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 coa.r.s.e wire of few turns. If the potential of the system is to be lowered, the main current is pa.s.sed 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, ill.u.s.trates 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 ma.s.s of soft iron will by induction be polarized and will be rotated, although not necessarily synchronously, with the rotatory field. Any ma.s.s 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 t.i.tle "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 a.s.signed, 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.