise cathode potential above grid (tied to ground) and stabilize plate current;
Grid leak bias - diverting DC grid current through a high value grid resistor.
Alternator synchronization
The process of connecting an AC generator (alternator) to other AC generators is known as synchronization and is crucial for the generation of AC electrical power.
Operation Modes of Alternator
An alternator is an AC generator used mostly at power stations. The alternator is technically known as a synchronous generator. There are two modes of its operation:
Stand-alone operation: The alternator is operated isolated or connected to a DC supply via a rectifier. There is no need for synchronization in a stand-alone operation. An isolated example is a diesel engine-driven alternator used as a standby generator. An automotive alternator is used to recharge the battery with the aid of a three-phase diode rectifier.
Parallel operation: The alternator is connected with other alternators. This mode of operation takes place at power stations where many generators run in parallel. The generators are connected together via transformers and busbars and so a power network or grid is formed. The power grid is like a water pool and the generators are like pipes that supply the pool with water. The water is analogous to energy. The power grid is sometimes referred to as a power pool. The grid is also referred to as infinite busbar as far as synchronization is concerned. However, the infinite bus implies fixed voltage, frequency, phase sequence and phase angle. The grid is composed of different voltages and frequencies. Hence, it is technically said that an alternator is synchronized with the infinite bus.
Conditions
There are five conditions that must be met before the synchronization process takes place. The alternator must have equal line voltage, frequency, phase sequence , phase angle and waveform to that of the infinite bus.
In the past, synchronization was performed manually using three-lamp method. Nowadays, the process is automatically operated and controlled with the aid of synch relays and micro-electronic systems.
Operation Modes of Alternator
An alternator is an AC generator used mostly at power stations. The alternator is technically known as a synchronous generator. There are two modes of its operation:
Stand-alone operation: The alternator is operated isolated or connected to a DC supply via a rectifier. There is no need for synchronization in a stand-alone operation. An isolated example is a diesel engine-driven alternator used as a standby generator. An automotive alternator is used to recharge the battery with the aid of a three-phase diode rectifier.
Parallel operation: The alternator is connected with other alternators. This mode of operation takes place at power stations where many generators run in parallel. The generators are connected together via transformers and busbars and so a power network or grid is formed. The power grid is like a water pool and the generators are like pipes that supply the pool with water. The water is analogous to energy. The power grid is sometimes referred to as a power pool. The grid is also referred to as infinite busbar as far as synchronization is concerned. However, the infinite bus implies fixed voltage, frequency, phase sequence and phase angle. The grid is composed of different voltages and frequencies. Hence, it is technically said that an alternator is synchronized with the infinite bus.
Conditions
There are five conditions that must be met before the synchronization process takes place. The alternator must have equal line voltage, frequency, phase sequence , phase angle and waveform to that of the infinite bus.
In the past, synchronization was performed manually using three-lamp method. Nowadays, the process is automatically operated and controlled with the aid of synch relays and micro-electronic systems.
Indian locomotive class WAP-4
WAP-4 is one of the most important electric locomotives used in India. It is a highly powerful class capable of hauling 26 coaches at the speed of 160 km/h. It is also among the most widely used locomotive.
The locomotive was developed, after a previous class WAP-1 was found inadequate to haul the longer, heavier express trains that were becoming the mainstay of the Indian Railways network. It was introduced in 1994, with a similar bodyshell to the WAP-1 class, but with Hitachi traction motors developing 5000hp (5350 hp starting).
Electricals are traditional DC loco type tap changers, driving 6 traction motors arranged in Bo-Bo fashion. This locomotive has proved to be highly successful, with over 450 units in service and more being produced. Newer examples have been fitted with Microprocessor Controlled diagnostics, Static Convertor units (instead of arnos) and roof mounted Dynamic (Rheostatic) Brakes.
The locomotive can be seen in service across the electrified network of Indian Railways and is homed at 11 sheds (depots).
Design
The loco has a streamlined twin cab carbody design, with top-mounted headlamps. The first 150 or so units had the headlamp mounted at waist level, with the lights being mounted in a protruding nacelle. Later on the headlamps were placed in a recessed nacelle, and from road # 22579 onward, the headlamps were moved to the top. Newer locos also feature larger windshields, more spacious driver cabin with bucket type seats and ergonomic controls. The control panel also features a mix of digital and analog displays in newer units (all analog display in older versions).
The loco features higher power rated silicon rectifiers and indigenously-designed 5400kVA transformer coupled with Hitachi HS15250 traction motors. Starting power is 5,350 hp (3,990 kW), with 5,000 hp (3,700 kW) being supplied continuously.
Original units were weighed 120 tonnes, which was brought down to 112 tonnes through the usage of lighter material.
Some WAP-1 and WAP-6 units were rebuilt to WAP-4 specifications after replacing the bogies & electricals.
Performance
The WAP-4 class hauls 24 coach (1430 tonne) trains at 110 km/h. It is also used to haul the premier Rajdhani & Shatabdi Expresses at 130 km/h. In trials, the loco has achieved a top speed of 169.5 km/h, though Indian Railways limits its top speed to 140 km/h.
With a 24 coach passenger train, the acceleration time / distances are:
110 km/h - 338 seconds (6.8km)
120 km/h - 455 seconds (10.5km)
130 km/h - 741 seconds (20.5 km)
Starting Tractive Effort (Te) - 30800 kg/force
Polyphase system
A polyphase system is a means of distributing alternating current electrical power. Polyphase systems have three or more energized electrical conductors carrying alternating currents with a definite time offset between the voltage waves in each conductor. Polyphase systems are particularly useful for transmitting power to electric motors. The most common example is the three-phase power system used for most industrial applications.
Phases
Main articles: Phase and Phase shifting
In the very early days of commercial electric power, some installations used two phase four-wire systems for motors. The chief advantage of these was that the winding configuration was the same as for a single-phase capacitor-start motor, and, by using a four-wire system, conceptually the phases were independent and easy to analyze with mathematical tools available at the time. Two-phase systems have been replaced with three-phase systems. A two-phase supply with 90 degrees between phases can be derived from a three-phase system using a Scott-connected transformer.
A polyphase system must provide a defined direction of phase rotation, so mirror image voltages do not count towards the phase order. A 3-wire system with two phase conductors 180 degrees apart is still only single phase. Such systems are sometimes described as split phase.
Motors
Polyphase power is particularly useful in AC motors, such as the induction motor, where it generates a rotating magnetic field. When a three-phase supply completes one full cycle, the magnetic field of a two-pole motor has rotated through 360° in physical space; motors with more pairs of poles require more power supply cycles to complete one physical revolution of the magnetic field, and so these motors run slower. Nikola Tesla and Michail Dolivo-Dobrovolsky invented the first practical induction motors using a rotating magnetic field - previously all commercial motors were DC, with expensive commutators, high-maintenance brushes, and characteristics unsuitable for operation on an alternating current network. Polyphase motors are simple to construct, are self-starting, and have little vibration compared with single-phase motors.
Higher phase order
Higher phase numbers than three have been used. A common practice for rectifier installations and in HVDC converters is to provide six phases, with 60 degree phase spacing, to reduce harmonic generation in the AC supply system and to provide smoother direct current. Experimental high-phase-order transmission lines have been built with up to 12 phases. These allow application of Extra High Voltage (EHV) design rules at lower voltages, and would permit increased power transfer in the same transmission line corridor width.
Phases
Main articles: Phase and Phase shifting
In the very early days of commercial electric power, some installations used two phase four-wire systems for motors. The chief advantage of these was that the winding configuration was the same as for a single-phase capacitor-start motor, and, by using a four-wire system, conceptually the phases were independent and easy to analyze with mathematical tools available at the time. Two-phase systems have been replaced with three-phase systems. A two-phase supply with 90 degrees between phases can be derived from a three-phase system using a Scott-connected transformer.
A polyphase system must provide a defined direction of phase rotation, so mirror image voltages do not count towards the phase order. A 3-wire system with two phase conductors 180 degrees apart is still only single phase. Such systems are sometimes described as split phase.
Motors
Polyphase power is particularly useful in AC motors, such as the induction motor, where it generates a rotating magnetic field. When a three-phase supply completes one full cycle, the magnetic field of a two-pole motor has rotated through 360° in physical space; motors with more pairs of poles require more power supply cycles to complete one physical revolution of the magnetic field, and so these motors run slower. Nikola Tesla and Michail Dolivo-Dobrovolsky invented the first practical induction motors using a rotating magnetic field - previously all commercial motors were DC, with expensive commutators, high-maintenance brushes, and characteristics unsuitable for operation on an alternating current network. Polyphase motors are simple to construct, are self-starting, and have little vibration compared with single-phase motors.
Higher phase order
Higher phase numbers than three have been used. A common practice for rectifier installations and in HVDC converters is to provide six phases, with 60 degree phase spacing, to reduce harmonic generation in the AC supply system and to provide smoother direct current. Experimental high-phase-order transmission lines have been built with up to 12 phases. These allow application of Extra High Voltage (EHV) design rules at lower voltages, and would permit increased power transfer in the same transmission line corridor width.
Reservoir capacitor
A reservoir capacitor is a capacitor that is used to smooth the pulsating DC from an AC rectifier.
Performance with low impedance source
The above diagram shows reservoir performance from a near zero impedance source, such as a mains supply. As the rectifier voltage increases, it charges the capacitor and also supplies current to the load. At the end of the quarter cycle, the capacitor is charged to its peak value Vm of the rectifier voltage. Following this, the rectifier voltage starts to decrease as it enters the next quarter cycle. This initiates the discharge of the capacitor through the load.
Performance with significant impedance source
These circuits are very frequently fed from transformers, and have significant resistance. Transformer resistance modifies the reservoir capacitor waveform, changes the peak voltage, and introduces regulation issues.
Performance with low impedance source
The above diagram shows reservoir performance from a near zero impedance source, such as a mains supply. As the rectifier voltage increases, it charges the capacitor and also supplies current to the load. At the end of the quarter cycle, the capacitor is charged to its peak value Vm of the rectifier voltage. Following this, the rectifier voltage starts to decrease as it enters the next quarter cycle. This initiates the discharge of the capacitor through the load.
Performance with significant impedance source
These circuits are very frequently fed from transformers, and have significant resistance. Transformer resistance modifies the reservoir capacitor waveform, changes the peak voltage, and introduces regulation issues.
Electra the Electric Train
Electra the Electric Engine is a fictional character from the rock musical Starlight Express.
Electra is the newest train to enter the yard and, therefore, Control's newest toy. He comes with his own line of components that are loyal to him. He's rich, self-centered and sexy. His eyes are set on Pearl and they race in the heats until she chooses Greaseball. Electra then takes Dinah for the uphill Final after Greaseball dumps her. Dissatisfied with his performance, Dinah disconnects herself from him and he calls for CB to race with him for the re-run Final. In the end, he crashes along with Greaseball and CB.
Character
Electra is, from the moment he enters, seen as some sort of Prima Donna. Even before then, as we meet his components, we aretold he is a "megastar" and "rich, and cool". He also seems to think a lot of himself, and the subtext in his song (along with choreography and his character tick-overs) suggests he is AC/DC.
Inspiration
Electra's character is influenced by David Bowie, an androgynous Rock Star. He, along with Greaseball the Diesel, are the Ugly stepsisters in the original concept of the show being Cinderella for trains. He also reminds some of the Rum Tum Tugger from the musical Cats because both are vain, sexy and rebellious.
Musical numbers
Electra sings "AC/DC", his opening theme and also features in group numbers. In the Original London version he has a second solo "No Comeback", which he sings as he leaves in a rage after losing the Race final to Rusty. This song did not appear in any subsequent productions. He now sings "One Rock 'n' Roll Too Many" with Greaseball and Caboose.
Costume
Electra's costume is based on a palatte of electric blue, silver and red. He is patterned to resemble computer circuitry. His impressive mohawk is usually styled to resemble frayed wire, but when played by John Partridge his head was shaved and painted. His look bears a passing resemblance to other mechanoids, Transformers. As one of the most eye-catching designs, Electra is often used for advertising the show.
Electra's Components are designed to represent elements of a Superstar's entourage. Krupp is the Chauffeur and Bodyguard, represented in his uniformed hat, sunshades and gun holster. Wrench, as a mechanic, wears a welding apron and her hat represents the crane found on repair trucks. Purse wears pinstripes as suits an accountant. Joule's London design reflected her Animal Truck origin, her skintight red/grey costume and white bib resembling Bombalurina from the musical Cats. Volta wears elegant black, her hair styled into a fan, reflecting her nature of a Freezer Truck. All the component's costumes are designed on a restricted colour palatte of Red, Electric Blue, Silver/Grey and Black, this reinforces their relationship to Electra onstage.
In Starlight on Ice, Electra and the components' outfits change. Instead of being colorful, Electra is now bright "warning sign" yellow with a matching mohawk. His look is much sleeker and bears no resemblance to the original design. The components wear very similar outfits which are yellow and silver.
Electra's make-up
Electra's make-up is an assortment of blue, silver and red glittered stripes, patterns and lipstick, as well as a pair of long, silver eyelashes.In the London production, his look was less camp, and he had a black and white checkered pattern on one side of his face.
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