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Variable Frequency Drive working Principle, Advantages

01 Aug

Common VFD Terms

There are several terms used to describe devices that

control speed. While the acronyms are often used

interchangeably, the terms have different meanings.

Variable Frequency Drive (VFD)

This device uses power electronics to vary the fre-quency of input power to the motor, thereby con-trolling motor speed.

Variable Speed Drive (VSD)

This more generic term applies to devices that con-trol the speed of either the motor or the equipment

driven by the motor (fan, pump, compressor, etc.).

This device can be either electronic or mechanical.

Adjustable Speed Drive (ASD)

Again, a more generic term applying to both

mechanical and electrical means of controlling

speed.

This paper will discuss only VFDs.

VFD OPERATION

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.

AC sine wave

The rectifier in a VFD is used to convert incoming

ac power into direct current (dc) power. One rectifi-er 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 neg-ative. 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 Figure 2). Appropriately, the term “6 pulse” is

used to describe a drive with 6 rectifiers. A VFD

may have multiple rectifier sections, with 6 recti-fiers per section, enabling a VFD to be “12 pulse,”

“18 pulse,” or “24 pulse.” The benefit of “multi-pulse” VFDs will be described later in the harmon-ics section.

Rectifiers may utilize diodes, silicon controlled rec-tifiers (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.”

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 sev-eral 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 fre-quency 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

Example:

2-pole motor at different frequencies

3600 rpm = 60 hertz x 120 / 2 = 3600 rpm

3000 rpm = 50 hertz x 120 / 2 = 3000 rpm

2400 rpm = 40 hertz x 120 / 2 = 2400 rpm

BENEFITS OF VFD

As VFD usage in HVAC applications has increased,

fans, pumps, air handlers, and chillers can benefit

from speed control. Variable frequency drives pro-vide the following advantages:

• energy savings

• low motor starting current

• reduction of thermal and mechanical

stresses on motors and belts during starts

• simple installation

• high power factor

• lower KVA

Understanding the basis for these benefits will allow

engineers and operators to apply VFDs with confi-dence and achieve the greatest operational savings.

VFD Capacity Control Saves Energy

Most applications do not require a constant flow of

a fluid. Equipment is sized for a peak load that may

account for only 1% of the hours of operation. The

remaining hours of operation need only a fraction of

the flow. Traditionally, devices that throttle output

have been employed to reduce the flow. However,

when compared with speed control, these methods

are significantly less efficient.

Mechanical Capacity Control

Throttling valves, vanes, or dampers may be

employed to control capacity of a constant speed

pump or fan. These devices increase the head, there-

by forcing the fan or pump to ride the curve to a

point where it produces less flow (Figure 3). Power

consumption is the product of head and flow.

Throttling the output increases head, but reduces

flow, and provides some energy savings.

Variable Speed Capacity Control

For centrifugal pumps, fans and compressors, the

ideal fan (affinity) laws describe how speed affects

flow, head and power consumption (Table A).

When using speed to reduce capacity, both the head

and flow are reduced, maximizing the energy sav-

ings. A comparison of mechanical and speed control

for capacity reduction (Figure 4) shows that variable

speed is the most efficient means of capacity

control.

Efficiency

Low Inrush Motor Starting

Motor manufacturers face difficult design choices.

Designs optimized for low starting current often

sacrifice efficiency, power factor, size, and cost.

With these considerations in mind, it is common for

AC induction motors to draw 6 to 8 times their full

load amps when they are started across the line.

When large amounts of current are drawn on the

transformers, a voltage drop can occur2

, adversely

affecting other equipment on the same electrical

system. Some voltage sensitive applications may

even trip off line. For this reason, many engineers

specify a means of reducing the starting current of

large AC induction motors.

Soft Starters

Wye-delta, part winding, autotransformer, and solid-state starters are often used to reduce inrush during

motor starting. All of these starters deliver power to

the motor at a constant frequency and therefore must

limit the current by controlling the voltage supplied

to the motor. Wye delta, part winding, and auto-transformer starters use special electrical connec-tions to reduce the voltage. Solid-state starters use

SCRs to reduce the voltage. The amount of voltage

reduction possible is limited because the motor

needs enough voltage to generate torque to acceler-ate. With maximum allowable voltage reduction, the

motor will still draw two to four times the full load

amps (FLA) during starting. Additionally, rapid

acceleration associated with wye-delta starters can

wear belts and other power transmission

components.

VFDs as Starters

A VFD is the ideal soft starter since it provides the

lowest inrush of any starter type as shown in

Table B. Unlike all other types of starters, the VFD

can use frequency to limit the power and current

delivered to the motor. The VFD will start the motor

by delivering power at a low frequency. At this low

frequency, the motor does not require a high level of

current. The VFD incrementally increases the

frequency and motor speed until the desired speed is

met. The current level of the motor never exceeds

the full load amp rating of the motor at any time

during its start or operation. In addition to the bene-

fit of low starting current, motor designs can now be

optimized for high efficiency.

Table B

Comparison of Starter Types Based on Inrush

Easy Installation

Many pieces of equipment are factory shipped with

unit mounted VFDs that arrive pre-programmed and

factory wired. Motor leads, control power for auxil-iaries, and communication lines are all factory

wired. The VFD cooling lines on unit-mounted

chiller VFDs are also factory installed. The

installing contractor needs only to connect the line

power supply to the VFD.

High Power Factor

Power converted to motion, heat, sound, etc. is

called real power and is measured in kilowatts (kW).

Power that charges capacitors or builds magnetic

fields is called reactive power and is measured in

Kilovolts Amps Reactive (kVAR). The vector sum

of the kW and the kVAR is the Total Power (energy)

and is measured in Kilovolt Amperes (KVA)

(Figure 5). Power factor is the ratio of kW/KVA.

Motors draw reactive current to support their mag-netic fields in order to cause rotation. Excessive

reactive current is undesirable because it creates

additional resistance losses and can require the use

of larger transformers and wires. In addition, utilities

often penalize owners for low power factor.

Decreasing reactive current will increase power

factor.

Typical AC motors may have a full load power fac-

tor ranging from 0.84 to 0.88. As the motor load is

reduced, the power factor becomes lower. Utilities

may require site power factor values ranging from

0.85 to 0.95 and impose penalties to enforce this

requirement. Power factor correction capacitors can

be added to reduce the reactive current measured

upstream of the capacitors and increase the meas-

ured power factor. To prevent damage to the motor,

power factor correction capacitors should not exceed

the motor manufacturer’s recommendations. In most

cases, this results in maximum corrected values of

0.90 to 0.95.

The VFDs include capacitors in the DC Bus that per-

form the same function and maintain high power

factor on the line side of the VFD. This eliminates

the need to add power factor correction equipment to

the motor or use expensive capacitor banks. In addi-

tion, VFDs often result in higher line side power fac-

tor values than constant speed motors equipped with

correction capacitors.

Low Full Load KVA

Total Power (KVA) is often the limiting factor in the

amount of energy that can be transmitted through an

electrical device or system. If the KVA required by

equipment can be reduced during periods of peak

demand, it will help alleviate voltage sags, brown

outs, and power outages. The unit efficiency and

power factor are equally weighted when calculating

KVA. Therefore, equipment that may be equal or

worse in efficiency, but higher in power factor has

significantly lower KVA (Table C).

In this example, equipment with a higher power fac-

tor uses 15% less KVA while performing the same

job. This can lower electrical system cost on new

projects and free up KVA capacity on existing sys-

tems.

Table C

Power Factors and Energy Usage

Backup generators are typically sized to closely

match the load. Lowering KVA can reduce the size

of the generator required. When VFDs with active

front ends are used, the generator size can approach

an ideal 1:1 ratio of kW/KVA because the power

factor is near unity (1.0) and the harmonics pro-duced by the VFD are extremely low.

Lower KVA also benefits utilities. When the power

factor is higher, more power (kW) can be delivered

through the same transmission equipment.

HARMONIC DISTORTION AND INDUSTRY

STANDARDS

A discussion of the benefits of VFDs often leads to

a question regarding harmonics. When evaluating

VFDs, it is important to understand how harmonics

are provided and the circumstances under which

harmonics are harmful.

Harmonic Definition

In the United States, three-phase AC power typical-ly operates at 60 hertz (60 cycles in one second).

This is called the fundamental frequency

A harmonic is any current form at an integral mul-

tiple of the fundamental frequency. For example, for

60-hertz power supplies, harmonics would be at

120 hertz (2 x fundamental), 180 hertz, 240 hertz,

300 hertz, etc.

What Causes Harmonics?

VFDs draw current from the line only when the line

voltage is greater than the DC Bus voltage inside the

drive. This occurs only near the peaks of the sine

wave. As a result, all of the current is drawn in short

intervals (i.e., at higher frequencies). Variation in

VFD design affects the harmonics produced. For

example, VFDs equipped with DC link inductors

produce different levels of harmonics than similar

VFDs without DC link inductors. The VFDs with

active front ends utilizing transistors in the rectifier

section have much lower harmonic levels than

VFDs using diodes or silicon controlled rectifiers

(SCRs).

Electronic lighting ballasts, uninterruptible power

supplies, computers, office equipment, ozone gener-

ators, and other high intensity lighting are also

sources of harmonics.

Rocks and Ponds

Obviously, the magnitude of the contributing wave

forms has an effect on the shape of the resultant

wave form. If the fundamental wave form (60 Hz)

has a very large magnitude (5,000 amps) and the

harmonic wave forms are very low (10 amps), then

the resultant wave form will not be very distorted

and total harmonic distortion will be low. If the har-

monic wave form current value is high relative to

the fundamental, the effect will be more dramatic.

In nature, we see this effect with waves in water. If

you continually throw baseball size rocks into the

ocean, you would not expect to change the shape of

the waves crashing onto the beach. However, if you

threw those same size rocks into a bathtub, you

would definitely observe the effects. It is similar

with electrical waves and harmonics.

When you calculate harmonics you are calculating

the effect of the harmonics on the fundamental cur-rent wave form in a particular distribution system.

There are several programs that can perform esti-mated calculations. All of them take into account the

amount of linear loads (loads drawing power

through out the entire sine wave) relative to non-lin

ear loads (loads drawing power during only a frac-tion of the sine wave). The higher the ratio of linear

loads to non-linear loads, the less effect the non-lin-ear loads will have on the current wave form.

Are Harmonics Harmful?

Harmonics that are multiples of 2 are not harmful

because they cancel out. The same is true for 3rd

order harmonics (3rd

, 6th

, 9th

etc.). Because the power

supply is 3 phase, the third order harmonics cancel

each other out in each phase 3

. This leaves only the

5th

, 7th

, 11th

, 13th

etc. to discuss. The magnitude of the

harmonics produced by a VFD is greatest for the

lower order harmonics (5th

, 7thand 11th) and drops quickly as you move into the higher order harmon-ics (13th

and greater).Harmonics can cause some disturbances in electrical

systems. Higher order harmonics can interfere with

sensitive electronics and communications systems,

while lower order harmonics can cause overheating

of motors, transformers, and conductors. The oppor-tunity for harmonics to be harmful, however, is

dependent upon the electrical system in which they

are present and whether or not any harmonic sensi-tive equipment is located on that same electrical

system.

Understanding IEEE 519

IEEE (Institute of Electrical and Electronics

Engineers) created a recommendation for evaluating

harmonics. The IEEE-519 standard provides recom-mended limits for harmonic distortion measured at

the point of common coupling. The point of com-mon coupling is the point at which the customer’s

electrical system is connected to the utility.

Although the IEEE standard recommends limits for

both voltage distortion and current distortion, speci-fications that reference a 5% harmonic limitation are

generally referring to current distortion. In most

cases, if the current distortion falls within IEEE-519

requirements, the voltage distortion will also be

acceptable.

Determining compliance with IEEE-519 requires an

actual measurement of the system during operation.

Predicting compliance in advance often requires a

system study that accounts for all electrical equip-ment (transformers, wires, motors, VFDs, etc.) in

the system.

Introduction To Harmonic Terms

Total Harmonic Voltage Distortion – THD (V)

As harmonic currents flow through devices with

reactance or resistance, a voltage drop is developed.

These harmonic voltages cause voltage distortion of

the fundamental voltage wave form. The total mag-nitude of the voltage distortion is the THD (V). The

IEEE-519 standard recommends less than 5% THD

(V) at the point of common coupling for general

systems 69 kV and under.

Total Harmonic Current Distortion – THD (I)

This value (sometimes written as THID) represents

the total harmonic current distortion of the wave

form at the particular moment when the measure-ment is taken. It is the ratio of the harmonic current

to the fundamental (non-harmonic) current meas-ured for that load point. Note that the denominator

used in this ratio changes with load.

Total Demand Distortion – TDD

Total Demand Distortion (TDD) is the ratio of the

measured harmonic current to the full load funda-mental current. The full load fundamental current is

the total amount of non-harmonic current consumed

by all of the loads on the system when the system is

at peak demand. The denominator used in this ratio

does not change with load. Although TDD can be

measured at any operating point (full or part load),

the worst case TDD will occur at full load. If the full

load TDD is acceptable, then the TDD measured a

part load values will also be acceptable. To use ou

rock analogy, the full load fundamental current is th

size of our pond and the harmonic current is the siz

of our rock. (See Table D.)

short Circuit Ratio

Short circuit ratio is the short circuit current value of

the electrical system divided by its maximum load

current. Standard IEEE-519 Table 10.3 defines dif-ferent acceptance levels of TDD depending on the

short circuit ratio in the system. Systems with small

short circuit ratios have lower TDD requirements

than systems with larger short circuit ratios. This

difference accounts for the fact that electrical sys-tems with low short circuit ratios tend to have high

impedances, creating larger voltage distortion for

equivalent harmonic current levels. (See Table E.)

Mitigating Harmonics

Some utilities now impose penalties for introducing

harmonics onto their grid, providing incentives for

owners to reduce harmonics. In addition, reducing

harmonic levels can prevent potential damage to

sensitive equipment residing on the same system.

There are many approaches to mitigating harmonics.

Several commonly used methods are discussed here.

Line Reactors

Line reactors add reactance and impedance to the

circuit. Reactance and impedance act to lower the

current magnitude of harmonics in the system and

thereby lower the TDD. Line reactors also protect

devices from large current spikes with short rise

times. A line reactor placed between the VFD and

the motor would help protect the motor from current

spikes. A line reactor placed between the supply and

VFD would help protect the supply from current

spikes. Line reactors are typically only used

between the VFD and the motor when a freestand-

ing VFD is mounted more than fifty feet from the

motor. This is done to protect the motor windings

from voltage peaks with extremely quick rise times.

Passive Filters

Trap Filters are devices that include an electrical cir

cuit consisting of inductors, reactors, and capacitor

designed to provide a low impedance path to ground

at the targeted frequency. Since current will trave

through the lowest impedance path, this prevents the

harmonic current at the targeted frequency from

propagating through the system. Filters can be

mounted inside the drive cabinet or as free standing

devices. Trap filters are typically quoted to meet a

THD(I) value that would result in compliance with

IEEE-519 requirements if the system were other

wise already in compliance.

Active Filters

Some devices measure harmonic currents and

quickly create opposite current harmonic wave

forms. The two wave forms then cancel out, pre

venting harmonic currents from being observed

upstream of the filter. These types of filters general

ly have excellent harmonic mitigation characteris

tics. Active filters may reduce generator size

requirements.

VFDs Using Active Front End Technology (AFE)

Some VFDs are manufactured with IGBT rectifiers.

The unique attributes of IGBTs allow the VFD to

actively control the power input, thereby lowering

harmonics, increasing power factor and making the

VFD far more tolerant of supply side disturbances.

The AFE VFDs have ultra low harmonics capable of

meeting IEEE-519 standards without any external

filters or line reactors. This significantly reduces

installation cost and generator size requirements.

An AFE drive provides the best way to take advan-

tage of VFD benefits and minimize harmonics.

Multi-Pulse VFDs (Cancellation)

There are a minimum of six rectifiers for a three-

phase AC VFD. There can be more, however.

Manufacturers offer 12, 18, 24, and 30 pulse drives.

A standard six-pulse drive has six rectifiers, a

12-pulse drive has two sets of six rectifiers, an

18-pulse drive has three sets of six rectifiers and so

on. If the power connected to each set of rectifiers is

phase shifted, then some of the harmonics produced

by one set of rectifiers will be opposite in polarity

from the harmonics produced by the other set of rec-

tifiers. The two wave forms effectively cancel each

other out. In order to use phase shifting, a special

transformer with multiple secondary windings must

be used. For example, with a 12-pulse VFD, a

Delta/Delta-Wye transformer with each of the sec-

ondary phases shifted by 30 degrees would be used.

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3 Comments

Posted by on August 1, 2011 in Variable Speed Drive

 

3 responses to “Variable Frequency Drive working Principle, Advantages

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