CURVES AND CLASSIFICATION In the application of electric motors to their work the character istic to which the greatest attention must be paid is that of the variation of speed under load. A graph is drawn from standstill to the maximum speed which the motor attains, showing how the torque increases or decreases as the speed rises or falls. Such a graph or characteristic curve is called a speed-torque curve and illustrations are shown in figs. 1, 2, 3, 4. With reference to their speed-torque curves, motors are classified as follows: (a) constant speed; (b) varying speed; (c) adjustable speed; (d) multi-speed.
Constant speed motors run as nearly as possible to a fixed speed, regardless of change of load from no load to the maximum load which can be carried. A varying speed motor is one whose speed changes widely with load changes, in general running slowly of a relatively large number of turns of wire of relatively small cross section connected directly across the full line voltage as in A, fig. 5. It will be seen in this diagram that the field winding and the armature winding are in parallel across the supply voltage, or, using another expression, the field is shunted across the armature. In the series motor, whose connection is shown in B, fig. 5, the field coils are made up of a relatively small number of turns of relatively large cross section and the field coil is in series with the armature winding instead of being iii parallel with it as in A. In the series motor the same current flows through the field coils that flows through the armature winding, and hence it is doubly effective, once in magnetizing the field and once in producing under heavy loads and fast under light loads. Adjustable speed motors have the added possibility that by changing a rheostat or equivalent resistance control device the no load speed of a motor may be changed over a considerable range, but when once set for a given speed it will run practically at that same speed at any and all loads. Multi-speed motor is a term usually applied to alternat ing current motors, the connections to whose windings may be changed in such a manner as to change the number of magnetic poles and hence the speed of the motor. Like the adjustable speed motor, these motors, when connected for one speed, run practically constant at that speed, even under load changes.
Speed-torque Classification of Direct Current Motors.— The classification of direct current motors into shunt, series, com pound and compensated has reference to the manner in which the magnetizing coils on the field magnets are wound. This is shown
schematically in fig. 5. In the shunt motor the field coils consist torque in the armature. For this reason the torque in a series motor is proportional to the square or second power of the cur rent up to the point where the magnetic material of the motor is saturated with magnetic flux so that the field strength cannot be further proportionally increased. In the compound motor, whose connections are shown in C, fig. 5, the field poles are provided with a shunt coil and also with a series coil so that the motor partakes partly of the characteristics of a shunt motor and partly of a series motor. The compensated motor illustrated in D, fig. 5, is a special form of compound motor to which there is added a compensating winding in series with the armature winding which compensates for the magnetic effect on the field of the working currents flow ing in the armature. This compensating winding is placed in slots in the faces of the poles. The compensating winding decreases the sparking which occurs to some extent on the commutator of all commutating motors, and also makes possible a very flat speed torque curve with almost no change in speed from no load to maximum overloads. The curves in fig. 1, illustrate the behaviour of the different classes of direct current motors when the load on the motor is varied. The load on the motor in per cent of full load torque is plotted horizontally and the revolutions per minute vertically. The same rated torque and the same full load r.p.m. has been chosen for three motors wound and connected respec tively according to the schematic diagrams A, B and C, fig. 5. This full load point is shown as P in fig. i and the r.p.m. is 84o. In the case of the shunt motor, the speed is seen to be fairly steady, varying from 88o r.p.m. at no load, point R, to 835 r.p.m. at iso% load, point S. The shunt motor is therefore classified as a constant speed motor. In the case of a compound wound motor, the speed regulation is much wider and the speed changes from 1,15o r.p.m. at no load, point T, to 77o r.p.m. at 15o% load, point V. For this reason the compound motor is classed as a varying speed motor. In the case of the series motor, fig. 1, the change in speed with change in load is still greater, ranging from 1,600 r.p.m. at low load, point V, to 620 r.p.m. at iso% load, point IV. If the load were removed entirely the speed would rise still higher than 1,60o and the motor possibly damage itself. It is therefore neces sary on a series motor to make sure that some restraining load is provided at all times.