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A Variable Speed Pump Compared to a Centrifugal Pump

Abstract

Pumps and their functions are a necessity in industries and homes nowadays. Buildings and lands with distinct topographical characteristics need pumps to provide water to users. The basic function of a pump is to transfer liquid from a low-pressure area to a higher one. Without the application of a pump, it is impossible to move a liquid to a high-pressure zone because of the force that prevents the liquid to move. The old cliché goes that water, or liquid, seeks its own level. This can only be countered by means of a pump.

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There are different types of pumps that facilitate this operation. In this paper, we are concerned with centrifugal pumps and pumps applied with variable speed drive. How it works and which one is more applicable, whether in homes or industries, is the aim of this essay.

A variable speed pump is operated by a circuit consisting of a rectifier that converts alternating current (AC) to direct current (DC), and another circuit is known as an inverter circuit to transform the DC into a different version of the AC. Then, the AC produced by the inverter controls the speed of an electric motor. This is the operation of the variable frequency drive (VFD) which is also known as variable speed drive (VSD). By controlling the speed of the centrifugal pump through fluctuating current, we are able to control the capacity of the pump (Ramey, 2012). On the other hand, a centrifugal pump needs no speed drive and is only controlled by manipulating the input and output valve.

The variable speed pump

The VSD has an inverter circuit, which converts the DC back into the alternating current but in a varied frequency and voltage. This is done through the creation of a “high-frequency pulse-width modulated signal” (PWM), a circuit that controls the voltage that runs the motor by means of an on-off switch, applied at a certain speed (Jenkins, 2015). The pulse-wave modulation is applicable to other motors of different designs.

Makers of technology products often use complex technology to define the type of control and function of their particular drive. But the circuit applied to these drives does not considerably differ. A significant aspect of the efficiency of one product is its capability to provide control rotation and speed, especially during lower frequencies. The control functions are provided with the use of different algorithms applied to the microprocessor that controls the drive, which then defines the output rotation of the motor (Europump and Hydraulic Institute, 2004).

The algorithms need detailed information about the motor in order to make it functional to the full. Important features of the motor, such as volts, current, and base speed are incorporated in the drive configuration when the motor is being set up. Other considerations necessitate the drive to function a regular tuning on the motor to provide a gauge of its static and dynamic features. The drive circuit also regularly guards volts, amps, and frequency when operational and may grade the speed and position of the rotor, while an encoder is placed on the motor shaft and returned back to the drive.

Theoretically, a motor can work at any speed; this can be done according to the motor’s design, with a corresponding inverter voltage. A common configuration for a VSD applied to a motor is the use of a three-phase AC voltage, with the necessary rectification, i.e. from AC to DC and then back to DC again with the use of the DC/AC inverter. The AC inverter provides the pulse output voltage which drives the motor, and the motor in turn gives an inductance (Europump and Hydraulic Institute, 2004).

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While there are AC motors, there are also DC motors, technically termed DC commutator motors. This kind of motor has an armature, which is driven by a rectifier circuit, usually an abridged type rectifier. The AC motor is driven by an uncontrolled rectifier. The supply for a DC motor must be a constant voltage. A PWM converter is specifically required of “Brushless DC motors” (Europump and Hydraulic Institute, 2004 p. 64).

VFD applications enable designers to manufacture motors without “specific rotational speeds,” which was the traditional way of manufacturing rotators or motors (Europump and Hydraulic Institute, 2004, p. 65). With a VFD, a motor can be driven even at a higher speed. A VFD can be used for any new motor system. However, we have to be sure that a VFD is compatible with the motor if this motor is newly purchased.

The single pump system is a common application. This uses a sensor that activates the VFD or tells it to act on the signal. The pump will act accordingly, whether to provide a low or large volume of flow, while the pressure is maintained. This type of system is most common in water supply schemes, which necessitate a constant pressure while various flow rates are required by different users.

Pumps operated by VFDs can have several problems. For example, fluid stickiness is affected by the speed of the pump. Dilatants as an example will increase which will result in a need for more power. Another thing is the shift which can reach its critical speed once it is at its ideal speed. With too much load applied, the motor will give up and burn while on the operation (Girhard & Moniz, 2004).

Additionally, if you try to get the best of the pump, the shaft might lose its effective deflection. Over time, the pump may lose its effectiveness of allowing the fluid to pass through it due to erosion and rough action. Parts that have to be closely monitored include the piston rings, rod seals, or diaphragm. These parts have to be lubricated or replaced, depending on how they function in the pump.

Centrifugal pumps

On the other hand, a centrifugal pump is not applied with any type of control because of its low starting torque rod. There are special control forms provided to create more benefits. The owner should provide more care because of the motor’s positive displacement, wherein the needed power is proportional to the rotational speed. Moreover, a high-starting torque is needed especially for “progressive cavity pumps” (Europump and Hydraulic Institute, 2004).

The pump has a suction nozzle where the fluid first flows through. When the fluid arrives at the suction piping, it must have enough energy so that the pump can suck it. We must have close monitoring in the pump’s suction as this is where most of the problem occurs. The “Net Positive Suction Head” is required of the pump to perform its task of providing fluid flows (Bachus & Custodio, 2003, p. 12).

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NPSH is the event occurring during suction, and this important task of the pump occurs only if there is accurate piping, and the needed velocity is applied, to include the required temperature. All these support the required energy of the fluid in its entry into the pump. To sum it up, this energy forces the impeller to produce pressure to make the fluid flow. If the energy is not enough, then there is insufficient NPSH. The suction nozzle is larger than the release spout because of the required NPSH (Karassik & McGuire, 1998).

Practical application

From the information and discussion provided in this essay, the question is: which one is more applicable, or can provide practical application?

Traditionally, controlling the flow of a pump was managed through valve control. Speed control using VSD is more applicable nowadays since we use less energy with the VSD method. If there is a large amount of flow needed, such as in other pumping systems, several pumps are connected in parallel to provide the necessary action.

While there are problems with VFDs, pumps with readily integrated VFDs are more applicable for a number of reasons. New products and new improvements are introduced in the market. Some of these new products have integrated motor/VFD units, which means that the motor has a specially designed VFD in one package, ready for use. This is favorable for the users in a number of ways (Europump and Hydraulic Institute, 2004, p. 66):

  • The duly matched motor and VFD provides no hassles;
  • This single package is less costly for the users;
  • There is saving of installation time and materials;
  • No worries for over-voltage due to the short length of the cables, and the user is assured of long-lasting insulation;
  • Extra radiation is minimized since there is less output cable;
  • VFD can also be used for the motor cooling systems;
  • Since parts are appropriately connected and matched, noise and vibration is reduced;
  • It is easy to install in buildings or industry.

References

Bachus, L., & Custodio, A. (2003). Know and understand centrifugal pumps. Oxford, UK: Elsevier Limited.

Europump and Hydraulic Institute. (2004). Variable speed pumping: A guide to successful applications. New York, New York: Elsevier Inc.

Girhard, P., & Moniz, O. (2004). Practical centrifugal pumps: Design, operation and maintenance. London: Elsevier Ltd.

Jenkins, S. (2015). Variable frequency drives. Chemical Engineering, 122(9), 42. Web.

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Karassik, I., & McGuire, T. (1998). Centrifugal pumps. London, England: Chapman & Hall.

Ramey, J. (2012). Variable frequency drives for centrifugal pumps. Chemical Engineering, 119(12), 31-42. Web.

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