Brushless DC Electric Machine Amplifier
Abstract Generated abstract
This paper presents the design and experimental study of a brushless DC electromechanical amplifier intended to overcome limitations associated with commutator and brush assemblies. The proposed model is a single-armature, two-stage amplifier combining two electrical machines in one magnetic circuit, with semiconductor rectifiers used in the rotor and output circuits and design conditions specified to avoid unwanted inductive coupling and magnetic forces. Tests of external characteristics, amplification factor, and response speed showed adjustable characteristic stiffness, amplification of about 5,000 to 7,000, and response comparable to a transverse-field electromechanical amplifier. The authors conclude that the device has satisfactory characteristics and may be useful where sliding contacts are undesirable.
Full Text
BRUSHLESS ELECTROMECHANICAL DC AMPLIFIER
A. I. Skorospeleshkin, E. N. Podborskii,
E. K. Sviridova
Recommended by the scientific seminar of the departments of electrical machines and general electrical engineering
The significant shortcomings of commutator electromechanical amplifiers (EMAs), due to the presence of the commutator-brush apparatus, limit the areas of their application, do not allow electromagnetic loads and rotational speeds to be increased, reduce the reliability of machine operation, and in some cases make their use impossible. Therefore, the transition to brushless EMAs corresponds to the modern trend in the development of electrical machine construction.
Brushless EMAs are simpler to manufacture and repair, do not require careful maintenance, and are reliable in operation.
For this reason we designed and built a model of a brushless DC EMA.
The model is a single-armature two-stage amplifier, in the second stage of which excitation from the armature current of the first stage is used. Both stages are made in accordance with the conditions for combining two electrical machines in one magnetic circuit.
We shall explain the operating principle of the BEMA according to the circuit shown in Fig. 1. When the control winding is connected to a constant voltage, a control current \(I_y\) will flow through it, creating an m.m.f. \(F_y\). It creates a flux \(\Phi_y\) with the number of poles \(2p = 2\), stationary in space. Since the rotor is rotated by a drive motor at speed \(n\), an e.m.f. \(E_{p1}\) is induced in the rotor winding \(W_{p1}\) with \(2p = 2\). The winding \(W_{p1}\) is connected to a three-phase rectifier \(B_1\), assembled on semiconductor diodes. The load of the rectifier is the winding \(W_{p2}\) with \(2p = 6\). The rectified rotor current \(I_{p2}\) creates a flux \(\Phi_{p2}\), which rotates together with the rotor and induces an e.m.f. \(E_s\) in the stator winding \(W_s\). The winding \(W_s\) is connected to a three-phase rectifier \(B_2\). When the load is connected, a rectified current \(I_n\) will flow through it. In series with the load there is connected a current-feedback winding, called the compensating winding. The flowing-
Fig. 1.
the load current flowing through it creates an m.m.f. directed in accordance with \(F_y\). By regulating the resistance \(R_{\mathrm{sh}}\), one can obtain different stiffnesses of the external characteristics.
To combine two stages in one magnetic core, it is necessary:
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that there be no inductive coupling between the windings of the different stages.
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that the interaction of the two magnetic fields not create vibrations and forces of unilateral magnetic attraction.
To satisfy the first condition in a BEMA it is necessary and sufficient that there be no inductive coupling between the control winding and the excitation winding of the second stage. During operation of the BEMA, the flux \(\Phi_{p2}\) induces an e.m.f. in each coil of the control winding, but the sum of the e.m.f.s of all the coils is equal to zero, as is seen from Fig. 2.
The second condition will be satisfied if the inequality is fulfilled:
\[ p_1 \div p_2 \ne 1. \]
Experimental investigations were carried out for the principal characteristics:
a) external characteristics;
Fig. 2.
b) amplification factor;
c) speed of response.
The test results showed that external characteristics of the required stiffness can be obtained in the amplifier; the amplification factor reaches 5–7 thousand. A comparison of the time constants of the BEMU and the transverse-field EMU (EMU-12A) showed that they have the same speed of response.
The operating characteristics of a DC brushless electromechanical amplifier are determined by the combination of two electrical machines in a single magnetic circuit and by the presence of two rectifier units \(B_1\) and \(B_2\). The output stage of the brushless amplifier is designed and manufactured in such a way that the curve of the linear electromotive force has the form of a trapezoid. This makes it possible to obtain a rectified voltage with a minimal level of ripple if the rectifier at the output is assembled according to Larionov’s circuit.
The rotor rectifier is assembled according to a single-half-period circuit. It requires half as many diodes (in comparison with Larionov’s circuit) and makes it possible to obtain a larger rectified current for identical control signals.
Thus, studies of the DC brushless electromechanical amplifier show that it has satisfactory characteristics. Consequently, it can find wider application in comparison with commutator amplifiers, especially in those areas where the presence of sliding contacts is inadmissible. Work on the DC brushless electromechanical amplifier is continuing.