1. To demonstrate the operating characteristics of a three-phase induction motor
using the four pole squirrel cage induction motor.
2. To record the load characteristics N, I, cos 0, P2, s, and n as a function of
torque, (t) of a three-phase induction motor with squirrel-cage rotor for star
and delta connections.
3. To examine its performance characteristics.
LIST OF REQUIREMENTS
1. Voltage supply
2. Three-phase induction motor with squirrel cage rotor
3. Power factor meter
4. Watt meter
7. Pendulum machine
8. Display panel
9. Coupling cover
11.Shaft and cover
An AC motor is an electric motor that is driven by an alternating current. It consists of
two basic parts, an outside stationary stator having coils supplied with alternating
current to produce a rotating magnetic field, and an inside rotor attached to the
output shaft that is given a torque by the rotating field.
There are two types of AC motors, depending on the type of rotor used. The first is
the synchronous motor, which rotates exactly at the supply frequency or a
submultiples of the supply frequency. The magnetic field on the rotor is either
generated by current delivered through slip rings or by a permanent magnet.
The second type is the induction motor, which turns slightly slower than the supply
frequency. The magnetic field on the rotor of this motor is created by an induced
DEVICE 2 POLES
Three-phase AC induction motors
Three phase AC induction motors rated 746 W (1.000 hp) and 25 W (left),
with smaller motors from CD player, toy and CD/DVD drive reader head traverse (9
V battery shown, at bottom center, for size comparison).
Most common AC motors use the squirrel cage rotor, which will be found in
virtually all domestic and light industrial alternating current motors. The squirrel cage
refers to the rotating exercise cage for pet animals. The motor takes its name from
the shape of its rotor "windings"- a ring at either end of the rotor, with bars
connecting the rings running the length of the rotor. It is typically cast aluminum or
copper poured between the iron laminates of the rotor, and usually only the end rings
will be visible. The vast majority of the rotor currents will flow through the bars rather
than the higher-resistance and usually varnished laminates. Very low voltages at
very high currents are typical in the bars and end rings; high efficiency motors will
often use cast copper in order to reduce the resistance in the rotor.
In operation, the squirrel cage motor may be viewed as a transformer with a
rotating secondary. When the rotor is not rotating in sync with the magnetic field,
large rotor currents are induced; the large rotor currents magnetize the rotor and
interact with the stator's magnetic fields to bring the rotor almost into synchronization
with the stator's field. An unloaded squirrel cage motor at rated no-load speed will
consume electrical power only to maintain rotor speed against friction and resistance
losses; as the mechanical load increases, so will the electrical load - the electrical
load is inherently related to the mechanical load. This is similar to a transformer,
where the primary's electrical load is related to the secondary's electrical load.
This is why, for example, a squirrel cage blower motor may cause the lights in
a home to dim as it starts, but doesn't dim the lights on startup when its fan belt (and
therefore mechanical load) is removed. Furthermore, a stalled squirrel cage motor
(overloaded or with a jammed shaft) will consume current limited only by circuit
resistance as it attempts to start. Unless something else limits the current (or cuts it
off completely) overheating and destruction of the winding insulation is the likely
In order to prevent the currents induced in the squirrel cage from
superimposing itself back onto the supply, the squirrel cage is generally constructed
with a prime number of bars, or at least a small multiple of a prime number (rarely
more than 2). There is an optimum number of bars in any design, and increasing the
number of bars beyond that point merely serves to increase the losses of the motor
particularly when starting.
Virtually every washing machine, dishwasher, standalone fan, record player,
etc. uses some variant of a squirrel cage motor.
If the rotor of a squirrel runs at high speed, the flux in the rotor at any given
place on the rotor would not change, and no current would be created in the squirrel
cage. For this reason, ordinary squirrel-cage motors run at some tens of rpm slower
than synchronous speed, even at no load. Because the rotating field (or equivalent
pulsating field) actually or effectively rotates faster than the rotor, it could be said to
slip past the surface of the rotor. The difference between synchronous speed and
actual speed is called slip, and loading the motor increases the amount of slip as the
motor slows down slightly.
The speed of the AC motor is determined primarily by the frequency of the AC
supply and the number of poles in the stator winding, according to the relation:
Ns = 120F / p
Ns = Synchronous speed, in revolutions per minute
F = AC power frequency
p = Number of poles per phase winding
Actual RPM for an induction motor will be less than this calculated synchronous
speed by an amount known as slip, that increases with the torque produced. With no
load, the speed will be very close to synchronous.
The slip of the AC motor is calculated by:
S = (Ns − Nr) / Ns
Nr = Rotational speed, in revolutions per minute.
S = Normalised Slip, 0 to 1.
As an example, a typical four-pole motor running on 60 Hz might have a nameplate
rating of 1725 RPM at full load, while its calculated speed is 1800 RPM.
PART A (WYE)-CONNECTION
1. Established the connections for recording the load characteristics in star (Y)
connection according to the current diagram shown in the figure 2.3.
2. Before that, we must adjusted the operating elements of the control units as
shown in table 2.1.
Operating elaments Value
Type of Power 1000 W
Operating switch On position Off
Switch “ N const. const.” position const
Switch “torque range”position 10 Nm
Switch”speed range” position 3000
3. The control unit with master switch was switch on. The reset button then pressed.
Note that the red LED light should not be alight anymore. Otherwise the mistake
might had occurred during the set-up. Check or the following:
I. The coupling hoop guard is missing.
II. The hoop guard for the shaft and cover is missing.
III. The jack plug for the motor temperature control has not been plugging.
IV. The motor is too hot.
4. The motor was started by increasing the voltage supply gradually until it was
reached its rated voltage. Measured the require quantities at no load. Enter the
measured value into Table 2.2. switch the function selector from “off” to
Nconstant , The corresponding greed LED light up. ( the speed of the
pendulum machine automatically adjusted to the motor frequency). Then
adjusted the given load at the control units of the pendulum machines by
pressing the down button. When exceeding the selected value, press the up
button. Read the corresponding values measured. Enter the measured values
into the Table 2.2. Measurements was continued with other values.
5. Used those equations to calculated the values of:
The power output, =
=(2 x N)/60
The apparent power,S=1.732(V/ )
The efficiency, =( )x100%
The slip,s=[( ]x 100%
6. Plotted the graph showing N,I, cos , , and torque in star (Y) connection in
PART B: (DELTA) CONNECTION
1. The motor was connected in delta connection. Connected terminals U1-
W2, V1-U2 and W1-V2.
2. Conducted the experiment as in a Part A, under procedure 3 and 4. The
calculated and measured value was recorded in Tables 2.3.
3. Plotted a graph showing N,I, cos , , and torque in delta ( connection in
Graph 2.5. Indicated the rated torque clearly.
Parameter Measured Value
Torque, (Nm) 0.0 0.5 1.0 1.5 2.0 2.5
Voltage (V) 400 400 400 400 400 400
Current, (A) 1.8 1.8 1.8 1.8 1.8 1.8
Power factor (cos ) 78.46 75.52 72.54 69.51 66.42 63.26
Speed, N (rpm) 1495 1494 1490 1487 1480 1480
Power input, (W) 80 100 120 140 160 190
Apparent power, S(VA) 1.247k 1.247k 1.247k 1.247k 1.247k 1.247k
Power output, (W) 0.00 78.23 156.0 233.6 310.0 387.5
Efficiency, (%) 0.00 78 130 167 194 204
Slip, s 0.3 0.4 0.67 0.87 1.3 1.3
Parameter Measured Value
Torque, (Nm) 0.0 0.5 1.5 2.0 2.5 2.5
Voltage (V) 230 230 230 230 230 230
Current, (A) 2.8 2.8 2.8 2.8 2.8 2.8
Power factor (cos ) 78.46 75.52 72.54 69.51 66.42 60
Speed, N (rpm) 1496 1493 1490 1487 1482 1480
Power input, (W) 80 100 120 140 180 190
Apparent power, S(VA) 1.115k 1.115k 1.115k 1.115k 1.115k 1.115k
Power output, (W) 0.00 78.2 156.0 233.6 310.4 387.5
Efficiency, (%) 0.00 78 130 167 172 204
Slip, s 0.27 0.47 0.67 0.87 1.2 1.3
D.F. Warne,(2000). Newnes Electrical Engineer’s Handbook, Newnes.
Rusnani Ariffin and Mohd Aminudin Murad, (2010).Laboratory Manual Electrical
Engineering Laboratory 2 EEE 240.University Publication Centre (UPENA).
Stevenson, William D., Jr. (1975). Elements of Power Systems Analysis. McGraw-
Hill electrical and electronic engineering series (3rd
Ed.). New York:
Paul E. Tippens, (2007). Physics (7 th Ed.).McGraw Hill International Edition.
http://www.tpub.com/neets/book1/chapter1/1g.htm retrieved on 18 march 2013
TABLE OF CONTENT
Objectives and Equipments