Variable Frequency Drives

Published On: May 16, 2024By Categories: Engineering Your Business, Pumps and Water Systems

Part 4a. Concepts and applications.

By Ed Butts, PE, CPI

During the past three months, we introduced variable flow and head systems and the various types of variable speed drives for pumping systems. With this column and the one next month, we will continue the series with an overview on the most popular method: variable frequency drives.

Introduction

Figure 1. Open loop and closed loop VFD control.

Although variable speed drive systems are comprised of various methods of motor and pump speed regulation, for our purposes, we shall generally limit our discussion to electronic, low-voltage (less than 600VAC), three-phase variable frequency drives (VFDs). Medium-voltage VFDs will also be briefly discussed, albeit less than low voltage units.

VFDs are a remarkable and often perfectly matched control device for most pumping applications that adhere to the affinity laws. However, they are not a panacea or without potential issues and problems. Water system designers and engineers must recognize VFDs’ potential strengths as well as their limitations and implement them with consideration of these operating characteristics.

Although VFDs are currently still more expensive than most comparable fixed-speed three-phase motor controls, the increased cost comes with benefits. Here are just a few of the many possible benefits of using a VFD.

Figure 2. True and VFD modified sinewaves.

Improved Power Factor: This is achieved using direct current (DC) bus capacitors within the VFD. The DC bus capacitors provide the reactive current to the motor needed to induce the rotor’s magnetic field. Therefore, the power drawn from the input supply line will consist of the real power only with the voltage and current almost perfectly in phase with power factors up to 97%.

The benefits of improving the power factor include avoiding power factor penalties and demand charges from the utility and reducing the current on the distribution network.

Extended Motor and Pump Life: This is from controlling the motor’s starting and running current while moderating the starting and stopping functions, thus reducing the degree of shock and motor and pump wear.

This can lead to lowered replacement costs by reducing the frequency in which a unit cycles and providing a smoother operating transition. However, long VFD motor feeder lengths can result in reflected waves, plus the slower shaft speed of the motor can cause cooling issues for submersible motors and totally enclosed fan-cooled (TEFC) enclosures. These are potential issues which must be addressed.

Figure 3a. Simplified six-pulse VFD schematic and waveforms.

Speed and Flow/Pressure Control: A VFD can produce a varied output frequency, which in turn can be used to control the speed of a motor and pump’s output to match its needed demand as well as controlled starting and stopping ramping speeds to control undue stress and system water hammer. Due to the impact of the affinity laws, flow and pressure delivery rates can be controlled with precision throughout a wide operating performance band. This can be performed using an open loop or closed loop control method (Figure 1).

For the open loop control method, the volts per hertz (V/Hz or V/f) output to the motor is controlled independent of feedback from the motor. This can be done using linear or custom nonlinear output curves. To ensure the VFD provides accurate speed control, a tuning mode can be used to compensate for speed as needed.

For the closed loop control method, the VFD will directly monitor and control the voltage and current of the motor by utilizing a pressure or flow feedback signal (i.e., process variable) from an external analog sensor.

Reduced Energy Consumption and Starting Stress: Energy consumption is lowered by reducing the speed for motors that do not need to continually run at 100% speed and output load (pump). This benefit is often the single most important and valuable benefit and can provide more cost savings than other typical motor starting or variable flow/head control methods.

Figure 3b. VFD components and flowchart.

This is because of how the affinity laws work for centrifugal pumps, which was discussed in last month’s Engineering Your Business. These affinity laws express mathematical relationships between flow, head, and horsepower variables involved in pump performance and are useful for predicting the effect of speed on pump performance.

Based on the affinity laws, a 50% reduction in rotational speed will typically reduce the input power used to 12.5% of that at 100% speed. Even a slight change in the pump’s rotational speed to 90% full speed will reduce the power required to 73% of that required at full speed. This obviously means that even small reductions in motor and pump speed can still provide a major potential in energy savings.

In addition, VFDs provide inherent soft-starting characteristics to motors, reducing accelerating stress on the motor and pump components.

Phase Conversion Capability: When properly applied, a VFD can be used to convert single-phase to three-phase power. This combines the roles of a motor control with phase conversion and eliminates the need for rotary or static phase converters.

When used as phase converters, the single-phase line will theoretically draw 173% greater power than the motor amperage from each line. However, after adding internal losses and an overload factor, this value typically rises to approximately 185% of the motor’s full load current per line.

For an added margin of safety, doubling the motor’s current for design is generally recommended and is what should be planned and anticipated. Thus, for a 30 HP, 460 VAC, 3-phase motor rated at 40 amps, the two single-phase line currents should be estimated to draw 40 amps × 2.0 or 80 amps per line.

VFD Waveform Shapes

Figure 4. VFD power conversion train.

VFDs, also known as inverters, do not generate power. Rather, they transform power from a standard AC waveform to DC and then back to a modified AC power waveform. They use solid state components with no moving internal parts other than perhaps a cooling fan.

Most inverters are available in the modified sinewave version, which is the least expensive way to manufacture an inverter. However, the waveform generated does not replicate true utility power, but rather an approximate version of it. The quality of this waveform will vary greatly between inverters, depending upon the type and model of the inverter.

Higher quality inverters are now available at reasonable prices that are labeled as true sinewave inverters. The true sinewave inverters still use solid state circuitry to create this modified waveform, but advances in technology now allow that waveform to be virtually equal to utility grade power.

Figure 5a. VSI power flow train.

So, what’s the difference? Figure 2 shows the two waveforms, examining the utility power true sinewave first. As the sinewave shows, the peak voltage is around 170 volts, but it only touches this value for a brief instant. Electrical equipment needs exposure to that voltage for a longer time in order to use it. Therefore, typical household electrical equipment is designed to run on 120 volts RMS voltage. (RMS is root mean square and is a true measurement of power.)

All electronic voltmeters and test instruments will generally measure the RMS voltage. This RMS value is measured at a lower point than the peak voltage on the waveform. This is determined by squaring the value to make sure it is always positive, averaging it, then taking the square root of the average to compensate for squaring it to begin with. The modified sinewave generally follows the path and intensity of the true sinewave, but in a square waveform.

Operating Principle of VFDs

Figure 5b. CSI power flow train.

A typical pulse type of variable frequency drive operates on the fundamental principle of power conversion, using three separate power conversion sections: AC power initially enters from the source and is initially converted to DC power in the rectifier section. The converted DC power then flows through a filtering capacitor section over a positive (+) and negative (–) DC bus. Finally, the power flows through the inverter section where it is inverted back to a modified form of AC power and thereafter delivered to the motor.

This train is illustrated on a simplified six-pulse VFD flowchart and waveforms in Figure 3a. Each separate voltage conversion generates pulsations of different waveforms to simulate a sinusoidal wave, similar in shape to an actual AC power cycle. These components are illustrated on the simplified flowchart in Figure 3b.

For water pumping applications, input AC power is ultimately converted to mechanical output power using this same type of train. As illustrated in Figure 4, standard, three-phase 60 Hertz sinewave of AC power is supplied to the VFD, which then converts the power internally to a DC bus voltage and then back to a square wave AC output to the motor to provide variable speed mechanical output power.

This process is highly efficient with typical total losses of 5% or less. There are two basic functional torque delivery types of VFDs in common use: constant torque and variable torque.

Constant torque units are used to drive loads needing consistent torque such as conveyor belts, presses, and other similar units where the horsepower and torque requirement does not necessarily vary with the driven speed. In constant torque applications, the load, and therefore the horsepower, will not usually decrease substantially at lower speeds. For these applications, VFDs are primarily beneficial for their ability to precisely vary and control the speed of a process and not necessarily for energy savings.

Variable torque units are mainly used to drive loads in which the power demand varies with speed, such as fans and pumps. According to the affinity laws, the horsepower demand will vary as the cube in direct proportion to the speed of the process.

In addition to the torque output, there are three basic electronic types of VFDs in common use. The first, voltage source (or voltage) inversion (VSI) or (VVI) drives (Figure 5a) have a higher power factor and produce less harmonic distortion than the second most common type, current source inversion (CSI) (Figure 5b), which are non-regenerative in operation.

In a VSI drive, the DC output of the diode-bridge converter stores energy in the capacitor bus to supply stiff voltage input to the inverter. The CSI types have been successfully used in signal processing and industrial power applications for several years. The CSI style is the only type that has regenerative power capability. In other words, they can absorb regenerative power from the motor and deliver it back into the power supply.

The CSI types provide a clean current waveform but require large and expensive inductors in their construction and can cause a condition known as cogging (i.e., a continually pulsating movement that occurs during motor rotation) below a frequency of 6 Hz. In a CSI drive, the DC output of the SCRbridge converter stores energy in a series-inductor connection to supply stiff current input to the inverter.

CSI drives can be operated with either pulse-width-modulated (PWM) or six-step waveform outputs. Consequently, CSI and VSI drives have not been widely used for industrial or water-related three-phase motor applications.

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This concludes the first installment on the concepts and fundamentals of variable frequency drives. Next month, we will wrap up the series with an examination of the third and most common type of VFD: pulse-width-modulated VFDs, their applications, and the available control methods.

Learn How to Engineer Success for Your Business
 Engineering Your Business: A series of articles serving as a guide to the groundwater business is a compilation of works from long-time Water Well Journal columnist Ed Butts, PE, CPI. Click here for more information.

Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at epbpe@juno.com.

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