Pumps & Systems - September 2007

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

400.004200.0

380.004190.0

360.004180.0

340.004170.0

320.004160.0

300.004150.0

280.004140.0

Current (A)

12:00.906

12:00.806

260.004130.0

12:00.706

12:00.606

12:00.506

240.004120.0

12:00.407

12:00.307

220.004110.0

12:00.207

12:00.107

200.004100.0

12:00.007

11: 59.907

180.004090.0

11: 59.807

11: 59.707

160.004080.0

11: 59.607

11: 59.507

Voltage (V)

11: 59.407

140.004070.0

11: 59.307

11: 59.207

120.004060.0

11: 59.107

11: 59.007

100.004050.0

11: 58.907

11: 58.807

80.004040.0

11: 58.707

11: 58.607

11: 58.507

60.004030.0

11: 58.407

11: 58.307

40.004020.0

11: 58.207

11: 58.107

20.004010.0

11: 58.007

11: 57.907

0.004000.0

11: 57.807

11: 57.707

11: 57.607

11: 57.507

11: 57.407

11: 57.307

11: 57.207

11: 57.107

11: 57.007

11: 56.907

11: 56.807

11: 56.707

11: 56.607

11: 56.507

11: 56.407

11: 56.307

11: 56.207

11: 56.108

11: 56.008

Cur. Ave.CHs Volt. Ave.CHs

Figure 2. Startup test #2 (dry impeller, EL = negative 3.5-ft). Motor pulls to synchronous speed after about 3 seconds.

Focused Time : 06/22/07 11:11: 56 - 06/22/07 11:12:01 Display Cycle : Meas. Cycle(1WAVE)

Current , DC Voltage - 1 (06/22/07 11:11: 56 - 06/22/07 11:12:01)

400.000.10

380.00

360.000.09

340.00

320.000.08

300.00

280.000.07

260.00

240.000.06

Current (A)

220.00

180.00

140.00

100.00

60.00

20.00