STABLE OPERATION OF A DC MOTOR WITHOUT A STABILIZING WINDING

V. A. IVANNIKOV, I. A. MILORADOV

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

The rapid development of semiconductor technology is gradually reducing the need for the dc machine (d.c.m.) as a source of direct current, and the principal consumer of d.c.m.’s is becoming the widely regulated electric drive. In this connection, the range of speed regulation by field weakening should be regarded as one of the most important characteristics of dc motors. The upper limit of speed regulation, if the mechanical strength of the machine and the increasing difficulty of ensuring continuity of the sliding contact are not considered, is limited, as a rule, by instability and ambiguity of the speed characteristics and by deterioration of commutation. Therefore, in dc motors with speed regulation by the field, in order to ensure stable operation and unambiguous speed characteristics, a light series winding is used, or a winding also placed on the main poles but supplied by the voltage on the compensating winding and the interpoles [1].

A common shortcoming of the indicated methods of ensuring stable operation is the presence of an additional winding. This causes additional expenditure of copper and, in some cases, an increase in dimensions. In addition, the presence of a series winding in reversible motors, because of the additional switching in the armature circuit during reversal, considerably complicates motor control.

Stable operation of separately excited motors can also be ensured without special windings. For this it is sufficient to connect the field winding in series-opposition with the interpole winding, as shown in Fig. 1. With such a connection

Fig. 1

winding, an increase or decrease in the current of the armature circuit causes a change in the voltage on the winding of the auxiliary poles, which in this case acts in agreement with the supply source of the excitation winding. Under the action of this voltage, an additional current appears in the excitation circuit, the magnitude of which is determined by the total resistance of the excitation circuit and the voltage on the winding of the auxiliary poles. The additional current in the excitation winding varies in proportion to the current of the armature circuit and, consequently, the action of the additional magnetomotive force along the axis of the main poles is equivalent to the presence of a series excitation winding (in the case under consideration, an aiding one).

Fig. 2. Speed characteristics of a DC motor P 63, \(P = 3\ \text{kW}\), \(U = 220\ \text{V}\), \(325\text{–}2600\ \text{rpm}\),
— without a stabilizing winding,
– – – with a stabilizing winding

Figure 2 shows the natural speed characteristics of a P63 DC motor with valve-mechanical commutation and independent excitation. The characteristics were taken with a light stabilizing winding under the usual connection of the excitation winding, and without a stabilizing winding with series-opposing connection of the excitation windings and auxiliary poles. As follows from the figure, in the latter case the speed characteristics also have a sufficient slope for stable operation of the motor over the entire range of rotational speeds, and their single-valuedness is somewhat better, especially at the upper speed limit.

Figure 3a gives the speed characteristics obtained on the same machine at different values of the additional resistance in the cir-

Fig. 3. Speed characteristics of the P 63 dc motor: a) — \(W_{\mathrm{d}}=\mathrm{const}\), b) — \(R_{\mathrm{add}}=\mathrm{const}\)

... of the field winding circuit \((R_{\mathrm{add}})\) and with a constant number of turns of the interpole winding \((W_{\mathrm{d}})\), connected in series with the field winding. In Fig. 3,b the characteristics were obtained at \(R_{\mathrm{add}}=\mathrm{const}\) and different \(W_{\mathrm{d}}\). As can be seen, when \(R_{\mathrm{add}}\) or \(W_{\mathrm{d}}\) is varied, the slope of the speed characteristics also changes: with an increase in \(R_{\mathrm{add}}\) and a decrease in \(W_{\mathrm{d}}\), the stiffness of the characteristics increases; with a decrease in \(R_{\mathrm{add}}\) and an increase in \(W_{\mathrm{d}}\), it decreases; that is, with series-opposing connection of the field winding and the interpole winding, any slope of the motor speed characteristics can be obtained.

In our opinion, such a connection of the field windings and interpoles is especially effective in machines with regulation of rotational speed over wide ranges \((1:6,\ 1:8)\) by field weakening. In this case, along with compounding, compensation is provided for the demagnetizing action of the reaction of the commutating currents and for the excessive weakening, at the upper rotational speed, of the interpole field; thereby, in addition to stability, the single-valuedness of the speed characteristics and commutation are improved.

In reversible machines, during reversal it is necessary to preserve the opposing direction of the currents of the armature and the field winding. This will not create major difficulties in machine control, since the power of the field circuit usually amounts to \(2\text{–}5\%\) of the power consumed by the machine.

REFERENCES

1. I. N. Rabinovich, I. G. Shubov. Design of DC Machines. “Energiya,” 1967.