For pumps to operate, they need some type of driver, and the most popular driver is the electric motor.

It is estimated that 60 percent of all electric motors are used to drive pumps. There are a lot of different motor sizes and types available; however, most industrial applications use standard NEMA B design, AC motors, either single- or three-phase powered. These are normally open drip proof (ODP), totally enclosed fan cooled (TEFC) or explosionproof (EXP) construction. Other common construction includes washdown, marine and inverter duty.

Determining the horsepower requirement for a pump usually is easy. If the application has a controlled flow rate, or a maximum flow rate is known, the horsepower rating is selected from the pump curve. If the flow rate is variable or not known, and the flow may run out to a high rate, select the horsepower rating based on the maximum power the pump can require. The horsepower rating selected should be enough to provide the power necessary for the operating condition without overloading the motor. Other factors affecting the horsepower requirement include viscosity, specific gravity, suspended solids and temperatures.

While pump motors can produce power above their rating for a short time, continued operation at that level will cause increased internal temperatures.

While motors can produce a substantial amount of power above their rating for a short time, continued use will cause increased temperatures, resulting in failure in the windings. NEMA motors carry a service factor (SF) rating, which identifies the amount of continuous overload the motor is capable of producing without damage. A 1.15 service factor indicates the motor is capable of 15 percent more load than its rating. Small motors may have a service factor up to 1.8 or higher. Of course, operating pump motors within their service factor rating is acceptable. But, if the horsepower required is between motor ratings, always go to the next higher power rating.

Oversizing the motor horsepower rating with respect to the pump requirement will not increase the pump performance. AC motors run at designed speeds dependent upon the number of motor poles and frequency; for example, 60 Hz speeds are:

  • 6-pole, 1,200 rpm.

  • 4-pole, 1,750 rpm.

  • 2-pole, 3,450 rpm.

Under normal load conditions, speeds will drop up to 5 percent. The motor only will generate the power required to drive the load that is placed on it, so it is most efficiently utilized when the load is closest to its rating.

Most motors are capable of being wired for two distinct voltages. Typical single-phase voltage is 115/230 V and three-phase is 230/460 V. It is extremely important to make sure the motors are wired for the proper incoming voltage. Improper wiring will result in motor failure. When using three-phase power, it also is important to check the wiring because improper connection will result in the pump running backward, resulting in poor performance. Low voltage conditions are defined as incoming voltage lower than 10 percent of the rated voltage. Low voltage will cause excessive temperature rise in the motor and significantly reduce the life of the motor. This condition is common during high usage periods for the power supplier or when the installation is a long distance from the incoming power and high line losses occur.