International Islamic University,
Islamabad
Electrical Machines LAB
EXPERIMENT # 6:
Demonstrate the Voltage/Frequency Speed Control of
Induction Motor
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EXPERIMENT # 6:
Demonstrate the Voltage/Frequency Speed Control of
Induction Motor (Scalar Control)
Theory
The speed of an induction motor depends upon two factors:
1. The number of motor poles
2. The frequency of the applied power
AC motor speed formula:
P
f
N
120
Where f is the applied frequency and P is the number of poles of motor.
The scalar control method is based on varying two parameters simultaneously. The speed can be
varied by increasing or decreasing the supply frequency, but this results in change of
impedances. The change of impedance eventuates the increase or decrease of current. If the
current is small, the torque of the motor decreases. If the frequency decreases or voltage
increases, the coils can be burned or saturation can occur in the iron of coils. To avoid these
problems, it is necessary to vary the frequency and the voltage at the same time. In this way, the
occurring disadvantages of changing frequency and voltage can be compensated. According the
to the equation of induced voltage the V/Hz constant control gives constant flux in the stator.
* * * 44 . 4
,
N
f
V
RMS ind
Where
= magnetic flux linkage in the stator
N= No. of turns
= constant of coil
The speed torque equation of the induction motors can be used to determine the voltage-torque
and frequency torque functions.
s
R
I
r
r ind
2
2
3
Where
ind
= the torque of the motor in air gap
= the mechanical angular speed
r
I = the rotor current
r
R = the rotor resistance
It can be seen from the above equation that the relationship between torque and frequency is
inverse, while voltage is directly proportional to torque.
Torque speed control can be solved by the linear variation of the two parameters
~
2
2
2 f
V
~
f
V
While this control method the torque is accessible in all operating points upto the nominal speed
and the motor can operate over the nominal speed.
In the overspeed range, the torque of the motor will decrease in inverse proportion to the
increasing frequency because voltage cannot be higher than the maximum value.
Volts-per-Hertz Ratio
This term describes a relationship that is fundamental to the operation of motors using adjustable
frequency control. An AC induction motor produces torque by constant will enable the motor to
produce full load torque . Below base speed, this is accomplished maintaining a constant voltage
to frequency ratio applied to the motor when changing the frequency for speed control. If this
ratio rises as the frequency is decreased to reduce the speed of motor, the motor current will
increase and may become excessive. If it reduces as the frequency is increased, the motor torque
capabilities will decrease. There are some exceptions to this rule which are described below.
Above base speed, this ratio will decrease when constant voltage is applied to the motor. In these
cases, the torque capabilities of the motor decrease above base speed. At approximately 30 Hz
and lower, the volts per ratio is not always maintained constant. Depending on the type of load,
the voltage may be increased to give a higher ratio, in order for the motor to produce sufficient
torque, especially at zero speed. This adjustment is called voltage boost. At base speed and
below, the Volts-per-Hertz ratio can be adjusted to minimize motor current when the motor is
lightly loaded. This adjustment which lowers the voltage to the motor, will reduce the
magnetizing current to the motor. Consequently, the motor will produce less torque which is
tolerable.
Procedure:
1. Connect the apparatus as shown in the detailed connection diagram.
2. First switch on the field supplies of DC separately excited motor and also turn on the
field supply of three phase alternator, setting some initial values.
3. Next turn on the single phase supply for single phase induction motor and press the start
switch so that after start of motor centrifugal switch throws the auxiliary winding out of
circuit.
4. Next increase the supply of DC separately excited generated, which will in turn increase
the armature voltage of DC separately excited motor and as a result its speed will
increase.
5. Since the DC separately excited motor is mechanically coupled with three phase
alternator its output voltage will rise.
6. Three phase alternator is electrically connected with three phase induction motor and due
to rise of its voltage and frequency it will start to rotate.
7. Keep on adjusting the field supplies of DC machines and alternator until there is 50Hz
input to the three phase induction motor.
8. Set the maximum applied voltage of three phase induction motor to 190V. Now the
Volts/Hz ratio will be 3.8.
9. Keeping the Volts/Hz ratio constant change the frequency to 50, 45, 40, 35, 30 and 25 Hz
in steps and take relevant observations in the table given below.
10. Discuss the observations and also the scalar control method of Induction motor speed.
