by Ray Hardee (Engineered Software, Inc.)
March 10, 2015

Last month's column (Pumps & Systems, February 2014) reviewed the pump selection process and described how many users oversize pumps by adding design margins to account for unknown conditions present during the pump specification process. Design margins are similar to insurance, providing protection for unknown conditions expected in the life of the plant. This column will explore how oversizing a pump affects the motor driving the pump. It will also examine the adverse effects that occur when a pump is no longer operating at its best efficiency point (BEP) for extended periods of time and situations in which a design margin increases cost of ownership.

Drive Operations

Induction motors used to drive pumps are efficient in converting electrical energy into mechanical energy. A motor is 92 to 96 percent efficient depending on its classification and frame size. The efficiency is relatively constant from 50 to 100 percent of full load, with only a slight drop off when operating down to 25 percent of full load. Figure 1 shows a typical efficiency graph for a totally enclosed fan-cooled (TEFC) motor at a range of sizes operating at varying loads.

Figure 1. The motor efficiency for a variety of motor sizes and percent of full load power (Graphics courtesy of the author)Figure 1. The motor efficiency for a variety of motor sizes and percent of full load power (Graphics courtesy of the author)

The problem associated with lightly loaded alternating current (AC) induction motors is their effect on power factor. To understand the way electrical energy is converted to fluid energy using motors to drive a pump, consider the components and operation of a motor.

An electric motor consists of a stator and a rotor. The AC power supplied to the stator generates a rotating magnetic field within the motor's stator windings. When the rotor's windings are exposed to the stator's rotating magnetic field, an electric current is generated in the rotor's windings. This results in a magnetic field within the rotor. The interaction between the stator's rotating magnetic field and the rotor's magnetic field causes the motor shaft to rotate. The motor shaft is connected to the pump shaft, resulting in rotation of the pump's impeller. Increasing the flow rate through the pump requires more power from the motor. A rise in power requires increased current to the motor stator, resulting in a rise in the motor's electrical load.

The amount of electrical power used in the motor to rotate the pump can be determined by measuring the voltage across the stator and the electrical current through the stator winding. The amount of power supplied to the motor is referred to as real power (P) and is measured in watts.

Because the source of electrical power is alternating current and the motor's stator consists of a conductor wound around a stator, an induced load in the motor develops. The result of the induced load is referred to as reactive power (Q) and is measured in volt-amperes reactive (VAR). The reactive power caused by the stator coil lags in relation to the real power by 90 degrees. This reactive power produces no useful work.

The reactive current simply moves between the generator and the motor across the electrical grid. The product of the real power and the reactive power is called apparent power (S) and is measured in volt-amperes.

The power factor of the motor is defined as the ratio of the real power over the apparent power and is expressed as the cosine of the angle φ between the real power and apparent power. Power factor ranges between 1, all real power, and 0, all reactive power. The value is unitless. The power factor of a fully loaded motor is indicated on the motor's nameplate.

The real power varies with the load on the motor, but the reactive power is based on the construction of the motor. Figure 2 shows that a reduction in real power causes an increase in the angle φ.

Figure 2. The relationship between real, reactive and apparent power along with the power factorFigure 2. The relationship between real, reactive and apparent power along with the power factor

As a result, a lightly loaded induction motor has a reduced power factor.

The real power is used to drive the motor. The reactive power simply flows between the motor and the generator supply electricity from the utility. The resulting apparent power flows through the power grid.

The electric utility must design the transmission lines depending on the apparent power, but the generator only measures the real power. Figure 3 shows how a motor's power factor varies depending on the motor frame size and percent of full-load horsepower.

Figure 3. Motor power factor as a function of motor loading for a range of motor sizesFigure 3. Motor power factor as a function of motor loading for a range of motor sizes

Figure 3 shows that a lightly loaded motor has a very low value of power factor. An industrial customer with many lightly loaded motors will either have a power factor surcharge added to their utility bill or will need to purchase and install hardware to correct the plant's low power factor. An increased power expense is one example of the added costs of excessive design margin.

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