The Application of VFDS Provide Irrigation Solution

8/7/2019 The Application of VFDS Provide Irrigation Solution

1/4

By Donna Hoover

he application of Variable Fre-

quency Drives (VFDs) in turf irri-

gation systems is a hot topic. A

VFD is an electronic device thatchanges the frequency of the AC power

going to a motor, thereby varying the

motor speed. When attached to a

pump, the change in motor speed

affects the flow and pressure of the

pump discharge.

VFD Advantages

ENERGY SAVINGS

Any time the demand is lower thanthe design point of the system, VFDs

reduce the motor speed (and there-

fore horsepower), resulting in a reduc-

tion in energy. The amount saved

depends on the design of the system,

actual demand and other factors.

REDUCTION IN WATER HAMMER

& LIFE EXTENSION OF THEIRRIGATION PIPING

This occurs because VFDs can be con-

figured to ramp the motor up and

down in a controlled manner.

ELIMINATION OF THE PRESSURE

CONTROL VALVE

The valves historically have required

excessive maintenance (though there

has been improvement in recent

designs). Additionally, control valves

reduce the pump discharge pressure

5-10 PSI, perhaps resulting in the

need for a larger pump.

L

L

L

12 PIPELINE


2/4

www.fisstate.org 13

VFD Disadvantages

ENVIRONMENTAL SENSITIVITIES

The forces of nature in certain geo-

graphic areas may be of concern when

considering the use of VFDs. In Flori-

da, for instance, heat, humidity andlightning are natural phenomena that

can affect the performance of VFDs.

Since most VFDs are rated to operate

in ambient temperatures up to 104

F, in Florida auxiliary cooling would

be necessary. For drives mounted in

an industrial enclosure, an air condi-

tioner or air/water heat exchanger

would suffice. There are also drives

that can be mounted outside. These

are produced by some VFD manufac-

turers in NEMA 4 packages. They

should include a fan to cool the drive

when running. It is necessary that

they be shielded from direct sunlight.

Surge / lightning protection presents

another potential challenge. Since

the printed circuit boards inside VFDs

can be sensitive to surges, intermit-

tent problems or catastrophic failurescan occur. Well-engineered VFD sys-

tems include a line reactor or other

device to guard against surges.

SERVICE ISSUES

Since many VFD problems can be dif-

ficult to trace, it is often necessary to

call in a highly trained drive specialistto make repairs. When a VFD fails, it

can take a week or more for the

problem to be diagnosed and a new

VFD ordered and installed. (Larger

VFD systems do come with a bypass

that enables the system to be run in

manual if the VFD fails).

L

L


3/4

14 PIPELINE

Ideal VFD ApplicationsSome excellent situations for applying a

VFD in a turf irrigation pump system are:

Any pump station with dramaticallyvarying flow rates over a wide flow

range, the VFD can be a better regu-lating device than a control valve.Care must always be taken to achieveevenly balanced flow requirementsacross the irrigation zones.

Some conventional pump stations uti-lizing jockey or pressure maintenancepumps The pumps keep the mainline

pressurized, offsetting the weeping offield valves. Jockey pumps reduce thewear and tear on the main pumpwhen low flows are required. A VFDon the main pump in lieu of a jockeypump may enable the pump to bet-ter operate at low flows.

Booster pumps for potable water or irri-

gation. The incoming pressure and dis-charge demand can vary greatly basedon time of day and other variables.

Pump stations with higher horsepowerpumps. They are better candidates forenergy savings than smaller stations.

Specifying a VFDPump Station

Once it has been determined that aVFD is appropriate for the application, thedesigner should consider the following:

Is the system designed to maintainconstant pressure? The pump stationshould be designed to maintain aconstant discharge pressure. In a welldesigned VFD system, a pressuretransducer provides a control feed-back. The VFD, when properlytuned, maintains constant pressure, ineffect matching pump output to

actual demand in the field. If a feed-back signal from a pressure transduc-er is absent, most of the advantagesof a VFD are lost.

How is the temperature limitation ofthe VFD being handled? If the VFD isto be installed in an industrial enclo-sure, an air conditioner or air/water

heat exchanger should be specified.If the VFD comes in a self-containedNEMA 4 package, it can be mountedunder the pump enclosure or on anopen skid. In that case, the VFDshould include a cooling fan and alsobe shielded from direct sunlight.

What electrical protection is being

offered? The pump station manufac-turer must include a separate devicefor disconnect, ground-fault/short-cir-cuit protection, and surge / lightningprotection as required by the Nation-al Electrical Code, the VFD manufac-turer, and best design practices.

Does the pump station include a VFD

for each motor? For stations withmultiple pumps, there should be anindividual drive for each motor. Manymanufacturers include one VFD andtransfer the motor across the linewhen the next pump starts. This isdone to reduce cost, but will result inreduced energy savings and unbal-anced loads between the pumps.

L

L

L

L

L

L

L

L


4/4

What's WrongWith ThisPicture?

What's WrongWith ThisPicture?There are at leastfour obviousproblems withthis pump inletconnection . . .Can you see them?

For the answersturn to page 32.

www.fisstate.org 15

What pump controllogic is provided?The pump stationshould be designedto not only maintaina constant discharge

pressure, but also toretire the pump on ano-flow signal.Without this logic thesystem will cyclebetween high andlow operating setpoints, and the life ofthe motor and piping

will be affected. To provide the no-flow retirement, as well as loss ofprime and low pressure shutdown sequences, the pump stationshould include a programmable logiccontroller (PLC) or an integrated logiccard within the VFD.

The benefits of using VFDs in many

turf irrigation applications are notewor-thy. Thoughtful consideration should begiven, however, to each individual situa-tion to warrant the use of a VFD. It is of

major importance to properly design thesystem to ensure optimum efficiency andto reap the rewards a VFD promises.Time spent is money saved!

H o o v e r P u m p i n g S y s t e m s h a s b e e n m a n u f a c t u r i n g i r r i g a t i o np u m p s t a t i o n s f o r t h e F l o r i d a m a r -k e t f o r 25 y ea r s . Fo r mo re i n f o rma-t ion, p lease contact George Newman

or Kev in Cava io l i a t Hoover PumpingSys tems , 954-971-7350. Em a i l :g e o r g e @ h o o v e r p u m p i n g . c o m o r k e v i n @ h o o v e r p u m p i n g . c o m .

L

Documents

The Application of VFDS Provide Irrigation Solution