© ABB Inc. - MV Drives, North AmericaFebruary 8, 2009 | Slide 1

Gas/Electric Partnership Conf. XVIIVFD Compressor Drives

Christian Bruderer – ABB Inc., Medium Voltage Drives - North America

© ABB Inc. – MV Drives, North AmericaFebruary 8, 2009 | Slide 2

Gas/Electric Partnership Conference XVIIVFD Compressor DrivesAgenda

Introduction

Compressor Basics

The most common torque load types

Centrifugal torque speed curve example

Summary – Determine the application requirements

© ABB Inc. – MV Drives, North AmericaFebruary 8, 2009 | Slide 3

Gas/Electric Partnership Conference XVIIVFD Compressor DrivesIntroduction

A variable frequency drive system is typically sized for the system nominal point. Most compressors are driven via a gearbox and a 4 pole motor.

Therefore the nominal point of such a system is defined at 60Hz, or 1800rpm. The motor and the VFD are designed to reach nominal power at 1800rpm.

The centrifugal compressor has a quadratic load characteristic (torque is proportional to the square of the speed), it is assumed that the power will be reduced with the cube of the speed, for any speed below 1800rpm.

However, the reality is often different-

The compressor is designed for several design points. Many of these points are likely below nominal speed. It often occurs that the VFD system needs to provide more torque than the rated torque atthese below rated speed conditions.

© ABB Inc. – MV Drives, North AmericaFebruary 8, 2009 | Slide 4

Gas/Electric Partnership Conference XVIIVFD Compressor DrivesCompressors Basics

Centrifugal compressors approximate the constant head – variable volume machine

Reciprocating compressors are constant volume – variable head machines

Axial compressors are low head, high flow machines

With variable speed, the centrifugal compressor can deliver:

constant capacity at variable pressure

variable capacity at constant pressure

A combination variable capacity and variable pressure

© ABB Inc. – MV Drives, North AmericaFebruary 8, 2009 | Slide 5

Gas/Electric Partnership Conference XVIIVFD Compressor DrivesThe most common torque load types

The most common load type is the quadratic torque.

The power is cubically proportional to the speed.

Applications

Centrifugal pumps / compressor and fans

A constant torque load type is typical when fixed volumes are being handled.

The power increases linear with the speed

Applications

Recip compressors, conveyors

0

0.2

0.4

0.6

0.8

1

1.2

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Speed [rpm]

To

rqu

e /

Po

we

r [p

u]

P = k * n3

M = k * n2

0

0 .2

0 .4

0 .6

0 .8

1

1 .2

0 20 0 400 6 00 800 1 000 1200 140 0 1600 180 0 2 000

Sp e e d [ r p m ]

To

rqu

e / P

ow

er [

pu

]

C o ns tan t T o rq ue

P = k * n

© ABB Inc. – MV Drives, North AmericaFebruary 8, 2009 | Slide 6

0

0.2

0.4

0.6

0.8

1

1.2

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Speed [rpm]

Torq

ue

/ P

owe

r [p

u]

P = k * n3

M = k * n2

Gas/Electric Partnership Conference XVIIVFD Compressor DrivesCentrifugal compressor torque speed curve example

Constant torque

Nominal point

Constant motor power

region

© ABB Inc. – MV Drives, North AmericaFebruary 8, 2009 | Slide 7

Gas/Electric Partnership Conference XVIIVFD Compressor DrivesSummary - Determine the application requirements

What are the torque requirements?

Motor torque (not power) is usually the decisive factor

Torque requirements establishes current requirements

Torque requirements

Constant vs variable torque

Define as many operating points as possible

What is the speed requirement?

Maximum speed

Minimum speed

© ABB Inc. – MV Drives, North AmericaFebruary 8, 2009 | Slide 8