Test Submersible Wastewater Pumps & Motors Frequently to Accommodate for Changing Conditions

In last month’s column (Pumps & Systems, May 2014), the drawdown analysis portion of the pump and motor field test spreadsheet was reviewed. The calculations performed by that section of the spreadsheet provided the flow and total dynamic head produced by the pump during testing. This month, the motor testing section will be reviewed.

Motor Testing Importance

Motor testing is important for two reasons. First, it provides the information required to calculate horsepower at the test point. Horsepower allows for the calculation of pump efficiency at the test point. Then the tested head, flow, horsepower and hydraulic efficiency can be compared with those on the manufacturer’s pump curve. Pump efficiency also allows for monitoring the cost per thousand gallons pumped each time a test is performed. During motor testing, operational data that helps identify unhealthy conditions that can lead to premature failure can also be collected. Important components include:

• Motor temperature
• Loading
• Actual supply voltage

Unbalanced phase voltage and current can also influence submersible pump reliability. A small voltage unbalance may lead to a current unbalance that is 6 to 10 times greater. Unbalanced currents increase the stator operating temperature, which reduces insulation life. Phase unbalance is a primary cause of premature motor failure in the U.S.

Horsepower Calculation

Figure 1 shows the motor test portion of the submersible wastewater pump and motor testing spreadsheet. Like the pump test portion, the gray cells are entered data, and the yellow cells are the calculated results. The equations on the right show how certain results were calculated. The information required includes phase voltage and current, motor nameplate efficiency, power factor (PF), and the power cost per kilowatt hour (kWh). Voltage and current are measured with a high-quality voltmeter and clamp-on ammeter. The spreadsheet instructions detail the optional use of kilowatt and PF meters. The phase voltage and current should be measured at the beginning of the pump down cycle to provide an accurate horsepower calculation. They should also be measured on the motor side of the contactor. Contactor corrosion cannot be detected when voltage is measured on the line side. Before or after the pump down test, measure the phase voltage on the line side with the pump off. If phase voltage unbalance occurs with the pump off, the supply side can be identified as the source of the unbalance. As seen in Figure 1, the calculated voltage unbalance was 1.1 percent with the motor off. My July and August columns will explain the effect of voltage variation and phase unbalance in detail. The average voltage and current are used to compute the motor horsepower at the test point. The test-point horsepower and hydraulic horsepower (assuming 100-percent pump efficiency) are used to compute the actual pump efficiency at the test point. The cost per thousand gallons pumped is calculated using the equation shown at the bottom right in Figure 1. Submersible pumps and motors should be tested annually. Changes in system conditions can have a significant impact on the pump’s operating point. This can result in a reduced life for several components—including seals, wear rings and bearings. Off-best-efficiency-point operation will also increase the cost per thousand gallons pumped. Unbalanced phase voltage can occur at any time. Just because it was balanced a year ago does not necessarily mean it will remain balanced this year. My field test spreadsheet is available on the “Pump Evaluation, Selection & Testing Tools” page at www.PumpEd101.com.