AUTONOMOUS ALTERNATING-CURRENT SOURCE WITH STABLE FREQUENCY

Sh. S. Roiz, A. I. Skorospeshkin

(Presented by the scientific seminar of the departments of electrical machines and general electrical engineering)

In electric-power systems used in transport and in aviation, the drive of synchronous generators (SG) has a variable rotational speed. In such systems, alternating current of stable frequency is obtained by installing, between the prime mover and the SG, a constant-speed drive (CSD), i.e., a device that ensures a constant generator speed when the rotational speed of the prime mover varies.

The presence of a CSD leads to an increase in weight characteristics and to a reduction in the reliability of the power-supply system. In this connection, much attention has recently been devoted to creating alternating-current systems of constant frequency without a CSD.

Progress in the field of semiconductor technology makes it possible to regard the synchronous generator—frequency converter as the most promising system.

The brushless regulated-frequency electrical-machine units (EMUs) that we are developing can be successfully used as an autonomous source of alternating current of stable frequency. The most acceptable circuit of such a source is shown in Fig. 1.

Fig. 1. Circuit of a brushless regulated-frequency EMU

The proposed system consists of an asynchronous generator (windings $W_2$ and $W_3$) and an excitation winding $W_1$, fed from a controlled

semiconductor commutator (SPC). The field winding and the windings of the asynchronous generator are made with the same number of poles and are placed in the same slots.

The operating principle is as follows. The field winding \(W_1\), with the aid of the SPC, creates a magnetic flux rotating at a constant speed. The excitation magnetic flux induces an emf in the windings of the asynchronous generator. The emf of the output winding \(W_2\) varies with a frequency determined by the SPC. The presence of the SPC in the circuit makes it possible to obtain a stable frequency of high accuracy over a wide range of variation in the rotational speed of the prime mover.

For the most efficient operation of the asynchronous generator over a wide range of variation in the rotational speed of the prime mover, an additional resistance \(R_p\), regulated according to a definite law, is inserted in the rotor circuit.

The energy diagram (Fig. 2) for operation of the commutatorless electric-machine amplifier in the mode of a stable-frequency generator is constructed on the basis of the equation

\[ P_{\mathrm{m}} = P_{\psi 2} + P_{\psi 3}, \tag{1} \]

where \(P_{\mathrm{m}}\) is the mechanical power on the shaft of the prime mover,

\(P_{\psi 2}\) is the electromagnetic power of the output winding \(W_2\),

\(P_{\psi 3}\) is the electromagnetic power of winding \(W_3\).

The presence of feedback diodes in the SPC circuit makes it possible to transfer part of the power from the shaft to the input, which leads to a reduction in the input power. The powers \(P'\) and \(P'_{\psi 2}\) in the energy diagram are components of the output power \(P_{\psi 2}\). Their magnitude is determined by the parameters of the windings, the load, and the output frequency. The reactive power \(Q_{\mathrm{oy}}\) is the converted power \(P_y\) consumed from the dc network.

Fig. 2. Energy diagram

Experimental investigation of a breadboard model \((2p_1 = 8)\) was carried out at an output frequency \(f_{\text{out}} = 100\ \text{Hz}\). This was due to the fact that the speed of the prime mover varied from 1500 to 3000 rpm. A further increase in speed is limited by the design implementation of the entire system.

The variation of the powers of the prime mover \(P_{\mathrm{m}}\), the output \(P_{\psi 2}\), the excitation \(P_y\), and the efficiency as a function of the rotational speed of the prime mover is shown in Fig. 3.

It follows from Fig. 3 that the excitation power \(P_y\) amounts to 10–15% for a speed-variation range of \(2:1\). The efficiency, as the rotational speed increases, has a steeply decreasing character. This is due to the fact that the slip power \(P_{\psi 3}\) increases. The output power \(P_{\psi 2}\) remains unchanged as the rotational speed of the pri-

primary engine. The form of the output voltage (Fig. 4) is satisfactory over the entire range of variation of the speed of the primary engine.

Fig. 3

Fig. 4. Form of the output voltage

As a result of the investigations carried out, the following conclusions may be drawn:

1. A brushless EMU of regulated frequency can be successfully used as an autonomous source of stable frequency when the rotational speed of the primary engine is variable.

2. The excitation power is comparatively small, which makes it possible to substantially reduce the power of the UPC, the most expensive device.

3. The form of the output voltage is satisfactory over a wide range of variation of the rotational speed of the primary engine.