-,1D
E X P ERI MENT
NO . 3
SHUNT GENERATOROUTPUTPOI-ARITYAND BUILD.UP
PU R P OS E :
To discover the effect that the direction of rotation has, and the effect that the direction of
the field flux has, on the output polarity of a self-excited shunt generator.
PE R F OR MA N C E
OB JE C T IV ES:
Upon successful completion of [,his saperipent,
the student witl be able to:
1. Connect a DC shunt generator correctly so that voltage will build up.
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2. Flash the freld of a DC generator to rever.sethe polarity of the terminal voltage.
B R I E F IN G:
A voltage is induced into a conductor as it cuts the flux lines of a magnetic field. The
poiarity of this induced voltage depends on two thinp.
First is the direction in which the
conductor is moving. Second is the direcr.cn in which the magnetic lines of force are pointing.
In a DC Generator it is the conductors that make up the armature coil that cut through the
flreld as the armature is made to rotate by what is called a prime mover. The magnetic flreld
through which the conductors cut, comes from the pole pieces around which the field coil is
wound. Initially, in a self-excited generator, the magnetic field is present only becauseof the
residual magnetism in the pole pieces. There is no current flow through the field coil until the
generator starts generating.
So we see two factors at work determining the direction of the field flux. First is the polarity
with which pole piece was last magaetized. This determines the direction of the residual flux.
Second is the terminal connections of the field coil to the armature. If the current that begins
to flow in the field coil (when the armature starts generating) tends to produce the same
magnetic polarity in the pole pieces as the residual magnetism, there's no problem. Each
increment of additional current increasesthe mmf and thus the strength of the field. And this
causesmore voltage to be generated until the greater reaches rated voltage,
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However, if the direction of the magnetic field produced by the field current is opposite from
that of the residual magnetism, the generator cannot build up to rated voltage. There are
three ways this could be corrected. First is to reverse the prime mover. This revenes the
polarity of the generated voltage, so the magaetic polarity of the current-prrcduced magnetic
field matches that of the residuai magnetism. The second is to reverse the connections between the armature and field coil.
This, too, reverses the mag:retic polarity of the field. However, both of these caseshad the
effect of reversing the polarity of the output termiuals. The third method lets you keep the
same direction of rotation and the same terminal polarity. It is called "flashing the field'. The
idea is to apply full excitation voltage acrossrthe field for a short time so that the residual
magnetism in the pole pieces will end up with the opposite magnetic polarity.
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MACHINESREQUIRED:
DM-100ADC Machine Operaringas a generator.
lv1FM-100Singie-phaseMotor as the prime mover.
POWERREQUIRED:
115 volts AC, single-phase.
0-150volt variableDC, 1 amp.
METERSREQUIRED:
0-150volt DC voltmeter.
ADDITIONALMATERIALREQUIRED:
MGB-100-DGBedolare.
PROGRAMPLAN:
Step 1.
Place the two machines on the bedplate with the motor on the left and the generator on the right.
Step 2.
Couple the machines tightly using the nrbber coupling. Be sure the mbber coupling frts snugly inside both flanges.
Step 3.
Clamp the machines tightly and place the coupling guard over the coupling.
3-2
Step 4.
Connect the single-phase motor to the 120 volt AC supply as showa in Figure 3-1.
Connect the voltmeter to the armatur€ terminals.
WDG.
START
WDG.
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MAIN
ST
SW
A R M A TU R f
] 2O V AC
ROTOR
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Figure$l
Step 5.
Have the instructor check your connections to be sure they are conect.
Step 6.
Turn ON the main AC circuit breaker and turn ON the circuit br.eakerswitch that
starts the motor.
Step 7.
At this point, the shunt freld is not connected. Therefore any magnetic field
present in the generator is solely due to the residual magnetism of the pole pieces.
Record the terrninal voltage and indicate the direction of rotation on Table 3-1 of
TEST RESLI.LTS. Consider the direction of rotation as viewed fr.om the outboard
(extreme right hand) end of the generator.
Step 8.
Siep 9.
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Turn OFF the motor circuit bresker switch and the main AC circuit breaker
switch.
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Connect the generator's shunt field in series with its rheostat and connect this
series combination in parallel with the armatur€, as shown in Figure 3-2a.
Step 10. T\rrn the generator's field rheostat knob fully clockwise to its maximum resistance
position.
3-3
r.
t
Step 11. T\rrn ON the main AC and motor circuit breaker switch.
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Step 12. Slowly turn the field rheostat knob counterclockwise, decreasing its resistance. If
the output voltage of the generator starts increasing, meaning that the residual
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magnetism in the shunt freld has the same polarity as the output of the generator,
proceedto Step 16.
AR M A TURE
ARMATURE
Figure&2a
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Figure 3-2b
Step 13' If the output voltage of the generator starts to decreasein Step 12, meaning that
the shunt field has the opposite polarity a-sthe output of the generator, turn OFF
the circuit bresker switches.
Step 14. Disconnect the shunt field and reconnect it with the oppo.sitepolarity as shown in
Figure 3-2b.
Step 15. Go back to Step 11.
Step 16. Continue turning the field rheostat knob counterclockwise until ttre output voltage
of the generator reads 125 volts DC.
Step 17. R€cord the generator connections and armature & shunt field oolaritv on Table 3-1
of TEST RESULTS,
Step 18. Turn OFF circuit breaker switches.
t
t
t
t
t
t
t
t
t
t
t
q
3-4
tr
FI-ASHINGTHE FIELD
Step 19. Disconnectthe shunt field and rheostat fmm the armature.
Step 20. Connect the shunt field and rheostat to the 0-150 volt DC power supply, as shown
in Figure 3-3, with the OPPOSITE polarity from the way it was in Step tZ.
Step 21. T\rrn the knob of the DC supply fully clockwise to its maximum output.
Step 22. Turn ON the main AC circuit breaker switch and the 150-volt supply circuit
bresker switch. Leave ON for 15 seconds;then turn them both off.
Step 23. Connect the shunt field and rheostat to the armature with the same connectlons
you had in Step 17 except for reversingthe voltmeter leads from Step 23,
Step 24. RepeatSteps 1 L, 12, 16,1?, and 18.
Step 25. Disconnectall leads.
TEST RESULTS:
Sttr'r
DIRECTION
N O.
OF ROTATION
POLARIry
AND
CONNECTIONS
CONDITIONS
OF TEST
Generatedvoltagedue
to residualmagnetism.
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TABLE 3.1
3-5
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Volts
Properconnectionof
shuntfieldto cause
build-up.
Volts
125
Generatedvoltage after
flashingthe fieldwith
reversedpolarity
Volts
o
1 50 v
SHUN T
rIELD
VARIABLE V CLTAGE
DC SUPPLY
Figure&3
D E . B R IE F IN G:
What two thinp determined the polarity of the generated voltage?
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2.
Why is it important that the current in the generator produce a magnetic field
having the same direction as the residual magnetism?
If you revensed the motor's direction of rotation,
why would you have to reverse
the field connections of the generator?
4.
Why doesn't a self excited generator start producing full rated voltage as soon as
the armature starts to turn?
5.
What effect does "flashing the field' have on the direction of the residual magnelic
field?
3-6
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whv?
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O U I C K QU IZ :
l.
Voltage begrns to be generated in a self-excited generator because of:
a. Residual current in the field coil.
b. R.esidualvoltage in the armature.
c. R.esidualmagaetism in the field pole pieces.
2.
The initial generated voltage causescurrent to flow in the field coil, which:
a. Eliminates the magnetic fieid of the generator.
b. Strengthens the magnetic fieid of the generator.
c. Has no effect on the magnetic field of the generator.
3.
You cau reveree the polarity ofa self excited generator by:
a. Reversing its direction of rotation oniy.
b. Reversing its field connections only.
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c. Reversing both its field connections and direction of rotation.
4.
If the direction of rotation is producing the wrong polarity but you can't change the
direction of rotation, you can r€verse polarity by:
a. Flashing the fieid only.
b. Flashing the fieid and reversing field connections to armature.
c. Reversing the field connections to the armature oniy.
5.
If the voltage doesn't build up and it doesn't matter which terminal is positive and
which is negative, the simpiest change you can make is:
a. Reverse the connections between the fieid and armature.
b. Reverse the voltmeter leads.
c. Reverse the direction of the prime mover.
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