Download Experiment 5: Principle of 3 phase induction motor

Basic Electrical Technology (DET 211)
Laboratory Module
EXPERIMENT 5
PRINCIPLE OF 3 – PHASE INDUCTION MOTOR
Objective:
To understand the structure and principle of 3 – phase induction motor and to control the
rotary direction
Introduction:
The induction motor (Figure 1) is an AC motor commonly used at homes in fans,
compressor and in some certain office appliances such as photocopy machine. An
induction motor has the same physical stator as synchronous machine, with a different
rotor construction. There are two different types of induction motor rotors which can be
placed inside the stator. One is called a squirrel cage rotor, while the other is called a
wound rotor.
Figure 1: Induction motor
A squirrel cage induction motor rotor consists of a series of conducting bars laid
into slots carved in the face of the rotor and shorted at either end by large shorting rings.
This design is referred to as a cage rotor because the conductors, if examined by them,
would look like one of the exercise wheels that squirrel run on.
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Figure 2: two types of induction motor
The speed of induction motor is dependent on motor design. The synchronous
speed (the speed at which the stator field rotates) is determined by the frequency of
input ac power and the number of poles in the stator. The relationship between poles (P),
frequency (f) and synchronous speed (ns) is:
ns 
120 f
P
rpm
The difference between the synchronous speed (ns) and the rotor speed (nr) is called the
slip and is expressed as a percentage of synchronous speed.
slip , s 
n s  nr
 100%
ns
The rotor speed may be expressed as
nr  1  s   ns rpm
where s is expressed as a decimal.
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At standstill, when the rotor is at rest, the operating field sweeps the rotor bars at
its maximum rate. Under those conditions, the generated voltage in the rotor circuit will
be maximum and determined by the number of turns on the rotor. As the field revolves, a
back EMF is generated in the stator winding which is nearly equal to the impressed
voltage. It thus follows that at standstill the flux sweeps the stator turns at the same rate
as those in the rotor. This means that the induced voltages in the rotor and the stator
turns on per phase basis are related by the turns ratio, as is the case in a transformer
between primary and secondary. It also follows that the frequency of the rotor – induced
voltages equals the line frequency when the rotor is at rest. In this condition the slip is
s  1.0 or 100%. As the slip decreases, the rate at which the flux sweeps across the
conductors decreases proportionately and the rotor EMF becomes
ER  s  EBR
and the rotor frequency is
fR  s  f
where ER  rotor – induced voltage at slip, s
E BR  blocked rotor – induced voltage per phase
f R  rotor frequency
Equipments:
1) Starting resistor (NO – 5302)……………………………………………………..1 set
2) Power supply (NO – 5306)………………………………………………………..1 set
3) Auto driving unit (NO – 5311)………………………………………………… …1 set
4) AC Voltage / Ammeter (NO – 5307)……………………………………….….....1 set
5) DC Voltage / Ammeter (NO – 5308)…………………….……………………….1set
6) Machine field frame (NO – 5310)……………………………………………..….2 set
7) 3 phase machine graphic board (NO – 5314)……..……………………………1set
8) Rotary converter graphic (NO – 5315)……………………………………….….1 set
9) 3 – pole rotor (A04)……………………………………………………………...…1set
10) Cage rotor (A06)…………………………………………………………………..1set
11) Wide pole piece for field winding (A10)………………………………………....5set
12) Field winding / 300 turns (A13)…………………………………………………..3 set
13) Field winding / 700 turns (A14)……………………………………………….….2 set
14) C Type brush holder set (A18)…………………………………………………..1 set
15) Rotor fixture (A20)…………………………………………………………….…..2 set
16) Magnetic pole fixture (A21)…………………………………………………...….5 set
17) Fixing bolt (A22)………………………………………………………………..….6 set
18) 8mm spanner (A24)……………………………………………………………….1set
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Procedure:
1. Design and constructed the circuit based on single-line diagram as shown in
Figure 3.0. Make sure you use the correct items with their respective parts numbers.
2. Make sure you use the field winding/300 turns (A13) for the induction motor and
the field winding/700 turns (A14) for the rotary converter. (Refer to Figure 3).
Figure 3 Circuit diagram of induction motor
3. Wiring up your set – up accordingly. Before turning ON the power, perform these
steps:
I. Adjust the speed control knob of the Auto Driving unit (NO-5311) to the
position of MIN.
II. Set the starting resistor (NO-5302) to the position of maximum value (50) .
(Request your instructor to confirm your work before turning on the power supply)
4. Turn ON the power supply. Slowly, set the speed until 1200 rpm on CW direction.
Then, adjust the starting resistor, R f until you get starting voltage, V 2  5V
Measure and record the value of starting current, A2 , starting voltage V 2 and field
resistance R f . Observe the rotor direction.
5. Adjust the starting resistor slowly until the starting voltage, V 2  6V . Record the
value of starting current, A2 and rotor speed, nr in Table 1.
6. Repeat procedure no 6 for the starting voltage of 6.5V, 7V, 7.5V and 8V.
7. Turn OFF the power supply. Change the switch direction of the driving unit to
CCW. Observe the rotor direction.
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8. Turn all the switches OFF and disconnect the connecting code after complete the
experiment. Disassemble the equipments, parts and store in the designated places.
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EXPERIMENT 5
PRINCIPLE OF 3 – PHASE INDUCTION MOTOR
Name: ______________________________
Group: ____________
Matric No: _______________
Date: ______________
RESULTS:
Starting Voltage, V =_______________V
Starting current, I =________________A
Field Resistance, R f  ____________
Speed, N =_____________________ rpm
3. Table 1
Starting Voltage (V2)
(Volt)
Starting Current (A2)
(Ampere)
Rotor Speed, nr
(rpm)
4. Plot a graph of rotor speed vs starting voltage.
5. Plot a graph of rotor speed vs starting current.
6. Explain your graphs.
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Name: ______________________________
Group: ____________
7.
Matric No: _______________
Date: ______________
How was the rotor direction when the switch direction in CW and CCW? Explain.
8. Calculate the synchronous speed if a 3-pole motor supplied from a three – phase
60Hz supply. The rotor speed at full load is your max rotor speed value from your
data.
9. What is the slip of your induction motor? (refer to Question 8)
10. What is the rotor frequency? (refer to Question 9)
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Name: ______________________________
Group: ____________
Matric No: _______________
Date: ______________
DISCUSSION:
CONCLUSION:
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