Bulletin 100AC-EX Experiment Manual for AC Motors
EXPERIMENT NO. 8
DIRECTION OF ROTATION OF SINGLE-PHASE MOTORS
PURPOSE:
To discover the factors affecting the direction of rotation and speed of a single-phase motors.
BRIEFING:
The rotor of a single-phase squirrel-cage induction motor has a core made of steel laminations. These laminations are stacked into what looks like a solid core. Imbeded in the outer
edge of the core are aluminum bars, short circuited by a ring at each end. These are called
squirrel cage bars or rotor bars. Together they are known as the rotor winding or the squirrelcage winding.
The stator windings produce alternating magnetic poles when energized with AC. As the
rotor bars cut through the stator’s field, alternating current is induced into the bars. This
current sets up a rotor field that interacts with the stator’s field to produce a torque on the
rotor. This torque is in the same direction that the rotor is already turning.
The problem is in getting the rotor turning in the first place. If you simply apply power to
the main field of an induction motor, the rotor will not turn. AC in the stator induces AC in
the rotor, all right, but there is equal and opposite torque produced. Then, if you spin the
rotor, the bars cut through the stator field. This unbalances the torques, adding to that in the
direction of spin and subtracting from the torque in the opposite direction.
What must be done to get the motor started is include an auxiliary winding on the stator
halfway between the poles of the main field. This produces the effect of a moving stator field.
Current in the auxiliary field reaches its peak at some time before current in the main field.
Direction of rotation is from an auxiliary pole to the adjacent main pole haying the same
polarity. The speed at which the stator field moves (the synchronous speed) is proportional to
both frequency and the number of poles. The equation is:
Synchronous speed =
Frequency
x 60
no. of pairs of poles
where the speed is in RPM, frequency is in hertz (cycles per second). The figure 60 is used to
conver t seconds to minutes.
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Bulletin 100AC-EX Experiment Manual for AC Motors
A four-pole motor, like Hampden’s MFM-100, has a synchronous speed (S) of 1800 RPM,
when a voltage of 120 volts, 60 hertz is applied.
S = 60/2x60 = 3600/2 = 18OO RPM
The rotor can never travel at synchronous speed, however. There must be relative motion
between the field and the rotor so that induction may take place. When the rotor is at rest,
this relative motion is solely due to the rotating field. When the rotor is turning, it is the
difference between the field’s speed and the rotor speed that produces the induced rotor currents. This difference is called slip.
Finally, we saw that after the rotor began turning, the torque producing rotor current was
induced simply from the rotor cutting through the main stator field. The auxiliary field isn’t
needed any more. Therefore, a centrifugal switch, which operates from rotor speed, takes the
auxiliary winding out of the circuit when the rotor reaches about 75% rated speed.
PERFORMANCE OBJECTIVES:
Upon successful completion of this experiment, the student will be able to:
1. Explain the operation of an induction motor.
2. Reverse the direction of a split-phase motor and compute slip.
MACHINE REQUIRED:
MFM-100 Single-Phase Squirrel-Cage Induction Motor
POWER REQUIRED:
Fixed l$ AC Supply
HT-1OOJ Tachometer
MGB-100-DG Bedplate
8-2
Bulletin 100AC-EX Experiment Manual for AC Motors
step 1.
Clamp the motor on the bedplate. Install coupling guards.
step 2.
Connect the motor as shown in Figure 8-l
00
I
I
I
START
WDG.
1
lQ MAIN
WDG.
ROTOR
Is A C I N P U T
N
Figure 8-1
step 3.
Turn ON the main AC and the motor’s circuit breakers.
step 4.
Noting the direction of rotation as that viewed from the right-hand end, indicate
the direction on Figure A of test results.
step 5.
Turn OFF the motor.
Step 6.
Interchange the connection to the start (auxiliary) winding as shown in Figure 8-2
ROTOR
10 AC
INPUT
0
I
I
START
WDG.
1
N b-Rlr‘
Figure 8-2
8-3
MAIN
WDG.
Bulletin IOOAC-EX Experiment Manual for AC Motors
step 7.
Start the motor. Indicate the direction of rotation on Figure B of TEST RESULTS.
Step 8.
Turn OFF the motor.
step 9.
Interchange the connections to the main winding as shown in Figure 8-3.
@I
I
2
4
START
WDG.
ROTOR
0
lc3
AC INPUT
Figure 8-3
Step 10. Start the motor.
SULTS.
Indicate the direction of rotation on Figure C of TEST RE-
Step 11. Turn OFF the motor.
Step 12. Interchange the connections to the start (auxiliary) winding, as shown in Figure
8-4, putting them back the way they were.
@M
I
I
I
I
START
WDG.
2*
1
ROTOR
MAIN
WDG.
AC
Figure 8-4
8-4
Bulletin 100AC-EX Experiment Manual for AC Motors
step 13. Start the motor. Indicate the direction of rotation on Figure D of TEST RESULTS.
Step 14. Turn OFF the motor and the main AC supply.
step 15. Interchange the incoming power leads.
Step 16. Turn ON the main AC supply and the motor. Indicate the direction of rotation on
Figure E of TEST RESULTS.
step 17. Measure the speed of the unloaded motor. Record in TEST RESULTS.
step 18. Turn OFF all circuit breaker switches. Disconnect all leads.
Figure A
Figure B
~~ ~,-I)
Figure D
M O T O R
Figure C
S P E E D
=
Figure E
DE-BRIEFING:
1*
What was the effect of interchanging leads from the main to the auxiliary windings
(Step 5)?
8-5
Bulletin 100AC-EX Experiment Manual for AC Motors
2.
Explain what to do to reverse the direction of a single-phase squirrel-cage induction
motor and why it works.
3.
Slip speed is the difference between synchronous speed and actual rotor speed. What
is the slip speed of the test motor running unloaded.
4.
Percent slip is the ratio between slip speed and synchronous speed (% Slip = Slip
Speed/ Synchronous Speed x 100). What is the percent slip of the test motor running
unloaded?
5.
Explain why an induction motor must have slip in order to run.
QUICK QUIZ:
1.
The direction of rotation of a single-phase motor is:
a) From the main pole to the adjacent auxiliary pole having the same magnetic polarity.
b) From the auxiliary pole to the adjacent main pole having the same magnetic polarity.
c) Either direction. It is impossible to predict.
2.
To reverse a single-phase motor:
a) Interchange incoming power leads.
b) Interchange connections between main and start windings.
c) Reverse connections to the rotor.
8I)6
Bulletin 100AC-EX Experiment Manual for AC Motors
3.
A single-phase induction motor needs:
a) An auxiliary winding to start.
b) An auxiliary winding to run.
c) An auxiliary winding for both starting and running.
4
l
An induction motor must run:
a) At synchronous speed.
b) Faster than synchronous speed.
c) Slower than synchronous or rotor speed.
5.
Slip is the term used to describe:
a) The sum of synchronous and rotor speeds.
b) Either synchronous or rotor speed.
c) The difference between synchronous and rotor speeds.
8-7