Centrifugal pumps driver limits

Pumps are driven by either fixed speed or variable speed motors. Variable speed motors are becoming increasingly common. How can be controlled by varying the pump speed with the motor and thus eliminating parasitic energy losses across a flow control valve. However, because of the extra electronic components needed for a variable speed AC motor, 95 percent of the motors we work with are fixed speed alternating current motors. 1 will only discuss fixed speed, alternating current (AC) motors in this chapter. 

Direct current (DC) motors are sometimes used in process plants. It is easy to manipulate the speed of a DC motor. But direct current is almost never available on a process unit. Our motors at home are mainly alternating current motors operating at a fixed speed. 

Copyright 2008 by The McGraw-Hill Companies, Inc. Click here for terms of use. 

A process motor in an American, Canadian, or Latin American plant is likely to run at either 1750 to 1800 rpm or 3500 to 3600 rpm. In other locations, a motor will likely run at 1500 rpm or 3000 rpm. In the United States, we use alternating current at 60 Hz cycles per second. In Europe, they use alternating current at 50 Hz cycles per second. As the alternating current cycles 20 percent slower in Europe, the motor spins 20 percent slower. 

At home, our motors are single phase. At the plant, the motors are usually three phase. There is an important difference between a single and a three phase motor. The three phase motor can also be used as an electric power generator. The single phase motor we use at home, without modification, cannot be used to generate electric power. The difference is important in that 1 once used this fact to avoid tripping off a critical process pump, as related in the following story. 

There are three types of limits on centrifugal pumps  

A process motor in an American, Canadian, or Latin American plant is likely to run at either 1750 to 1800 rpm or 3500 to 3600 rpm. In other locations, a motor will likely run at 1500 or 3000 rpm. 

In the United States, we use alternating current at 60 Hz cycles per second. In Europe, they use alternating current at 50 Hz cycles per second. As the alternating current cycles 20 percent slower in Europe, the motor spins 20 percent slower. 

---

If we cannot open the discharge control valve of a centrifugal pump 100 percent before the FLA point is reached, then we say that the pump is driver-limited. This is a frustrating problem for plant operators, and clearly reflects poor design practice in undersizing the motor driver horsepower. 

Pumps (centrifugal, reciprocating) Fluid density, capacity, dynamic head, type, driver, operating condition limits, material of construction... 

---