INDUCTION MACHINE
Introduction
Apparatus
The 3-phase induction motor, directly
connected to the electrical supply network is by far the
most widely used, albeit least flexible, drive system used
in industry.
AC machine windings are usually simpler than
dc machine windings. In this motor the 3 stator phase
windings are electrically separate and have all ends
brought out (6 terminals). See figure 1.
* Induction motor, slip- ring type, 1.5kW, 380V, 4 pole.
* Torque dynamometer, TERCO swinging frame dc
machine, 2kW.
* Load Resistor, 220V, 3kW, 20Ω.
* Ammeter(MI), 6A.
* Voltmeter(MI), 500V.
* Centre zero ammeter(MC), unscaled ±5A.
* Frequency meter, Frahm vibrating reed type.
* WARNING *
DO NOT START/STOP THE
INDUCTION MOTOR ON LOAD
Procedure
Figure 1
As the stator winding currents sequentially rise to their
maximae, the stator is never de-energised. Rather, the
position where maximum current is found moves around
the periphery. Current density may be viewed as a
moving wave. The resultant pattern of flux also rotates.
This flux also links the rotor circuits and induces
voltages and currents therein, hence INDUCTION
motor.
In a majority of machines(squirrel cage) the
rotor coils are shorted on themselves and no rotor
terminals are needed. In this machine, the internally star
connected rotor windings are brought out through sliprings and brushes to 3 terminals. A main purpose in
bringing out the rotor circuits is to allow addition of
resistance in order to reduce the level of induced
current, but this is not for investigation today. Instead
the rotor will be shorted directly.
1 CHECK RATINGS
The rating plate lists leading data for the
machine. Copy out this data. From it and knowing that
the local line voltage is 380V, decide whether the stator
windings must be connected in star or in delta. An
incorrect decision would in practice lead to the
destruction of the motor from overheating.
2 TESTS WITH MACHINE DISCONNECTED FROM
TORQUE DYNAMOMETER
Decouple the machine from the dynamometer.
Make sure it is clamped to the bedplate.
2.1 Direct on-line start at rated voltage.
This is the normal starting procedure for
induction machines. It is a bit severe on a slip-ring
induction machine designed to see rotor resistance
during starting. Make only one such start.
Use the fixed three phase supply. Short the
rotor terminals. Make a clear diagram of your proposed
connections, which should incorporate one ammeter.
Represent the motor as in figure 1. Have the diagram
checked by your demonstrator before switching on.
Take care to observe the correct phase sequence to the
motor. Use the standard RED/YELLOW/BLUE colour
code for the 3 phases. Note the surge of starting current
and rapid acceleration.
Reference
Slemon and Straughan, "Electric Machines", section 5
Objectives
1. Familiarisation.
2. Measurement of torque speed characteristic.
2.2 Measurement of no-load currents.
In order to allow adjustment of the voltage to
rated or other target values, and also to reduce the
starting current, the non-isolated variable 3-phase
supply is used for all the remaining tests. Make a new
diagram of connections and have it checked. Start the
M8.1
motor by smoothly increasing the voltage. Always reset
the variac to zero before switching out. Bear in mind
however, that in practice induction motors are not
started in this way.
Measure the average stator current from the
bench supply ammeters (average of 3) at rated voltage.
This is the induction motor "no load current". Record
what fraction it is of the rated current.
2.3 Measurement of no-load slip frequency and no-load
speed.
For any p-pole ac machine
ωs = ωm
p
+ ωr
2
where ωs is the system electrical angular velocity, ie 50
Hz expressed in rads/S, ωm is the mechanical speed of
rotation, ωr is the slip angular velocity, this is the
angular velocity of the currents induced in the rotor.
Slip is often expressed as a fraction of system frequency
and slip s is defined as:
s =
ωr
ωs
Hence
ωm
2
= (1- s) ω s
p
when slip is zero the synchronous mechanical speed
ωsm is
ω sm = ω s
2
p
ie the mechanical speed is synchronised with the system
speed.
To measure the slip connect the centre zero dc
ammeter to measure the current in one rotor phase, the
rotor still "seeing" a 3-phase short circuit. By timing its
slow movement with a watch over say, 30 seconds ,
measure the electrical frequency. This is the slip
frequency ωr which will be very low indeed at no-load
and rated voltage.
Measure the slip frequency at 380V and 190V.
What is the no-load slip of this machine at rated
voltage? Compute the mechanical speed from the slip at
380V and 190V given that the synchronous mechanical
speed for a 4-pole machine is 50/2 rev/S or 1500 rpm.
2.4 Tests with open circuit rotor.
Remove the short circuit from the rotor. Will the
machine turn? (Two of the test machines will even
though there is no current in the rotor windings.
Hysteresis in the rotor iron causes the flux to lag behind
the rotating stator mmf giving a small torque. See
Slemon, p.501, "Hysteresis motors".)
Now connect the frequency meter and voltmeter across
rotor terminals. Bring the voltage as high as possible
without causing the shaft to turn. Note the rotor
frequency and see how it varies when the shaft is turned
by hand, with or against the direction of the rotating
flux.
With the rotor locked, the induction machine
acts like a transformer. The stator windings may be seen
as the primary, the rotor windings as the secondary. The
transformation ratio is the ratio of stator to rotor voltage.
Measure it. Does the transformation ratio accord with
the rating-plate data?
2.5 Single Phase Excitation
With the rotor open-circuited excite one phase
winding only of the stator at 220V (phase-neutral!). This
will produce a pulsating rather than a rotating mmf.
Measure the rotor voltage and note its dependence on
angular position. You should see 4 maximae per
revolution. This is a way of confirming that the machine
is wound 4-pole, and also allows you to think of the
windings as having angle dependent mutual inductance.
3 TORQUE SPEED CHARACTERISTIC
(see curve in Slemon p.399)
Couple the motor to the torque dynamometer
and reclamp. Check that the couplings are not in metallic
contact and that the shafts are aligned (misalignment
causes wear and vibration). Check that the spring
balance is zeroed. Connect the shunt regulator for the
dynamometer field supply. Connect the 3 kW load to the
armature. Replace the short circuit of the induction
motor rotor, retaining the centre-zero dc ammeter. (cf.
figure 2)
Draw a complete circuit diagram of this circuit,
connect up and have it checked by your demonstrator.
P = Tω m
where P = Mechanical power and T = Torque.
First run up the machine with the dynamometer
field not excited. Is the rotation direction correct as
indicated by the arrow on the machine? How would you
reverse it? Run the motor in the reverse direction. The
dynamometer frictional torque is of course a load on the
machine. Do not use this as the no-load reading.
Calculate the rated torque at rated speed from the
nameplate data. Have this confirmed by your
demonstrator. Ensure that the machine is rotating in the
correct direction. Take readings of torque versus ω from
zero to rated torque by adjusting the dynamometer field
current, calculating ω from s as in 2.3.
M8.2
Figure 2
To measure the remainder of the torque speed curve
without exceeding the machine ratings the test must be
conducted at reduced voltage. The curve has the same
profile at this voltage but must be scaled by a factor of
2
the square of the voltage ratio (380/Vs) . This allows the
torque speed characteristic to be measured in the range
0 to rated rpm.
Remove the dc ammeter from the rotor circuit
for these readings and replace the ammeter with a short
circuit. Set and maintain the supply voltage (LINE) at
Stator
line voltage
(V)
Torque
maintain at
380V
0
.
.
.
(Nm)
100 volts. Take readings of torque and speed as the
dynamometer load is increased at regular speed intervals
down to 500 rpm. As the load is increased the induction
motor stator currents rise rapidly. Do not exceed 150%
rated current. Do not maintain overloads unnecessarily.
Remember, torque measurements taken at reduced
voltage must be scaled as shown in the table below.
Tidy up all leads and switch off bench supply and
instruments after experiment.
Adjusted
torque
(Nm)
Speed
Tactual
compute
from slip
ωm =(1-s)ωsm
(rpm)
Trated
maintain at
100V
.
.
.
.
.
2
Tactual(380/100)
speed from
rpm meter
M8.3
Requirements for report
Questions for discussion
* No theory required except as requested for discussion.
Concise answers are required to the questions in the
discussion.
* Present neat clear diagrams of all circuits used.
1. Explain what you understand by rotor slip. What is
the value of s at standstill and at synchronous
mechanical speed? Write down the equation relating
synchronous speed to the electrical supply frequency
and the number of poles.
* Present all calculations and results clearly.
* For the discussion it is sufficient to answer the
questions below.
2. Comment on the torque speed characteristic. Indicate
the full load torque, starting torque and synchronous
mechanical speed.
3. Why does excitation of just one sinusoidally
distributed stator winding fail to get the machine to
rotate? What in principle, is the minimum number of
stator phases necessary to get motor action?
M8.4