VFD vs Liquid Rheostat Controls 1

[joefiegl]

For a variable speed pumping application, what are the main advantages of variable frequency drive (VFD) over liquid rheostat controls? Are there any advantages of liquid rheostats over VFDs?

Any help would be much appreciated. Thanks.

 

[DanDel]

I'm not sure, but I think you are talking about a wound-rotor motor with the rotor slip rings connected to conductors in a saline-water tank. The conductors are lowered into the tank to decrease the secondary resistance and speed up the motor. If this is true, the main difference (besides the fact that the tank method is REALLY obsolete) is that the VFD, with the proper motor, can run at practically any speed (typically down to 50% of rating) for any length of time, can be programmed to run based on the load, the current, or other parameters. VFDs are also much smaller and more dependable. Don't you have to keep adjusting the water and the position of the conductors in the tank? There are several other options for a wound-rotor motor which are newer than the tank and which do not require a VFD, the 'static-stepless' control among them.

 

[nbucska]

The advantage of the liquid reostat -- unmatched by any
other technology -- is that it can't fail instantaneously
so with regular maintenance it is very reliable.
On the other hand it constantly looses water, so maintenance
is a must.
<nbucska@pcperipherals.com>

 

[dpc]

The only advantage of the liquid rheostat is that you probably already have one.

The VFD will be much more efficient and provide better speed/level control. It should be more reliable as well.

The existing wound rotor motor can be used with a VFD by shorting out the rotor windings. But you might be better off with a new squirrel-cage motor since the efficiency would probably be better and you can get an inverter-duty motor that will hold up better.

Not many new liquid rheostat systems being installed these days.