Bulletin IOOAC-EX Experiment Manual for AC Motors
EXPERIMENT NO. 1
SATURATION CURVE OF AN ALTERNATOR
PURPOSE:
To discover the effect of saturation on the terminal voltage of an alternator.
BRIEFING:
Alternators are designed to run at a specific speed (known as synchronous speed) to produce
voltage at a specific frequency. That’s why they are referred to as “synchronous alternators”.
Alternators are driven at synchronous speed by a prime mover. Typical prime movers are
diesel engines, jet engines, steam turbines, hydroturbines, and DC motors.
The field coil of a synchronous alternator is wound on a rotor, which has salient poles. The
laminated iron core is called the “spider”. The armature coils are imbeded in slots on the
stator. As the rotor (field) is driven, its magnetic field sweeps around inside the housing. This
moving field is cut by the turns of the armature coil of the stator. This induces a voltage into
the armature coils.
The amount of voltage induced depends on two things: (1) the speed of the rotor and (2)
the strength of the magnetic field. Magnetic field strength, in turn, depends on the amount of
current passing through the field coil. As the current increases, so does the field strength up
to a point. That point we call saturation. When the spider becomes magnetically saturated,
further increase of field current produce little or no further increase of field strength. Thus,
th ealternator’s terminal voltage levels off.
PERFORMANCE OBJECTIVES:
Upon successful completion of this experiment, the student will be able to:
1 . Describe ho wvoltage is generated in an armature and the effect of field current.
2. Perform saturation tests on alternators.
DM-1OOA DC Machine operat i n g as a motor
SM-100-3A Synchronous Machine operating as an alternator
l- l
Bulletin 100AC-EX Experiment Manual for AC Motors
O-125 volt variable DC, S amps
O-150 volt variable DC, 1 amp
O-300 volt AC voltmeter
O-1 amp DC ammeter
MGB-100-DG Bedplate
SLA-1OOD Strobe-Tachometer
PROGRAM PLAN:
Couple and clamp machines securely.
step 1
Place the two machines on the bedplate.
Install guards.
step 2.
Connect the DC machine as a self-excited shunt motor, as shown in Figure l-l. Do
not turn the power ON yet.
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Bulletin 100AC-EX Experiment Manual for AC Motors
Step 11. Record the exact value of field current and terminal voltage in TABLE 1-l. Note
that the voltage across each of the three armature coils is the same, since they are
being produced by the same field.
Step 12. Repeat steps 10 and 11 for the following values of alternator field current: 0.4, 0.6,
0.8, and 1.0 amps.
Step 13. Slowly decrease the excitation voltage until the ammeter reads approximately 0.8
amps. Record the exact value of field current and terminal voltage in TABLE 1-2.
Step 14. Repeat step 13 for the following approximate values of field current: 0.6, 0.4, 0.2,
and 0 amperes.
Step 15. Turn OFF all circuit breaker switches. Disconnect all leads.
TEST RESULTS:
INCREASING FIELD CURRENT
FIELD CURRENT
.
TERMINAL VOLTAGE
TABLE 1-1
DECREASING FIELD CURRENT
FIELD CURRENT
TERMINAL VOLTAGE
TABLE 1-2
DE-BRIEFING:
1. .
Using the data you recorded in TABLE l-l, plot a curve of Terminal Volts versus
Field Amperes on the graph provided. Label this curve INCREASING.
2.
Using the data you recorded in TABLE 1-2, plot a curve of Terminal Volts versus
Field Amperes on the same graph. Label this curve DECREASING. Note: The reading taken at 1.0 amperes is a plotting point on both curves.
1-4
Bulletin 1OOAC-EX Experiment Manual for AC Motors
3.
What effect did saturation have on terminal voltage?
4.
From your observations would you regard residual magnetism in the rotor core an
important or unimportant factor?
What led you to this conclusion?
5.
Why is it good to use low hysteresis material for the rotor core?
QUICK QUIZ:
1.
Induced (generated) voltage in an alternator results from:
a. A stationary field and moving conductors.
b. A moving field and stationary conductors.
c. Both moving field and moving conductors.
2.
You cannot have a self-excited alternator without rectifying the AC output, because:
a.
The field poles must be constant North-South.
b . There is not enough residual magnetism to begin the generating process.
c.
3.
Armature coils must have DC applied to them.
Below the saturation point, as field current increases, terminal voltage:
a. Increases in direct proportion.
b . Decrease-s in direct proportion.
c.
Stays the same.
l-5
4.
Above saturation, increases in field current produce:
a. Larger increase8 in terminal voltage.
b. Smaller increases in terminal voltage.
c. No difference in terminal voltage.
5.
When running at a constant speed, the terminal voltage of an alternator can be
changed by:
a. Reversing the polarity of the field.
b. Changing the direction of rotation.
c. Changing field current.
320
280
240
160
8 0
0.2
0.4
0.6
FIELD
1-6
0.8
AMPS
1.0
1.2