For some types of forced commutation, the thyris to r is reverse biased for a specified time, and must then block forward voltage. Under these circumstances, other fac to rs which will be discussed under forced commutation must be taken in to account. The literature on the physics, characterization, rating, and application of thyris to rs is voluminous. The most comprehensive reference is: F. Gentry, et al. The his to ry of thyris to r development up to is interestingly summarized in: J.
Harnden, Jr. Blocking Voltage Ratings for Commercial Thyris to rs. The design of commercial thyris to rs involves tradeoffs between power controlling capability and speed. In an optimum device, both can be increased simultaneously only by increasing the silicon area. The development of larger and larger devices accounts for much of the progress that has been made in thyris to rs, but the largest area that is feasible at any specified time is limited by the technology of growing silicon crystals of adequate quality, by the differential expansion between silicon and package materials, and by the heat density which can be withdrawn through the package.
Thyris to rs having a silicon diameter of 50mm are now readily available, and mm devices are being developed. Although the subject of thyris to r design tradeoffs and their interactions with circuit design tradeoffs is quite complex, some simple relationships between the voltage, current, and turn-off time ratings can be illustrated.
A survey of high-power commercially available thyris to rs is summarized in Figure 3.
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The interaction between the ratings of state-of-the-art thyris to rs is plotted in Figure 3. V RM, with t q shown as a running parameter. Microseconds 50 20 10 5 Figure 3. Most devices cluster about the line whose equation is: Thus we have deduced an empirical relationship between the power controlling capability and the speed of commercially available thyris to rs.
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Rated Power Capability for Commercial Thyris to rs. Circuit Functions Performed by Type 2 Switches Type 2 switches allow forward current to be turned on at arbitrary times but do not permit arbitrary turn-off or commutation. The basic circuits for all of these functions are the same, since all are based on the switching matrix. The circuits are distinguished primarily by which switches are omitted from the general matrix and by the timing of the switching control signals, and secondarily by the presence of freewheeling diodes, various interconnections of transformer windings, ect.
These circuits have an obvious relationship to diode rectifier circuits and will be discussed first. If each thyris to r receives a firing pulse at the instant when its anode becomes positive with respect to its cathode, no change occurs and the circuit operates exactly as it did as a diode rectifier. This change is easily implemented by building a controllable phase delay in to the switching control signal genera to r. Remember that all four simplifying assumptions are imposed on the analysis of this circuit.
In particular, the DC current is assumed to be constant over each cycle. If firing of one set of thyris to rs is delayed, commutation is also delayed and the other set must continue conduction in the meantime. If the AC source reverses polarity, the forward bias on the thyris to rs to maintain this conduction will be supplied by the kickback voltage of the filter inductance.
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The voltage and current waveforms are shown in the figure for this case. The ripple voltage appearing across the filter inductance is shaded, and as before its average value over a full cycle must be zero. Therefore the average load voltage is: It is seen that the phase delay has reduced the average load voltage from the corresponding value for a diode rectifier. Therefore the converter has an increasingly lagging power fac to r at the AC terminals as the output DC voltage is reduced.
Since the DC current is constant, the instantaneous power dissipated in the load resis to r is the same throughout each half cycle. However, the power interchange between the other parts of the circuit changes continually. When V out exceeds V d, the voltage across the inductance is positive, and both the inductance and the load resis to r are absorbing power from the AC source.
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When V out drops below V d, the inductance starts to supply part of the power to the resis to r. Then when the source voltage reverses polarity, the inductance supplies instantaneous power to the load resis to r and to the AC source. As with the one-pulse diode rectifier having an inductive load, the AC source receives instantaneous power during part of each cycle even through the net power flow over a cycle is from the AC source to the load resistance. Therefore the current in the load resis to r also drops to a very low value, ideally zero.
Actually this case is only of hypothetical interest with a passive load because as soon as the current drops below the holding current, the thyris to rs refuse to fire. But now assume that the passive load is replaced by an active load which is independently capable of maintaining a constant flow of current greater than the holding current.
This current must be in the positive direction or the thyris to rs will not be able to carry it. For the sake of discussion, assume that the load is a DC current source of value I d. The AC source current is still a square wave of amplitude I d, but now it is displaced 90 o from the AC voltage. Therefore, both the average DC voltage and the net power are zero. The instantaneous power flow from the AC voltage source to the DC current source during one part of the cycle is just balanced by the instantaneous power flow in the opposite direction during another part of the cycle.
In other words, since the average DC voltage has reserved polarity but the average DC current has not, operation has entered Quadrant IV. If this limit is exceeded, the thyris to r which is commutating off does not have sufficient time to regain its blocking ability before it is subjected to forward voltage. As a result, the converter permits the average DC voltage to be controlled and the net power flow to be reversed, neither of which is possible in a rectifier using only Type 1 switches. V out Figure 3. A converter at each end of a DC transmission line links it to two AC systems.
The AC systems need not be synchronized in fact, they can operate at different frequencies , and power can be caused to flow in either direction. Much literature exists on HVDC converters and system considerations. Introduc to ry articles include: F.
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Kimbark, Direct Current Transmission, vol. I, Wiley- Interscience; However, in many applications it is desirable for the output to be a linear function of an input control signal. At the heart of this method is a circuit known as a zero crossing detec to r. This circuit compares two input signals and emits a pulse when they cross each other that is, when they are instantaneously equal. One input to the zero crossing detec to r is the input control signal, V i. Then pulses will appear at the output of the zero crossing detec to r at: The pulses which occur when the sinusoidal signal crosses V i in a negative-going direction can then be used to initiate one set of firing pulses.
In this analysis we have assumed that the input voltage, V i, is a constant. Because of the nonlinearities inherent in the switching action, a more exact analysis which is valid for rapidly varying inputs requires advanced techniques such as describing functions or z-transforms.
These techniques are described in the following references: G.
De and A. IEE, vol. Sucena-Paiva, R. Hernandez and L. Hazell and J.
Hence the linear analysis given in Section 10 is frequently valid. The basic elements of a closed-loop speed control supplying the armature of a DC mo to r are shown above. The inside current-limiting loop serves to override the outside speed-control loop if necessary to prevent the current from exceeding a safe value. Otherwise this inside loop may be ignored. The analysis of the dynamics of the speed-control loop can be performed with standard linear techniques, such as the Laplace transform.
The applied armature voltage, V d, is sometimes used as an approximate indica to r of speed where precise speed control is unnecessary. Better regulation is achieved by means of a tachometer which generates a feedback voltage proportional to armature speed. In this case, the transfer function of the feedback path is a constant: Examination of the loop gain, GH, reveals that this is a second-order Type 0 or proportional control system.
Such systems are analyzed in any textbook on feedback control. Neer and R. PAS- 93, pp. Bezoid, et al. IA-9, pp. IGA-5, pp. Two-Pulse Semiconverters Figure 3.