plied to the armature conductors, it will be apparent, after a
little study, that the current in a given conductor must reverse
in direction in exactly the time taken for the conductor to pass
from one pole to an adjacent pole in order that the mechanical
force on each conductor may always be in the same direction as
In other words, the motor must run at the same frequency as
the alternator which supplies it with power, and for this reason
the motor is called the synchronous motor. When the synchro-
nous motor is started, it must, in general, be brought up to syn-
chronous speed by some outside mechanical means (an auxiliary
motor), that is, to the same frequency as the alternator, and the
motor must further be connected to the supply mains at that in-
stant when the electromotive force of the armature (called
counter electromotive force after the machine is running as a
motor) is equal to that of the alternator. This operation is termed
After being synchronized, the motor continues to run at syn-
chronous (same frequency) speed at all loads, unless, due to an
overload its speed be momentarily brought lower than synchro-
nous speed, in which case it stops and must be started as before.
Note that the synchronous motor is a constant speed motor and
that its speed cannot be adjusted.
Very often the synchronous motor forms a part of what is
termed a synchronous converter (or rotary converter), namely, a
(110 Volts 60 Cycles A. C.)
Fig. 26. Synchronous motor. Note that the main switch is to be
thrown in before the synchronizing switch, and the latter not until the
conditions for synchronizing are fulfilled. The lamp should be 220
volts, or two 110-volt lamps may be used in series.
machine arranged with collector rings at one end and a commu-
tator at the other end of the armature, each connected to the
same armature winding. The common use of the rotary con-
verter is to operate it as a synchronous motor from the collector
ring side, for example, in a railway sub-station, and to supply
direct current to the railway line from the commutator end of
the machine. The rotary converter can, however, be used to run
as a direct current motor and supply alternating current, in
which case it is sometimes referred to as an inverted rotary con-
ALTERNATING CURRENT 95
Current Supply. 110 or 220 volts Alternating Current, and
110 or 220 volts Direct Current (for field excitation).
Apparatus Required. (1) An alternator to be run as a motor;
(2) field rheostat; (3) incandescent lamp to be used for syn-
chronizing; (4) speed indicator; (5) two ammeters; (6) watt-
meter; and (7) voltmeter.
Order of Work. 1. Connect the alternator for synchronous
motor operation as shown in Fig. 26.
2. Bring up the motor to speed by an auxiliary motor (or by
direct current if a rotary converter is used), and adjust the volt-
age to the value of the supply mains. Determine, by the lamp,
when the conditions for synchronizing are realized and connect
the motor to the line. Disconnect the driving motor (or the di-
rect current supply to the rotary converter as the case may be).
3. Measure and record the speed of the motor, and observe the
number of poles and the frequency of the supply alternator.
4. "With the synchronous motor in operation (unloaded) keep
the applied electromotive force constant, and measure and record
the armature volts and amperes, field amperes and watts deliv-
ered to the armature with various field currents, above and be-
low that value giving minimum armature current.
Written Report. 1. Explain in detail the necessary conditions
2. Compare the speed of the alternator as calculated from the
observations in item 3, Order of Work, with the observed speed of
3. Just why must the current in a given synchronous motor
conductor reverse each time the conductor passes from a north to
an adjacent south pole?
4. In a direct current motor, the field is of course furnished by
direct current and although the machine, as a whole, is supplied
with direct current, the commutator causes the current in the
armature conductors to be alternating. Explain just how the
mechanical force in the direct current shunt motor armature con-
ductors is always automatically in the direction of motion.
5. For each of the sets of observations in item 4, Order of
Work, calculate the apparent watts (El) input to the armature
and the power factor (true watts as indicated by the wattmeter
96 LABORATORY MANUAL
divided by the apparent watts). Explain any variations in the
power factor for the different values of field current.
6. Why is the armature current greater when the field current
is above or below that value which gives minimum armature cur-
Alternators in Parallel.
See the Theory under Experiments 19, 20 and 28 in the Man-
Theory. The conditions to be met in the operation of direct
current generators in parallel must also be met in the parallel
operation of alternators, namely, the polarity of the terminals
connected to a given bus bar and the voltage must be the same,
and, further, the two machines must have the same frequency.
This means that if one alternator is connected to the bus bars,
and a second one is to be connected in parallel with the first, the
second machine must be synchronized with the first, like the case
described in Experiment 28. Further, if the two machines have
the same number of poles, they must obviously run at the same
speed to have the same frequency.
The conditions to be met in throwing one alternator in parallel
with another may be summarized as follows :
(a) Each machine must have the same terminal voltage.
(b) The two terminals to be connected to the same bus bar
must be of the same polarity at each instant, namely, they must
be of the same phase.
(c) The machines must have the same frequency.
Current Supply. 110 or 220 volts Direct Current (for field ex-
citation), and the Alternating Current from the alternators as-
Apparatus Required. (1) Two alternators; (2) lamp for syn-
chronizing; (3) speed indicator; (4) lamp bank to be used as a
load for the bus bars; (5) three ammeters, one for each machine
and one for the total bus bar load; and (6) two voltmeters.
Order of Work. 1. Arrange the machines and instruments as
shown in Fig. 27. Starting up the machines, bring them to nor-
mal speed and voltage, and connect one machine to the bus bars
(two lengths of wire). Synchronize the second machine with the
first and connect it to the bus bars at the proper instant as indi-
cated by the lamp (used for synchronizing).
Fig. 27. Parallel operation of alternators. A voltmeter, not shown
in the diagram, is to be available for measuring the voltage of the in-
2. Load the machines from the bus bars until each delivers its
full rated load (or some fraction of full load), making needed ad-
justments of load between the machines with the field rheostats.
Observe and record the current delivered by each machine and by
the bus bars.
3. Same, for % the value of current used in item 2, leaving
the field rheostats untouched in the positions as in item 2.
98 LABORATORY MANUAL
4. With each machine delivering about ^2 its rated load to the
bus bars, throw all the load to one of the machines and discon-
nect the unloaded machine from the bus bars.
5. Start the machines, and load the bus bars until each ma-
chine is delivering, say, full load (or some fraction of full load).
Raise the field current in one of the machines and lower it in
the other, keeping the bus bar voltage constant. Observe and re-
cord the effect on (a) the armature currents of the two machines
and their arithmetical sum; (b) the total load current from the
bus bars; (c) power output (El in the case of lamps) ; and (d)
power delivered by each machine.
Written Report. 1. Explain briefly just why the three condi-
tions for throwing two generators in parallel must be met, as de-
scribed under the Theory.
2. Why are these conditions fulfilled when the synchronizing
lamp is dark (or light, depending on which method was used) ?
3. Do the machines share the total bus bar load equally when
the load is reduced to half value in item 3, Order of Work? Ex-
4. Why is it necessary to reduce the load on the one machine
to zero before disconnecting it from the bus bars ?
5. What would be the effect if one of the machines were discon-
nected from the bus bars while delivering its share of the total
bus bar load ?
6. From the observations obtained in item 5, Order of Work,
state and explain the effect produced on the armature currents
of the two machines and their arithmetical sum ; the total load
current from the bus bars; the power output (total) ; and the
power output from each machine, as the field currents are ad-
Study of the Mercury Arc Rectifier.
The object of this experiment is to afford the opportunity for
observing the construction and operation of the Mercury Arc
Theory. The function of this device is to make a uni-direc-
tional (one direction) current from alternating current. Its
ALTERNATING CURRENT 99
principal uses are in connection with arc lighting systems where
direct current arc lamps of the Magnetite or Metallic Flame type
are used (not operative on alternating current circuits) and
where the advantages of distribution of the power by alternating
current make it an economy to install this auxiliary piece of ap-
paratus for transforming the alternating to direct current ; also
where storage batteries are to be charged (with direct current)
and alternating current is the only available supply.
The principle of operation depends on the fact that mercury
placed in a vacuum bulb with one terminal in contact with the
mercury and one terminal above the line of the mercury, permits
current to flow through it in one direction only. An ingenious
device permits both alternations in each cycle to be used in con-
nection with the rectifier.
Current Supply. 110 or 220 volts Alternating Current.
Apparatus Required. (1) Mercury arc rectifier; (2) lamp
bank to be used as a load ; (3) two ammeters; (4) two voltmeters ;
and (5) a wattmeter.
Order of Work. 1. Connect the rectifier switch to the alternat-
ing current supply mains and arrange to load the outfit with the
lamps, inserting an ammeter and voltmeter on each side of the
rectifier, and a wattmeter on the alternating current side.
2. Start up the rectifier and connect in the alternating cur-
rent supply. Turn on lamps up to the capacity of the rectifier,
and observe and record the current and voltage on each side of
the rectifier and the watts on the alternating current side. Note
carefully the tilting device for starting the rectifier action.
3. Same, for %, % and % loads, in turn.
Written Report. 1. Describe briefly the action of the Mercury
2. From the observations in items 2 and 3, Order of Work, cal-
culate the efficiency of the rectifier at full load and at %, % and
% loads, also the power factor for each load on the alternating
current side and the relation of the alternating voltage to the di-
rect current voltage in each case.
100 LABORATORY MANUAL
3. What regular maintenance item must be considered in the
operation and up-keep of the rectifier ?
4. Although the current can flow in one direction only through
the rectifier bulb, the statement was made in the Theory that
both alternations in each cycle are utilized. Explain how this is
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