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EXPERIMENT
NO. 6
LOAD CHARACTERISTICSOF A SEPARATELY.EXCITED
SHUNT
GENERATOR
PURPOSE:
To discover the effect that loading ha-s on the terminal voltage of a separately-excited DC
shunt generator mnning at rated speed and with rated field excitation.
PERFORMANCE
OBJECTIVES:
Upon successful completion of this experiment, the student will be able to:
1. Perform load tests on generators and compute voltage regulation.
2. Explain the behavior of a separately-excited shunt generator.
BR I E F IN G:
In Experiment 2, you increased the freld excitation voltage until the freld pole pieces became
magnetically saturated. And you saw that saturation caused a change in the ratio between
terminal voltage and field current.
In Experiment 5, you discovered that the terminal voltage increased as the g€nerator wa-s
driven at a faster speed. In both Experiment 2 and Experiment 5, the generator was not
loaded. That is, the generator did not supply power to a load. Just because there is a voltage
generated in the ananature, this do€sn't mean that there is a current flow in the ar:nature.
Current flows in the ar:nature only when there is an electrical load connected to the terminais.
Then, since current is the same in every part of a series circuit, the load current is the arnatur€ current.
The curent drawa by the load is inversely proportional to the load resistance. In this experiment you will maintain the speed at which the geuerator is driven at 1800 revolutions per
minute. AIso you will hold the separate excitation supply at a constant value, below saturation. Then you will vary the resistive load on the generator. Because both the freld strength
and speed are eonstaut, there will be no change in the voltage generated. There will, however,
be a change in terminal voltage.
6-l
Voltaee rezulation
is a term used to indicete how much the terminal
tween no load and full load.
If there is only a smnll change from no load to full ioad, the
generator is said to have good voltage reguiation.
poor voltage regulation.
voitage changes be-
Voltage regulation
If there is a big change, the generator has
is usually expressd
in percent.
It is computed
using the followrng equation:
vo YortageReguration: \|'p
where
x 1oo
VNt = Terminal voltage at uo load
VFL = Terminal voltag,ewith rated load applied
MACHINESREQU]HED:
DM-100A DC Machine operating as a generator
DYN-100-DM Dynamometer operating as a moto.-
P O W E R RE QU IR E D :
0-150 voit variable DC, 1 amp
0-125 voit variable DC. 5 amps
M E T E RS REQU IR E D :
0-150volt DC Voiuneters(2)
O-0.5amp DC Ammeter
0-2.5 amp DC Ammeter
ADDITIONALMATERIALREOUIRED:
RL-100A Resistance Load Bank
MGB-1OO-DGBedplate
Sl,{-100D Strobe Tachometer
PRO GRAM P I.A N:
NOTE:
The DYN-100-DM Dynamometer is being used as the prime mover for this experiment.
It is to be operated as a DC shunt motor and will be referred to as simply
the motor.
a.)
S to -
l
Piace the two machines on the bedplate with the
motor on the left and the generator on the right.
Step 2'
Couple the machines using the rubber coupling. Be sure the couplinp f,rtssnugiy
inside both flanees,
Step 3.
Clamp the machines tightly to the bedpiate. Place the coupling guard over the
coupling and the shaft guards over the motor and geuerator shafts.
Step 4'
Make the motor connectiousshown in Figure 6-1 but do not tum the power
ON
yet' T\rra the kaob of the 0-125 variabie DC suppiy fully countercirckwise
to its
zero position. T\rrn the motor's field rheostat knob fully counterclockwise to
its
minimum resistance position.
DC M O TO R
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2.54
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0-12svDC I
vARtAgLE
SUPPL'r'
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1.5A
FigureF1
Step 5'
Make the generator connections show:r in Figure 6-1 but do not turn the power
ON
yet. Turn the generator's freld rheostat knob fuliy clockwise to
its maximum resrstance position. Turn the krrob of the 0-150 voit supply fully counterclockwise
to its
zero position.
Step 6'
Have someone check your connections to be sure they are conect.
Then turn ON
the main AC, the 0-150 volt DC, the 0-125 voit DC, and the
motor circuir breakers.
Step T
Piug in the SLA-100D Strobe-tachometer and set its knob to read 1800 rom
Step 8.
Slowly turn the knob of the I25 volt suppiy fully clockwise to its maximum ourput
position. The motor should now be running.
Step 9.
With the Strobe-tachometer directed at the motor shaft, turn the motor's field
rheostat knob clockwis€ until the motor is rotating at 1800 rpm.
Step 10. Push all of the toggie switches on the RL-100-A Resistance Load Bank to the
dowaward (OFF) position. This is the nnoload'condition.
Step L1. Slowly increasethe output of the 0-150 volt DC supply to 125 volts,
Step 12. T\rrn the generator's field rheostat knob counterclockwise until the generator terminai voltage is 130 volts.
Step L3. Recheck the speed aud make any adjustment necessary to the motor's field rheostat knob to bring the motor back to 1800 rpm.
Step 14. Read the field current, the armature (load) current, and the terminal voltage of the
generator. R€cod these values in Table 6-1 of TEST RESIILTS.
Step 15. On the RL-100A ResistanceLoad Ban\ push the toggle switches upward ou legs 1
and 2. This loads the generator with resistances 1 and 2 in parallel.
Step 16. Repeat Steps 13 and 14.
Step 17. On the RL-100A ResistanceIoad Bank, push the toggle switches upward on legs 3
and 4. This loads the generator with resistances I, 2, 3, and 4 in parailel.
Step 18. Repeat Stetrs13 and 14.
Step 19. On the RL-100A ResistanceIoad Bank, push the toggle switches upward on lep 5
and 6. ThG loads the generator with resistances I, 2, 3,4, 5, and 6 in parallel.
Step 20. Repeat Steps 13 and 14.
Step 21. On the RL-100A ResistanceLoad Ban\ push the toggle switches upward on legs 7
and 8. This loads the generatorwith resistancesL,2, 3, 4, 5, 6, 7, and 8 in parallel.
Step 22. Repeat Steps 13 and 14.
6-.1
Step 23. On the RL-100A ResistanceLoad Bank, push the toggle switches upward on lep g
and 10. This loads the generatorwith resistances7, 2, 3, 4, E, 6, 7, 8, g, and 10 in
parallel.
Step 24. RepeatSteps 13 and 14.
Step 25. On the F.L-100A Resistanceload Ban\ push the toggle switches upward on leg 11.
This loads the geueratorwith resistauc€s
Ir 2,3,4,5, 6,7r 8,9, L0, and 11 in
parallei.
Step 26, Repeat Steps 13 and 14.
Step 27. Turn OFF all circuit breaker switches. Disconnect all leads.
TEST RESULTS:
TERM.
CURRENT CURRENT VOLTS
FIELD
NO LOAD
U
130
RES.1&2INPARALLEL
RES.1, 2, 3 & 4 IN PARALLEL
BES.1, 2, 3, 4,5 & 6 IN PARALLEL
RES.1,2,3, 4,5, 6, 7 & 8 IN PARALLEL
RES.1,2,3,4,5, 6, 7, 8, 9 & 10 IN PARALLEL
RES.1,2,3,4,5, 6, 7, 8, 9, 10 & 11 IN PARALLEL
TABLE S1
D E- B R I E F IN G:
1.
On the graph provided, use the data you have recorded in Table 6-1 to piot a curve
showing how terminal voltage varies with load (armature) current.
2.
From your curye, determine the terminal voitage when the generator is delivering
the full load current of 2.0 amperes. This is Vp1. Compute percent voltage regulation from the equation:
voYoltageRegulation
= E$"]"
6-5
x 1oo
3.
Did the tenninal
voltage increase or decresse when additional
load was placed on
the generator?
4,
For this separately excited generator did equal changes in load result in equal or
unequal changes in terminal voltage? Explain if this was expected and why.
5.
Would you consider a separately-excited shunt generator to have good regulation or
poor regulation? Explain what led you to this conclusion.
Q U I C KQ U I Z :
1.
A separately excited shunt generator has constant:
a. Field Strength.
b. Load Current.
c. Terrninal Voltage.
2.
The current drawn fi.om a generator by the load:
a. Is also its field current.
b. Has no relation to internal generator currents.
c. Is also its armature current,
3.
The terminal voltage of a separately-excitedshunt generator:
a. Goes up as load increases.
b. Goes down as load increases.
c. Stays the same as load increases.
o-c)
If the terminal voltage had dropped more, the generator:
a. Would have had a poor€r voltage regulation.
b. Would have had a better voltage regulation.
c. Wou-ldhave had the same voltage regulation.
Field excitation current in a separatery-excitedshunt
generator:
a. Goes up as the load increases.
b. Goes dowlr as the load increases.
c. Stays the same as the load increases.
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