Name of Experiment:
Study of the characteristics of Poly-phase Induction Motor (squirrel rotor type)
Objective:
• To Study the construction of 3-phase induction motor.
• To study the different starting methods of 3-phase induction motor and also study how to
reverse the direction of rotation in a 3-phase induction motor.
• To draw the performance characteristics of 3-phase squirrel cage induction motor by
conducting load test.
Theory:
Induction motor is one of the most important machines, which is used in Industrial and
Domestically applications. These motors are classified into different types namely (i). Squirrel
cage Induction motor (ii). Slip ring Induction motor. Where the first one is most preferable
because of its simple construction and its performance characteristics. In the squirrel-cage rotor,
the rotor winding consists of single copper or aluminium bars placed in the slots and short-
circuited by end-rings on both sides of the rotor. In the wound rotor, an insulated 3-phase winding
similar to the stator winding and for the same number of poles is placed in the rotor slots. The
ends of the star connected rotor winding are brought to three slip rings on the shaft so that
connection can be made to it for starting or speed control.
The supply is connected to the stator and the rotor received power by induction caused by the
stator rotating flux, hence the motor obtains its name –induction motor. The stator consists of a
cylindrical laminated & slotted core placed in a frame of rolled or cast steel. The frame provides
mechanical protection and carries the terminal box and the end covers with bearings. In the slots
of a 3-phase winding of insulated copper wire is distributed which can be wound for 2, 4, 6 etc.
poles.
This motor has normal starting torque and adjustable speed so that speed control can be achieved
easily. Normally direct on-line starter, star-delta starter and autotransformer starter are used to
start the motor. It works under the principle of Faraday’s law of electromagnetic induction.
Induction motor is simply an electric transformer whose magnetic circuit is separated by an air
gap into two relatively movable portions, one carrying the primary and the other secondary
winding. Alternating current supplied to the primary winding from an electric power system
induces an opposing current in the secondary winding, when latter short-circuited or closed
through external impedance. Relative motion between the primary and secondary is produced by
the electromagnetic forces corresponding to the power thus transferred across the air gap by
induction. A 3-phase induction motor consists of stator and rotor with the other associated parts.
In the stator, a 3-phase winding is provided. The windings of the three phase are displaced in
space by 120∘120∘. A 3-phase current is fed to the 3-phase winding. These windings produce a
resultant magnetic flux and it rotates in space like a solid magnetic poles being rotated
magnetically. Figure 1 shows the slip-ring type starting of induction motor. These motors are
practically started with full line voltage applied across the stator terminals. Starting current is
adjusted by introducing a variable resistance or rheostat in the rotor circuit. The rheostat
connected in star & the resistance being gradually cut out of the rotor circuit equally in each
phase as motor picks up the speed. By increasing the rotor resistance, the rotor current is reduced
at starting and the starting torque is also increased due to improvement in power factor.
The rotation of a Y or delta-connected motor can be changed by exchanging any two of the three
phases of the incoming voltage. Fig. 2 shows diagrams for a WYE-connected motor for clockwise
(forward) and counterclockwise (reverse) rotation. From these diagrams notice that T1 and T22
supply voltage terminals have been exchanged in the diagram for motor reversal. In industrial
applications, terminals T1 and T2 are generally switched by the contacts of the reversing motor
starter. These diagrams will be useful for installation connections and troubleshooting.
Fig-Slip ring 3-phase induction motor
1
2
1
Apparatus Required:
• 3 phase-Induction Motor 440v, 1500 RPM, 3HP
• Three phase supply
• Magnetic Circuit breaker (DOL Starter)
• Voltmeter
• Ammeter
• Insulated Combination Pliers
• Screw driver
Circuit Diagram:
Procedure:
1. Record the Name plate data.
2. Connections are given as per circuit diagram.
3. 3-Φ induction motor is started with DOL starter.
4. The no load readings are taken.
5.The motor is loaded step by step till we get the rated current and the readings of the voltmeter,
ammeter, wattmeter, spring balance are noted in the data table.
Data Table:
L i n e L i n e I n p u t S p r i n g Torque Speed Slip Outp Efficien Power
Voltage Current Power Balance (T) (N) (S) ut cy Factor
(IL) (Wi) reading Powe (η) (pf)
(VL) (s) r
(W)
400 0.3 190 25.6 60.64 1410 0.06 1 6 4 . 86.52% 0.9
4
Result:
1.The torque-speed characteristic curve of a squirrel cage induction motor was plotted.
2.The motor was observed to have nearly constant speed with varying load.
3.The efficiency was highest near full load.
4.Starting torque was low, confirming standard behavior of squirrel cage motor
5.The motor was observed to have nearly constant speed with varying load.
6.The efficiency was highest near full load.
7.Starting torque was low, confirming standard behavior of squirrel cage motors.
Answer the following questions:
1. Explain why the power factor of an induction motor is very low at starting.
Answer: At starting, an induction motor draws a large magnetizing current because there is no
back EMF (electromotive force). This magnetizing current lags the voltage by nearly 90°,
resulting in a very low power factor. Since the rotor is stationary, it behaves like a short-circuited
transformer secondary, drawing high reactive current.
2. Why are the rotor slots of an induction motor skewed?
Answer: Rotor slots are skewed to:
• Reduce magnetic hum and noise.
• Minimize torque pulsations (cogging).
• Ensure smoother and quieter operation.
• Improve starting torque by providing better magnetic coupling.
3. Why is an induction motor called an asynchronous motor?
Answer: An induction motor is called an asynchronous motor because its rotor does not
rotate at the same speed as the stator's rotating magnetic ield. Instead, it rotates at a
slightly lower speed, creating the necessary slip to induce current in the rotor.
4. Give the advantages of a wound rotor motor over a squirrel cage induction motor.
Also state the effect of rotor resistance on starting torque.
Answer: Advantages of a wound rotor motor:
• External resistance can be added to control starting current and torque.
• Higher starting torque compared to squirrel cage motors.
• Better speed control over a wide range.
Effect of rotor resistance on starting torque:
Increasing rotor resistance improves starting torque up to an optimal point. After this, further
increase may reduce efficiency and heating increases.
5. Why is the efficiency of a 3-phase induction motor less than that of a
transformer?
Answer:Efficiency is lower because:
• Induction motors have additional losses due to mechanical friction and windage.
• Rotating parts lead to bearing losses.
• There are slip losses in the rotor (I²R losses).
• Transformers are static devices and don’t suffer from these mechanical losses.
f
