Variable Frequency Drives

Understanding

“VFD”

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Connecting Engineering Students to Industries…

Variable Frequency Drive

INSTRUCTIONAL OBJECTIVES

What is VFD? How it Works?Induction Motor FactsVFD Practical Examples


What is VFD ?

Variable Frequency DriveUsed to control the speed of MotorAnd make the system intelligent & Saves energy

Speed of Motor directly Proportional toFrequency

rmp = 120f/p

F: FrequencyP: Number of Poles


Electric Motor Facts

Electric motors account for more than two-thirds of all energy

used in industry

63% of the energy consumed is linked to air & liquid flow

A typical 50HP industrial motor, with an efficiency of 90%, costs over

$25,000 to operate continuously over a year.


Energy Usage in Office Building

Other

17%

Ventilation

& Cooling

52%Lighting

31%

How Does VFD Works?


How Does VFD vary Frequency?A VFD, AC inverter, or electronic speed controller for AC motors converts the AC supply to DC using a rectifier (diode), then converts it back to a variable frequency by using a inverter bridge.

Switches

(IGBT’s)

VFD Block Diagram

Main

Power

Rectifier Capacitor

Motor

Lets see how we can save energy using

Variable Frequency Drives

FAN CURVE – BASIC CHARACTERISTIC OF A FAN

FLOW (Q)

PR

ESSU

RE

(P)

AT A GIVEN SPEED

At decreasing speed

FAN OPERATING POINT WITH LOAD

FLOW (Q)

PR

ESSU

RE

(P)

Operating Point

P = K X Q.Q

Now lets talk about varying the flow….

SYSTEM LOAD PROFILE

1

2

3

4

5

6

7

8

9

10

11

12

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95

CFM range

% T

IME

RA

NG

E

CFMDUTY CYCLE (% OF TIME)

100 % 10 %

80 % 40 %

60 % 40 %

40 % 10 %

Now let’s see the different ways of Air Flow Control…

Basic ON/OFF Control

OFFON

Q max

Average Flow rate: Q max x Ton/ (Ton + Toff)

SIMPLE METHOD

TEMPERATURE FLUCTUATION

HOME APPLIANCES

Outlet Damper Control

OUTLET DAMPER CHARACTERISTICS

% FLOW (Q)

% P

RES

SUR

E (P

)

CURVES SHIFTS AS DAMPER CLOSES

60 80 100

100

120

Energy = P X Q


% FLOW (Q)

% P

RES

SUR

E (P

)

CURVES SHIFTS AS DAMPER CLOSES

60 80 100

100

120

Energy = P X Q

Energy Saving = Difference in Area


CFM DUTY CYCLEHORSE POWER

WEIGHTEDHP

100 10 35 3.5

80 40 35 14.0

60 40 31 12.4

40 10 27 2.7

TOTAL AVERAGE HORSE POWER REQUIREMENT 32.6

VARIABLE SPEED FAN CONTROL

FLOW (Q)

PR

ESSU

RE

(P)

At decreasing speed100

50

20

1008050

VARIABLE SPEED FAN CONTROL

FLOW (Q)

PR

ESSU

RE

(P)

At decreasing speed100

50

20

1008050


WEIGHTEDHP

100 10 35 3.5

80 40 18 7.2

60 40 7.56 3.024

40 10 2.24 0.224

TOTAL AVERAGE HORSE POWER REQUIREMENT 13.948

VFD CHARACTERISTICS


WEIGHTEDHP

100 10 35 3.5

80 40 18 7.2

60 40 7.56 3.024

40 10 2.24 0.224

VFD CHARACTERISTICS



WEIGHTED HP

100 10 35 3.5

80 40 35 14.0

60 40 31 12.4

40 10 27 2.7TOTA

L A

VER

AG

E H

OR

SEP

OW

ER R

EQU

IREM

ENT

32

.6

TOTA

L A

VER

AG

E H

OR

SE

PO

WER

REQ

UIR

EMEN

T

13

.94

8

OUTLETDAMPER

VARIABLE SPEED

WEIGHTED HORSEPOWER

32.6 13.948

x KW/HP .746 .746

x HR/MONTH 730 730

= KWH/MONTH 17753 7596

x COST (INR) 2.5 2.5

= TOTAL COST (INR) 44382.5 18990

SAVING CALCULATION:

Saving per month on a single fan…?

Now Lets see the Practical Demonstration of

VFD

Important Parameters used in various Applications

Source of Frequency Command

• Master frequency determined by digital keypad • Master frequency determined by 0 to +10 V input on

AVI terminal • Master frequency determined by 4 to 20mA input on

ACI terminal • Master frequency determined by potentiometer on

digital keypad

Source of Operation Command

• Operation determined by digital keypad • Operation determined by external control terminals,

keypad STOP is effective• Operation determined by external control terminals,

keypad STOP is ineffective


STOP Method

• Ramp Stop• Coast Stop


Maximum Output Frequency

Range

0 ~ 400 Hz


Acc./ Dec. Time

Voltage / Frequency Setting for various Applications


Acc S CurveTo Smooth the Acc/ Dec. Operation

Range 0 - 7


Jog FrequencyTo adjust the Speed of Motor at Jog Mode

Jog Acc./ Dec Time

0.1 to 600.0 sec or 0.01 to 600.0 sec


Over–Voltage Stall PreventionOver-Current Stall Prevention during Acceleration


Operation Control of VFD

2 Wire Control (1)– On/Off & Direction from Toggle Button



2 Wire Control (2) – On/Off & Direction from Toggle Button



3 Wire Control – On/Off from Push Button & Toggle Button for Direction Control


Analog Output SignalAnalog Frequency Meter (0 to Maximum Output Frequency)

Analog Current Meter (0 to 250% of the rated AC drive current)

Analog Output Gain 0 ~ 200%


Bias Adjustment


Bias Adjustment


Bias Adjustment


Gain Adjustment


Gain Adjustment


Gain Adjustment


Second Source of Frequency Command

Keypad Up/DownAVI (0-10V)ACI (4-20mA)Keypad Potentiometer


Reverse Operation InhibitionParameter LockPLC Operation

Jog Acc./ Dec Time0.1 to 600.0 sec or 0.01 to 600.0 sec


VFD saves Energy & make Motor

Control Mechanism Intelligent

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Variable Frequency Drives