CASCADE BRUSHLESS SYNCHRONOUS MOTOR

A. I. Skorospelkin, M. L. Kostyrev, A. P. Felzing

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

Possessing high reliability and not requiring careful maintenance during operation, brushless synchronous motors (BSM) are finding ever broader application in various branches of the national economy, especially under conditions where the presence of sliding contact is inadmissible. At present, the effectiveness of using low-power BSMs (1.1–10 kW) has been demonstrated [1, 2]. However, existing brushless synchronous motors (claw-pole types with an externally closed magnetic circuit, with rotating semiconductor rectifiers) have a number of substantial shortcomings:

1) considerable overall dimensions and weight;
2) low starting torque;
3) insufficient reliability of motors with rotating rectifiers;
4) when supplied from a higher-frequency network (300–400 Hz), in order to obtain an acceptable speed it is necessary to increase the number of poles, which worsens the weight and energy-performance indices of the motor.

In connection with this, it is necessary to search for new, more advanced BSM circuits. We investigated the circuit of a cascade brushless synchronous motor shown in Fig. 1 [3].

Fig. 1. Circuit of a cascade brushless synchronous motor

The motor is a cascade consisting of two asynchronous machines combined in a single magnetic core. The primary winding of the second machine \((W_{p2})\) is placed in the rotor slots and is electrically connected to the secondary winding of the first machine \((W_{p1})\). In synchronous operation the role of the excitation winding is performed by the secondary winding \((W_{\mathrm{B}})\) of the second machine, placed in the stator slots. When the three-phase stator winding \((W_{\mathrm{C}})\) of the first machine is connected to the supply, a magnetic field arises in the air gap, rotating at the speed

\[ n_1=\frac{f_1}{p_1}, \]

where

\(f_1\) is the supply frequency,

\(p_1\) is the number of pole pairs of the first machine.

The cascade starts operating as an ordinary asynchronous motor with additional resistance inserted in the rotor circuit. In this case an emf with frequency

\[ f_{p1}=(n_1-n)p_1=f_1-n\cdot p_1, \]

is induced in the rotor winding \(W_{p1}\), where \(n\) is the rotor rotational speed.

Through winding \(W_{p2}\), connected to winding \(W_{p1}\) in such a way that the magnetic field it creates rotates opposite to the rotor, a current of the same frequency flows. The secondary winding of the second machine \(W_{\mathrm{B}}\) is connected to a direct-current source. The magnetic flux it creates induces in winding \(W_{p2}\) an emf with frequency

\[ f_{p2}=p_2\cdot n, \]

where \(p_2\) is the number of pole pairs of the second machine.

The cascade rotor will increase its rotational speed until a sufficient synchronizing torque arises and the motor pulls into synchronism. In this case the magnetic field of winding \(W_{p2}\) will be stationary relative to the stator, and the emf frequencies in windings \(W_{p1}\) and \(W_{p2}\) will be equal:

\[ f_1-np_1=n\cdot p_2. \]

Hence the synchronous rotational speed is

\[ n=\frac{f_1}{p_1+p_2}. \]

The synchronous speed of the cascade is determined by the supply frequency and the total number of pole pairs of the combined machines.

Studies of the operability of such a cascade brushless synchronous motor were carried out on a model made on the basis of an A0-32-6 asynchronous motor. Figure 2 presents the mechanical characteristics of the cascade \(n=f(M)\). It should be noted that, although the machine was not optimal, the cascade nevertheless operated stably at synchronous speed over a sufficiently wide range of load variation. The motor started and self-synchronized under load, withstood sudden load shedding and load application without falling out of synchronism. The circuit permits regulation of the cascade’s \(\cos\varphi\) (Fig. 3). However, the nominal power developed by the cascade in synchronous operation is approximately two times less than the nominal power of an asynchronous motor with the same rotational speed.

On the whole, from the results of the investigations carried out, the following conclusions may be drawn:

1. The cascade brushless synchronous motor develops considerably less power in comparison with an asynchronous squirrel-cage safeguarding?

Fig. 2. Mechanical characteristics of the motor:
a — asynchronous mode ($W_{p1}$ short-circuited),
b — synchronous mode

Fig. 3. Regulation of the motor $\cos \varphi$ at constant shaft load ($M = 0.11$ kgm)

with a commutator motor in the same dimensions at the same synchronous speed of rotation.

1. The increase in power in the synchronous mode is associated with an expansion of the region of stable operation of the cascade brushless synchronous motor, which can be obtained by an appropriate selection of the parameters and characteristics of the combined machines.

2. By regulating the degree of excitation, it is possible to vary the reactive component of the current consumed from the network.

3. Since the synchronous speed of the cascade is determined by the total number of pole pairs of the combined machines, a motor of this type is advantageous for use in electric drives of increased frequency (300, 400 Hz) when it is necessary to obtain a low rotational speed.

REFERENCES

1. V. M. Kutsevalov. Brushless synchronous electric motors for driving production mechanisms. Institute of Power Engineering, Academy of Sciences of the Latvian SSR, vol. 12, 1966.

2. Yu. F. Vaivars. Some economic comparisons of brushless synchronous and asynchronous motors. Institute of Power Engineering, Academy of Sciences of the Latvian SSR, vol. 16, 1963.

3. E. Mishkin. Poly-field Alternating—Current Induction Machines. U.S. Patent No. 2966623, 27. 12. 1960.