Clarence Edward Clewell.

# Laboratory manual. Direct and alternating current online

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Font size low resistance windings, while the shunt field winding, through
which a relatively small current flows, has a high resistance
winding.

Current Supply. 110 volts Direct Current.

Apparatus Required. (1) A compound generator; (2) pro-
tective resistance (lamp bank), and (3) a suitable ammeter for
the armature and series field winding; (4) an adjustable resist-
ance (field rheostat), and (5) a suitable ammeter for the shunt
winding; and (6) suitable voltmeters with flexible leads for the
three sets of observations.

14

LABORATORY MANUAL

Order of Work. Make a diagram of the exact connections em-
ployed in each of the following items, labelling each instrument
on the drawing with its laboratory number.

Supply Mains (110 Volts D. C.)

_a_

-1 J IV-

Voltmeter

Ammeter

Protective Resistance (Lamp Bank)

Fig. 5. Measurement of the resistance of an armature when station-
ary. In the case here shown, the voltmeter indicates the volts drop be-
tween the armature terminals.

1. With a suitable protective resistance (lamp bank) in series
with the armature (which is at rest) , and the ammeter as shown in
Fig. 5, observe the volts drop across the armature of the machine
for say 25, 50 and 75 per cent, of full load current (see name

No.

Percent, of Full
Rated Current

Amperes

Volts Drop

Resistance in Ohms
(To be Calculated Later)

1

2

3

Form 3.

plate on machine for normal current). Allow a brief interval
between each of these observations to note the effect of tempera-
ture rise on resistance. Use Form 3.

DIRECT CURRENT 15

2. Repeat with the series field winding in place of the arma-
ture.

3. With an adjustable resistance (field rheostat) in series with
the shunt field winding and the ammeter, observe the volts drop
across the field terminals for the normal field current (when the
normal machine voltage is impressed on the field) and for ten
and twenty per cent, below normal value. The shunt field cur-
rent is so small that a smaller ammeter will probably be required
than in items 1 and 2.

Written Report. 1. What is the resistance of the armature for
each of the three values of current ? What is the average ?

2. Same for the series field winding ?

3. Does the resistance increase for the higher values of current
in 1 and 2, and if so, why ?

4. What is the resistance of the shunt field winding for each
of the three values of current ? What is the average V

5. Why is Ohm's law applicable to the finding of the resist-
ance in each of these three cases ?

6. Would Ohm's law apply if the armature in item 1 were in
motion ? Explain briefly.

EXPERIMENT 3.
Voltage and Power Losses in Transmission.

See Articles 45, 46, 47, 61, 62 and 69 in the text book (Timbie's
Elements of Electricity).

The' purpose of this experiment is to gain a working knowledge
of the calculation of volts drop and power loss in the transmission
of electric power from one place to another.

Theory. In the transmission of electric power (El) over
longer or shorter distances, the current / determines the loss in
voltage and the loss of power for a given size of transmission
wire. Thus, if the resistance R of the two wires out and back be
known, the product RI indicates the volts drop in the line (out
and back), and the product RP indicates the power loss (watts)
in transmission. In the simple apparatus shown in Fig. 6 (see
Fig. 97, Timbie), the two wires leading from the supply switch

16

LABORATORY MANUAL

to the lamps serve on a small scale as transmission wires, conduct-
ing the electric current from the supply switch to the lamps.

Supply Mains (110 Volts D. C.)

r

JL

i [) '

X"N .

Main Switch

Voltmeter

-Oi

o
-o

o
o

Short Transmission Line

Fig. 6. Measurement of the voltage and power losses in a short
transmission line. The voltmeter, as here shown, indicates the volts
drop in one side of the lin'e only.

Current Supply. 110 volts Direct Current.

Apparatus Required. (1) Long wires across the r0om ar-
ranged as in Fig. 6; (2) a low and (3) a high reading voltmeter
with flexible leads; and (4) an ammeter.

i.

1
P

&

i

8

6
&

So

00

1
1

00

C

1 &

E- -2

8-2

|
1

<

VoHs Drop

These Line Calculations

C

1!

O o

Is

3
S*

1

03 e
^=J

|o

Resistance
in Ohms

o.

eg

la

1"

l

2

3

Form 4.

Order of Work. Make a diagram of the exact connections em-
ployed in each of the following items, labelling each instrument
on the drawing with its laboratory number.

DIRECT CURRENT 17

1. Measure the current in each wire separately, which connect
the lamps to the supply switch, also the volts drop in these two
wires (all the lamps being turned on), from which the resistance
R of the wires may be calculated by Ohm's law. Use Form 4.

2. Measure the supply voltage and the voltage at the lamp
terminals for a constant value of current (all the lamps turned
on), recording these observations in Form 4.

3. Repeat 1 and 2 when half of the lamps are turned off, using
Form 4.

Written Report. 1. Calculate the resistance R for each of the
wires in 1, Order of Work, and calculate the total RI (volts)
drop and the total RP (watts) loss in the two line wires.

2. Find the RI drop in the line wires for 2, Order of Work, by
subtracting the voltage at the lamp terminals from the supply
voltage.

3. Calculate the resistance R for each of the wires in 3, Order
of Work, and calculate the total RI (volts) drop and the total
RP (watts) loss in the line wires for half the lamps in operation.

4. How does the RI drop in 1 and in 2 compare ?

5. How does the RI drop and the RP loss in 1 and in 3 com-
pare ? Explain briefly.

EXPERIMENT 4.
Study of Measuring Instruments.

See Articles 57, 58, 59, 60, 61, 62, 63 and 64 in the text book
(Timbie's Elements of Electricity), also Articles 3 and 4 in the
introduction to the Manual.

The purpose of this experiment is to gain an understanding,
both as regards construction and operation, of the features of
electrical instruments for the measurement of current, voltage
and power.

Theory. The underlying principle on which a majority of in-
struments is based, is the mechanical force produced on a wire
carrying current when the wire is placed in a magnetic field.
This mechanical force is directly proportional to the current for

18

LABORATORY MANUAL

a given value of the magnetic field and for a given length of wire.
Hence, by the introduction of a balancing hair spring, the motion
of the pivoted wire and, hence, that of the attached needle or
pointer is proportional to the current strength.

In the ammeter the scale of the instrument is calibrated in am-
peres; in the voltmeter, the needle is actuated by the current
through the movable coil, but the scale is calibrated in volts (this
is possible since E=I/R where R is the constant resistance of the
instrument).

Fig. 7. Diagram for observing the mechanical force on an electric
wire when placed in a magnetic field.

In the wattmeter the magnetic component of the mechanical
force is proportional to the main current since the magnetism
may be considered as produced by the main current through a
stationary coil, while the movable coil carries a current value
which is proportional to the voltage E across the terminals of the
instrument, therefore, the deflection of the needle is proportional
to the product El. Hence, the wattmeter is a combined ammeter
and voltmeter.

In the watt-hour meter, the principle is that of a small motor
in which the force and the number of revolutions per second is
proportional to El, and the total number of revolutions in a given
time t and, hence, the displacement of the pointers on the dials

DIRECT CURRENT 19

is proportional to the product Elt. The watt-hour meter is used
in residence electric lighting work for recording the number of
watt-hours consumed per month.

Current Supply. 110 volts Direct Current.

Apparatus Required. (1) A length of small sized wire; (2)
a protective resistance (lamp bank) ; (3) a strong electromagnet;
(4) a Siemens Electrodynamometer ; (5) a board on which are
mounted the parts of a Weston instrument; (6) a Weston am-
meter (10 amperes range) ; (7) a Weston voltmeter (150 volts
range) ; (8) a wattmeter (5 amperes, 150 volts range) ; and (9)
a watt-hour meter.

Order of Work. Make a diagram of the exact connections em-
ployed in each of the following items which make use of the elec-
tric current, labelling each instrument on the drawing with its
laboratory number.

1. Connect the length of small sized loosely stretched wire in
series with an ammeter and a protective resistance (lamp bank),
also connect the electromagnet in series with a lamp bank and a
second supply switch as shown in Fig. 7. "With the current of
about five lamps flowing through the loose wire, quickly close
the supply switch of the magnet. Note the mechanical force on
the wire. Open and close the switch of the electromagnet a num-
ber of times in succession. Repeat with the current of one lamp
and of ten lamps flowing through the loose wire.

2. Connect the Siemens Electrodynamometer in series with a
lamp bank to the line and note the action of the movable coil
when a current flows through the instrument; reverse the cur-
rent and note the direction of the force as compared to the direc-
tion in the preceding case. (Caution: do not exceed the current
rating of the instrument.) Note carefully what produces the
mechanical force on the movable coil.

3. On the board containing the parts of a Weston instrument
note and make a simple sketch of the magnet and of the movable
coil. Look for these parts through the glass in the cover of one
of the Weston instruments.

4. Connect a lamp bank and the ammeter, voltmeter, watt-
meter and watt-hour meter as shown in Fig. 8. Turn on a por-
tion of the lamps and observe the reading of the watt-hour meter

20

LABORATORY MANUAL

before and say 15 minutes after turning on the lamps. Observe
and record the ammeter, voltmeter and wattmeter readings sev-
eral times during this 15 minute interval. Use Form 5.

Before disconnecting this apparatus open the supply switch,
reverse the terminals of the ammeter and close the switch mo-
mentarily. Note the effect 011 the direction of force as shown by
the movement of the needle.

Supply Mains (110 Volts D. C.)

^

A

r i ft

Ammeter

[ I (

Main Switch

Watthour
Meter

Wattmeter

Fig. 8. Measurement of the power supplied to a bank of lamps.
The flexible lead from the watthour meter is one voltage connection,
the other is made in the instrument to the wire carrying the main cur-
rent.

Written Report. 1. What effect was produced by the larger
as compared with the smaller current values through the loosely
stretched wire on the mechanical force in item 1, Order of Work ?
What was the function of the electromagnet in this particular
test?

2. What produces the mechanical force in the Siemens Electro-
dynamometer? What effect was produced on the direction of
this mechanical force by reversing the current through the in-
strument ? Why ?

3. From the results obtained under item 4, Order of Work,
calculate the watt-hours from the voltmeter and ammeter read-
ings, and from the wattmeter readings and compare each of these

DIRECT CURRENT

21

results with the number of watt-hours recorded by the watt-hour
meter.

4. If the commercial rate for electric power in residence light-
ing is 9 cents per 1000 watt-hours (9 cents per kilowatt-hour),
then, in a residence containing twenty 25-watt tungsten lamps,
how many hours can all the lamps be turned on per night for a
month of 30 days to make the bill equal \$3.60 for the month?

5. What effect was produced on the direction of throw of the
ammeter needle in item 4, Order of Work, when the connections
were reversed? Compare this result with that of the direction

|

S

1

1

Watthour Meter

Watthours for the Interval
(To be Calculated Later)

From Watthour

From Voltmeter

From. Wattmeter

i

2

3

Form 5.

of force on the Electrodynamometer when the current was re-
versed in its coils ? Explain the difference briefly.

EXPERIMENT 5.
Study of Fuses and Circuit Breakers.

See Article 36 in the text book (Timbie's Elements of Elec-
tricity), also Article 6 in the Introduction to the Manual.

The purpose of this experiment is to gain familiarity with the
make-up of various types of fuses, the elements of fuse blocks,
and the construction and operation of circuit breakers.

22 LABORATORY MANUAL

Theory. The simple fuse is an alloy with a relatively low melt-
ing point. A fuse is inserted in a circuit, usually one on each side
of the line, as a protection against overloads and short circuits,
which might otherwise injure the wires or appliances in the cir-
cuit. Fuses may roughly be classified either as open or enclosed.
In the enclosed type, commonly known as the cartridge fuse, the
fuse wire is surrounded with a fire-proof covering which prevents
the scattering of small particles of hot fuse metal in case of a
blow-out.

The distance between the terminals on standard fuses depends
on the voltage, that is, with higher voltages the fuse terminals
are made farther apart so as to reduce the likelihood of the cur-
rent arcing across the terminals when the fuse is blown. It is
obviously very important in fusing up a circuit, to use a fuse
so rated as to protect the circuit against current values in excess
of the permissable limiting current for the circuit. Thus, if the
maximum allowable current is 10 amperes, the rating of the fuse
should not exceed this current value. Fuses are usually so rated
as to allow a small margin above the rating before the fuse act-
ually blows. If the circuit is opened by the blowing of a fuse,
an inspection will often show whether the blow-out has been due
to a simple overloading of the circuit or to a short circuit. If a
slight overload, the fuse is melted away without the appearance
of burns on the porcelain fuse block. If a short circuit, the
charred or burned appearance of the fuse block generally indi-
cates the fact. The inspection here mentioned refers mainly to
the open fuse.

The circuit breaker is an electromagnetic device which me-
chanically opens the circuit when the current exceeds the value
for which it is adjusted or set to open. The continual breaking
of the circuit between metal surfaces would soon cause excessive
wear due to the arcing, and to avoid this, the actual opening of
the circuit is between carbon blocks which may readily be re-
newed as necessary. "When closed, the current is conducted
through copper contacts, the breaker being so arranged that as
it opens the copper contacts separate first and then the carbon
contacts.

Current Supply. 110 volts Direct Current.

DIRECT CURRENT 23

Apparatus Required. (1) A low and a high voltage fuse
block; (2) a ceiling rosette; (3) several lengths of 3 to 5 ampere
fuse wire; (4) a cartridge fuse for low current and one for heavy
current; (5) an Edison fuse plug; (6) several typical circuit
breakers; (7) an adjustable rheostat; (8) a lamp bank; (9) foot
rule; and (10) an ammeter.

Order of Work. 1. Make a simple sketch of the fuse blocks for
low and for high voltage and of the ceiling rosette, showing the
fuse as well as the line terminals, and give dimensions on the
sketch. Insert fuses in each of the three, and have them checked
by the Instructor.

2. Connect the lamp bank in series with the adjustable rheo-
stat and the ammeter through a fuse block to the supply switch.
Starting with zero current, gradually increase the current until
the fuse blows, observing the value of the current just before the
circuit is broken. Eepeat this test with the fuse entirely clear
of the porcelain block except at the terminals, also when the
fuse rests against the porcelain throughout its length. (The fuse
block may be equipped with a small sized fuse wire for this test,
a 3 or 5 ampere fuse being sufficient.)

3. Make a simple sketch of a low and a heavy current cartridge
fuse. Take out and replace each of these fuses, observing care-
fully how they make contact at the terminals. Measure the sup-
ports and the length of the fuse in each case, recording the di-
mensions on the sketches.

4. Make a simple sketch of an Edison fuse plug and its recep-
tacle with dimensions.

5. Sketch two types of circuit breakers. Note how they are
adjusted for opening the circuit at various current values, and
note the carbon blocks between which the circuit is broken, and
the copper contacts between which the current flows when the
breaker is closed.

6. Arrange to supply current to a bank of lamps through
an ammeter and one of the circuit breakers. Set the breaker for
a nominal current value and gradually increase the current by
turning on the lamps until the breaker opens. With the lamps
turned on attempt to close the breaker. With the lamps still
on, open the main switch, close the breaker and then close the
main switch. This is the regular procedure in closing a circuit

24 LABORATORY MANUAL

breaker. (Caution: Do not exceed the capacity of the ammeter
in this test.)

Written Report. 1. What difference was observed in the dis-
tance between fuse terminals in the fuse blocks for low and high
voltage? What is the object of the longer distance for the higher
voltage ?

2. In item 2, Order of Work, was there a fixed value of current
at which the 3 or 5 ampere fuse melted? What effect did the
fuse touching or not touching the porcelain have on the current
value at which the fuse melted ? Explain briefly.

3. What advantage has the cartridge and Edison fuse over the
open fuse? Has the open fuse any advantages ? What provision
is made in the cartridge fuse for heavy currents ?

4. State briefly the operation features of the circuit breaker.
Why are carbon contacts better than copper at the opening point
of the breaker ? Why is the breaker arranged so that the current
flows through copper contacts rather than carbon after it is
closed ?

5. What effect was noticed when the attempt was made to close
the breaker in item 6, Order of Work, before opening the main
switch ?

6. What distinguishes the blowing of a fuse or the opening of a
circuit breaker under a gradual increase of current on the one
hand, or by the sudden application of a heavy current on the
other hand ?

EXPERIMENT 6.
Study of Motor Starting Boxes.

See Articles 133, 136, 137, 138, 139, 143 and 145 in the text
book.

A majority of the laboratory tests involve the starting of a
motor. The purpose of this experiment is to afford an oppor-
tunity for a study of the physical make-up of common types of
starting boxes and of the principles of motor starting.

Theory. The armature winding of a motor has a very low re-
sistance and if connected to 110 or 220-volt mains directly at
starting an excessive current will flow, which is apt to damage the

DIRECT CURRENT

25

machine. As in all other cases where low resistance devices are
connected to the 110 or 220-volt mains, a protective resistance
must always be connected in series with the armature of a motor
before connecting it to the supply mains. After applying the
power to the motor it speeds up and in so doing a counter voltage
is set up in the armature which acts like resistance opposing the
flow of current from the supply mains. Hence, as the speed in-

Supply Mains (110 Volts D. C.)

A

1 I M

Ammeter

[] I

Main Switch

AVWMAA,

Fig. 9. Simple connection for starting a shunt motor. Note that
the shunt field circuit is entirely independent of the armature adjust-
able resistance. A voltmeter is to be connected across the armature
terminals.

creases, the protective resistance may be reduced and finally cut
out entirely, since at normal speed the counter voltage nearly
equals the applied voltage and the relatively small difference or
resultant voltage sends only a moderate current through the
low resistance of the armature.

The simplest form of starting resistance consists of a pro-
tective rheostat in series with the armature as shown in Fig. 9.
In this diagram note that the shunt field winding is connected
directly to the supply mains through a field rheostat and that the

26 LABORATORY MANUAL

starting resistance has nothing whatever to do with the field cir-
cuit. (The field rheostat merely serves to vary the field current
during the operation of the motor if desirable.) Other conveni-
ent modifications are made as explained in the text book articles
referred to, but the underlying principle in all these starting
boxes is as shown in Fig. 9.

If the field circuit is opened or if the field current is greatly
reduced while a motor is in operation, the counter voltage is
either reduced to zero or greatly lessened and the resistance of
the armature being low, an excessive current is apt to damage
the machine. As a precaution, therefore, never open the field cir-
cuit when a motor is running, and always start with all the re-
sistance of the field rheostat cut out so that the counter voltage
of the armature may quickly reach its normal value.

Current Supply. 110 volts Direct Current.

Apparatus Required. (1) An adjustable resistance which
will carry the armature current of the motor to be tested, without
overheating; (2) a simple "no voltage" or "no field" release
starting box; (3) a starting box with "no voltage" and "over-
load" release; (4) ammeter and (5) voltmeter. (Note: The
current usually used in starting an unloaded motor is larger than
its normal running current, hence, an ammeter must be selected
that will carry the maximum starting current.

Order of Work. 1. Connect the adjustable resistance and the
ameter in series with the armature of the shunt motor assigned,
while the shunt field winding is to be connected directly to the
supply mains through a field rheostat as shown in Fig. 9. (Cau-
tion: Be sure that the field circuit is always connected to the
supply mains when current is being delivered to the armature.)
With all the armature rheostat resistance cut in and all the field
rheostat resistance cut out, throw in the supply switch and grad-
ually cut out the armature resistance entirely. Note the current
just at starting and after the motor is running at full speed. Al-
so note the voltage across the armature. Record these observa-
tions on the data sheet and make a diagram of the exact connec-
tions labelling each instrument on the drawing with its labora-
tory number.

DIRECT CURRENT 27

2. Repeat with a portion of the field rheostat resistance cut in,
noting carefully any difference in the ammeter and voltmeter

3. Take the cover off one of the starting boxes and trace out the
connections of the armature portion of the resistance. Make a
diagram of these connections labelling each part in terms of the
function it plays in the starting of the motor. Note also any
other parts of the starting box involved.

4. Make a general inspection of the "no voltage" and the
"overload" release types of starting boxes, trace their circuits
and determine their functions, ascertaining such points as may
not be clear, from the Instructor.

Written Report. 1. Why was the armature current higher at
starting than after the motor was running at normal speed in
item 1, Order of Work? What determines the initial current?
Explain the difference in the voltmeter reading at starting and at
normal speed in this same item.

2. In item 2, Order of Work, what effect was noticed on the
starting current and voltage with the increased field rheostat re-
sistance ? Explain briefly. What da you conclude from this ob-
servation as to the proper way for setting the field rheostat at
starting?

3. What is the underlying principle of all starting boxes ?

4. What is the function of the "no field" or the "no voltage"
release in a starting box ?

5. Same for the "overload" release?

EXPERIMENT 7.

Study of Electric Lamps.

See Articles 214, 215, 218 (two articles), 219, 220 and 222 in
the text book.

The object of this experiment is, first, to gain a working knowl-

Online LibraryClarence Edward ClewellLaboratory manual. Direct and alternating current → online text (page 2 of 8)