RESULTS OF A STUDY OF THE OPERATION OF AN ASYNCHRONOUS IMPACT GENERATOR WITH AN INDUCTIVE LOAD

Yu. A. Romanov, G. A. Sipailov, A. V. Loos

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

A complex engineering problem that arises in conducting research connected with high-energy magnetic fields is the creation of power sources that would ensure the transfer of considerable energy into an inductive load over a relatively short interval of time.

The choice of one or another storage device is determined mainly by its technical capabilities from the standpoint of obtaining the required values of energy, magnetic-field intensity, and pulse duration. If, for technical reasons, the use of different types of storage devices becomes possible, then in this case the choice must be determined by economic considerations. Thus, for energies measured in tens of millions of joules, mechanical storage is the most economical. For example, the cost of a capacitor storage device for an energy of 100 million J at a voltage of 10–15 kV is approximately 250 million rubles, whereas that of an electric-machine storage device is 2 million rubles [1].

However, the lower limit for the expedient use of an electric-machine storage device, as capacitors improve, is shifting toward higher energy values. Thus, at present, when choosing storage devices for energies of up to 5–10 million J, a number of researchers prefer to use capacitors, arguing in doing so that they are more economical in comparison with existing impact generators [2].

The intensification of research in various branches of modern physics requires a further increase in the energy concentrated in a single pulse. In this connection, the development and improvement of energy storage devices satisfying the increased requirements constitutes one of the most important problems of power engineering.

In recent years, in developing electric-machine energy storage devices, along with synchronous and unipolar generators, researchers have begun to turn to the asynchronous machine. Asynchronous generators with a squirrel-cage rotor are simple in design, reliable in operation, and relatively inexpensive. When capacitance is connected to an asynchronous machine, self-excitation of the latter is possible, with an increase in the amplitude of current and voltage until a steady-state mode is reached.

Such an asynchronous generator with capacitor self-excitation can be used both for exciting a controlling magneto-

for the field of charged-particle accelerators, as well as in autonomous power sources for special equipment.

The operation of an asynchronous generator in a pulsed mode has not been studied sufficiently. In this connection, the problem arises of determining the fundamental possibility of operating a single-phase asynchronous generator in a pulsed mode, studying the influence of various parameters on the energy delivered to the load, and comparing its energy characteristics with those of a synchronous generator.

The normal operating mode of an asynchronous impact generator is sudden connection to the load at the moment when the voltage across the capacitor passes through zero, and disconnection at the very first passage of the current through zero. The nature of the electromagnetic processes occurring when the generator is suddenly connected to a load whose inductance is close in value to the generator’s subtransient inductance is described by differential equations formulated for the stator and rotor circuits. Studies of the influence of various parameters on the operation of an asynchronous impact generator were carried out on a mathematical model of an asynchronous impact generator [3]. In this case, the following parameter values, in relative units, were taken as the initial values:

\[ \begin{aligned} x_{ad}&=1; & x_{\sigma s}&=0.04; & x_{\sigma r}&=0.01;\\ R_s&=0.005; & R_r&=0.005; & \omega_r&=1. \end{aligned} \]

Here all designations correspond to those adopted in [3].

Below are the calculation results, which make it possible to identify the influence of certain generator parameters on the energy transferred to the load.

1. Investigation of the influence of the stator active resistance on the magnitude of the energy delivered by the generator to the load in the absence of capacitance. The capacitance was disconnected in order to eliminate its influence on the operation of the generator.

The method for solving the differential equations describing the electromagnetic transient process of operation of the asynchronous generator consists in the following: at the first stage the generator is excited—

Fig. 1a. Influence of the active resistance of the stator of an asynchronous generator on the energy transferred to the load: \(R_r=0.005\); 1 — \(R_s=0\); 2 — \(R_s=0.005\); 3 — \(R_s=0.01\); 4 — \(R_s=0.02\)

of the generator; for this purpose, in the mathematical model initial conditions are applied to one of the integrators. In the present case it was assumed that \(u_{c(0)}=0.05\,u_n\), where \(u_n\) is the rated voltage of the generator.

The process of excitation of the asynchronous generator, modeled on the AVM, is recorded visually by an oscillograph. The increase in magnetic flux is monitored at the output of functional block 1BH in Fig. 2 [3]. When the resultant flux in the air gap reaches its rated value \(\psi_\delta=1\), the load is switched on and the capacitance is disconnected at the instant when the voltage across the capacitance is equal to zero. The active resistance of the stator was varied within the limits \(R_s=0;\ 0.005;\ 0.01;\ 0.02\). Figure 1a gives results showing the degree of influence of the active resistance of the stator winding on the character of the change in the energy transferred to the load, \(Q_n=f(x_n)\); here, the unit adopted is the energy values corresponding to the rated data of the generator over the same interval of time.

As follows from the graph, an increase in the active resistance of the stator leads to a decrease in the energy transferred to the load and to a shift of the maximum of \(Q_n=f(x_n)\) toward larger resistances; i.e., when choosing \(x_n\), in the general case for \(R_s\ne0\), one should take \(x_n>x_d''\), since the matched regime occurs when

\[ x_n \simeq z_r = \sqrt{R_s^2 + x_d''^{\,2}}. \]

To determine the influence of the time constant of the rotor winding \(T_r\) on the transfer of energy to the load, the active resistance of the rotor was varied within the following limits: \(R_r=0;\ 0.005;\ 0.01;\ 0.02\). The procedure for carrying out the investigations is analogous to that described above. The calculation results are presented in Fig. 1b. As in the first case, an increase in the rotor resistance reduces the energy in the load, with a shift of the maximum of \(Q_n=f(x_n)\) into the region of larger resistances. This is explained by the fact that a decrease in the time constant of the rotor winding leads not only to an increase in the supertransient resistance of the generator, but also to more rapid damping of the rotor magnetic field, as well as penetration of the armature flux into the circuits of the rotor winding and, correspondingly, to a decrease in the generator emf.

Fig. 1b. Influence of the parameters of the rotor winding of an asynchronous generator on the energy transferred to the load: \(R_s=0.005;\ 1\)—\(R_r=0;\ 2\)—\(R_r=0.005;\ 3\)—\(R_r=0.01;\ 4\)—\(R_r=0.02\).

1. The influence of the capacitance in the generator stator circuit on the energy delivered to the load is shown in Fig. 2a. The capacitive reactance was varied within the generator self-excitation zone. The initial value of the generator voltage remained constant, corresponding to the rated flux in the air gap of the machine. To satisfy this condition, constant disturbances corresponding to a normally excited machine are applied to all integrators of the structural circuit. From the calculated data \(Q_{\text{н}}=f(x_{\text{н}})\) presented in Fig. 2a it follows that increasing the capacitance of the capacitor bank leads to an increase in the load energy. However, the increase in the load energy occurs by the amount of the energy of the capacitor bank. These studies were carried out at a constant generator speed, without taking into account the influence of saturation on the magnitude of the stator leakage inductive reactance.

Fig. 2a. Influence of the excitation capacitance on energy transfer to the load: \(R_s=R_r=0.005\); \(1 — X_c=0.3\); \(2 — X_c=0.1\); \(3 — X_c=0.05\); \(4 — X_c=0.025\); \(5 — X_c=0.01\).

Along with calculations on the mathematical model, experimental studies were performed on a prototype generator. In particular, a comparison was made of the synchronous and asynchronous operating modes of the machine with a pulsed load. The parameters of the asynchronous generator were as follows: \(R_s=0.436\ \Omega\); \(R_r=0.185\ \Omega\); \(x_d=13.75\ \Omega\); \(x_{\sigma s}=0.8\ \Omega\); \(x_{\sigma r}=0.74\ \Omega\).

As follows from Fig. 2b, an increase in the capacitance in the generator circuit is accompanied by an increase in the load energy, which agrees with the calculations performed. At a small excitation capacitance, the load energy of the asynchronous impact generator is less than the load energy of the synchronous impact generator. It should be noted that, with an increase in the energy transmitted to the load, the energy of the capacitor bank also increases in percentage terms. Thus, for

\[ C=150\ \mu\text{F}, \qquad Q_c=0.12\,Q_{\text{н}}, \]

\[ C=300\ \mu\text{F}, \qquad Q_c=0.19\,Q_{\text{н}}, \]

\[ C=400\ \mu\text{F}, \qquad Q_c=0.21\,Q_{\text{н}}, \]

where \(Q_{\text{н}}\) is the load energy.

Fig. 2b. Experimental studies of a mock-up model of the impact generator: — asynchronous generator; – – – synchronous generator; 1 — \(C=150\ \mu\text{F}\); 2 — \(C=300\ \mu\text{F}\); 3 — \(C=400\ \mu\text{F}\)

Fig. 3 shows an oscillogram of the load current \(i_{\text{н}}\) and the generator voltage \(u_c\) of an experimental model of an asynchronous impact generator.

Fig. 3

Thus, the studies carried out on the operation of an asynchronous single-phase machine in generator mode with a pulsed load have shown the fundamental possibility of generator operation; moreover, in terms of the energy delivered to the load, in some cases it is not inferior to the synchronous impact generator, and in terms of the quality of the output voltage and the use of active materials, the contactless asynchronous generator can fully compete with synchronous generators in autonomous pulsed systems.

LITERATURE

1. E. G. Komar. Charged Particle Accelerators. Atomizdat, Moscow, 1964.

2. F. K. Arkhangel’skii, A. M. Stolov. Application of impact generators for producing pulsed magnetic fields with large energy reserves. Electrophysical Apparatus, issue 4. Atomizdat, 1966.

3. Yu. A. Romanov, G. A. Sipailov. Analysis of the operation of an asynchronous impact generator using a digital computer. Izvestiya TPI, vol. 212, 1970.