Pumping Prescriptions
Dr. Lev Nelik, P.E., APICS
Transient Startup Problems for
Pumps with Synchronous Motors
The influent raw wastewater pumps at the wastewater processing plant.
Al
p
parge wastewater processing
lant experienced continual
roblems with its influent raw
wastewater pumps for several years.
These pumps are rated at 70,000-
gpm, 24-ft head and driven by 500-
hp, 4000-V, 225-rpm Westinghouse
brushless synchronous motors, 57.5-
amps steady state rated current.
Influent water level varies
between EL = negative 6-ft and EL =
4-ft. Pump volute/impeller centerline
is at elevation EL = negative 2-ft, and
impeller tips barely touch the water at
elevation EL = negative 4-ft.
The issue reported by operators
involved difficulty starting these units
(tripping on high amps) at high water
levels (with impellers flooded), but no
trips at low flow levels (impellers dry),
although the pumps would then have
trouble catching prime.
We were asked to troubleshoot
these units to identify the source of
the problem and recommend corrective actions. Since the main issue being
reported was the difficulty starting a
Figure 1. Startup Test #1 with partially flooded impeller, EL= negative 1.5-ft, i.e. water
above impeller centerline. Main amps and volts plotted. Motor trips after about 5 seconds
on high amps.
