The case of the direct current adjustable-speed motor is illus trated by the dotted curves of fig. 1. This is shown to be a shunt motor provided with a rheostat in the shunt field, which permits the full load speed to be adjusted between a range of 64o r.p.m. and 1,040 r.p.m. When adjusted for one full load speed, the change in speed with changes in load is small, and is comparable, as shown by the two dotted curves, with the curve of the straight shunt motor. The speed-torque curve of a compound compensated motor can be varied in accordance with the desired application, but in general the motor is classified as a constant speed motor. The speed-torque curve would be similar to that of the shunt motor shown in fig. 1. Consideration of the curves in fig. 1 leads to the conclusion that where constant speed is required at all loads, a shunt or compound compensated motor would be used. As a matter of practice, nearly all modern shunt motors have a light series field coil to prevent the speed actually rising at heavy loads due to the demagnetizing effect of the armature cur rent on the magnetic fields. Where heavy loads must be started and moved slowly and where lighter loads must be moved at higher speeds, a series motor is used or a very heavily compounded motor. Compound motors are used in general industrial work to drive tools such as punch presses and shears which are equipped with heavy flywheels. They are advantageous here for two reasons; first, because the starting duty is heavy, due to the weight and inertia and, second, because their dropping speed curve as load comes on allows energy to be drawn from the fly wheel to meet the sudden severe peak. After the peak load passes, the motor accelerates again and restores to the flywheel the borrowed energy. In the case of all direct current motors there is a direct relation between the speed and the supply voltage applied to the armature, as was inferred in the definition of counter electromotive force. Advantage has sometimes been taken of this to secure varying or adjustable speed characteristics, either by placing an external resistance in series with the armature or by changing the supply voltage applied. The resistance method is objectionable because it wastes some power, but the variable voltage scheme is efficient and finds an important application in the control of large high-speed passenger elevators. As an addi tional desirable characteristic a direct current motor may be so controlled that it will act as a generator while the load is being brought to rest, and thus considerably lessens the duty and the wear on the mechanical brakes. This is particularly useful in the case of series and compound motors, as in the case of railway motors. This feature is called dynamic braking.
Table I. gives some typical applications for the different classes of direct-current motors.
motors on the main roll drives of steel mills and similar applica tions, a system has been worked out by using auxiliary machines of the commutator type which gives a true adjustable speed characteristic for the whole combination. This arrangement in volves too much complication and expense to justify it in case of comparatively small units. Aside from this rather special application, the broad legitimate field of alternating current mo tors is in constant speed and varying speed applications. So great is the mechanical simplicity and ease of installation and main tenance of the squirrel-cage rotor induction motor that it is doing a major percentage of the world's work in general industrial applications where constant speed is required, coupled with com Speed Classification of Alternating Current Motors.— Many attempts have been made to develop an adjustable speed alternating current motor which would compare favourably with its direct current competitor, but up to the present time this cannot be said to have been accomplished. If it were readily feasible to vary the frequency of the alternating current supply this could be accomplished. but since such variation is attended with too much complication this field is practically monopolized by the direct current motor. In the case of very large induction paratively infrequent starting duty.
The typical constant speed alternating current motor is the synchronous motor, a diagram of which is shown in fig. 6, which, as has been explained, has an unvarying speed at all loads so long as the frequency of the alternating current supply remains con stant. The synchronous motor in structural detail originally ex actly duplicated an alternating current generator but as so de signed had very little starting torque and could not be applied where compelled to start under load. Later an additional wind ing similar to that of a squirrel cage rotor induction motor was added to the rotor, and at the present time synchronous motors have sufficiently good starting characteristics to permit their use on many industrial applications formerly covered by induction motors. The speed-torque curve of a synchronous motor is a
straight line at synchronous r.p.m. If loaded beyond the capacity of its torque to hold it in step the motor stops entirely and comes to rest. The synchronous motor has a very desirable character istic from the standpoint of the supply system, in that it can be made to draw from the line a magnetizing current for its stator winding, which is said to be leading the supply voltage in time phase and which, therefore, compensates for the magnetizing current of induction motors on the same system, which current is said to be lagging. Expressed in a practical way, if a supply system is entirely loaded up with induction motors so that its generators cannot take on any more load, it may be possible to install additional synchronous motors and develop additional mechanical power without adding to the electrical current of the generator. So valuable is this characteristic that at times syn chronous motors are run on a system without any mechanical , load, merely to furnish this the so-called leading current. When so used, a synchronous motor is called a synchronous condenser.
The induction motor may be either a constant-speed or a varying speed motor as may be seen in fig. 6. In the squirrel cage rotor type, a constant speed motor has a comparatively low elec trical resistance in the copper bars and short-circuiting rings of the rotor winding as shown in Plate, fig. 2. If a varying speed characteristic is desired, the electrical resistance of the rotor winding is increased. The speed-torque curve for a constant speed squirrel cage rotor induction motor is shown in fig. 2. It will be noticed that the torque at start is about Li times normal full load running torque and that as the motor speeds up the torque increases until maximum torque is reached and then de creases until the motor runs steadily at a torque corresponding to the load. Unlike the synchronous motor, which runs at all times at synchronous speed, or cycles X 120 , it will number of magnetic poles be noted that the induction motor falls away slightly from syn chronous speed and, at full load, runs, for example, at 3% to 5% below synchronous speed. This departure from synchronous speed is characteristic of the induction motor and is called slip. In the case of the varying speed squirrel cage rotor induction motor with high electrical resistance in its rotor winding the slip will be much greater, at rated full load torque and the speed torque curve will be typified by that of fig. 3. In this case it will be noted that maximum torque occurs at start and that the torque decreases as the speed increases. Also, that the slip at full load is 25% or much greater than in the case of the constant speed motor. Such a speed-torque curve is desirable where heavy loads are to be started, perhaps frequently. Moreover, the varying speed motor has the further desirable characteristic that the current required in starting is less. On the other hand, it is less efficient when running continuously by a percentage represented by the increased slip. The phase wound rotor induction motor has a coil winding on the rotor of relatively low electrical resistance, and the terminals of the windings are brought out to collector rings on the shaft as shown in Plate I., fig. 7. Contact is made with these rings by means of carbon blocks or brushes, so that it is possible to insert or cut out resistance external to the motor and in series with the windings. In this manner a very flexible control is secured, since a number of different speed-torque curves can be obtained for the same motor, depending on the amount of this external resistance. This is shown by fig. 4 in which the speed torque curve 7 would be that with all resistance cut out of the circuit, and curves 1 to 6 inclusive those with varying amounts of external resistance. From this it might appear possible to get adjustable speed characteristics, but this is not the case for the reason that a change in load represented by X' on curve 7 pro duces a slight change in speed represented by Y' but the same change in load on X on curve I produces a considerable change in speed shown by Y. From this it appears that on curve 7 the motor could be rated as constant speed but on all other curves it would be varying speed and the motor is so applied. In con nection with all induction motors, the torque developed varies as the square of the line voltage applied.