Pumps & Systems, September 2007
A large wastewater processing plant experienced continual problems 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 flooded pump, a radial hydraulic thrust or high power (low head at initially unfilled conduit) was initially suspected as a root cause.
However, after extensive review of the pump hydraulics, it was determined that horsepower was non-overloading across the entire curve and hydraulic radial thrust was not excessive for the design. There were no signs or sound of internal rubbing, or any other mechanical pump-related abnormalities.
A more likely possibility appeared to be motors or motor controls. Synchronous motors have certain advantages over asynchronous units, but are more complicated, particularly with regard to controls. A synchronous motor starts following this sequence:
Initially, it operates as a typical induction (asynchronous) motor, with a magnetic field induced within the rotor armature by the rotating stator field. As the rotor accelerates and approaches close to synchronous speed, a field DC voltage is applied by the controls to the rotor (typically via brushes), at which point the rotor pulls into synchronism.
For the motors at this plant, the exciter (rotor field) coil current is 37.7-amps at 125-V field DC voltage. The timing of the rotor field application is critical, typically around 95 percent of the synchronous speed.
To pinpoint the root cause, a detailed transient analysis study of the startup was conducted. Motor amps, volts and DC field volts were recorded using a clamp-on power meter with graphics capability via download to a PC computer. Amps were taken from the CT (60:1 ratio) transformers and volts from the VT (35:1 ratio) transformers.
As can be seen from Figure 1, after completing a normal cycle of the initial in-rush locked-rotor current, amps swing to 390-amps (390/57.5 = 6.8 times, which is not unusual), the rotor has trouble synchronizing, and both amps and volts begin to swing wildly until the controls trip the unit on high amps after set time.
Dry-pump startup traces (see Figure 2) look similar to wet-pump startup #1, with amps and volts fluctuating wildly also, but somehow the rotor eventually manages to get close to synchronous speed and does not trip. The pump, however, remains dry, unable to catch prime.
The unit is restarted again, but this time the field DC volts are also recorded and plotted (see Figure 3).
Per Figure 3, a start button is pushed at the time t =11.56.6 sec, and rotor field DC is applied at t = 11.57.0 sec, i.e. 0.4 seconds after startup. At low water level, the dry rotor accelerates quickly, unburdened by the additional inertia of the water mass. Despite field DC voltage being applied prematurely and causing a wild fluctuation of amps, the rotor approaches and pulls into synchronous speed quickly (within less than 3 seconds), before the control timer would otherwise trip.
In contrast (see Figure 2, wet rotor), the impeller, surrounded and filled by water, is prevented from accelerating quickly and thus tripped by timers on high amps before the motor RPM approaches close to synchronous speed.
Once transient behavior was understood, corrections to the controls were made mainly by applying DC volts 5 seconds after startup, instead of the initial setting of 0.4 seconds (see Figure 4a).
The swings of amps are now essentially gone, with the rotor pulling to synchronous speed when it nears 95 percent of the synchronous speed (see Figure 4b).
With the corrective action applied to the controls, the pumps now operate satisfactorily, with no startup trouble.