CONTACTLESS ADJUSTABLE INDUCTION MOTOR

A. I. Skorospelshkin, M. L. Kostyrev, E. F. Obergan

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

One of the means that makes it possible to regulate the speed of a contactless induction motor and to improve its power factor consists in introducing into the secondary circuit an e.m.f. with slip frequency. In existing adjustable induction motors this e.m.f. is introduced by means of an auxiliary commutator machine, which may be built in (the Schrage–Richter motor) or made as a separate machine (induction-commutator cascades). Recently, static ionic and semiconductor frequency converters have also been used for this purpose.

Fig. 1. Diagram of a contactless adjustable induction motor.

In the proposed motor (Fig. 1), the e.m.f. is introduced into the rotor by means of an additional cascade 1 with a controlled semiconductor commutator (CSC). The motor has no sliding contacts and is recommended for operation in explosive and dusty environments, as well as under high-altitude conditions.

The rotor windings \(W_{p1}\), \(W_{p2}\), and the stator winding \(W_{c2}\) are ordinary three-phase windings. The control winding \(W_y\) is of the armature type of a direct-current machine. Each of its sections has a tap connected to a cell of the CSC.

Both cascades can be combined in a single magnetic core. In this case the ratio of the numbers of pole pairs, the shortening of pitches and the distribution of sections, as well as the order of connection of the coil groups in a phase, should be selected so that forces of unilateral magnetic attraction and transformer coupling between the cascades are absent [1].

Combining the cascades makes the machine compact and provides a substantial saving of structural materials.

The pulse sensor (PS) is made in the form of a salient-pole magnet on the rotor and a coil with a magnetic core on the stator. It generates pulses for the commutator.

In the case of a two-pole first cascade, the control winding is supplied from a direct-current source through diametrically opposite cells and creates a magnetic-field wave sinusoidally distributed around the circumference in the gap.

All cells, except for two diametrically opposite ones, are permanently locked. Each pulse arriving at the controlled commutator opens two opposite cells following the cells opened by the preceding pulse. The previously opened cells are closed, for example, by means of commutating capacitors.

This leads to a displacement of the control-field wave by one slot division. With a continuous sequence of pulses, the control field will rotate at a speed \(n_y\).

Tests of the commutator using silicon controlled rectifiers of the KU201L type showed that it operates steadily with a purely inductive load—armature-type windings—in the frequency range from 0 to 400 cps. The limiting value of the commutated current depends almost not at all on frequency and, for the tested circuit, was about 5 A at a section inductance of 4 mH.

If power is applied to winding \(W_{c2}\), then in the absence of a control signal the motor starts and operates as an ordinary asynchronous motor with synchronous speed

\[ n_2=\frac{f}{p_2}\;[\text{rev/sec}]. \]

\(p_2\) — number of pole pairs of cascade II,

\(f\) — mains frequency.

In the presence of a control signal in winding \(W_{p1}\), an e.m.f. \(E_{p1}\) is induced by the field of cascade I with frequency

\[ f_p=(n_y \pm n)p_1, \]

\(p_1\) — number of pole pairs of cascade I,

\(n\) — rotor speed \([\text{rev/sec}]\).

Equality of the frequencies of the main e.m.f. \(E_{p2}\) from the field of cascade II and the additional e.m.f. \(E_{p1}\) is achieved by controlling the controlled commutator through the circuit for forming pulses from the pulse sensor rotating synchronously with the rotor. If \(E_{p1}\) is in phase with \(E_{p2}\), then the motor increases its speed until the rotating torque is balanced by the load torque. When the polarity of the control current is changed, the motor will reduce its speed. In the general case the rotor speed is

\[ n=n_{\mathrm{з}}\left(1-S\pm\frac{\dot{E}_{p1}}{E_{p2}}\right), \]

\[ S=\frac{n_2-n}{n_2}. \]

The slip \(S\) depends on the load torque. When \(E_{p1}\) is shifted in phase relative to \(E_{p2}\), an e.m.f. component appears that causes a decrease or increase of the reactive current in the rotor, and consequent-

... in the stator as well. To increase the motor’s $\cos \varphi$, it is necessary to ensure that $E_{p1}$ leads $E_{p2}$ in phase. This can be achieved by rotating the sensor relative to winding $W_{p2}$ during adjustment. By shifting the sensor pulses in time by means of a trigger circuit, a high $\cos \varphi$ can be ensured over a wide speed range.

The proposed motor is a double-fed machine. Investigations show that its dynamic and static characteristics are determined primarily by the law of regulation of the emf introduced into the rotor [2].

By introducing feedback, for example with respect to rotor acceleration, one can obtain a wide range of stable speed regulation and mechanical characteristics of the required stiffness up to the synchronous mode. The possibility of contactless control, according to a specified law, of the amplitude and phase of the emf introduced into the rotor at slip frequency is an important advantage of the proposed circuit.

REFERENCES

1. V. S. Novokshenov. Investigation of an asynchronous brushless frequency converter. Dissertation, Tomsk Polytechnic Institute, 1960.

2. A. S. Sandler. Regulation of the rotational speed of asynchronous motors. Energiya, 1966.