General Issues in the Development of Electric Drives for Modern Metal-Cutting Machine Tools

L. I. Gandzha

The postwar period is characterized by a significant change in metalworking technology at domestic enterprises and, in connection with this, by an increased output of special metal-cutting machine tools. In the coming years the range of such machines as automatic and semi-automatic machines, precision lathes with a large center height of up to 1.5 m, heavy bar machines, four-spindle semi-automatic lathes, heavy vertical lathes, boring machines, planing machines with a table width of up to four meters, etc., must increase considerably.

The need to provide for the great construction projects of communism sets before the machine-tool industry a very responsible task: to master in the near future the production of heavy unique machine tools, such as longitudinal planers with a table length of up to 15 m, vertical lathes with a diameter of the workpiece to be machined of up to 7 m, boring machines, and others.

The standardization of the machine tools that must be mastered in the coming years is determined mainly by the following circumstances.

1. The broad introduction of automatic machine-tool lines, which must also be mastered in other branches of industry besides automobile manufacturing. Statistics show that the introduction of automatic machine-tool lines reduces the number of workers by a factor of 5 and increases productivity by more than a factor of 7.

2. A broad transition in metalworking to high cutting speeds. The use of hard alloys for cutting tools has overturned the standards in machine building and has made it possible to raise cutting speeds to 700 m per minute, that is, by more than a factor of 10.

In connection with the above, it may be considered that machine-tool building has taken the path of creating new designs with increased speeds, powers, and rigidity. In essence, a complete renewal of the machine-tool fleet is taking place. The new tasks in the field of machine-tool building require designers to resolve a number of most serious issues, the principal ones of which reduce to the following.

1. Experience shows that further reduction of the machining time during processing no longer leads to an increase in productivity. The latter is now limited by auxiliary time. It is therefore necessary to create on the machine such mechanisms and structural units as would provide a significant increase in productivity by reducing auxiliary time.

2. Operating experience shows that the number of stoppages and the related decrease in productivity due to the machine’s electrical equipment is estimated at less than 1%. Therefore, the design development of the machine must be directed chiefly toward eliminating possible stoppages during operation that are associated with malfunctions of the mechanical part of the machine.

1. The transition to high machining speeds requires the creation of such designs in machine-tool building in which the possibility of vibrations arising at high spindle speeds would be eliminated.

2. The possibility of vibrations arising at high cutting speeds requires resolving the question of stepless, continuous speed control. Here one may proceed along the line of creating continuous regulation by means of electric and hydraulic drives, as well as by creating an appropriate mechanical design. However, all the advantages with respect to continuous regulation evidently remain with electro-automation.

3. At high spindle speeds the no-load losses of the machine increase so greatly that the efficiency of the machine drops sharply. In order to increase the latter, an even greater simplification of the machine kinematics is evidently required. At the same time, the question of a rational type of lubrication also requires resolution.

4. The operator’s control over the machining process and his influence on individual mechanisms in heavy, unique machines must be simplified as much as possible. To achieve this by means of mechanical connections between the machine components and the control station is practically impossible. A successful solution to this question in this area can be achieved only through the further development of electro-automation.

In light of the foregoing, the electric drive of modern metal-cutting machine tools must change to a considerable degree, since one cannot speak of any successful machine design without taking into account the properties of its drive; new requirements imposed on the mechanical part of the machine inevitably impose new requirements on its electric drive as well.

Here it is necessary first of all to note the tendency toward an even deeper development of multi-motor electric drive. The desire to simplify the kinematics of the machine as much as possible is accompanied by an increase in the number of electric motors used to drive its individual units. The number of electric machines also increases because the requirement of continuous speed control compels the use of transverse-field electric-machine amplifiers, regulating exciters, generators, and so forth.

As a general characteristic of the electric drives of modern machine tools, the following examples may be cited.

1. A longitudinal planing machine, model 7288F, has up to 23 electric machines with automatic control. The main electric drive of this machine has a power of 150 kW, while the total installed power of all auxiliary electric machines is about 230 kW.

2. A heavy boring machine, intended for boring products up to 7 m in diameter and for simultaneous drilling and grinding of the product, is equipped with twelve individual electric drives with a total installed power of about 200 kW.

3. A unique centerless lathe, intended for machining tubular blanks weighing up to 3000 kg, with a length of 5500 mm and a diameter of up to 360 mm, and equipped with a 220 kW main-drive motor and 26 alternating-current electric motors with a total installed power of about 200 kW.

In heavy machines, the kinematics can be substantially simplified by dispensing with lead screws and shafts and replacing them with synchronous-following devices. Thus, on a powerful screw-cutting lathe of 1500 × 30000 mm, the necessary connection between the thread-cutting carriage and the faceplate is provided by means of a special electric power synchronous transmission of about 2 kW, including measuring selsyns (transmitter and receiver), an electronic device for measuring the angle between the selsyns, and an actuating electric drive. Experi-

experimental study of this system showed its good operation both in static and in dynamic conditions.

With the aid of a synchronous-follow-up system, the question of the feed drive on a lathe of type 1682 has also been resolved.

Turning to a general characterization of the drives for the main and auxiliary motions, the following should be noted. The spindle drive is implemented according to the generator–motor system when sufficiently wide limits of speed regulation and low absolute cutting speeds are required; such are: the drive of a boring lathe, requiring speed regulation within the limits of \(1:20\), the drive of the table of a longitudinal planing machine, requiring speed regulation within the limits of \(1:10\), etc. If the limits of speed regulation are considerably smaller (\(1:3\) to \(1:4\)), then a separately excited motor, with speed regulation by changing the magnetic flux, may be used as the drive. Such a drive is used in some vertical boring mills, together with two or three mechanical stages of speed regulation.

At low cutting speeds of up to \(60\ \mathrm{m/min}\), the assignment of the most advantageous cutting conditions is of substantial importance from the standpoint of machine-tool productivity, which is why the generator–motor system has become widespread for regulation in a range up to \(1:20\).

The situation is different at high cutting speeds, on the order of \(700\ \mathrm{m/min}\). The same absolute value of the speed change at high cutting speeds has a small relative magnitude and already has practically no effect on the productivity of the machine tool. In this connection, the need for wide regulation of spindle speed may in a number of cases disappear, and from this standpoint the use of an asynchronous motor as the spindle drive, instead of the generator–motor system, is very probable. It is clear that in each individual case the advisability of one or another drive system must be carefully substantiated. Thus, in the transition to high cutting speeds, the electric drive of the main motion can be considerably simplified.

Opposite tendencies are observed in the electric-drive systems for feeds. With the same machine power, the amount of feed must decrease as cutting speed increases; and since the speed of displacement of auxiliary mechanisms in idle running and in setup modes must be greatest in order to reduce auxiliary time, it is natural to expect that the speed-regulation range of the feed mechanisms must be considerably expanded in the transition to high spindle speeds. This tendency is also observed in the newest machine tools, in which the feed-speed regulation range according to the generator–motor system has values of \(1:80\), \(1:100\), \(1:200\), and even reaches \(1:2000\) with combined speed regulation according to the generator–motor system.

Achieving such wide limits of speed regulation has become possible by complicating the automatic-control circuits of the feed motors. The complexity of these circuits is explained by the need for precise stabilization of the speed of the feed motor when operating at low speeds. Figure 1 presents the natural mechanical characteristic of a separately excited motor used as the drive of feed mechanisms and controlled according to the generator–motor system.

The indicated characteristic, expressed by the equation

\[ n = \frac{U_n}{C'_e \cdot \Phi_n} - \frac{I \cdot r_0}{C'_e \cdot \Phi_n}, \tag{1} \]

assumes the constancy of \(r_0\), \(U_n\), and \(\Phi_n\) under changes of the load current \(I\), and is to a certain extent idealized, since equation (1)

does not take into account the change in \(r_{\partial}\) as the temperature of the motor windings rises under load, the armature reaction, etc. At high motor speeds, when operating on the natural characteristic, the deviations of \(n\) from the natural characteristic caused by the nonconstancy of \(r_{\partial}\), \(\Phi_{н}\), and \(U_{н}\) are in practice so insignificant that they need not be taken into account. Therefore, at high speeds characteristic (1) may be regarded as quite stiff, since

Fig. 1

the speed drop due to the ohmic voltage drop, expressed by the second term of equation (1), has a small relative magnitude. A different situation is observed with deep speed control and when the motor operates at low speeds. If the feed drive has a control range of \(1:1000\) (Fig. 1), then when operating at the lowest speed it will run at a speed of \(2\ \text{rpm}\) if its rated speed is \(2000\ \text{rpm}\).

At such a low speed, any insignificant deviations of \(U_{н}\), \(r_{\partial}\), etc. from the specified values will strongly change the specified lowest speed, which may be inadmissible from the point of view of the accuracy of operation of the feed motor.

Thus, with deep speed control of the motor, the control circuits must automatically compensate for all deviations of the parameters of equation (1) from the specified ones; that is, they must constitute an automatic-control system with regulators characteristic of it, negative feedbacks, etc.

Since the accuracy of stabilization of the motor speed depends primarily on the voltage at the brushes of the generator in the generator–motor system, and the latter changes owing to the voltage drop in the armature winding of the generator as the load current changes, owing to hysteresis, and owing to the voltage drop in the brush contact, which depends on speed, such an automatic-control system must have feedbacks for generator voltage, for the hysteresis loop, and for the voltage drop at the brushes. In addition, feedback must be provided to compensate for the change in \(r_{\partial}\) with the rise in temperature of the motor winding, and in a number of cases also feedbacks for motor current and speed when it is necessary to maintain the specified values of the indicated quantities. It should be borne in mind that extension of the control range within the limits indicated above is possible by eliminating the residual magnetism of the generator, which limits the control range. This is achieved, as is known, by using special bridge circuits for excitation of the generator of the generator–motor system, with the armature of the regulating exciter connected into the diagonal of the bridge.

The presence of the feedbacks listed above and of elements of bridge circuits is characteristic of modern automatic-control systems for the motors of feed mechanisms. Regulating exciters and cross-field electric-machine amplifiers (EMAs) have found wide application as regulators used in automatic-control systems for feed drives.

Furthermore, for the convenience of servicing machine tools, it proves necessary to concentrate the control of the mechanisms at the place where the worker is located. In heavy engine lathes and vertical boring mills there are several such points from which the machine must be controlled; sometimes the control of the machine is even brought out onto the faceplate (vertical boring mills). In all these cases a large number of trolleys is required for transmitting commands from the control station to the motor, which, first, considerably complicates the circuit and, second, creates sections in the control circuit that are vulnerable in operation. In this connection there is a tendency toward changing over to such control circuits as would permit the transmission of up to ten commands over two or three wires.

Another type of drive that has undergone proper development and makes it possible to obtain mechanical characteristics of proper stiffness and stable regulation over a range of up to 1:80 is the ionic drive—ELIR (electron-ion regulator). This drive makes it possible to obtain both stiff mechanical characteristics and excavator-type characteristics; it ensures constancy of current strength both during operation of the drive and during its starting. There is no doubt that this type of drive has a great future, and its further improvement should proceed along the line of developing drives of medium and high power, and also along the line of creating ionic drives using ignitrons and mercury rectifiers. It should be noted that the use of this drive is limited to a considerable extent by the insufficient quality of the ion-electronic apparatus (gasotrons and thyratrons) with respect to its durability.

The squirrel-cage induction motor with increased slip should find fairly wide application in metal-cutting machine tools. It can be successfully used in the following cases.

1. For the drive of screw-cutting automatics, characterized by a high frequency of switching operations and reversals. The use of a motor with increased slip is advisable here in order to reduce starting losses and ease the thermal conditions of the motor.

2. For the drive of high-productivity machines in which it is necessary to shorten the starting and reversing time.

3. For the drive of auxiliary mechanisms of metal-cutting machine tools requiring a relatively large starting torque or operating “against a stop” (clamping of a column in radial-drilling machines).

Fig. 2

1. When it is required to increase the stability of operation of the electric drive under expected load shocks and voltage fluctuations.

To fulfill the last condition, a mechanical characteristic of the motor of the type shown in Fig. 2 is desirable.

For a motor with increased slip having such a characteristic, the method of regulating the motor speed by changing the voltage applied to the stator should be recognized as expedient to a certain extent. With such a mechanical characteristic there is no branch of unstable operation; therefore the possibility of the motor “tipping over” when the voltage is reduced is eliminated. As apparatus, po-

...reducing the voltage on the stator when the motor is supplied from a network with constant voltage, a saturable reactor may be used.

Finally, it should be noted that, in connection with the transition to high cutting speeds, exceptionally wide application should be found for the asynchronous motor supplied with high-frequency current.

At present, our electrical-machine-building industry has mastered an asynchronous motor rated at 18,000 rpm, with power from 1.5 to 3.5 kW. The main difficulty in further increasing the speed is the design of the supports, since the ball bearings produced in series are suitable for speeds of 25,000–30,000 rpm. However, work at ENIMS has shown that at speeds up to 50,000 rpm, angular-contact bearings with a textolite separator of the 6000 series can quite well be used. Motors from 0.5 to 1.5 kW, manufactured by ENIMS with such bearings, operate reliably under production conditions at 50,000 rpm.

The principal trends outlined above in the development of the electric drive of modern metal-cutting machine tools pose a whole series of serious tasks also for the general theory of electric drive. Electric-drive theory must satisfy the urgent needs of practice in the field of further, more in-depth investigation of multi-motor drives, and must develop the theory of transient regimes of systems with wide speed regulation. In doing so, in addition to the mechanical and electromagnetic inertia of the electric drive, the thermal inertia of the drive must already be taken into account; as was shown above, this is very important at the lower limits of speed regulation. In the field of electric drive, further and more in-depth work must also be carried out on the theory of stability of complex electromechanical systems. The latter problems should be regarded as especially urgent, since without the theory of automatic control and without the theory of stability it is now impossible to design correctly modern complex, multi-motor electromechanical systems. Research in this field should concern chiefly nonlinear systems.

Special attention should also be given to the further development of the theory of electrical-machine amplifiers, both with self-excitation and with a transverse field, as well as to the search for new forms of electrical-machine amplifiers. Electrical-machine amplifiers are already finding wide application in electromechanical automatic-control systems; meanwhile, far from everything has yet been done for a clear understanding of the electromagnetic processes that take place in them.