Pumps & Systems - September 2007

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.

As always, a quiz question to our readers: Is there one more important test missing here to prove the point more definitely? (Hint: Examine the steady-state amps). As

always, your correct answer will get you a winning ticket to our next Pump School training session.

P&S

Focused Time : 06/29/07 11:11: 28 - 06/29/07 11:11: 34 Display Cycle : Meas. Cycle(1WAVE)

Current , DC Voltage - 1 (06/29/07 11:11: 28 - 06/29/07 11:11: 34)

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

200.000.05

180.00

160.000.04

140.00

DC Voltage - 1 (V)

120.000.03

100.00

80.000.02

60.00

40.000.01

20.00

11: 33.815

11: 33.615

11: 33.415

11: 33.215

11: 33.015

11: 32.815

11: 32.615

11: 32.416

11: 32.216

11: 32.016

11: 31.816

11: 31.616

11: 31.416

11: 31.216

11: 31.016

11: 30.816

11: 30.616

11: 30.416

11: 30.216

11: 30.016

11: 29.816

11: 29.616

0.000.00

11: 28.016

11: 28.216

11: 29.416

11: 29.216

11: 29.016

11: 28.816

11: 28.416

11: 28.616

Cur. Ave.CHs Analog Input - 1 CH1

Figure 4a. Startup test #4 (correct application of the DC field).

Focused Time : 06/29/07 11:11: 28 - 06/29/07 11:11: 34 Display Cycle : Meas. Cycle(1WAVE)

Current , Voltage (06/29/07 11:11: 28 - 06/29/07 11:11: 34)

Dr. Nelik (aka “Dr. Pump”) is president of Pumping Machinery, LLC, an Atlanta-based firm specializing in pump consulting, training, equipment troubleshooting, and pump repairs. Dr. Nelik has 30 years experience in pumps and pumping equipment. He has published over 50 documents on pump operations, the engineering aspects of centrifugal and positive displacement pumps, and maintenance methods to improve reliability, increase energy savings, and optimize pump-to-system operations. With questions, comments, or to attend his Pump School, please contact www. PumpingMachinery.com.