VFD Help


Thread Starter


We have a groundwater project in house that has eight wells, the farthest is located 2800 feet from the control house. Based on On/Off motor control,we derated the field wiring accordingly. The pumps range from 1 to 7.5 hp, with the 7.5 unit being the farthest away. What type of drives would be the best for this application, and does anyone see any possible pitfalls/problems with the application.


John Waalkes

I like GE's AF300 "G" series, it's been good to us. The only "drawback" is that we have never got the drive to automatically reset. The GE reps say it can be done...

Do you really want to run the output from the VFD that far? Check with your drive rep first.

As for things to watch out for.

1) Add a reactor to the output of the VFD. Because of PWM, drives that are far away (and this easily qualifys) can get their windings damaged (we burnt up four motors before we figured that one out -- so beware)

2) Consider adding an external fan to the motor. At lower speeds the internal fan doesn't do such a good job of keeping things cool.

3) Oversize your drive, running that far in a metallic raceway is going to choke a good portion of the power going to the motor. The reactor on the load side of the drive should help out a lot on this, but I would still oversize the thing.

I've seen 1 hp drives that wouldn't even budge the motor because the load side of the drive was run in ~50' of conduit.

That's all that I can think of offhand...


A colleague forwarded me your posting about your well field application.
I hope you don't mind that I respond.

There can be problems with long cable runs between the VFD and the motor
due to the potentially high voltage spikes generated by the VFD. The
phenomenon is sometimes called a "standing wave effect", and is akin to a
"series resonant tank circuit" in which the steep leading edge of the PWM
waveform creates high voltage spikes from the VFD, which are made larger
by the long run of low impedance cable -- these spikes can be over 1500V
(or higher!) by the time they reach the motor winding.

Danfoss VFDs are designed to limit the effects (by "soft gating" the
switching of the IGBT's and a switching pattern designed to produce a
sinusoidal waveform), and consequently have longer "out of the box" cable
runs (500 to 1000 ft.) than most of our competitors (typically < 150 ft.
with no built-in filtering). However, with runs in excess of 1000 ft.,
you should be considering the use of a "long lead" filter such as the
Danfoss LC filter or the TCI KLC dv/dt filter. They do not cost very much
and will do a great deal to preserve the motor's insulation system. The
TCI filter can be found at www.transcoil.com, and Danfoss filters can be
ordered with Danfoss drives. I don't recommend the use of the Danfoss LC
filter with other manufacturer's drives as they are carefully engineered
to match up with the characteristics of the drives they are paired with.
Either filter should be mounted on the output side of the drive, as close
to the drive as possible. I have used both filters with very good results.

It will also be wise to make sure that you size the cable for these long
runs in accordance with the manufacturer's recommendations. Too large a
cable will create more problems than it solves! The minimum cable size is
determined by code, and the max cable size should be listed in the VFD
catalog or manual, or contact the VFD manufacturer's applications
department. You should also use motors with "inverter duty" or "inverter
rated" stamped on their nameplates. These will typically be manufactured
using the Phelps Dodge Thermaleze Q Squared or some other type of
corona-resistant kapton coated magnet wire, and will therefor be more
resistant to the damaging effects of high voltage spikes which are
produced by the PWM wafeform.

Jim Mail
Danfoss Water & Wastewater
Northeast Regional Manager
[email protected]

Johan Bengtsson

Does this mean you can't keep on/off control of the motor (as I
assume by your post was intended from the start)?
Do you really need VFD:s on all the pumps?

/Johan Bengtsson

P&L, Innovation in training
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: [email protected]
Internet: http://www.pol.se/