Motors for Pumps
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.
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.