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 poles2. The frequency of the applied power

AC motor speed formula: N=120 f

P

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.

V ind ,RMSf

=4 .44∗N∗φ∗ξ

Where

φ = magnetic flux linkage in the stator

N= No. of turns

ξ = constant of coil

The speed torque equation of the induction motors can be used to determine the voltage-torque and frequency torque functions.

τ ind=3

2ωI r

2 R rs

Where

τ ind = the torque of the motor in air gap

ω= the mechanical angular speed

I r= the rotor current

Rr = 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

τω ~

V 2

2πf 2~

Vf

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.