The Standard Electrical Dictionary / A Popular Dictionary of Words and Terms Used in the Practice of Electrical Engineering

by T. O'Conor Sloane

Fig. 244. ELECTRIC MORTAR.
Motor, Compound or Compound Wound.
A motor which has two windings on the field magnets, one in parallel
with that on the armature, the other in series therewith, exactly as in
a compound dynamo. (See Dynamo, Compound.)
Motor, Differential.
A differentially wound motor; with a compound wound field, whose series
coil and shunt coil are wound in opposition to each other. It is
virtually a compound wound dynamo. (See Dynamo, Compound Wound.)
Motor, Electric.
A machine or apparatus for converting electric energy into mechanical
kinetic energy. The electric energy is generally of the dynamic or
current type, that is to say, of comparatively low potential and
continuous or virtually continuous flow. Some electrostatic motors have,
however, been made, and an influence machine can often be operated as a
static motor.
Electric motors of the current type may be divided into two
classes--direct current and alternating current motors.
Direct current motors are generally based on the same lines of
construction as dynamos. One of the great discoveries in modern
electricity was that if a current is passed through a dynamo, the
armature will rotate. This fact constitutes the principle of the
reversibility of the dynamo.
383 STANDARD ELECTRICAL DICTIONARY.
Motors built on the dynamo model may be series wound, shunt wound, or
compound wound, or of the magneto type, in the latter case having a
fixed field irrespective of any current sent through them. The field may
be produced by an electro-magnet separately excited and unaffected by
the current sent through the motor.
A current passed through a magneto or motor with separately excited
field will turn it in the direction opposite to that required to produce
the same current from it were it worked as a generator.
A current passed through a series wound motor acts exactly as above.
Both these facts follow from Lenz's law, q. v.
A current passed through a shunt wound motor acts oppositely to the
above. The direction of rotation is the same as that required to produce
a current of the same direction. This is because the field being in
parallel with the armature the motor current goes through the magnet
coils in the direction the reverse of that of the current produced in
the armature when it is used as a dynamo. Hence this also carries out
Lenz's law.
The compound wound motor acts one way or the other according as its
shunt or series winding preponderates. The two may exactly balance each
other, when there will be no motion at all. The series connections of a
compound wound dynamo should therefore be reversed, making both series
and shunt work in unison, if the dynamo is to be used as a motor.
The general principles of the electric motor of the dynamo, or
continuous rotation type, can only be outlined here. The current passing
through the field magnets polarizes them and creates a field. Entering
the armature by the brushes and commutators it polarizes its core, but
in such a way that the north pole is away from the south pole of the
field magnet, and the same for the south pole. Hence the armature
rotates. As it does this the brushes connect with other commutator
sections, and the poles of the armature are shifted back. This action
continues indefinitely.
Another class of motors is of the reciprocating type. These are now very
little used. (See Motor, Reciprocating.)
One valuable feature of continuous rotation electric motors is the fact
that they absorb energy, to a great extent proportional in amount to the
work they have to do. The rotation of the armature in the field of the
motor involves the cutting of lines of force by its coils. This
generates an electro-motive force contrary in direction to that
producing the actuating current. The more rapid the rotation the greater
is this counter-electro-motive force. The motor armature naturally
revolves faster with diminished resistance to the motion of the
armature. This increases the counter-electromotive force, so that less
energy is absorbed. When the motor is called on to do work, the armature
rotates more slowly, and the counter-electro-motive force diminishes, so
that the machine absorbs more energy. (See Jacobi's Law.)
384 STANDARD ELECTRICAL DICTIONARY.
Motor Electro-motive Force.
The counter-electro-motive force of a motor. (F. J. Sprague.)
A motor rotates in virtue of the pull of the field magnet upon the poles
of the core of its armature. In responding to this pull the windings of
the armature cuts lines of force and hence generates a
counter-electro-motive force, for which the above term was suggested.
Motor-Generator.
A combined motor and generator used to lower the potential difference in
a portion of a circuit, e. g., that part within a building.
A motor-generator is a dynamo whose armature carries two commutators,
with two separate windings, one of fine wire of many turns, the other of
coarse wire of few turns. If the potential of the system is to be
lowered, the main current is passed through the fine winding. This
causes the armature to turn motor-fashion, and a potential difference is
generated by the rotation of the large coils in the field. This
potential difference is comparatively low and by properly proportioning
the windings may be lowered to as great a degree as required.
The same apparatus may be inverted so as to raise potential difference.
It acts for continuous current systems as the induction coil transformer
does for alternating current systems.
Synonym--Continuous Current Transformer.
Motor, Multiphase.
A motor driven by multiphase currents. It is arranged in general terms
for distribution of the multi phase currents in coils symmetrically
arranged around the circle of the field. These coils are wound on cores
of soft iron. A rotating field is thus produced, and a permanent magnet
or a polarized armature pivoted in such a field will rotate with the
field, its poles following the poles of the rotatory field.
The cut, Fig. 245, illustrates the principles of action of a four phase
current motor, connected to a four phase current dynamo or generator.
The generator is shown on the left hand of the cut and the motor on the
right hand. In the generator the armature N S is supposed to be turned
by power in the direction shown by the arrow. Each one of the pair of
coils is wound in the reverse sense of the one opposite to it, and the
two are connected in series with each other, and with a corresponding
pair in the motor. The connection can be readily traced by the letters A
A', a a' for one set of coils and B B' b b' for the other set.
385 STANDARD ELECTRICAL DICTIONARY.
For each rotation of the armature two currents, each in opposite
direction, are produced in A A', and the same is the case for B B'.
These currents which have an absolutely constant relation of phase, and
which it will be seen alternate four times for each rotation of the
armature, regulate the polarity of the field of the motor. The resultant
of their action is to keep the poles of the field magnet of the motor
constantly traveling around its circle. Hence the armature N S of the
motor, seen on the right hand of the cut, tends to travel around also
its north and south poles, following the south and north poles of the
rotatory field respectively.

Fig. 245. FOUR-PHASE CURRENT GENERATOR AND MOTOR.
It is not essential that the armature should be a magnet or polarized.
Any mass of soft iron will by induction be polarized and will be
rotated, although not necessarily synchronously, with the rotatory
field. Any mass of copper, such as a disc or cylinder, will have
Foucault currents induced in it and will also rotate. The only
components of such currents which are useful in driving the motor are
those which are at right angles to the lines of force and to the
direction of motion. A very good type of armature based on these
considerations is a core of soft iron wound with insulated copper wire
in one or more closed coils; and so wound as to develop the currents of
proper direction.
Such an armature is used in the Tesla alternating current motor. An
efficiency of 85 per cent. has been attained with some of the Tesla
motors.
Motor, Prime.
A machine used for producing mechanical motion against resistance. It
may operate by converting heat or any other form of kinetic or potential
energy into mechanical energy of the moving type. A steam-engine and a
water-wheel are examples of prime motors.
Motor, Reciprocating.
The early type of motor depending upon reciprocating motion, such as the
motion of a coil in a solenoid. These were based upon the lines of a
steam engine, and have been abandoned except for special purposes where
reciprocating motion is especially required, as in the case of rock
drills.
386 STANDARD ELECTRICAL DICTIONARY.

Fig. 246. RICORDON'S RECIPROCATING MOTOR.
In the cut, B is an electro-magnet; A is an armature; E a pole piece.
The current enters by the springs, b b, and by commutation is supplied
and cut off alternately, thus maintaining a reciprocating movement of
the armature and rotation of the fly-wheel.
Synonym--Pulsating Motor.
Motor, Series.
A motor whose winding on the armature is in series with the winding on
the field. It is similar to a series dynamo. (See Dynamo, Series.)
Motor, Shunt.
A motor whose winding on the armature is in parallel with the winding on
the field magnets. It is similar to a shunt wound dynamo. (See Dynamo,
Shunt.)

Fig. 247. MULTIPLE ARC CONNECTION.
Multiple.
A term expressing connection of electric apparatus such as battery
couples, or lamps in parallel with each other. In the ordinary
incandescent lamp circuits the lamps are connected in multiple.
Synonym--Multiple Arc.
387 STANDARD ELECTRICAL DICTIONARY.
Multiple Arc Box.
A resistance box arranged so that the coils may be plugged in multiple
instead of in series. Such can be used as a rheostat, as the resistance
can be very gradually changed by putting the coils one by one into
parallel with each other. Thus by adding in parallel with a 10 ohm coil
a 10,000 ohm coil the resistance is decreased to 9.999001 ohms, and thus
the resistance can be very slowly changed without sudden stops or abrupt
changes.
[Transcriber's note: The correct value is 9.99001]
Multiple Series.
Arrangements of electric apparatus in a circuit in a number of series,
which minor series are then arranged in parallel. The term may be used
as a noun, as "arranged in multiple-series," or as an adjective, as "a
multiple-series circuit."
Fig. 248. MULTIPLE SERIES CONNECTION.
Multiple Switch Board.
A switch board on whose face connecting spring jacks or other devices
are repeated for the same circuits, so that different operators have
each the entire set of connections repeated on the section of the board
immediately in front of and within their reach. This multiplication of
the same set of connections, giving one complete set to each operator,
gives the title "multiple" to the type of switch board in question. The
typical multiple switch board used in telephone exchanges is the best
example of this construction. The calling annunciators of the
subscribers are distributed along the bottom of the board extending its
full length. To each operator a given number is assigned, all within
reach of the right or left hand. This gives five or six feet length of
board to each, and an operator only responds to those subscribers within
his range. But anyone of his subscribers may want to connect with any of
the others in the entire central station. Accordingly in front of each
operator spring jacks are arranged, one for each of the entire set of
subscribers connected in that office. The operator connects as required
any of the calling subscribers, who are comparatively few, to any one of
the large number served by the central station. Thus the entire set of
subscribers' spring jacks are multiplied over and over again so as to
give one set to each operator.
388 STANDARD ELECTRICAL DICTIONARY.
Multiple Wire Method for Working Electro-magnets.
A method for suppressing sparking in working electro-magnets
intermittently. The magnet core is wound with a number (from four to
twenty) of separate layers of fine wire. A separate wire is taken for
each layer and all are wound in the same direction, from one end to the
other of the space or bobbin without returning. The ends are then joined
so as to bring all the wires in parallel. The effect of this is that as
the coils vary in diameter the time constants of each is different from
that of the others, the coefficient of self-induction being less, and
the resistance being greater for the coils farthest from the central
axis. Thus the extra currents run differently in the different coils,
and only a comparatively small spark can be produced owing to the
division of forces thus brought about.

Fig. 249. DIAGRAM ILLUSTRATING MULTIPLE WIRE WORKING.
Multiplex Telegraphy.
Any system of telegraphy transmitting more than four messages
simultaneously over a single wire. Properly it should apply to all
transmitting more than one, but conventionally has the above restricted
meaning, distinguishing it from duplex and quadruplex telegraphy.
Multiplying Power of a Shunt.
When a resistance is placed in parallel with a galvanometer on a circuit
the following relation obtains. Let s and g equal the resistances of the
shunt and galvanometer respectively, S and G the currents in amperes
passing through them, V the potential difference between their common
terminals, and A the whole current in amperes. Then we have
A = ( (s + g ) / s ) * G
and ( (s + g ) / s ) is termed the multiplying power of the shunt, as it
is the factor by which the current passing through the galvanometer must
be multiplied by to produce the total current.
Muscular Pile.
A species of voltaic battery, often termed Matteueci's pile, made up of
alternate pieces of muscle cut longitudinally and transversely
respectively. The different pieces represent the elements of a battery,
and their difference of potential is naturally possessed by the pieces.
Myria.
A prefix; one million times. Thus myriavolt means one million volts.
[Transcriber's note: Contemporary usage is mega, as in megavolt.]