Relay-Pulse Voltage Regulator for Direct Current Generators
Abstract Generated abstract
This paper presents a relay-pulse voltage regulator for direct-current generators using semiconductor switching elements to control excitation current. It describes a circuit based on a blocking oscillator measuring element, amplification stages, and a power transistor switch, then derives expressions for maximum, minimum, average, and ripple values of the field current under pulsed excitation. Experimental tests on a PPN-27 converter-type DC generator compare calculated and measured ripple factors and average excitation currents under no-load and full-load conditions. The results indicate voltage regulation accuracy of about ±1% over a 0 to 100% load range, transient duration not exceeding 0.1 seconds, and close agreement between the analytical formulas and experimental data.
Full Text
Relay-Pulse Voltage Regulator for DC Generators
A. I. Zaitsev, S. A. Kitaeva
(Presented by the scientific seminar of the Electromechanical Faculty)
The reliability of modern radio-engineering and electronic equipment, and the electrical characteristics of devices, are largely determined by the constancy of the supply voltages. Therefore, an important place in the automation of production processes is assigned to the creation of voltage regulators for direct- and alternating-current generators.
The most important characteristics of voltage regulators are the accuracy of voltage maintenance, speed of response, range of regulation, and reliability in operation. Improving these characteristics is an urgent task of electrical engineering.
Recently a number of works have been published on the creation of various versions of voltage regulators using semiconductor devices operating in a pulse mode [1—5].
However, questions of the theoretical investigation of generator excitation systems using pulsed semiconductor elements have not yet been covered sufficiently fully. Excitation systems containing semiconductor elements have their own distinctive features and require theoretical investigation.
This article proposes a version of a voltage regulator for a direct-current generator using semiconductor elements and employing the principle of pulse regulation of the excitation current. The regulator circuit is shown in Fig. 1.
The measuring element of the regulator is made on the basis of a blocking oscillator and a measuring bridge. The blocking oscillator has two inputs and contains windings of positive and negative feedback. To one of the inputs, through diode \(D_7\), a stabilized signal from a silicon Zener diode is applied; to the second input—through diode \(D_8\)—there arrives a signal proportional to the voltage at the generator armature. The blocking oscillator does not generate if the input signals at diodes \(D_7\) and \(D_8\) are equal in magnitude. To amplify the signals of the measuring device, preliminary amplification stages using triodes \(PT_4\) and \(PT_3\) are employed. The amplification stage on triode \(PT_4\) is made with a transformer input. The final amplification stage contains the power triode \(PT_1\) and the feedback transformer \(TOS1\). The load of triode \(PT_1\) is the generator excitation winding. Power to the excitation winding is supplied through triode \(PT_1\), winding \(w_p\) of the feedback transformer. Uninter—
the load current is ensured by diode \(D_2\) and the feedback winding \(w_{\mathrm{v}}\) of transformer TOS1. Triode PT\(_1\) is controlled by triode PT\(_2\) and the feedback winding \(w_{\mathrm{oc}}\). All triodes in the circuit operate in switching mode.
Fig. 1.
The regulator operates as follows. After supply voltage is applied to the final stage of the amplifier and to the measuring element, a signal is fed to the input of the measuring blocking oscillator through diode \(D_8\), causing the latter to generate. The high-frequency pulses of the blocking oscillator are transformed through transformer \(T_p\) into the base circuit of the preamplifier PT\(_4\), and, amplified by amplifier PT\(_5\), are applied to the input of the control triode PT\(_2\) of the final amplifier. Triode PT\(_2\) opens, which corresponds to the open state of the power triode PT\(_1\). Current flows through the field winding; the final stage of the amplifier operates with a large duty factor, and the generator is excited. As soon as the generator voltage reaches the setpoint voltage (the setpoint is varied by means of resistor \(R_{13}\)), diode \(D_8\) opens and the blocking oscillator stops generating. As a result, triode PT\(_2\) closes, the duty factor of triode PT\(_1\) decreases, the generator field current decreases, and the voltage at the generator armature drops. The decrease in voltage at the generator armature leads to a reduction of the signal at diode \(D_8\) of the measuring device; an error signal appears, diode \(D_8\) is blocked and \(D_7\) opens, and the blocking oscillator begins to generate. In this case the control triode opens, which leads to an increase in the duty factor of triode PT\(_1\). As a result, the generator field current increases, and the generator voltage is restored to the specified value.
In the described version of the voltage regulator, a system of pulse regulation of the field current is used, in which the field winding is supplied from a source with constant voltage through a periodically opening and closing semiconductor switch. A simplified circuit for supplying the field winding is shown in Fig. 2.
In considering the processes associated with the establishment of the field current, the following assumptions are adopted:
1) the diode shunting the field winding is ideal;
2) the losses in the steel caused by the pulsating current are small in view of the insignificant pulsations;
3) armature reactions are absent;
4) the inductance of the field winding is constant when the field current changes;
5) the active resistance and inductance of windings $w^p$ and $w^b$ of transformer TOS1 are neglected because of their smallness;
6) the value of the bias resistance is neglected, since $r_c \ll R$.
Fig. 2.
Rectangular voltage pulses with amplitude $u$, repetition periods $T_1$ and $T_2$, and duty factors $\gamma_1$ and $\gamma_2$ are applied to the field winding. The different duty factor of the pulses is due to the pulsed mode of operation of the control triode PT$_2$, to which rectangular pulses with repetition period $T$ and duty factor $\gamma$ are applied. During the period when triode PT$_2$ is open, the output stage generates pulses with duty factor $\gamma_1$ and period $T_1$; when PT$_2$ is closed, it generates pulses with duty factor $\gamma_2$ and period $T_2$. The duty factors of the pulses $\gamma_1$ and $\gamma_2$, and the repetition periods $T_1$ and $T_2$, are determined by the parameters of the output stage of the amplifier [5].
The repetition period $T$ and their duty factor $\gamma$ are determined by the switching parameters of the measuring element and depend on the properties of the system itself.
The voltage diagram on the generator field winding is shown in Fig. 3.
Let us consider the processes by which the current is established in the generator field winding. Since voltage pulses of different duration and with different repetition periods are applied to the field winding, the process of establishing the current is considered for two time intervals— for the interval during which pulses with $\gamma_1$ and $T_1$ are applied to the field winding, and for the interval during which pulses with duty factor $\gamma_2$ and repetition period $T_2$ are applied to the field winding.
The processes of establishing the field current in the time intervals
\[ nT \leq t \leq T(n+\gamma) \]
and
\[ T(n+\gamma) \leq t \leq T(n+1), \]
where $n$ is an integer, are described in detail in [6].
On the basis of the expressions given in [6], it is possible to determine the maximum and minimum values of the current, the current ripple factor, and the full range of current ripple.
The maximum value of the current $i_{\max}$ during operation of the regulator is determined as
\[ i_{\max} = \frac{(1-e^{-\beta_1 \gamma_1})u} {(1-e^{-\beta_1})R}, \tag{1} \]
where
\[ \beta_1=\frac{T_1}{T_{\mathrm{n}}}, \qquad T_{\mathrm{n}}=\frac{L}{R}, \]
\(T_{\mathrm{н}}\) is the time constant of the excitation winding.
The full amplitude of the pulsations of the excitation current is
\[ \Delta i = \frac{U}{R} + \left\{ \frac{u \cdot e^{-\beta_2 \gamma_2}\left[1-e^{-\beta_2(1-\gamma_2)}\right]\left(1-e^{-\beta_2/K_2}\right)} {R\left(1-e^{-\beta_2}\right)} -\frac{U}{R} \right\} e^{-\beta_2(1-\gamma_2)} . \tag{2} \]
In this equation
\[ \beta_2=\frac{T_2}{T_{\mathrm{н}}}, \]
\(K_2\) is the number of pulse intervals with duty factor \(\gamma_2\),
\(U\) is the supply voltage of the final amplifier.
The minimum value of the current in the excitation winding is
\[ i_{\min}=i_{\max}-\Delta i = \]
\[ = \frac{U}{R} \frac{ (1-e^{-\beta_1\gamma_1})(1-e^{-\beta_2}) -\left[1-e^{-\beta_2(1-\gamma_2)}\right] \left[1-e^{-\beta_2(1/K_2)}\right](1-e^{-\beta_1}) } {(1-e^{-\beta_1})(1-e^{-\beta_2})}. \tag{3} \]
The pulsation coefficient is defined as the ratio of the maximum steady-state current to the minimum value:
\[ \chi=\frac{i_{\max}}{i_{\min}}= \frac{(1-e^{-\beta_1\gamma_1})(1-e^{-\beta_2})} { (1-e^{-\beta_1\gamma_1})(1-e^{-\beta_2}) -\left[1-e^{-\beta_2(1-\gamma_2)}\right] \left[1-e^{-\beta_2(1/K_2)}\right](1-e^{-\beta_1}) }. \tag{4} \]
When the amplitude of the current pulsations \(\Delta i\) is small, the process of establishing the current in the excitation winding is characterized sufficiently completely by a continuous curve—the smooth component—drawn through the points corresponding to the mean value of the current in each time interval \(T\).
The mean steady-state value of the current is
\[ I_{\mathrm{av}}=I\gamma_1\gamma+I\gamma_2(1-\gamma), \tag{5} \]
where \(\gamma_1\) and \(\gamma_2\) are the duty factors of the pulses generated by the final stage of the amplifier;
\(\gamma\) is the duty factor of the pulses supplied to the control triode of the final stage of the amplifier.
Thus, the mean steady-state value of the current is directly proportional to the pulse duty factor of the final stage of the amplifier and to the duration of the pulses supplied to the final stage.
Fig. 3. Oscillograms of the excitation current and the voltage on the excitation winding at \(I_{\mathrm{н}}=100\%\).
An experimental study was made of a voltage regulator for a DC generator of converter type PPN-27.
The principal parameters of the circuit are indicated in the circuit diagram shown in Fig. 1. All experimental studies of the regulator were carried out under established thermal conditions. In the studies, the accuracy of regulation was determined when the load varied from 0 to 100%, as were the magnitude of the ripple factor of the excitation current and the response speed of the regulator.
The nature of the variation of the excitation current and of the voltage on the excitation winding is illustrated by the oscillograms shown in Fig. 3.
Using equations (1, 3, 4, 5), the ripple factors of the excitation current and the value of the average steady-state current at various load values were determined. The results of the calculated and experimental data are given in Table 1.
Table 1
| No. | Regulator operating mode | $\beta_1$ | $\beta_2$ | $\gamma_1$ | $\gamma_2$ | $\gamma$ | $K_2$ | $z$, calc. | $z$, exp. | $I_{\mathrm{av}}$ (A), calc. | $I_{\mathrm{av}}$ (A), exp. | Discrepancy between exp. and calc., %, $z$ | Discrepancy between exp. and calc., %, $I_{\mathrm{av}}$ |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | No-load operation | 0.04 | 0.015 | 0.5 | 0.1 | 0.65 | 2 | 1.13 | 1.17 | 4.0 | 3.8 | 2.35 | 5.55 |
| 2 | Load 100% | 0.035 | 0.02 | 0.6 | 0.1 | 0.7 | 3 | 1.22 | 1.21 | 4.5 | 4.6 | 0.82 | 3.85 |
Conclusions
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The developed voltage-regulator circuit makes it possible to obtain an accuracy of voltage maintenance, when the load varies from 0 to 100%, of ±1%.
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The maximum duration of the transient process during load shedding and load application is no more than 0.1 sec.
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The proposed regulator circuit can be used both for direct-current generators and for alternating-current generators.
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Comparison of the experimental data with the values calculated from the derived equations gives only a small discrepancy.
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