ALTERNATORS

The main difference already noticed between the D. C. generator and the A. C. generator, called for shortness' sake an alternator, is that one has a commutator and the other a collector.

There are, however, differences in construction which must be noticed.

The highest voltage for which D. C. generators are wound is 1,200, this being the lowest voltage for which alternators are wound, while for railroad work 11,000 is the usual and 22,000 not uncommon.

This makes the problem of proper insulation for A. C. armatures more difficult. To make the work easier, instead of having the field stationary and armature revolve, as in most D. C. generators, in alternators the field revolves and the armature is stationary.

The field is fed with D. C. at 250 volts pressure, and is easy to insulate even though subjected to the mechanical strains of rapid motion and the lack of plenty of space.

The armature being stationary, there are no mechanical strains, also weight being no objection, plenty of space can be given to insulation.

This type of construction is shown in Figs. 1 and 2. Fig. 1 shows a revolving field of 18 pairs of poles, or 36 poles. Current is led in through a collector. This field revolves inside of the stationary armature (Fig 2), whose windings are fully exposed to the cooling effect of the air. This armature needs no collector, for the terminals of the winding are attached to leads which come out of the base at one side. (In the Fig. at right side.)

Ventilation of the field and armature is accomplished by means of air ducts, as is well shown in Fig. 3.

The magnetism in all armatures fluctuates and reverses in polarity as it passes or is passed by the poles;

cases of exciter trouble the storage battery furnishes the field current.

The field coils occupy about 50 per cent of the surface of the field bore, because when their inner edges are

FIG. 2.

tight together their outer edges are apart, due to the larger circumference at the pole pieces, and because some interpolar space must be left to prevent excessive leakage from pole to pole.

Only 50 per cent of the armature bore is wound, for otherwise the coils. would be so wide that they would extend over into the field of a wrong pole piece. If one side of a coil is under a N-pole the other side should be under a S-pole. Then the two E. M. F.'s induced add together. Should the coil be so wide as to extend over to the next N-pole any E. M. F. induced by that pole would be subtracted.

There is then on the ordinary alternator half of the armature empty. Such a machine is called a Single Phase Alternator.

It occurred to some inventor that an entirely separate winding could be put on between the coils of the original winding and be connected to its own collector. The current was to be led to a different circuit, but it soon became evident that it was better to make of the four wires from

the alternator a three-wire circuit by joining two of them inside the armature and leading out three wires to the switch board. Such an alternator is a Two Phase Alternator.

Of course, the capacity of the machine is not doubled, because from a single phase alternator is drawn enough current to heat it to the safe limit. From a two phase alternator we do the same thing. The reason we gain in capacity is because in a single phase machine the heating is concentrated, while in the two phase machine it is evenly distributed all over the armature.

Even in a two phase alternator there is a portion of the armature not used for winding and there was still a desire to reduce the number of line wires. This led to the Three Phase Alternator.

The three armature windings of the alternator are connected together at one point and the other ends to

Q. What is full load of dynamo? Of motor?

A. It is the pressure the generator or alternator delivers at its own terminals. The voltage of motor is the voltage which should be applied to its terminals in order to develop full horse power.

Q. What is full load current of dynamo? Of motor?

A. Full load current of a dynamo is that current which may be drawn steady for 24 hours without causing any part of machine to exceed a safe temperature, i. e., 150° Fah. This applies to factory motors. If a railway motor, it is the current which passing through motor for one hour as it runs on the blocks in testing room, will cause it to rise to a temperature of 212° Fah. We mean, of course, that the hottest part shall be no hotter than 212° Fah.

Q. What is meant by the rating of a dynamo? Of a motor?

A. The product of full load current multiplied by voltage expressed in kilowatts is rating of a dynamo. The actual mechanical horse power developed at the pinion of the motor as tested in shop. The gearing increases the power applied to and reduces the speed of the car wheels.

Q. What is armature core?

A. The sheet iron body which carries the armature winding and conducts the flux from pole piece to pole piece.

Q. What is armature spider?

A. The casting consisting of hub and arms which supports armature

core.

Q. What are binding wires?

A. They are narrow bands of phosphor bronze wire placed around armature every three or four inches to help bind winding to core. They rest on strips of mica and are sweated with solder all around.

Q. What are commutator seg

ments?

A. The commutator segments or bars are the copper pieces of which the commutator is built.

Q. What insulation is there between conductors of winding?

A. The double cotton covering of each wire makes four thicknesses between conductors.

Q. What is the air gap?

A. It is the air space between armature and pole pieces. In dynamos it is made as small as possible for efficiency. In motors it is not made too small because this tends to make machine spark due to the weak field. In D. C. series motors it is from 8 to 1/4 of an inch, in A. C. series motor it is smaller, say, 1/10 to 8 inch. The larger the air gap of a motor the more the bearings may wear before there is danger of armature rubbing against lower pole pieces.

Q. What are field spools?

A. The brass shells on which the field coils are wound.

Q. What is the commutator?

A. It is a series of copper bars placed parallel to shaft, insulated from each other and from the frame of machine. Each is connected to the winding and current flows from winding through them to brushes. It at the same time reverses the connections between the brushes and winding at the proper times so that the brush always collects current.

Q. What is a collector or slip rings?

A. A collector consists of two or more rings of copper placed around the shaft and insulated from it and each other. Each is connected to a part of the winding. The brushes. rest on the rings. They are used to collect current from a revolving armature style of alternator, to feed current into armatures of rotary converters or the revolving fields of alternators. The collector has no corrective influence and passes on the A. C. or D. C. current exactly as it receives it. Single phase machines have two rings; two, three and six phase machines have three rings.

Q. Is there a difference between. no load and full load voltage of dynamos?

A. Yes. A shunt dynamo gives highest voltage at no load and lowest at overloads; the series dynamo gives lowest at no load and highest at full load. The compound dynamo is a combination of series and shunt and gives some voltage at all loads. Fig. 8 will make this clear. An alternator acts like a shunt dynamo.

Q. What is a field rheostat?

A. It is a resistance in the field circuit which can be varied to change the current, and hence the field strength. This alters the voltage of dynamo.

A. The greater the field and armature current the greater the power.

Q. What is a ring winding?

A. One which passes over and under around the core, a space being left between shaft and core to accommodate winding.

Q. What is a drum winding? A. One where all winding is on the outer surface of core.

Q. Upon what does does sparkless commutation of current depend? A. (1) The more commutator bars the better, there being less voltage and therefore tendency to spark between bars. The average railway

motor has from 100 to 125 bars on commutator. (2) The fewer the amperes turns on the armature in comparison to the ampere turns on the field the less sparking. (3) The more turns short circuited by the brush when touching two or more bars at once the greater the tendency to spark.

Q. What is a shunt field?

A. One whose coils are placed as a shunt across the brushes. It carries a small current.

Q. What is a series field?

A. One which carries the main or nearly all the main current and is placed in series with the armature. A small strip of resistance metal is used sometimes to divert a portion of the main current from the series field.