Semiconductor Voltage Regulator for Inductor Generators

A. I. Zaitsev, B. L. Zelenov

(Recommended by the scientific seminar of the Electromechanical Faculty)

With the development of semiconductor technology, it has become possible to design semiconductor voltage and frequency regulators for both AC and DC generators. This makes it possible to create fast-acting systems for automatic regulation of voltage and frequency. Especially promising is the use of transistors in pulse automatic-control systems, in which the transistors operate in switching mode. In this case their utilization factor increases to 30–50, and the circuit becomes relatively insensitive to the spread in the parameters of semiconductor devices. In pulse automatic-control systems, the error signal is quantized in time, and control of the actuating device is carried out at discrete instants of time by pulses whose parameters (magnitude, duration, polarity, phases) depend on the magnitude of the error signal. Most often, pulse automatic-control systems employ pulse elements with pulse-amplitude modulation (PAM) and pulse-width modulation (PWM). In the latter, the duration of the pulses depends on the magnitude of the error signal.

This article describes an automatic voltage-control system for inductor generators. The system consists of a phase-compounding unit and a semiconductor voltage regulator containing the following components:

1. Measuring element (m.e.).
2. Regulating device.
3. Power-supply unit.

Measuring Element

The measuring element combines two functions: the function of measuring the generator output voltage and the function of a pulse-width modulator, which is undoubtedly an advantage over the measuring elements considered in [2]. The operating principle of this measuring element is as follows.

The reference voltage produced by the voltage stabilizer is compared with a sawtooth voltage generated by the sawtooth-voltage former. The voltage difference

(which has a sawtooth form) is fed to the input of transistor PT₃. Depending on the amplitude of the sawtooth voltage, the time during which transistor PT₃ remains in the closed state changes; that is, a pulse-width method of regulation is implemented without an explicitly expressed pulse-width modulator.

Fig. 1. Voltage corrector for the ATT-20-400 unit.

The sawtooth-voltage shaper consists of the measuring transformer T.I., rectifier bridge B₁, capacitor C₁, and resistor R₂.

In principle, a half-wave rectifier circuit may be used; however, in this case the frequency of the sawtooth voltage will be equal to the frequency of the generator output voltage, which may lead to the occurrence of low-frequency modulation of the generator output voltage and to deterioration in the accuracy with which the disturbing effect is compensated. The static error of the measuring element is determined by the expression:

\[ K_{\text{st}}=\frac{1}{f R_2 C_1}. \tag{1} \]

The voltage regulator is built on transistor PT₁ and voltage stabilizer ST₁. Temperature compensation of the regulator is carried out by means of diodes D₁–D₄.

The output voltage of the stabilizer is equal to the reference voltage with an accuracy up to the voltage \(U_{\mathrm{eb}}\) of transistor \(PT_1\). The load of the stabilizer is resistance \(R_4\), which at the same time also serves to change the setting of the reference voltage.

Regulating Device

The regulating device consists of:

1. A preliminary amplifier, assembled on transistors \(PT_3\) and \(PT_5\).
2. An intermediate amplifier on transistor \(PT_4\).
3. A final amplifier on transistor \(PT_2\).

The preliminary amplifier serves to amplify the mismatch signal of the measuring element. When the mismatch signal is equal to zero, transistor \(PT_3\) is opened by the negative of the reference voltage. In the presence of a mismatch signal, \(PT_3\) and \(PT_5\) operate in a switching mode.

To improve the pulse edges in the preliminary amplifier, feedback through resistance \(R_5\) is used.

For better matching of the amplifier stages, an n-p-n conductivity transistor is used as the intermediate amplifier. The load of the final amplifier, made on transistor \(PT_2\), is the magnetizing winding of the voltage transformer of the phase compounding unit. For reliable cutoff of transistor \(PT_2\) in the absence of an input signal, rectifier \(B_3\) is used.

Principle of Operation of the Voltage Regulator

The initial excitation of the generator is effected from transformer \(TR\) and diode \(1B\). Under the action of this pre-excitation, the generator is excited to a voltage amounting to \(10 \div 15\%\) of nominal. After the generator has been excited to \(10 \div 15\%\) of nominal, the phase-compounding unit comes into operation, forcefully exciting the generator to the nominal value.

The semiconductor voltage regulator performs two functions:

1. It serves for forced excitation of the generator during the starting process of the unit.
2. It serves to remove the forcing of the excitation and to maintain the generator voltage at a specified level under any disturbing influences.

Forced excitation of the generator is achieved by the fact that the voltage regulator does not act until the voltage on the generator is less than nominal. Indeed, if the voltage on the generator is less than nominal, a minus is applied to the base of transistor \(PT_3\). In this case transistor \(PT_3\) is open, and \(PT_5\) is closed. Transistor \(PT_4\) is reliably closed by the negative bias voltage (rectifier \(B_6\)). Transistor \(PT_2\) is also reliably closed by the positive of the bias source at rectifier \(B_3\). In this case an insignificant current flows through the magnetizing winding, determined only by the value of resistance \(R_3\), and it has an insignificant magnitude. In this case the voltage at the output of transformer \(TN\) is maximum, and the generator is forcefully excited. As soon as the generator voltage reaches the nominal value, a mismatch signal appears at the output of the measuring element, and the larger it is, the greater the de-

deviation of the generator voltage from the nominal value. Positive sawtooth pulses are applied to the base of transistor PT\(_3\). Transistor PT\(_3\) is cut off for the duration of the pulse; that is, the time during which the transistor is in the cut-off state depends linearly on the magnitude of the mismatch signal produced by the measuring element. At the same time transistors PT\(_5\), PT\(_4\), and PT\(_2\) are turned on. A current flows through the magnetizing winding, the magnitude of which is determined by the duration of the conducting state of transistor PT\(_2\), and ultimately by the magnitude of the mismatch signal. Application of current to the magnetizing winding leads to a reduction in the generator voltage; in this process the magnitude of the mismatch signal decreases, the duration of the cut-off state of transistor PT\(_3\) decreases, and the rate of decrease of the generator voltage decreases. In this way the generator voltage is maintained at the specified level. When the load is switched on and off, the voltage regulator operates similarly.

Recommendations for Calculating the Circuit Parameters

It is advisable to adopt the following calculation procedure:

1. Calculation of the final amplification stage.
2. Calculation of the intermediate amplification stage.
3. Calculation of the preliminary amplification stage.
4. Calculation of the measuring element.
5. Calculation of the power-supply unit.

When calculating the final stage, the switching frequency of the power transistor is selected. In doing so, the following must be taken into account:

a) The switching frequency should be as low as possible. This facilitates the operating conditions of the transistor, but, in the case of using alternating-current generators, low-frequency modulation of the generator output voltage may occur. Therefore the switching frequency should be increased.

b) An excessive increase in the switching frequency leads to an increase in the power dissipated in the transistor, i.e., the transistor operating conditions worsen. It is recommended that the switching frequency of the power transistor be taken equal to or greater than the frequency of the generator output voltage.

The switching period can be determined from the expression

\[ T=\frac{\lg \chi}{1-\gamma}\cdot T_{\text{n}}, \tag{2} \]

where \(\chi\) is the specified ripple factor of the current at a given pulse duty factor \(\gamma\).

\(T_{\text{n}}\) is the time constant of the magnetizing winding of transformer TN.

The pulse duty factor \(\gamma\) is determined from the expression

\[ \gamma=\frac{(r_{\text{on}}+R_3)\cdot i_{\text{on}}\cdot r_{\text{on}}-r_{\text{on}}\cdot U_{\text{p}}}{U_{\text{p}}\cdot R_3}, \tag{3} \]

where \(r_{\text{on}}\) is the resistance of the magnetizing winding of TN;

\(i_{\text{on}}\) is the current of the magnetizing winding;

\(U_{\text{p}}\) is the supply voltage of the final stage;

\(R_3\) is the resistance shunting the emitter–collector junction of transistor PT\(_2\).

Resistor \(R_3\) is used in order to reduce the voltage across

transistor PT\(_2\). In calculating the measuring element, the value of resistance \(R_2\) is selected.

In order to increase the static accuracy of regulation, resistance \(R_2\) should be chosen large; however, its value is limited by the base current of transistor PT\(_3\).

In practice, resistance \(R_2\) is taken as \(10 \div 20\) kΩ. \(C_1\) is determined from expression (1).

Results of Testing the Voltage Regulator

According to the circuit presented, a prototype voltage regulator was manufactured and tested under laboratory conditions.

The voltage regulator was tested on a unit with a power of 20 kW, \(U_g = 230\) V, \(f_g = 400\) Hz. During the tests, in order to improve the dynamics of the system, it became necessary to introduce flexible feedback using capacitors \(C_8\), \(C_9\) and resistor \(R_{12}\).

The capacitance of capacitor \(C_8\) is on the order of \(10 \div 15\) μF; that of capacitor \(C_9\), \(0.1 \div 0.5\) μF. Resistance \(R_{12}\) is selected within the range of several tens of kΩ.

Main test results.

1. The thermal drift of the generator voltage after 4 hours of operation of the converter at 100% load was \(+0.4\%\).
2. The accuracy of maintaining the generator output voltage in all modes was no worse than \(-1\%\).
3. The voltage regulator does not increase the modulation of the generator voltage produced by the converter.
4. The duration of the transient process is no more than 0.3 s.
5. The voltage dip and surge during load application and shedding do not exceed 15%.

Fig. 2. Oscillogram of the generator output voltage when the load is increased from 0% to 30%.

Fig. 3. Oscillogram of the generator output voltage when the load is increased from 30% to 100%.

Main Conclusions

1. Owing to the use of a pulse-type automatic-control system, the circuit proved to be noncritical with respect to the spread of transistor parameters.

Fig. 4. Oscillogram of the generator output voltage when the load is dropped from 100% to 0.

1. The voltage regulator operated stably at an ambient temperature of \(+50^\circ\text{C}\). In the case of a higher ambient temperature, the use of silicon transistors is desirable.

2. The voltage regulator can be used on single-phase and three-phase current units with a power of up to 50 kW.

Fig. 5. Oscillogram of the generator output voltage when the load is dropped from 30% to 0.

1. With a unit power of up to \(8 \div 10\) kW, the voltage regulator can be used without a B.F.C.

References

1. G. M. Vedeneev, V. S. Moin. Semiconductor voltage regulator for an alternating-current generator, VEP, No. 7, 1961.

2. Development of automatic voltage and frequency regulators for alternating-current generators using semiconductors, Department of EAPU, TPI, pts. I, II, III, IV, 1963–1964.