EXPERIMENTAL STUDY OF A MODEL IMPACT GENERATOR WITH LONGITUDINAL–TRANSVERSE EXCITATION

V. I. ANDREEV, K. A. KHORYKOV

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

The efficiency of converting the kinetic energy of the rotor of an impact generator into electromagnetic energy transmitted to the load is determined primarily by the magnitude of the machine’s excitation magnetic flux. In order to increase the magnetic flux in impact generators, as a rule, excitation forcing is carried out before the stator winding is connected to the load. The forcing effect can be intensified if an additional excitation winding is placed on the rotor in the part of the pole pitch free from the main excitation winding. In this case, the axis of the additional excitation winding is displaced by an angle \(\pi/2\) relative to the longitudinal axis of the rotor. The additional excitation winding is supplied from an independent voltage source by voltage pulses of large amplitude. In order to reduce losses in the steel and increase the rate of rise of the excitation flux, the rotor must be made laminated. There should be no damper winding along the transverse axis of the rotor. The resulting excitation flux \(\Phi_o\) is formed as the geometric sum of the main flux \(\Phi_d\) and the additional flux \(\Phi_q\). As the flux along the transverse axis increases, the resulting flux \(\Phi_o\) is displaced through a certain angle in the direction of rotor rotation or, conversely, depending on the direction of the flux \(\Phi_q\) (Fig. 1). When the fluxes \(\Phi_d\) and \(\Phi_q\) are equal, the resulting flux increases by \(\sqrt{2}\) times and is displaced from the \(d\)-axis by an angle \(\pi/4\).

Fig. 1. Construction of the resultant excitation flux

Longitudinal–transverse excitation makes it possible to form a steeper front of the current pulse of the impact generator. In this case the working process proceeds as follows. The main flux of the generator \(\Phi_d\) is excited and, at the moment when the flux \(\Phi_d\) is fully linked with the stator winding (the stator winding is single-phase), the generator is connected to the load and, simultaneously, or even with a certain lead, power is supplied to the additional excitation winding. The rapid rise of the transverse excitation flux, leading the longitudinal flux of the main winding, causes the amplitude of the load-current pulse to shift toward smaller ...

time. Figure 2 presents the curves of the emf of the stator winding \(e_0\) and the current of the matched inductive load under longitudinal–transverse excitation \(i_{n0}\) in the case of equality of the fluxes \(\Phi_d=\Phi_q\). For comparison, the curves of the emf \(E_d\) and the load current \(i_{nd}\) are shown with a dashed line only for a single longitudinal excitation of the machine. Increasing the current amplitude by \(\sqrt{2}\) times leads to a twofold increase in the energy delivered by the generator to the load. The steepness of the pulse front increased by a factor of 2.5; moreover, the pulse front approached a linear form, while the current pulse as a whole has a quasi-triangular shape.

Fig. 2. Curves of the emf of the stator winding \(e_0\) and the current of the matched inductive load \(i_{n0}\) under longitudinal–transverse excitation, in the case of equality of the fluxes along the longitudinal and transverse axes (solid lines). Curve of the emf \(E_d\) and the load current \(i_{nd}\) only with a single longitudinal excitation of the machine (dashed line)

The theoretical propositions set forth above were tested on a model of an impact generator constructed on the basis of an asynchronous motor with a phase-wound rotor of type MTO-12-6. Two phases of the three-phase rotor winding were used as the single-phase armature winding.

The armature voltage from the slip rings is supplied to an inductive load through a high-speed switching device \(T_1\), Fig. 3. The choice of the rotor winding as the armature winding of the generator is due to the fact that, on the phase-wound rotor of a standard asynchronous motor, the neutral point is not brought out and there are three slip rings; in this connection difficulties arise in forming two independent excitation windings. As the main excitation winding, phase of the stator winding \(C_1—C_4\) was adopted, and as the additional excitation winding, the series-connected phases \(C_2—C_3\). The positions of the stator and rotor windings in Fig. 3 correspond to the instant of switching on the load.

The generator drive is provided by an asynchronous motor. The parameters of the windings of the model generator and the results of the experiments are presented in Table 1.

Fig. 3. Diagram of the experimental setup

Table 1

Note. \(x_{fd}=x_f+x_{ad}\).

Indicative is the dependence of the time constant of the additional excitation winding, and consequently of the rate of increase of the flux along the transverse axis, on the saturation of the magnetic circuit of the machine. At the same voltage, the current in the additional excitation winding reaches its steady-state value 15 msec after voltage is applied to the winding when the main excitation winding is open, and after 7 msec if a current of 30 A flows through the main excitation winding. The experimental investigations show the possibility of a rapid increase of the excitation flux along the transverse axis.

Figure 4 presents oscillograms of the impact current of the generator under longitudinal–transverse excitation \((i_{ed}=30\ \text{A},\ i_{eq}=42.5\ \text{A})\). The current pulse has a quasi-triangular shape with a steep leading edge.

Fig. 4. Oscillogram of the impact current under longitudinal–transverse excitation.

On the basis of the experiments carried out, the following conclusions may be drawn:

1. Longitudinal–transverse excitation makes it possible to use the rotor of the impact generator more fully and thereby to increase its power.

2. Longitudinal–transverse excitation makes it possible to form current pulses of quasi-triangular shape with a steep leading edge.