technical seminar on vfd.docx

VARIABLE FREQUENCY DRIVE

CHAPTER 1: INTRODUCTION

In today’s industrial and commercial world, variable frequency drives (also known as

VFD’s, Frequency Drives, AC motor controllers, and Inverters) are becoming a more and

more important method of speed control. They are used for all but the most demanding speed

control applications. Large rolling mills are among the few places where DC motors and

controls are still used. One of the reasons for this is that AC motors and their associated

controls are much more reliable than the DC motors and the associated controls that to a

large extent they have replaced. In current market VFD’s ranging from ¼ hp to 30,000 hp are

available for use.

In 1960 the first inverter were made and had limited applications due to small size

and reliability in 1980’s when high power transistor were available, larger inverters were

made and many more applications opened up. Reliability and Mean Time between Failures

(MTBF) was still a problem. All of these earlier devices used linear amplifiers and controls

for their basic operation. Small potentiometers and dip switches were used to set their

operating characteristics. In the 1990’s digital controls began to be used more and more in

Inverters. Solid state devices were developed that allowed higher voltage and current ratings.

Inverters are used in larger motors. Micro processors made the Inverter a versatile device.

For many applications, the Inverter can be removed from a packing box, wired to a motor,

and turned on and operated without additional set up.

The motors that are usually controlled by VFD’s are induction motors. A three phase

induction motor is one of the simplest power conversion devices ever made. It has one

moving part. If the motor has ball bearings, then an induction motor with ball bearings does

have more than one moving part. VFD’s are very simple, and hence very reliable. They have

a winding on the stator, or part that stands still, and a winding on the rotor, or the part that

turns. When voltage is applied to the stator, a voltage is induced (Hence – induction motor)

in the stator coil. This causes a current to flow in both the stator and the rotor. The design of

the motor is such that the magnetic fields of the two currents act against each other to cause a

force on the rotor and make it rotate.

Efficiencies of over 90 % and power factors of over 80 % are common at full load.

Some larger motors have power factors of up to 90 % when fully loaded. However, lightly

Department of Electrical and Electronics,SDM IT


loaded AC induction motors typically have low efficiency and low power factor.

CHAPTER 2: VARIABLE FREQUENCY DRIVE

Motors are designed to run at a constant speed. However, motor drive systems are

often operated at part or variable load. In particular, fans and pumps can have highly irregular

load profiles. This means, the motors on these systems either run at constant speed by passing

the excess capacity, or use some form of capacity regulation such as dampers, valves, or inlet

guide vanes, all of which are very inefficient.

System output can be controlled by adjusting the speed of the motor using one of

different types of Adjustable Speed

• Variable Frequency Drives (VFDs)

Motion control is required in large number of electrical and domestic applications like

rolling mills, fans, paper machines, textile mills, pumps, robots etc. System employed for

motion control is called drives. Drives employing electric motors are known as electrical

drives.

SOURCE

POWER MODULATOR

SENSING UNIT

CONTROL UNIT

MOTOR

LOAD

BLOCK DIAGRAM OF AN ELECTRIC DRIVE



SOURCE

POWER MODULATOR

SENSING UNIT

CONTROL UNIT

LOAD

VFD

MOTOR

BLOCK DIAGRAM OF AN ELECTRIC DRIVE WITH VFD

In this block diagram, load is usually a machinery to accomplish a given task, e.g. fan,

pumps, robots, drills etc. A motor having speed-torque characteristics and capabilities

compatible to load requirement is chosen. Power modulator are used to modulate flow of

power from the source to motor in a manner that required by the load, during transient

operations, converts electrical energy of the source in the form of suitable to motor and also

selects the modes of operation of the motor. Power modulator is as desired, it may also

generate commands for the protection of power modulator and motor. Input command signal

adjust the operating point of drive, forms an input control unit. In this basic block diagram a

variable speed drive can be placed so as to adjust the speed of the motor as required by the

load.

VFDs of higher efficiencies are easier to control, require less maintenance, and have

become the drive of choice in the majority of applications. In addition, speed control is

generally the most energy-efficient flow control technique because it requires the least

amount of energy to meet the given load. VFDs have widespread acceptance in the industry

therefore we will focus our attention on these drives. Within the VFD family, there are several

types available, each with its own practical application. All perform the job of controlling the

motor speed by varying electrical voltage and frequency in response to an electric feedback

from the end use. Depending on the application, VFDs, when applied correctly, can reduce

energy consumption by more than 50 percent.



Induction motors find applications in low to high power applications and synchronous

motor drives are employed in high power and medium power drives. The permanent magnet

synchronous motor and brushless dc motors are being considered for replacing dc servo

motors for fractional hp range. As the trend exits, applications of ac drives will continue to

grow.

The speed of an Ac motor is determined for the most part by two factors. The applied

frequency and number of poles.

N=120f / P

Where:

N=RPM

f=frequency

P=number of poles

Some motor such as in typical paddle fan have the capability to switch poles in and

out to control speed. In most cases however, the number of poles is constant and the only way

to vary the speed is to change the applied frequency changing the frequency is the primary

function of VFD. We must remember that the impedance of the motor is determined by

inductive reactance of winding and given by

X L= 2∏fL

X L= inductance in ohms

f = line frequency

L = inductance

This means that if the frequency applied to the motor is reduced, the reactance and therefore

impedance of the motor is reduced. In order to keep current under control we must lower the

applied voltage to the motor as the frequency is reduced. This is where we get the phrase volt

per hertz. The most common method of controlling the applied voltage and frequency is with

a pulse width modulated ‘PWM’ technique. With this method, a dc voltage is applied to the



motor windings in time controlled pulses in order to achieve current that approximates a sine

wave for the desired frequency. IGBTs are the latest technology and often the ability to

switch the PWM very fast. This allows several thousand pulses to be applied in one cycle of

the applied motor frequency. More pulses in a given cycle result in a smoother current wave

form and better performance.



CHAPTER 3: OPERATION OF VFD

Understanding the basic principles behind VFD operation requires understanding the

three basic sections of the VFD: the rectifier, dc bus, and inverter.

The voltage on an alternating current (ac) power supply rises and falls in the pattern

of a sine wave (see Figure 1). When the voltage is positive, current flows in one direction;

when the voltage is negative, the current flows in the opposite direction. This type of power

system enables large amounts of energy to be efficiently transmitted over great distances.

2

1

0 90 180 270 360

- 1

-2

Fig 1. AC SINE WAVE

The rectifier in a VFD is used to convert incoming ac power into direct current (dc)

power. One rectifier will allow power to pass through only when the voltage is positive. A

second rectifier will allow power to pass through only when the voltage is negative. Two

rectifiers are required for each phase of power. Since most large power supplies are three

phase, there will be a minimum of 6 rectifiers used (See fig 2). Appropriately, the term “6

pulse” is used to describe a drive with 6 rectifiers. A VFD may have multiple rectifier

sections, with 6 rectifiers per section, enabling a VFD to be “12 pulse ,” “18 pulse,” or “24

pulse.”



Fig 2. VFD (existing technology)

Rectifiers may utilize diodes, silicon

controlled rectifiers (SCR), or

transistors to rectify power. Diodes are the simplest device and allow power to flow any time

voltage is of the proper polarity. Silicon con-trolled rectifiers include a gate circuit that

enables a microprocessor to control when the power may begin to flow, making this type of

rectifier useful for solid-state starters as well. Transistors include a gate circuit that enables a

microprocessor to open or close at any time, making the transistor the most useful device of

the three. A VFD using transistors in the rectifier section is said to have an “active front end.”

Fig 3. Basic block diagram of

VFD

After the power flows through the rectifiers it is stored on a dc bus. The dc bus

contains capacitors to accept power from the rectifier, store it, and later deliver that power

through the inverter section. The dc bus may also contain inductors, dc links, chokes, or

similar items that add inductance, thereby smoothing the incoming power supply to the dc



bus. The final section of the VFD is referred to as an “inverter.” The inverter contains

transistors that deliver power to the motor. The “Insulated Gate Bipolar Transistor” (IGBT) is

a common choice in modern VFDs. The IGBT can switch on and off several thousand times

per second and precisely control the power delivered to the motor. The IGBT uses a method

named “pulse width modulation” (PWM) to simulate a current sine wave at the desired

frequency to the motor.

Motor speed (rpm) is dependent upon frequency. Varying the frequency output of the

VFD controls motor speed:

Speed (rpm) = frequency (hertz) x 120 / no. of poles



CHAPTER 4: TYPES OF VFD

VOLTAGE SOURCE INVERTER (VSI):

Variable frequency and variable voltage supply for IM control can be obtained from

VSI. VSI allows a variable frequency to be obtained from a dc supply. It can be operated as a

stepped wave inverter or pulse width modulated inverter. When supply is dc, variable dc

input is obtained by connecting a chopper between dc supply and inverter. When supply is ac,

dc input voltage is obtained by connecting a controlled rectifier between supply ac and

inverter. A large electrolytic capacitor is connected in dc link to make inverter operation

independent of rectifier or chopper and to filter out the harmonics in dc link voltage.

Harmonics are reduced, low frequency harmonics are eliminated, associated losses are

reduced and smooth motion is obtained at low speeds also when inverter is operated as pulse

width modulated inverter. Since output voltage can be controlled by PWM, no arrangement is

required for variation of input dc voltage, inverter can be directly connected when supply is

dc and through a diode rectifier when supply is ac.

VSI induction motor drives

V = m Vd / 2√ 2

Where m is modulation index.

CURRENT SOURCE INVERTER (CSI):

In current source inverter, inverter behaves as current source due to the presence of large



inductance in dc link. For a given speed, torque is controlled by varying dc link current by

changing the inverter voltage. Therefore, when supply is ac, a controlled rectifier is

connected between the supply and inverter and when supply is dc, a chopper is interposed

between the supply and inverter. The maximum value of dc output voltage of fully controlled

rectifier and chopper are chosen so that the motor terminal voltage saturates at rated value.

CSI induction motor drives

COMPARISON OF CSI AND VSI:

The relative advantages of CSI and V SI are

● CSI is more reliable than VSI because

1 Condition of two devices in the same leg due to commutation failure does not

lead to sharp rise of current through them.

2 it has inherent protection against short circuit across motor terminals

● Because of large inductance in dc link and large inverter capacitors, CSI drives has

higher cost, weight and volume, lower speed range and slower dynamic response.

● The CSI drive is not suitable for multimotor drives. Hence, each motor is fed from its

own inverter and rectifier. A single converter can be used to feed a number of VSI

motor systems connected in parallel. A single VSI can similarly feed a number of



motors connected in parallel.

CHAPTER 5: ENERGY SAVING WITH VFD

Rising energy prices create a need for energy-efficient motor control solutions. To

help reduce energy losses, process engineers are turning to variable frequency drives (or

adjustable speed drives) as an alternative to fixed speed controllers and throttling devices

such as dampers and valves.

Advancements in drive technology, careful selection of the hardware and power

system configuration, and intelligent motor control strategies produce improved operating

performance, control capability and energy savings.

Things to consider when choosing a motor control solution include peak-demand

charges, operating at optimized efficiency, power factor, isolation transformer cost and losses,

regeneration capabilities, synchronous transfer options and specialized intelligent motor

control energy-saving features.

Optimize Power Usage:

Centrifugal loads offer the greatest potential for energy savings by using variable frequency

drives (VFDs) to control speed. Energy consumption in centrifugal fan and pump

applications follows the affinity laws, which means that flow is proportional to speed,

pressure is proportional to the square of speed, and horsepower is proportional to the cube of

speed. That means if an application only needs 80 percent flow, the fan or pump will run at

80 percent of rated speed and only requires 50 percent of rated power. In other words,

reducing speed by 20 percent requires only 50 percent of the power.

● Beat peak-demand charges

VFDs also affect peak-demand electricity prices that the utility charges to companies

that exceed a preset limit, such as when industrial motors started across-the-line draw

large peaks of current. VFDs help reduce the peaks by supplying the power needed by the

specific application, and gradually ramping the motor up to speed to reduce the current

drawn



● Power Factor

Power factor and how it affects displacement and harmonic distortion is another

important consideration in drive selection. Drives that are near-unity true power factor

translate to reduced energy use.

● Regeneration

Some VFD applications enable users not only to save energy, but to regenerate power,

which can be routed back to the system or sold to a utility for additional revenue..

● Multiple Motors drive

Another way to reduce energy costs is the synchronous bypass method which uses only

one VFD to start and synchronize multiple motors by transferring a load from one source to

another by matching the voltage waveform frequency, amplitude and phase relation between

the two sources. Using a VFD to start a motor, bring it up to speed and then synchronize it,

causes a reduction in full-load current and optimizes the process.

● Extra Energy-Saving Potential

Intelligent motor control solutions including high-efficiency variable frequency drives are

an important part of an energy savings program. But not all drives have the same capabilities.

Software features and programmability can further contribute to a drive’s energy savings

potential by reducing inrush current requirements.

Programmability - Users can program their VFD to adjust the total acceleration time and

current limit and adjust the speed to the load requirement. Current limit on drives is normally

set between 105 and 110 percent, whereas using the across-the-line starting method produces

current limits of approximately 650 percent.

SGCTs – Advances in power semiconductor switches like SGCTs (symmetrical gate-

commutated thyristors) are designed for high-voltage operation and ensure the lowest



switching and conduction losses while maintaining a high switching frequency.

Power Optimization – Power optimizing features optimize the power usage when

operating fans and pumps by adjusting the required voltage to the application. This reduces

losses for improved motor and drive efficiency.

Communication software – Software features enable torque limit and integrated

architecture through communication connectivity between the drives, starters and soft starters

for greater control and optimizatio



CHAPTER 6: APPLICATIONS

The three major areas where drives and motor are applied are

Variable torque application – When a low torque is needed at low speed and high

torque at high speed. These applications are generally centrifugal loads such as fans, pumps

and blowers. These are good targets of energy savings.

Constant power application – When a high torque is required at low speed and low

torque at high speed. It includes machine tools and traction.

Constant torque application- When the same amount of torque is required at low

speed as at high speed. Power is directly proportional to speed. They include conveyors,

mixers, screw feeders, extruders and positive displacement pumps.

Lifts – Energy saving when regenerative breaking is involved.

Transport- Where all the vehicles, train, ship etc involves motor, VFDs are used for

speed control.

The latest developments in technology and successful development of electronic drives (AC

drives) for cage motors have resulted in the following benefits:

1 Availability of full load torque from standstill

2 Absence of torque fluctuations at low speed.

3 Ability to hold a set speed, regardless of load torque variation

4 Ability to control the rate of increase & decrease of speed

5 Dynamic response.



CHAPTER 7: ADVANTAGES AND LIMITATIONS

Advantages:

● Soft starting

● Precise speed and torque control

● Wide speed control torque

● High reliability and availability

● Low audible noise

● Capability of speed reversal / regenerative breaking

● Flux optimization

● Power loss ride through

● Energy saving

Limitations:

● AC drives not reliable as dc drives

● Cost of AC drives are more

● Presence of harmonics may heat the motor

● Separate filter circuits may be required to reduce harmonics



CONCLUSIONS:

For precise process control and wide speed range, to provide good running and transient

performances variable frequency drives are natural choice. The report describes functioning

of such a drive. It is very essential to know the applications of them. The consumer is using

them enormous. Hence, this paper concludes that variable frequency drive plays an in

important role in speed control of ac electric motors and also VFDs are simple, reduced

thermal stress on motor, most energy efficient, reduced noise levels, energy savings, soft

starting and stopping are the key features of the VFD.



BIBLIOGRAPHY:

1 Gopal k.Dubey, Fundamentals of electrical drives, , Narosa Publishing house, II

edition

2 Bimal K Bose, Modern power electronics and AC drives

3 M. H Rashid , Power electronics, Pearson India, II edition

4 Bimal K Bose,The past, present and future of Power Electronics, IEEE industrial

magazine vol.3

5 Marcel Dekker ,Solid state AC motor controls, ,New York

6 Carrier drive technology Syracus, Variable frequency drive technology, New

York

7 www.wikipedia.org

8 nptel.iitk.ac.in

9 www.ab.com


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