(Speed Control of Induction Motors Using VFDs)

Ali Hassan

(Electrical Department)

List of Contents

● VFD● Main Components of VFD● VFDs used in SML Bhone(Boilers)

● How Energy is saved using VFD● Advantages● Drawbacks

VFD

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

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 polesSpeed =

Main Components of VFD

➢ Rectifier

➢ DC Bus

➢ Inverter

HOW SPEED IS CONTROLLEDUSING 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.

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.

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.

VFD Control Circuit

VFDs used in SML Bhone(Boilers)

VFDs used in Boilers at ID, FD motors and bagasse feeders.

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.

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

Drawbacks

➢ Initial cost of installing VFDs can be expensive

➢ VFDs can produce harmonic distortion

➢ Special shielded cable is used

THANKS