LOW-POWER BRUSHLESS DC GENERATOR

G. A. Sipailov, A. B. Tsukublin

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

The reliability of the brush-and-commutator unit of DC generators is the principal reason why, in certain special installations—for example, in transport [1] and in other fields of technology—synchronous generator–rectifier (SG–R) systems are used as sources of direct current. Most brushless DC generators currently in use employ, as the power-supply unit for the rectifier circuit, a synchronous generator with a sinusoidal emf waveform. This circumstance leads to the quality of the resulting DC voltage of brushless DC generators being substantially lower than that of ordinary commutator generators. Therefore, in order to obtain minimal ripple of the DC voltage in a number of installations, special measures have to be adopted. In particular, in some cases an increased number of generator phases is used [2], rather complex cascade rectification circuits are employed [3], smoothing filters are installed [4], and so on.

Analysis of the methods listed for reducing ripple in the rectified voltage has shown that they lead either to reduced utilization of the power-supply unit (increasing the number of phases), or to complication of the circuit or an increase in the number of rectifier elements. Naturally, such measures are inadmissible in systems with restricted dimensions and weight, where questions of utilization of the power-supply unit are of primary importance.

When designing low-power brushless DC generators, in a number of cases the installation of a smoothing inductive-capacitive filter is completely inadmissible, since its dimensions substantially increase the size of the entire installation.

One method of obtaining high-quality voltage in a synchronous generator–rectifier system is to select an appropriate emf curve shape that, under load, ensures minimal ripple of the rectified voltage. At the same time, an indispensable condition is also the preservation of a high utilization factor of the synchronous generator.

Both of these conditions are satisfied fairly well if a three-phase synchronous generator with a trapezoidal emf curve is adopted as the power-supply unit, together with a bridge circuit for the rectifier block.

To compensate for voltage dips appearing under load, a special system of windings on the rotor is proposed, ensuring-

giving a trapezoidal shape to the generator voltage under load.

Design and Principle of Operation

The schematic diagram of a low-power brushless direct-current generator with a low level of output-voltage ripple is shown in Fig. 1. The stator stack of generator 1 differs in no way from the stator of ordinary low-power synchronous generators.

Fig. 1.

A concentrated three-phase winding with full pitch (2) is placed in the stator slots. The winding phases are connected in star. If it is impossible to make a concentrated winding, a distributed winding may be used; however, in this case the number of slots per pole and phase should not be chosen greater than two, since this would lead to a considerable complication of the shape of the inductor poles. The stator phases are connected to a bridge rectifier 3, which is the generator output.

The generator rotor 4 is of salient-pole construction; however, the pole width is chosen not according to the design rules for salient-pole machines, but from the condition of obtaining the required shape of the emf curve.

A distinctive feature of the rotor is that two windings are arranged on each of its poles: the main winding 5, which creates the main magnetic flux of the machine, and an auxiliary winding 6, connected to the generator output. The auxiliary winding is chosen so that the flux it creates induces an emf in the stator winding that would compensate the voltage dips when the generator is loaded by the rectifier. In addition, to compensate the armature reaction on the rotor along the transverse axis, a compensating winding 7 may be placed, although its use in salient-pole machines is not obligatory.

To eliminate the influence of the higher harmonic of the armature reaction on the excitation winding, a damper system is installed on each pole.

For balancing the rotor and ensuring symmetry of the windings, insulating spacers 8 are installed on the rotor. The rotor windings are supplied in the usual manner through rings and a brush assembly. In this case the current in the auxiliary and compensating windings varies according to a law determined by the output current through a regulat—

also device 9, as well as by the leakage reactance of the stator winding of the generator.

The need for an additional winding compensating voltage dips under load can be explained if one considers the commutation process of switching the valves.

The causes of pulsations in the rectified voltage of a synchronous generator–rectifier system with a trapezoidal emf (Fig. 2a) under load are the transient electromagnetic processes of valve switching. When phase A is disconnected and phase B is connected, the anode inductance (the leakage inductance of the stator winding) prolongs the transient process of current rise (phase B) and decay (phase A). Inductive emfs appear in the phases:

\[ \text{emf} = -L_{\phi}\frac{di}{dt}, \]

where \(L_{\phi}\) is the phase inductance, and \(i\) is the instantaneous value of the phase current. These emfs reduce the voltage of the phase being switched into operation, creating a dip in the rectified voltage (Fig. 2b). The depth of the dips (pulsations) is determined by the magnitude of the commutation current, as well as by the magnitude of the anode inductance.

Fig. 2.

Consequently, at the moment of switching it is necessary to have such an emf as to compensate the dips in the phase voltage.

Let us consider how the emf of a synchronous generator should vary if the aim is to obtain, from a three-phase bridge rectifier loaded with an inductive-active load \((L_{\text{н}} = \infty)\), an ideally smoothed voltage. In considering this question, we shall proceed from the fact that the armature reaction of the generator has no influence on the form of the generator emf.

From the theory of operation of multiphase bridge rectifiers it is known that the rectified voltage, with ideally smoothed rectifi—

can be written in the repetition interval \(\dfrac{\pi}{m}\) by two equations:

1) during the period of current commutation from phase A to phase B

\[ U_d=\frac{e_A+e_B}{2}-e_C-\frac{3}{2}I_d r'; \tag{1} \]

2) in the noncommutation time interval, with phases B and C operating,

\[ U_d=e_B-e_C-2I_d r' \tag{2} \]

where \(I_d\) is the average value of the rectified current;
\(r'\) is the total active resistance of the generator phase and the forward resistance of the rectifying element.

If it is assumed that in the noncommutation interval the condition at \(\omega t=\gamma \div \dfrac{\pi}{m}\) is satisfied,

\[ -(e_C-I_d r')=e_B-I_d r'=U=\mathrm{const}, \tag{3} \]

then the appearance of ripple in the rectified voltage is possible only as a result of the commutation process. Therefore, in order to obtain an ideally smoothed rectified voltage it is necessary, at the instant of commutation, to ensure at \(\omega t=0 \div \gamma\)

\[ \frac{e_A+e_B}{2}-e_C-\frac{3}{2}I_d r'=2U=\mathrm{const}. \tag{4} \]

Taking into account that the generator provides a symmetrical system of e.m.f.’s, i.e.,

\[ e_A(t)=e_B\left(t+\frac{2\pi}{m}\right)=e_C\left(t-\frac{2\pi}{m}\right) \]

and confining ourselves to consideration only of the two-to-three-valve operating mode of the rectifier, \(\gamma<\dfrac{\pi}{m}\), it can be established that \(e_C\) in expression (4), taking (3) into account, is determined as

\[ e_C=-(U+I_d r') \tag{5} \]

at \(\omega t=0 \div \gamma\). Consequently, taking this into account, at \(\omega t=0 \div \gamma\)

\[ \frac{e_A+e_B}{2}-\frac{I_d r'}{2}=U=\mathrm{const}. \tag{6} \]

Assuming, for simplicity of calculation, that the e.m.f. of the phase being disconnected changes linearly during the commutation period,

\[ e_A=-\frac{U}{\beta}(\omega t-\beta), \]

we find the law of variation of the e.m.f. of phase B:

\[ e_B=-\frac{U}{\beta}(\omega t+\beta)+I_d r' \tag{7} \]

Thus, in order to obtain minimal ripple of the rectified voltage of a bridge three-phase rectifier, the synchronous generator must provide an e.m.f. whose waveform is described by the following equations:

\[ \omega t=-\beta \div 0,\quad e(\omega t)=\frac{U}{\beta}(\omega t+\beta); \]

\[ \omega t=0 \div \gamma,\quad e(\omega t)=\frac{U}{\beta}(\omega t+\beta)+I_d r'; \]

\[ \omega t=\gamma \div \frac{2\pi}{m},\quad e(\omega t)=U+I_d r'; \]

\[ \omega t=\frac{2\pi}{m}\div \frac{2\pi}{m}+\beta,\quad e(\omega t)=-\frac{U}{\beta}\left(\omega t-\frac{2\pi}{m}-\beta\right). \]

The form of the emf for this case is shown in Fig. 3, a. Taking into account the armature reaction of the synchronous generator, which affects not only the magnitude but also the form of the phase emfs, the calculated emf curve will be distorted and will take the form shown in Fig. 3, b. To create such an emf,

Fig. 3.

an additional winding on the field pole is required. The size of the additional winding over the pole width is determined by the value of the commutation angle, which may be calculated from the approximate formula

\[ \gamma=\arccos\left(1-\frac{I_d x_s}{\dfrac{4\pi}{m\beta}U\sin\dfrac{\pi}{m}}\right), \]

where \(x_s\) is the leakage inductive reactance of the generator.

Thus, even with an inductive load, a sufficiently good DC voltage can be obtained using only a three-phase bridge rectifier.

Such a generator may find application in systems requiring-

operating on direct current, in which, because of the operating conditions, the presence of a brush-and-commutator assembly and filtering elements is inadmissible.

References

1. A. F. Krogerus, Ya. K. Shinka. A contactless direct-current generator. National Economy of Soviet Latvia, No. 5, 1959.

2. G. A. Kovalkov. Rectifier generator. Branch of VINITI, Academy of Sciences of the USSR, No. Э-59-11/4, 1959.

3. G. G. Fedoseev. Rectifiers and Stabilizers. State Publishing House “Iskusstvo,” 1960.

4. N. A. Levin. Smoothing inductive-capacitive filters for rectifiers. Dissertation, LETIS, Leningrad, 1962.