Fluke 289 Application Note Keeping Things Flowing With Instruments

: Fluke Fluke-289-Application-Note-805140 fluke-289-application-note-805140 fluke pdf

Open the PDF directly: View PDF .
Page Count: 3

Application Note

From the Fluke Digital Library @ www.fluke.com/library

Control equipment includes

120 V control panels and about

20 Allen-Bradley PLC-5s commu-

nicating between buildings via

Data Highway Plus (DH+) with

fiber-optic converter modules

and within buildings using the

Data Highway network’s twi-

naxial Blue Hose cable. The PLCs

control the equipment through

24 V Remote I/O, 4-20 mA inputs

and outputs, and LAN to a SCADA

system with information about

flows, levels, pressures, tem-

peratures, conductivity, chlorine

content, pump speeds, and so

forth.

Plant equipment includes a

large number of 480 V motors

running pumps and blowers.

Motor controls include 700 hp

reduced-voltage starters, 400 hp

soft starts, and variable fre-

quency drives (VFDs) ranging

from 1 hp to 500 hp.

To keep the motors running

the facility has a preventive

maintenance (PM) program, a big

portion of which involves peri-

odic measurements to get ahead

of any trend that might go in the

wrong direction.

Operator: Instrument technician Bill

Moreno for Palm Beach County Water

Utilities Department’s Southern Region

Water Reclamation Facility

Measurements: VFD output filtering,

Min/Max frequency measurements,

phase-to-phase voltage imbalance,

motor insulation resistance checks

Tools: 87 DMM, 289 DMM,

1520 Insulation Resistance Tester

Testing

Functions

Case

Study

Palm Beach County, Florida,

with a population of 1,274,013

(2006 figures), produces a lot

of wastewater. To handle it

the Palm Beach County Water

Utilities Department’s Southern

Region Water Reclamation Facil-

ity operates around the clock to

process up to 35 million gallons

per day. About half of that is

recycled for use in the County’s

Reclaimed Water Program, mostly

to irrigate water to golf courses

and residential lawns in the

surrounding areas, and some to

feed the nearby Wakodahatchee

Wetlands. Another recently-

constructed 100-acre wetland is

available to recycle another two

to three million gallons per day.

The treatment facility employs

close to 30 people, half in opera-

tions and half in maintenance,

including two electricians, two

instrument technicians, four

mechanics, two general mainte-

nance workers and a supervisor

of maintenance.

The facility has its own 4160-

volt power distribution system for

its nine or ten buildings spread

over 130 acres; each building

has redundant transformers that

step the 4160 V down to 480 V

for use.

Keeping things flowing

with Fluke instruments

An overhead view

of the Palm Beach

County Water Utilities

Department’s Southern

Region Water

Reclamation Facility.

2 Fluke Corporation Keeping things flowing with Fluke instruments

On the VFDs, says instrument

technician Bill Moreno, those

measurements include monitor-

ing the incoming voltage and

tracking both the secondary

voltage out to the motors and the

amperage to provide a refer-

ence for troubleshooting. Moreno

also does periodic insulation

resistance checks on the motors.

“We’re finding that’s not always

as necessary,” he says, “as sensi-

tive as the drives are getting, but

as a practice I still keep track of

the megs historically.” At one

time, he used a hand-cranked

instrument for this, but for the

past five years has been using

a Fluke 1520 MegOhmMeter.

Another of the staff electricians

uses a Fluke 1503 Insulation

Resistance Tester.

Chasing an intermittent

fault

Up until recently, the instrument

Moreno used most frequently

was a Fluke Model 87 digital

multimeter (DMM), which he

found especially useful in solv-

ing an elusive problem with a

motor driven by a 500 hp vari-

able frequency drive. The motor

would run fine for 12 or 24

hours, then begin indicating an

intermittent short circuit. Pre-

liminary tests suggested that the

problem might be either in the

VFD or a short in the motor. The

VFD manufacturer was leaning

toward the motor, so Moreno did

an insulation resistance check on

it. “Every time we checked the

motor,” he says, “it was reading

80, 100 megs.” While that might

seem a little low to some people,

he continues, “it’s an outside

motor, it’s very humid down

here, so... we don’t get a real

high reading.” And the reading

was relatively constant when

compared with earlier readings

taken as part of the preventive

maintenance program.

He then called in the motor

company, which ran a surge

test on the motor and found it

healthy—a good thing, Moreno

says, because pulling out a

500 hp motor is a lot of work.

That left the drive, but how to

prove it?

The tech support people from

the drive company suggested a

diode test, which showed that

the diodes were good. The next

suggestion was to isolate the

drive from the motor and check

to see if the phase-to-phase volt-

ages were within 1 to 2 percent

of each other. Isolating the motor

was simple: just de-energize the

output contactor. But Moreno

didn’t have any way to measure

phase-to-phase voltage. There

was a power analyzer in stock,

but it measured only phase-to-

ground voltages, which were no

help.

He then got an idea: He gath-

ered up three Fluke 87 DMMs,

stuck them to the panel using

their ToolPak accessories, and

connected them between phases.

He then ran the drive at 25, 50,

75 and 100 percent, and found

“about a 3 to 4 percent voltage

imbalance.” He reported this

back to the drive manufacturer,

which concluded the problem

was a bad board in the drive

and put one on order. Problem

solved, without having to pull out

the motor.

Looking for a new meter

While the 87 DMM is a fine and

versatile instrument, Moreno had

been feeling the need for some-

thing with a recording function

and a low-pass filter for more-

accurate voltage and frequency

measurements on VFDs and other

electrically noisy equipment. He

decided to order a Fluke 289

True-rms Industrial Logging

Multimeter with TrendCapture.

The Green Cay Wetlands recycle about two to three million gallons of

water daily.

3 Fluke Corporation Keeping things flowing with Fluke instruments

Solving a tough vibration

problem

The new 289 DMM proved its

worth when Moreno was faced

with a puzzling problem. A

500 hp motor that drives a pump

had just come back from the

motor shop, where it was rebuilt

and supposedly balanced. But

when connected to the pump

and started up it would get to

about 95 % of full speed and

then trip out from vibration.

Trying to find the cause of the

tripping, Moreno looked first at

the 120 V ac control circuitry

for the VFD, which is mounted

in a panel next to the VFD itself

and connects to things like low

water level cutout floats and a

vibration/temperature monitoring

system. But the voltage readings

didn’t make any sense. Even

when the enabling relay was

open there was still about 90 V

on the line. What did that mean,

and where was it coming from?

The 120 V control lines share

a duct with 480V power wiring.

Perhaps that was the source of

the mysterious voltage. Moreno

then switched his 289 DMM to

the LoZ mode, which puts a low

impedance across the meter’s

input terminals and is intended

to eliminate ghost voltages. As

soon as he did this, the stray

voltage vanished. None of his

previous meters had come with

the LoZ feature, and tracking

down the stray voltage would

have been a real challenge, but

with the 289 DMM it was easy.

One problem was solved,

but the motor was still tripping

out. So Moreno started to look

for the exact speed at which

the tripout occurred. He con-

nected his 289 DMM to the

output of the VFD that drove the

motor and engaged the meter’s

low-pass filtering function. The

output of a VFD has all sorts of

odd waveshapes and harmonics

extending into the kHz region

that make it quite difficult to get

an accurate frequency reading

with a conventional DMM. With

the low-pass filter turned on,

the 289 sees only the power-

frequency components of the

output (see below) and gives an

accurate frequency reading.

Fluke Corporation

PO Box 9090, Everett, WA 98206 U.S.A.

Fluke Europe B.V.

PO Box 1186, 5602 BD

Eindhoven, The Netherlands

For more information call:

In the U.S.A. (800) 443-5853 or

Fax (425) 446-5116

In Europe/M-East/Africa +31 (0) 40 2675 200 or

Fax +31 (0) 40 2675 222

In Canada (800)-36-FLUKE or

Fax (905) 890-6866

From other countries +1 (425) 446-5500 or

Fax +1 (425) 446-5116

Web access: http://www.fluke.com

©2009 Fluke Corporation.

Specifications subject to change without notice.

Printed in U.S.A. 1/2009 3433667 AW-EN-N Rev A

Modification of this document is not permitted

without written permission from Fluke Corporation.

Fluke. Keeping your world

up and running.®

Moreno also used the min/

max frequency function with

time stamp to determine the

exact frequency at which the trip

occurred, and finally discovered

that the problem wasn’t the

motor itself, but the way it was

used. The pump is located about

20 feet below the motor, and it

turned out that even though the

motor itself was properly bal-

anced, the combination of pump

and motor was not. The answer

was to balance the pump and

motor together.

Moreno has also used the 289

DMM to check the calibration

on a valve actuator, by measur-

ing the unit’s 4-20 mA feedback

signal - the first time he had

used that option. “Once I got it

set up it’s been working like a

champ,” he reports. Moreno’s

overall impression? “This meter

has been very nice; I’ve been

pleased with it.”

The low-pass filter feature allows the Fluke 289 DMM to accurately measure VFD output.