This document discusses variable frequency drives (VFDs) and how they are used to control the speed of induction motors. It describes the main components of a VFD - the rectifier, DC bus, and inverter - and how they work together to convert incoming AC power to DC and then invert it back to AC at a variable frequency that controls motor speed. The document notes that VFDs are used in boilers at a company to control induced draft fans, forced draft fans, and bagasse feeders. It explains that VFDs provide energy savings of 25-30% by consuming only the power needed, with a return on investment period of 6 months to 2 years. Advantages include efficient motor speed control while

2. (Speed Control of Induction Motors Using VFDs)
Ali Hassan
(Electrical Department)

3. List of Contents
●
VFD
●
Main Components of VFD
● VFDs used in SML Bhone(Boilers)
●
How Energy is saved using VFD
●
Advantages
●
Drawbacks

4. VFD
➢
A Variable Frequency Drive (VFD) is
used to control the speed of the induction
motor, through changing the input
frequency and voltage.

5. Normally, Motor designed speed formula is
frequency x 120
Example:
➢
4-pole motor at different frequencies
➢
50 hertz x 120 / 4 = 1500 rpm
➢
40 hertz x 120 / 4 = 1200 rpm
no. of poles
Speed =

6. Main Components of VFD
➢ Rectifier
➢ DC Bus
➢ Inverter

7. HOW SPEED IS CONTROLLED
USING VFD?
Rectifier
➢
The rectifier in a VFD is used to convert incoming A.C power into D.C
➢
One rectifier diode will allow
power to pass through only
when the voltage is positive.
➢
A second rectifier diode will
allow power to pass through
only when the voltage is
negative.

8. DC Bus
The DC bus contains capacitors and inductors, it accepts power
from the rectifier and absorbs the ac ripple and delivers a smooth
DC voltage.

9. Inverter
The inverter contains IGBT that invert DC power into AC and deliver it
to the motor.
The IGBT can switch on and off several thousand times per second
and precisely control the power delivered to the motor.
The IGBT uses “pulse width modulation” technique (PWM) to
simulate a voltage sine wave at the desired frequency to the motor.

10. VFD Control Circuit

11. VFDs used in SML Bhone(Boilers)
VFDs used in Boilers at ID, FD motors and bagasse feeders.

12. How Energy is Saved Using VFD
➢
VFD consuming only the power
that’s needed.
➢
As a result, a large amount of
energy is saved.
➢
Energy savings vary from 25 to 30
percent.
➢
Resulting in a return on investment
of six months to two years.

13. Advantages
➢
Provides the most efficient means of
motor speed control
➢
Reduces the thermal and mechanical
stresses on the motor
➢
Improved efficiency of motor
➢
Smooth starting
➢
Increase power factor
➢
Control process variables such as flow

14. Drawbacks
➢ Initial cost of installing VFDs can
be expensive
➢ VFDs can produce harmonic
distortion
➢ Special shielded cable is used

15. THANKS