electromotive force must have both a strong field and many
turns upon the armature, so that both the fundamental stresses
may be large. As the field is generally strong—to secure
economy of iron-low-voltage and high-voltage machines differ
principally in the number of armature turns. For variable speed,
this latter factor being fixed, field strength and applied electromotive
force are the factors easily altered, and most of the speed
variation is accomplished by changing one or both of them.
Torque, neglecting field distortion, is at a maximum when the
current is the greatest possible at the given applied voltage—that
is, when the motor is at rest. With a small armature resistance
this current is generally far too great for convenience; hence the
motors are usually started with a rheostat in series with the
winding if the current is not limited by the generator itself. The
torque then depends on the sum of the resistances in circuit, and
can be made just sufficient to start the motor under the required
load. By the same device the motor can run at reduced speed,
although with a considerable loss of energy in the rheostat; it is
indeed, as a rule, difficult to get effective speed variation in motors
of any kind without serious loss of energy. The field can be
changed within wide limits only by a considerable increase of the
iron in the magnetic circuit, the applied electromotive force
cannot usually be varied except by increasing the resistances in
circuit, and the number of armature turns cannot be varied without
complication, although the effective number can be modified
by shifting the brushes, probably at the expense of sparking.
Altogether, if the speed variation demanded be more than 15 or
20%, it causes, in one way or another, considerable expense and
trouble, particularly if each speed must be closely held irrespective
of load. No large change in absolute speed can readily be
made without considerable change in the percentage variation
of speeds at various loads. Practically, the best results are obtained
from motors of very low armature resistance, in which the
field or the applied electromotive force, or both, are varied. The
whole problem is nearly identical with the production of constant
potential or constant current from generators driven at constant
speed, and is solved by similar means. For any one absolute
speed a generator can be made to give constant potential, nearly
irrespective of load, by compound winding. Similarly, a motor
may give a very nearly constant speed at constant potential by a
differential winding in series with the armature, weakening the
field as the armature current rises. This device, however,
obviously increases the energy required for magnetization, and
decreases the effective torque at starting. Practically, the best
continuous-current motors can be made to hold their speed to
within 1 or 2% from no load to full load. Commercial machines,
however, generally vary from 5 to 10% in speed. With respect
to the direction or rotation of a motor, the torque changes sign
with a change of sign in either field or armature current, but not
with a change of sign in both. The input of the motor is numerically
equal to the product of the current and the applied
electromotive force, while the output is determined by the
product of the current and motor electromotive force; hence
the .efficiency of the motor as a transformer of energy is the
ratio between these two quantities. The output is a maximum
when the applied electromotive force is double the motor
electromotive force, and the efficiency is a maximum when the
motor and applied electromotive forces are substantially equal.
At the point of maximum output the speed is that sufficient to
reduce the current to one-half its static value. No motor is
worked at or near this point, except momentarily, on account of
the low efficiency and severe heating in the armature. These
theoretical values are slightly modified in practical machines by
the small miscellaneous losses subject to independent variations.
The practical output of electric motors is limited in machines of normal design by the temperature they can safely endure. As a rule the working temperature, which is commonly reached only after six hours or more of continuous running, should not rise more than 40° to 50° F. above the temperature of the surrounding air. In case of traction motors and others subjected to occasional severe overloads, separated by periods of rest or of subnormal load, the temporary rise of temperature tolerated may be much higher, say 60° to 75° F., after a run of an hour or so. The temperature of the air is assumed at 70° F. in most cases, and the temperature of the motor-windings is preferably ascertained by the rise in electrical resistance due to the heating. Thermometers can seldom be so applied as to measure the full heating effect.
The actual output obtainable from a motor structure of given dimensions under these conditions with respect to heating depends chiefly upon the practicable rotative speed of the armature, since the chief losses are proportional to the torque, while the mechanical output at given torque is approximately proportional to the speed. Most makers utilize a single structure for several standard motors varying in speed and output, a 15 h.p. machine at, say, 1200 r.p.m. becoming a 10 h.p. at 800 r.p.m. or a 20 h.p. at 1600 r.p.m. There is no practically fixed relation between the rating and the speed, although it is approximately linear, for in winding the same carcass for different speeds the ratings are settled rather by commercial convenience than by exact determinations. Motors generally have approximately the same efficiencies as the corresponding sizes of generators. Small motors, say from 1 to 5 h.p., are commonly of 70–80% efficiency at full load, medium sized machines of 5 to 50 h.p. about 80 to 90%, and the larger sizes run up to 95% or thereabouts. In the effort to get low-speed motors without immoderately increasing the cost they are generally dropped a little in efficiency and allowed to run hotter than if wound for higher speeds.
The weight of motors per h.p. of output is therefore very variable. In machines of medium size and speed it is likely to be 50 to 75 ℔ per h.p., falling to 30 or 40 in large or specially high speed machines, and rising to 80 or 100 ℔ in small or very low speed motors. High-voltage motors, particularly if small, lose somewhat in relative output on account of the space taken up by the necessary insulation.
In all ordinary motors the magnetization of the iron is, for economy of material, pushed high; and hence the field, even at heavy loads, is fairly stable and the conditions of commutation remain good. When, however, motors are designed to stand severe overloads, or to admit of a wide range of speed regulation by varying the field strength, the commutation is likely to be unstable, and severe sparking may result. To meet this condition the commutating-pole motor—really a recrudescence of an old idea—has been introduced on a considerable scale. In this construction auxiliary pole pieces, excited by series coils from the motor circuit, are set midway between the ordinary field poles. The office of these poles is to neutralize the magnetomotive force due to the armature winding, thus checking field distortion, and also to ensure the proper reversal of the current in the armature coil directly under the brush. Of the total magneto-motive force due to the windings of the commutating pole, the major part, perhaps three-fourths, is devoted to the former work and the remainder to the latter, the proportion varying widely according to the design of the motor. The result of this construction is excellent, sparkless commutation being ensured over a wide range of load and field strength. The commutating-pole motor is intrinsically more expensive and slightly less efficient than the ordinary type, but for the particular kind of service it is designed to perform is extremely effective. It gives promise of especial value in high-voltage traction motors.
(A) 1. Separately excited Motors are interesting principally on account of the very efficient method of speed regulation possible by their use. In this method the field of the motor is excited from the supply mains, and the armature current is furnished by a motor-generator running at constant speed. A rheostat in the shunt field of the latter element enables the applied electromotive force to be varied to any desired extent, and hence the working motor can be given full torque at any speed up to that assigned by the maximum value of the electromotive force which can be applied to the armature. Moreover, if the armature resistance be small, the motor is fairly self-regulating at all speeds. The effect is rather startling since the motor may be giving a very great torque when it is merely turning over at a few revolutions per minute; and although the process is complicated, it leads to excellent results, and is widely used where delicate speed regulation is required.
