A New Method for Controlling the Speed of Slip-Ring Motors

I. G. KULEEV

Speed control of asynchronous slip-ring motors is carried out by inserting an active resistance into the rotor circuit or by connecting them in cascade. Speed control by the first method is associated with large losses and a reduction in efficiency. Control by the second method involves installing additional machines, with low utilization of each of them. In addition, speed control by the latter method takes place in steps. In view of this, the author set himself the task of finding a new method that would not have the above-mentioned disadvantages.

According to the proposed method, thyratrons are inserted into the rotor circuit of the motor (Fig. 1), a characteristic feature of which is a decrease in the ignition potential when the grid potential is increased. This feature of the thyratron is represented quite fully by the starting characteristics (Fig. 2), where the number I denotes the positive characteristic and the number II the negative one. According to the curves (Fig. 2), when the positive grid potential is increased, the thyratrons begin to pass current at a lower anode voltage. Applied to the rotor circuit of an asynchronous

Fig. 1

Fig. 2

motor, in whose circuit thyratrons are included, this circumstance should be understood as follows: with an unchanged rotational torque of the motor and an increase in the positive grid potential, the rotor e.m.f. must decrease. Since a smaller value of e.m.f. in the rotor winding of an asynchronous machine can be attained only at a higher speed, an increase in the voltage on the grid leads to an increase in the number of revolutions, and conversely, a decrease in the voltage on the grid leads to a decrease in the number of revolutions of the motor. Consequently, according to the author’s method, speed control

motors with slip rings is carried out as follows: two thyratrons are connected into the motor rotor circuit according to the circuit (Fig. 1), and the grid potential is varied.

When the positive grid potential is increased, the speed increases, and when it is decreased, the speed decreases. Good results are obtained in the case when the thyratrons have a positive characteristic. In this case, because the characteristic has a small slope over the entire section, the limits of speed regulation are considerably expanded. When thyratrons with a negative characteristic are used, the limits of speed regulation are so insignificant that the advisability of using this method is called into question.

A characteristic feature of speed regulation by the author’s method is that the speed regulation takes place practically without losses and with a greater degree of smoothness. Good smoothness of speed regulation is achieved by smoothly varying the grid potential.

As for the efficiency of the motor, its decrease as compared with the value when there are no thyratrons in the rotor circuit will be due to losses in the thyratrons \(P_T = \Delta U I_p\), where \(\Delta U\) is the voltage drop in the thyratrons, \(I_p\) is the rotor (anode) current. With a constant load torque \(M = c \Phi I_p\), the losses in the thyratron will be \(P_T = \Delta U\); since \(\Delta U = 15\text{ V} = \mathrm{const}\), the reduction in the motor efficiency is approximately

\[ \frac{\Delta U}{U_p}\,100\%. \]

Here \(U_p\) is the rotor voltage at no load. For \(\Delta U = 15\) V and \(U_p = 750\) V, the reduction in efficiency will be

\[ \frac{\Delta U}{U_p}\,100 = \frac{15 \cdot 100}{750} = 2\% \]

at all motor speeds. With such a small percentage decrease in efficiency, the cost of the thyratrons—and, for high-power installations, the cost of the mercury rectifiers that must be connected into the rotor circuit of the asynchronous motor—will undoubtedly be paid back fairly quickly.

In the electrical-machinery laboratory of TPI, speed regulation by the author’s method was carried out on a three-phase asynchronous motor DT-70: 70 watts, 1500 rpm, 220 volts. Thyratrons of type TG-235 were connected into the rotor circuit; their starting characteristic is shown in Fig. 3.

The dependence of the number of revolutions per minute on the grid potential—\(U_c\), at a load

\[ P = \frac{1}{2} P_n, \]

where \(P_n\) is the rated power of the motor, is shown in Fig. 4.

The motor was tested according to the circuit shown in Fig. 1. In order to expand the limits of speed regulation, it is necessary to connect one more thyratron into the rotor circuit, connecting its anode to the cathodes of the other two thyratrons, and its cathode to the third rotor ring. However, in this case, owing to losses in the additional thyratron, the efficiency of the installation decreases, and, owing to the change in the conditions of flow of the rotor current, nonuniformity of running appears. Connecting a larger number of thyratrons into the rotor circuit with pairwise opposing connection narrows the limits of motor speed regulation.

The author’s method may be applied for regulating the speed of slip-ring motors, both low-power and high-power. In the latter case it will be more economical. It is assumed that in high-power installations metallic mercury rectifiers with grids should be connected into the rotor circuit. As for the installation for regulating the potentials of the thyratron grids, it practically does not increase the cost of the power installation, since the power of this installation is small. In our case a reconstructed UM-40 was used (a machine of considerably

but of lower power). The reconstruction of the UM-40 machine consists in lengthening its shaft in order to mount on it six rings, to which three beginnings and three ends of the armature phases are connected, taken from the side opposite the commutator. The ends of the phases are connected together, and the beginnings are connected, through the corresponding resistances, to the grids of the thyratrons (Fig. 1).

Fig. 3

Fig. 4

Regulation of the voltage on the thyratron grids was carried out by changing the resistance inserted in the armature circuit of the single-armature converter OP (Fig. 1).

The proposed method has the following advantages over existing ones: 1) a higher efficiency of the installation and 2) ease of maintenance.