Brushless AC electric machine amplifiers
Abstract Generated abstract
A brushless alternating-current electric-machine amplifier is examined as a semiconductor-enabled alternative to collector-based designs. The paper describes a circuit using a compensating winding displaced by 90 electrical degrees to offset the transverse component of armature reaction, derives expressions for the output voltage and external characteristic under load, and compares calculated behavior with tests on a constructed model. The results indicate that complete transverse compensation makes the external characteristic linear under active load, while the measured gain factor exceeds 500 and the experimentally determined time constant is about 0.1 s. The authors suggest that the device can serve as an industrial high-frequency source with amplifier properties.
Full Text
A Collectorless Alternating-Current EMU
A. I. Skorospelishkin, E. N. Podborskii
(Recommended by the seminar of the Departments of Electrical Machines and General Electrical Engineering)
The present level of development of semiconductors makes it possible to develop collectorless electric-machine amplifiers. The circuit of such an amplifier is shown in Fig. 1. The principle of its operation is given in [1]. The difference between this amplifier and the BEMU described in [1] is that, in order to compensate the transverse component of the armature reaction caused by the load current, winding \(W_{pq}\) is used; it is shifted by 90 electrical degrees relative to winding \(W_{p2}\) and is supplied with direct current, the magnitude of which is proportional to the load current.
Fig. 1.
We have built a model and carried out an investigation of the principal characteristics of the amplifier: the external characteristics, the gain factor, and the speed of response. With linear current feedback, under active load, without compensation of the transverse component of the armature reaction, the external characteristics are presented in Fig. 2 (curves 1, 2). The nonlinearity of the characteristics is explained by the fact that, under active load, the armature reaction is directed along the transverse axis, while the current feedback increases the flux along the direct axis.
If it is assumed that the parameters of the amplifier are constant, and that the current, voltage, and e.m.f. vary with time according to a sinusoidal law, then the voltage at the output of the amplifier will be equal to:
\[ U_s = \sqrt{ \left(E_{s0}+R_{\alpha d}\frac{W_{ocd}}{W_y}K_{ocd}I_s\right)^2 + \left(R_{\alpha q}K_{ocq}I_s\right)^2 - I_s^2\left(X_{cx}\cos\varphi-r_s\sin\varphi\right)^2 } - I_sX_{cx}\sin\varphi-I_sr_s\cos\varphi, \tag{1} \]
where \(E_{s0}\) is the phase e.m.f. at no load,
\[ R_{\alpha d}=\frac{E_{s0}}{I_y} \]
is the magnetizing resistance along the longitudinal axis,
\(I_{ocd}=K_{ocd}I_s\) is the current in the feedback winding along the longitudinal axis,
\(R_{\alpha q}\) is the magnetizing resistance along the transverse axis,
\(K_{ocq}I_s\) is the current in the compensating winding along the transverse axis,
\(X_{cx}\) is the synchronous inductive reactance of the output winding,
\(W_{ocd}\) is the number of turns in the feedback winding along the longitudinal axis,
\(W_y\) is the number of turns in the control winding,
\(r_s\) is the active resistance of the output winding.
With complete transverse compensation,
\[ R_{\alpha q}K_{ocq}=\left(X_{cx}\cos\varphi-r_s\sin\varphi\right). \]
In this case the equation of the external characteristic is considerably simplified and takes the form
\[ U_s=E_{s0}+R_{\alpha q}\frac{W_{ocd}}{W_y}K_{ocd}I_s-I_sX_{cx}\sin\varphi-I_sr_s\cos\varphi. \tag{2} \]
For an active load \(\cos\varphi=1\), therefore longitudinal compensation is not needed. In this case we obtain
\[ U_s=E_s-I_sr_s. \tag{3} \]
For this case, the external characteristic shown in Fig. 2 by straight line 3 was obtained. Thus, with complete compensation of the transverse component of the armature reaction, the external characteristic becomes linear.
Fig. 2.
The amplification factor is one of the most important parameters of the amplifier and is evaluated by the ratio of the output and input powers.
\[ K_y=\frac{E_{SO}^{\Э} I_s+R_{ad}\frac{W_{ocd}}{W_y}K_{ocd}I_s^2\frac{X_{cx}}{X_{cxo}}s-X_{cx}\sin\varphi\, I_s^2-r_s\cos\varphi\, I_s^2}{U_y\cdot I_y}. \tag{4} \]
Figure 3 presents the dependences of the amplification factor on the load current, corresponding to the curves in Fig. 2. The obtained amplification factor (over 500) can be increased by optimizing the model.
Fig. 3
The response speed of the brushless EMU determines its operability in dynamic modes.
Figure 4 shows an oscillogram of the rise of the current in the control winding when it is suddenly connected to a DC voltage (curve 1), an oscillogram of the rise of the current in the excitation winding of the second stage (curve 2), and an oscillogram of the rise of the output voltage.
Fig. 4
The time constant, determined experimentally, is equal to \(T=0.1\) sec.
Thus, the amplifier presented can provide the required characteristics and find application in industry as a high-frequency source with amplifier properties.
LITERATURE
- A. I. Skorospeshkin, E. N. Podborskii, E. F. Obertan. Brushless electromechanical amplifier. Izvestiya TPI, vol. 145, 1966.