I don’t offer any technical discussions worthy of an electrical engineer or even an electrician in this article. I want to address issues arising during the writing of specifications and pump selection that can result in problems on the worksite. I’ll offer some “rule of thumb” guidelines that I have discovered while addressing some of these problems. Like all “rule of thumb” guidelines, these are generalizations, but they are sufficiently accurate to help us in this area.
1. MOST WW PUMPS ARE SINGLE VOLTAGE: there are several reasons for this that we will address in the subsequent rules, but the main point is that care must be taken when specifying a pump with an exact voltage/phase. The biggest problem is with 200v and 230v. There is a difference between motors wired for 230v or 200v ( 208v) single phase, and choosing the wrong pump for the voltage on site, will result in a greatly reduced motor ( and thus pump) life. The problem is increased with 3 phase, as a control panel with a specific AMP rating may exhibit nuisance tripping or offer no real protection for the pump if the pump is not operating at the design voltage.
2. PUMP CURVES CAN LIMIT PUMP SELECTION BEYOND PERFORMANCE: This stems from the fact that pump motors are matched to the impellers and the motor may become overloaded if the amount of work being done exceeds the design parameters. In order for a pump to receive a UL/CSA approval, the pump “ cannot overload anywhere on the published pump curve.” The most important quantity resulting from this rule is the FLA or Full Load Amp rating of the pump. This is the AMP reading taken at the maximum flow rate for a pump and is the AMP reading found at the far right end of the pump curve. If the curve starts at 5 ft TDH, then any application for that pump that does not have a TDH of at least 5ft will overload the motor. All pumps have a minimum TDH. With some pumps that can be as high as 30ft TDH.
3. THE AMP READING FOR A PUMP DECREASES AS THE TDH INCREASES: The pump produces less flow (GPM) as the head increases, so the pump is doing less work, and thus drawing less current ( fewer AMPS).
4. ROTATIONAL DIRECTION IS DETERMINED AT THE JOB SITE: Polarity of the electrical supply determines the rotation of a three phase pump. All 3 phase pumps must be checked for rotation at installation.
5. AMP DRAW IS THE BEST WAY TO DETERMINE ROTATION: Three phase wastewater pumps will pump liquid while running backwards. The best way to determine rotation is to take an AMP reading. This must be done while the pump is pumping the liquid. Three phase pumps will draw approximately 50% higher AMPS when running backwards. If the static head is relatively low, then you can compare your reading to the name plate FLA ( Full Load Amps) of the pump. If the static head is relatively high, the expected amp draw will be less than the listed FLA.
6. AIR LOCKED PUMP AND PLUGGED DISCHARGE HAVE SIMILAR AMP RATINGS: if a pump is air locked, the reading will be approximately 50% of the expected amp draw. This is also true if the discharge line is severely restricted of completely blocked.
7. VOLAGE, PHASE AND AMP RATING CAN AFFECT THE CHOICE OF CONTROL: Single phase pumps with a FLA of greater than 15 amps cannot use a variable leverl pump control switch, and must use a control panel. Virtually all three phase pumps require a control panel. some larger single phase pumps require starting components mounted in the control panel. (starting capacitor, run capacitor and relay).
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