coils are shunted to approximately half speed at some maximum
resistance.
The device that was used to vary the resistance seen by the
rotor was known as a “liquid rheostat.” Rube Goldberg died
in 1970, but I think he must have spent his later years helping
develop this machine. It consisted of a large tank that contained
an electrolyte (salt) solution and movable metal rods that were
wired to the rotor brushes.
When the rods were immersed deeper into the solution,
resistance was lowered due to increased surface area contact,
and motor speed increased. When they were raised, resistance
increased and speed decreased. A bubbler system determined
the wet well level and controlled a linear motor that raised and
lowered the rods in an attempt to keep the level constant. The
heat that was generated by the increased resistance was removed
from the electrolyte solution by a circulating pump and heat
exchanger.
Rube would have been proud because it was a complex
contraption and required considerable maintenance, but it
met its goal: dynamic level control and decreased pump starts.
The liquid rheostat was popular in the wastewater industry
throughout the 1970s and 1980s, and was still in use in the
early 1990s.
VFD Level Control
The liquid rheostat was all about controlling level for the purpose of reducing motor starts. Even though the required HP
was reduced at lower speeds, the heat generated by the higher
resistance tended to cancel any potential energy savings. Also, in
order to keep from stopping the
pump, it would often be forced to
run at lower than optimal speeds,
which resulted in low hydraulic
effi ciency.
Today, we can achieve level
control without these drawbacks
and save energy to boot. Let’s
compare a pump down and level
control application that uses the
same pump.
Figure 2 shows the performance of a 12-in non clog with
a manufacturer approved flow
range of 800-gpm to 5500-gpm
in a pump down application.
The red horizontal line at
22.5-ft is the static head seen by
the pump when the wet well is
full, and the brown line at 32-ft is
the maximum head at the end of
the pump down cycle. The cycle
begins at 4500-gpm and ends at
3000-gpm, and hydraulic efficiency is maintained at a healthy
77 percent to 78 percent across
the entire range of the pump
down cycle.
At first glance this appears
to be a pretty efficient operation.
All pumping occurs at or very
near BEP, and HP drops as we
approach the end of the cycle. But
if we divide the HP required at
each major flow point by the flow
in GPM at that point, we will gain
an entirely different perspective.
Figure 2
