MEASURING DEVICE FOR A CURRENT LIMITER FOR GENERATORS IN A HALF-WAVE RECTIFIER CIRCUIT

A. N. Korogodskii, A. D. Vlasov, V. A. Pleshkov, A. N. Shestakov

(Presented by the scientific seminar of the Departments of Electrical Machines and General Electrical Engineering)

Up to the present time, in motor-tractor electrical equipment, to protect the generator from overloads and the associated overheating of the armature winding, a contact-vibration current limiter has been used. Its measuring element is the coil of an electromagnetic relay, through which the measured current is passed, and a measuring spring that holds the relay contacts in the closed state. The actuating element of this current limiter is the contact pair of the relay, acting directly or through a transistor current amplifier on the current of the generator field winding. The current limiter described responds to the average value of the rectified current and, starting from some specified value of the load current, reduces the current in the generator field winding, thereby reducing the voltage at the generator output to such a value at which the load current does not exceed the specified value.

The method considered for protecting a generator from overload has a number of disadvantages, the principal ones being the low reliability of the contact pair and the instability of the measuring element.

When a transistor or thyristor voltage regulator is used, the current limiter is simplified: it may contain only a measuring device, while the regulating element may be the regulating element of the voltage regulator.

In a half-wave power rectification circuit, it is possible to create a very simple contactless measuring device based on measuring the voltage drop directly across the active resistance of the phase winding. In a generator whose phase windings are connected to rectifiers in a half-wave circuit, the phase voltage in the presence of load current contains a DC component equal in magnitude to the product of the DC component of the phase current and the active resistance of the phase winding. The latter is easy to show.

The magnitude of the voltage drop across the active resistance of the phase winding characterizes the thermal condition of the winding and can be used as an input signal for an automatic load-current regulation system. Since this quantity, in a generator whose phase windings are connected to the load in a half-wave circuit, can be measured directly at the phase terminals, it appears simplest and most expedient to construct the input device

systems for automatic regulation of load current on the principle of separating out the dc component of the phase voltage.

An indicating device for a current limiter for generators in a half-wave power-rectification circuit is considered; in this device, in order to simplify the circuit, reduce the power consumed, and eliminate moving contacts, the dc component of the phase voltage is fed to the device input, obtained either by using a filter or as the potential difference between the neutral point of the winding and an artificial neutral point.

Fig. 1.

In Fig. 1, a, a three-phase generator is shown in a half-wave rectifier circuit, in which the dc component of the phase voltage is separated out by means of a filter. In Fig. 1, b, the dc component of the phase voltage is taken between points \(O\) and \(O_1\).

Let us show that, when the resistances \(R_1, R_2, \ldots, R_m\) are equal and the phases are symmetrical, the potential difference between the neutral point of the winding \(O\) and the artificial neutral point \(O_1\) is equal to the dc component of the phase voltage, irrespective of the magnitude of the ballast resistances and of the ac component of the phase voltage.

Phase voltages

\[ U_1=\Delta U+\sum_{1}^{\infty} a_n \cos n\omega t+\sum_{1}^{\infty} b_n \sin n\omega t \]

\[ U_2=\Delta U+\sum_{1}^{\infty} a_n \cos n\left(\omega t+\frac{2\pi}{m}\right) +\sum_{1}^{\infty} b_n \sin n\left(\omega t+\frac{2\pi}{m}\right) \tag{1} \]

\[ \cdot\quad \cdot\quad \cdot\quad \cdot\quad \cdot\quad \cdot\quad \cdot\quad \cdot\quad \cdot\quad \cdot\quad \cdot\quad \cdot\quad \cdot\quad \cdot\quad \cdot\quad \cdot\quad \cdot \]

\[ U_j=\Delta U+\sum_{1}^{\infty} a_n \cos n\left[\omega t+(j-1)\frac{2\pi}{m}\right] +\sum_{1}^{\infty} b_n \sin n\left[\omega t+(j-1)\frac{2\pi}{m}\right]. \]

Let us denote the currents in the resistances \(R_1,\ldots,R_m\) respectively by \(i_1, i_2,\ldots i_m\).

We write the system of equations according to Kirchhoff’s laws, taking into account that

\[ R_1=R_2=R_j=R. \]

\[ \left. \begin{aligned} U_{00_1} &= U_1 + i_1 R,\\ U_{00_1} &= U_2 + i_2 R,\\ &\cdot\quad \cdot\quad \cdot\quad \cdot\\ U_{00_1} &= U_j + i_j R,\\ &\cdot\quad \cdot\quad \cdot\quad \cdot\\ U_{00_1} &= U_m + i_m R \end{aligned} \right\} \tag{2} \]

\[ \sum_1^m i_j = 0. \tag{3} \]

Adding equations (2), taking (3) into account, we obtain:

\[ U_{00_1}=\frac{1}{m}\sum_1^m U_j. \tag{4} \]

After substituting (1) into (4), we obtain:

\[ U_{00_1}=\Delta U. \tag{5} \]

Consequently, in the circuit of Fig. 1, b, the potential difference between the neutral point \(O\) and the artificial neutral point \(O_1\) is equal to the constant component of the voltage drop across the active resistance of the phase winding, which was what was required to be proved.

Figure 2 shows the circuit of the measuring device. The measuring device contains a comparison element (Zener diode ST) and a single-transistor amplifier. The capacitance \(C\) (not mandatory) serves to smooth pulsations of the input voltage, which may occur in the circuit of

Fig. 2

Fig. 1, b in the case of asymmetry of the phases or ballast resistances. In the initial state the transistor is open, and at the output of the device there is a voltage equal to the rated value. When the input signal becomes greater than the permissible \(\Delta U_{\max}\), breakdown of the Zener diode occurs, a cutoff base current appears, the triode closes, and the voltage at the output of the measuring device becomes equal to zero. The output of the current-limiter measuring device may be connected in parallel with the output of the measuring device of the voltage regulator, so as to use the regulating element of the voltage regulator for current limitation.

References

1. A. N. Korogodsky, V. A. Pleshkov, N. P. Bogdanets, A. N. Shestakov. Current limiter for a synchronous generator. Decision on the issuance of an inventor’s certificate under application No. 1231303/24-7.