Synchronous Sparkless Switch

N. P. Ryashentsev

Introduction

One of the progressive directions in the development of technology is the combination of the motor machine with the working machine. This question is solved most simply in electromagnetic mechanisms of reciprocating motion, where the working member is at the same time one of the component parts of the machine. At present, mechanisms with reciprocating motion of the working member are finding ever wider application (feeders, screens, vibrators, solenoid drives, hammers, impact-rotary drilling machines, etc.). However, many electromagnetic mechanisms, because of imperfections in their design and power-supply circuits, have not yet become widely used (solenoid hammers and impact-rotary drilling machines).

In work [1], Prof. A. N. Moskvitin points out that one of the main difficulties in creating solenoid hammers lies in the absence of a reliably operating interrupter. Owing to the large inductance of the hammer coil, when it is switched off an arc is formed at the switch terminals, which, at a high switching frequency, causes very rapid burning of the contacts. The methods used up to now to combat the occurrence of an arc have not been successful. It is known that a hammer with a separately located interrupter, or with an interrupter controlled by the striker, gave good results, but the contacts of the interrupter quickly burned under the action of the electric arc.

There were attempts to create synchronous alternating-current switches. In these designs the switching of the current occurred at the moment when it passed through zero. In this case the burning of the contacts was considerably reduced. However, the contacts still burned, since the moment when the current passes through zero is determined by the magnitude of the hammer load and by the oscillation of the rotor relative to the stator field, while the time for switching the supply of the hammer coils in the given circuit must be equal to zero. Improvement of the quality of the contacts, as well as reduction of the operating voltage, was not successful.

Subsequently, designs of a synchronous solenoid hammer of the “Synchronous” type and by K. N. Shmargunov [2] were proposed. Hammers of these designs operated on alternating current; rectifiers were used as switches. With such a power-supply circuit for the hammer coils, the striker made 3000 blows per minute and, with the existing designs, did not produce a sufficiently large impact energy. The hammer operates more effectively at a reduced blow frequency, since under these conditions the adverse effect of the counter-electromotive force of the striker, eddy currents, and hysteresis losses is smaller. In order to implement a hammer design with a blow frequency of 1500 per minute, it is necessary to find a new, reliably operating switch design that provides sparkless switching on and off of the supply to the hammer coils.

Synchronous Sparkless Switch

The principle of sparkless operation of the switch is based on switching the load supply on and off at the moment when the current in the rectifier is equal to zero (not counting the reverse current of the rectifier). The working element of the switch may be of the brush or cam type. Each type of switch has its own advantages and disadvantages. In the present work we do not set ourselves the goal of identifying the rational design of the switch, but wish to show the principle of operation of the switch and to determine the conditions for its sparkless operation. Let us consider the principle of operation of a brush-type sparkless switch. The number of brushes (or cams) of the switch and their mutual arrangement depend on the supply circuit of the coils of the solenoid hammer.

When the switch operates in the half-wave circuit shown in Fig. 1, a, its design will consist of the following main parts (Fig. 1, b)*: a synchronous electric motor (\(n = 1500\) rpm) 1, brushes 2 for switching on coil K1, sliding contact 3 receiving power from the rectifier (a bronze half-ring), sliding contact 4 (a bronze half-ring) for switching on coils K1 and K2, solder 5, insulating base 6, unsoldered half-rings 7, and brushes 8 for switching on coil K2.

With the positive half-wave of voltage \(U\), a current \(i_n\) flows through rectifier \(B\), the soldered half-rings of switch \(P\), and coil K1. The supply circuit of coil K2 will be open, since brushes 8 slide over the unsoldered half-rings 7. When the shaft of motor 1 is turned through 180°, the supply to coils K1 and K2 will change to the opposite, i.e., coil K2 will receive power through the soldered half-rings, while the supply circuit of coil K1 will be open.

* Student V. T. Lopatin took part in the manufacture of the switch.

To rotate the commutator slip rings, a single-phase synchronous motor 1 rated at 1500 rpm is installed. The arrangement of the soldered slip rings relative to the stator and rotor of the motor is shown in Fig. 1, c. During one revolution of the shaft of motor 1, two current periods can pass in phase A, but only the positive half-periods of the current are passed by valve B, as shown in Fig. 1, d.

Thus, if the soldered slip ring 3 and the brushes of the switch 2 and 8 are fixed relative to the rotor and stator of the motor as shown in Fig. 1, then, as the rotor rotates, the soldered slip ring 3 will close the brushes of the supply circuit of coil K1 at point a, i.e., before current begins to flow in phase A. The advance in switching on the load K1 by the commutator, and only afterward by the valve, makes it possible to avoid sparking at the commutator brushes during switching-on of the hammer coils.

Fig. 1. Sparkless brush commutator:
a—operation of the commutator in a single-half-period circuit; b—design of the brush commutator; c—arrangement of the commutator slip rings relative to the motor poles; d—developed diagram of the displacement of the soldered slip rings relative to the current-variation curves.

At point b, current begins to flow through the brushes, the slip rings, and coil K1. The rate of rise and fall of the current is determined by the circuit parameters. When the commutator operates according to the circuit shown in Fig. 1, the rate of change of current is determined by the inductive and active components of the circuit load. In deter-

parameters of the circuit, the choice of which is given below, switching off an active-inductive load by the valve occurs at point \(c\), and with an active load at point \(c'\), but the switch brushes leave the soldered half-rings only at point \(d\). Preliminary switching-off of the load by the valve makes it possible to ensure sparkless switching of the load by the brushes. We do not take into account the effect of the valve reverse current, since it is very small and in practice has no influence on spark formation.

Thus, for sparkless operation of the switch it is necessary that the angle of duration of valve operation be smaller than the angle of duration of the closing of the switch brushes \(\omega t_{\mathrm{k}1}\), \(\omega t_{\mathrm{k}2}\), and also that the load be switched on in advance by the switch

Fig. 2. Operation of the switch in a two-half-period circuit of three-phase current with a neutral conductor, with separate switching of phases \(A\) and \(B\).

\((a—b)\) and that the load be switched off in advance by the valve \((c—d)\). All the conditions for sparkless operation of the switch are illustrated clearly in Figs. 1 and 2.

When the switch operates according to the circuit shown in Fig. 2, the switch construction described above is supplemented by one more pair of soldered half-rings of phase \(B\), which are displaced relative to the soldered half-rings of phase \(A\) by 120 electrical degrees.

Influence of the load parameters on the period of sparkless operation of the switch and the choice of rational switch circuits

Operation of the switch in a half-wave rectifier circuit

In order to find the zone of sparkless action of the switch, it is necessary to find the dependence of the angle of duration of valve operation \(\Theta\) on the load.

When the switch and the valve operate in a half-wave rectifier circuit of single-phase current (Fig. 1), the processes during

changes in voltage according to a sinusoidal law are described by the differential equation

\[ L_{\mathrm{n}} \frac{d i_{\mathrm{n}}}{d t} + R_{\mathrm{n}} i_{\mathrm{n}} = u = \sqrt{2}U \sin \omega t, \tag{1} \]

where \(L_{\mathrm{n}}\) is the load inductance;
\(i_{\mathrm{n}}\) is the load current;
\(R_{\mathrm{n}}\) is the resistance included in the load circuit;
\(u\) is the instantaneous value of the voltage;
\(U\) is the maximum value of the voltage.

The load current of the hammer coils, flowing through the rectifier and the switch, for convenience of analytical expression may be represented as the sum of the steady-state current \(i'_{\mathrm{n}}\) in the absence of the rectifier and the free component of the current \(i''_{\mathrm{n}}\).

From the fundamentals of electrical engineering, the expression for these components is known:

\[ i'_{\mathrm{n}} = \frac{\sqrt{2}U}{z_{\mathrm{n}}} \sin(\omega t - \varphi_{\mathrm{n}}), \tag{2} \]

\[ i''_{\mathrm{n}} = c e^{-\frac{R_{\mathrm{n}}}{L_{\mathrm{n}}}t} = c e^{-\frac{t}{\tau_{\mathrm{n}}}}, \tag{3} \]

where \(\tau_{\mathrm{n}} = \frac{L_{\mathrm{n}}}{R_{\mathrm{n}}}\) is the time constant of the load circuit.

In equation (3), the integration constant is determined from the initial conditions of equality (1) at \(t = 0\) and \(i_{\mathrm{n}} = 0\):

\[ c = \frac{\sqrt{2}U}{z_{\mathrm{n}}} \sin \varphi_{\mathrm{n}}, \tag{4} \]

Then

\[ i_{\mathrm{n}} = i'_{\mathrm{n}} + i''_{\mathrm{n}} = \frac{\sqrt{2}U}{z_{\mathrm{n}}} \left[ \sin(\omega t - \varphi_{\mathrm{n}}) + e^{-\frac{t}{\tau_{\mathrm{n}}}} \sin \varphi_{\mathrm{n}} \right]. \tag{5} \]

In Fig. 3, a, curves are given for the variation of the load current and its components \(i'_{\mathrm{n}}\) and \(i''_{\mathrm{n}}\) during operation of the switch according to the circuit shown in Fig. 1, and with sinusoidal voltage of the power supply. From the graphical construction (Fig. 3) of equation (5) and from the oscillogram (Fig. 3, v), taken during operation of the switch, it is seen that the load current can flow through the rectifier and the switch for a time interval exceeding a half-period. The increase in the angle of the duration of operation of the rectifier is explained by the induction of a positive value of the self-induction emf

\[ e_L = - L_{\mathrm{n}} \frac{d i_{\mathrm{n}}}{d t} \]

during the decay of the load current \(i_n\) (3); therefore the current flows through the valve and the switch until \(u+e_L>0\).

The character of the variation of the voltage of the power-supply source \(u\), the emf of self-induction \(e_L\), and the load voltage (of the hammer coils) is shown in Fig. 3, b. From Fig. 3 it is seen that at \(\Theta=\omega t\) the valve is blocked because \(u+e_L=0\). Under the condition \(\Theta \leq \omega t \leq 2\pi\)

Fig. 3. Operation of the switch in a single-half-period circuit:
\(a\)—curves of variation of the load current and its components; \(b\)—curves of variation of the voltage of the power-supply source, the emf of self-induction, and the load voltage; \(v\)—oscillogram of the current variation.

no load current flows through the switch, since a reverse voltage is applied to the valve.

The interval of time during which the load current \(i_n\) of the valve is equal to zero (not counting the reverse current of the valve) will be called the sparkless switching period \(\alpha\). When the switch operates according to the circuit (Fig. 1), the sparkless switching period \(\alpha\) is expressed by the relation \(\alpha=2\pi-\Theta\). During this time interval the power supply to the hammer coils must be switched. From the beginning of the next period the cycle is repeated.

The dependence of the angle of the duration of operation of the valve on the ratio of the inductive component of the load to the active component is found from equation (5) for \(i_{\mathrm{н}}=0\).

\[ \sin(\Theta-\varphi_{\mathrm{н}})+e^{-\frac{R_{\mathrm{н}}}{L_{\mathrm{н}}}\Theta}\sin\varphi_{\mathrm{н}}=0. \tag{6} \]

The graphical expression of the dependence of the angle of the duration of operation of the valve,

\[ \Theta=f\!\left(\frac{\omega L_{\mathrm{н}}}{R_{\mathrm{н}}}\right), \]

calculated by formula (6) for operation of the rectifier in a single-half-period circuit (Fig. 1), is given in Fig. 4.

Fig. 4. Dependence of the angle of the duration of operation of the valve in a single-half-period circuit with an active-inductive load.

Sparkless operation of the switch will occur under the condition \(\alpha \geq 20^\circ\), where \(20^\circ\) is taken from the design data of the brush switch and is determined by the type of synchronous motor and by the choice of the design of the switch working member. From the diagram shown in Fig. 4 it is seen that, for \(\alpha \geq 20^\circ\), for sparkless operation of a switch operating according to the circuit shown in Figs. 1 and 2, it is necessary that the ratio of the inductive component of the load to the active component be no more than 12.5, i.e.,

\[ \frac{\omega L_{\mathrm{н}}}{R_{\mathrm{н}}}\leq 12.5. \]

When the switch operates according to the circuit for supplying the hammer coils shown in Fig. 5, the period of sparkless switching is expressed as

\[ \alpha=2\pi-\frac{3}{4}\pi-\Theta=\frac{5}{4}\pi-\Theta. \]

Under these conditions, sparkless operation of the switch is ensured when the ratio is

\[ \frac{\omega L_{\mathrm{n}}}{R_{\mathrm{n}}} \ll 1. \]

Operation of the Switch in a Two-Half-Period Circuit of Three-Phase Current with a Neutral Wire

For effective use of the copper and of the magnetic circuit of the hammer, and also to reduce hysteresis and eddy-current losses, the best circuit for supplying the hammer coils will be one that provides the maximum average value of the rectified current over a period with a minimum current amplitude for a given load power.

The supply circuit shown in Fig. 5, a, satisfies these conditions most fully. Figure 5, b shows an oscillogram of the current variation for this circuit.

Fig. 5. Operation of the switch in a two-half-period circuit of three-phase current with a neutral wire when phases A and B are switched jointly:
a—circuit; b—oscillogram of current variation.

The oscillogram of current variation for this circuit is shown. With such a supply circuit for the hammer coils, sparkless operation of the switch is ensured when switching a load of 200–300 W (at a voltage of 220 V).

In order to ensure sparkless operation of the switch with a considerable increase in load (by several tens of times), it is necessary to switch each phase on and off independently of the others. For this it is necessary that the switch operate according to the coil-supply circuit shown in Fig. 2. The proposed circuit provides the maximum average value of the rectified current and sparkless operation of the switch. In this case the design of the switch will have a larger number of brushes and soldered bronze half-rings (sliding contacts). The conditions for sparkless operation of the switch operating according to the circuit shown in Fig. 1 fully correspond to operation according to the circuit given in Fig. 2.

CONCLUSION

From the above it is evident that the proposed synchronous sparkless switch operates reliably and for a long time...

but only with quite definite parameters for each coil-supply circuit. From an analysis of the switch operating circuits for supplying the hammer coils (Figs. 1, 2, 5), it is evident that the circuit shown in Fig. 2 has the best sparkless switching properties at the maximum value of the rectified current and the minimum amplitude.

The proposed switch makes it possible to create a number of new designs of solenoid hammers with an impact frequency of 1500 impacts/min and less, which is very important for increasing the efficiency and impact energy of the hammer.

REFERENCES

1. A. I. Moskvitin. Electrical Machines of Reciprocating Motion. Academy of Sciences of the USSR, 1950.
2. K. N. Shmargunov. Electric Hammers. Mashgiz, 1950.
3. M. A. Rozenblat. Magnetic Amplifiers. Sovetskoe Radio Publishing House, 1956.
4. V. G. Komar. Operation of Semiconductor Rectifiers in Control Circuits. Gosenergoizdat, 1952.
5. M. A. Kaganov. Electron and Ion Converters. Gosenergoizdat, 1955.