THREE-STAGE AC ELECTROMECHANICAL AMPLIFIER WITH ADJUSTABLE FREQUENCY

K. A. KHOR’KOV, A. I. SKOROSPELKIN, Sh. S. ROIZ, E. F. OBERTAN

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

The proposed electromechanical AC amplifier with adjustable frequency (EMA-AF) consists of two amplification stages for the supplied signal, a frequency-control stage, a semiconductor commutator, and an asynchronous motor (Fig. 1). Structurally, the EMA-AF has two active-iron packets (Fig. 2). In one packet the input (I) and output (III) stages are combined; in the second, the intermediate (II) stage and the asynchronous motor are combined. The semiconductor commutator is placed in a separate unit. The schematic diagram of the amplifier is presented in Fig. 3.

Fig. 1. Block diagram of the amplifier.

Fig. 2. Structural diagram of the amplifier.

The input signal is supplied to the control winding of stage I, located on the stator of packet I. On the stator of this packet there may also be located one or several additional control windings and a compensating winding. The output winding of stage I is an $m$-phase AC winding operating into a valve load. The rectified current is supplied to the excitation winding of stage III, located on the rotor of packet II of active iron.

The amplified signal is taken from the stator winding of stage II, rectified, and supplied to the semiconductor-commutator unit. A description of the semiconductor commutator is given in [1]. From the output of the semiconductor commutator, the voltage is supplied to the excitation winding of stage III (packet I). The control winding of stage III is an ordinary lap armature winding of a DC machine, the taps of which go to the semiconductor commutator.

Fig. 3. Schematic diagram of the relay unit.

Visible inscriptions in the diagram:

• stator
• rotor
• IM
• 1st stage
• 2nd stage
• Block of the semiconductor commutator
• 3rd stage
• $OV_I$, $\varphi_1$
• $OV_{II}$, $\varphi_2$
• $OV_I$, $\varphi_3$
• $OV_{II}$, $\varphi_4$
• $U_{\mathrm{out}}$
• $f_{\mathrm{out}}$

The collector is capable of successively alternating the points of application of the controlled signal, whereby the control field is rotated in the direction of, or against, the direction of rotation of the amplifier rotor. Owing to this, the frequency of the emf in the output winding of the third stage, located on the rotor of stack I, is changed.

The windings located in one stack are made so as to satisfy the condition of combining the fields in one magnetic core.

The first two stages, as well as the third (with stationary flux \(\Phi_4\)), are synchronous machines. The use of two amplifying stages is due to the desire to avoid the appearance of additional rings on the rotor for supplying the signal to control windings I and II. The total amplification factor of both stages can reach several thousands.

The output voltage is taken from the slip rings of the rotor of stack I, from the three-phase output winding of the third stage.

The speed of rotation of the motor is 3000 rpm. With a constant control flux \(\Phi_4\) and with the number of poles of the output stage \(2p = 2, 4, 6 \ldots\), a voltage with a base frequency of 50, 100, 150 ... Hz can be taken from the rotor winding. The semiconductor collector makes it possible to regulate the frequency over wide limits:

\[ f_{\text{out}} = f_{\text{base}} \pm f_y. \]

Moreover, \(f_{y\max} = 100 \div 150\) Hz; that is, from the EMU-RCh one can obtain the signal voltage, amplified thousands of times, with a frequency ranging almost from zero to \(2 \div 3 f_{\text{base}}\). In addition, when supplied with power, the EMU-RCh can be used as a generator operating in a stabilized-frequency mode.

The last, third stage of the amplifier has an amplification factor of the order of \(10 \div 20\), depending on \(f_y\). This means that through the semiconductor elements of the collector it is necessary to pass power 10–20 times less than the power of the amplifier.

The proposed alternating-current amplifier with adjustable voltage frequency can be used as a power source for asynchronous motors in various automatic-control circuits.

REFERENCES

Sh. I. Lutigdze. Electric machines with a controlled semiconductor commutator, Elektroenergetika, No. 5, 1962.