Variable Frequency Drives

Published On: June 20, 2024By Categories: Engineering Your Business, Pumps and Water Systems

Part 4b. Concepts and Applications.

By Ed Butts, PE, CPI

We will conclude our two-part miniseries on the concepts and applications of variable frequency drives this month.

Pulse-Width Modulated (PWM) VFDs

Figure 1a. Pulse-width modulated (PWM) waveform.

The third and primary type of VFD control platform, pulse-width modulation (PWM) (Figure 1a), is commonly used in industry and water works service as an energy-saving motor speed control device.

The popularity is due to its excellent (high) input power factor and several other reasons: It does not cause motor cogging nor provide regenerative ability; it operates at higher power conversion efficiencies; and possesses a resultant lower investment and operational cost.

A PWM VFD consists of three basic components: the AC-to-DC conversion block (the rectifier), the filtering block (the DC bus), and the DC-to-AC conversion block where the frequency modulation takes place.

A PWM signal is a digital pulse, which means that it has two states: on and off (which correspond to 1 and 0 in the binary context) at a high frequency. As the relative on-time of the signal increases or decreases, so does the average voltage of the signal. This average voltage provides an equivalent lower power, while still maintaining full voltage for the on-state duration of the pulse.

Figure 1b. VFD switch frequency and pulse output.

The VFD switching frequency, illustrated in Figure 1b, refers to the rate at which the DC bus voltage is switched on and off during the PWM process. The on and off switching of the DC voltage is performed by insulated gate bipolar transistors (IGBTs). The PWM process utilizes the switching of the IGBTs to create the variable voltage and variable frequency output from the VFD for control of AC or DC motors.

The switching frequency, often called the carrier frequency, is defined using the electrical unit of hertz (Hz) and is generally in the kilohertz (kHz), Hz × 1000, range, typically ranging from 4 to 16 kHz or 4000 to 16000 on and off switches per second; the key determining factor of the switching frequency is the response rate of the load device.

Two key parameters control the PWM signal: the frequency of the switching and the relative duration of the on-time, called the duty cycle (Figure 2). The duty cycle of a PWM signal is the relative amount of time the signal will be on, expressed as a percentage. Therefore, if the duty cycle is 100%, the signal will be continuously activated. If lowered to 50%, the signal will be activated for half of the pulse and deactivated for the other half.

When controlling electric motors, the duty cycle is used to dictate the output power. For example, if a PWM controller produces an output voltage of 12 volts DC, a 50% duty cycle would provide the equivalent of 6 volts DC to power the load.

Figure 2. Pulse-width modulated VFD duty cycles.

The simplest converter is a full-wave diode bridge, which converts the incoming AC to a fixed DC voltage. This isn’t useful in a DC drive but works fine for an AC VFD.

Standard industrial drives, both AC and DC, use six rectifier devices to form a three-phase full-wave bridge. This type of converter is called a six-pulse design because it draws current in six distinct pulses from the AC line. Referring again to Figure 3a in the May 2024 issue, a typical six-pulse converter is comprised of six diodes in the rectifier section, which are similar in function to check valves used in piping systems.

Diodes allow current to flow in only one direction, the direction shown by the arrow in the diode symbol. For example, whenever Phase A voltage is more positive than Phase B or C voltages, that specific diode will open and allow current to flow. When Phase B becomes more positive than Phase A, the Phase B diode will open, and Phase A diode will close. The same is true for the three diodes on the negative side of the bus. Thus, there are six current pulses as each diode opens and closes in rapid succession.

This six-pulse VFD is the standard configuration for most electronic variable frequency drives. The number of pulses of a converter defines the number of commutations, which are used within one fundamental period to convert AC to DC. The most common commutations vary from six pulse (i.e., B6, based on 1-B6 circuit) to 36 pulses (B36, based on 6-B6 circuits).

The basic converter for a three-phase system is a B6 topology. It uses six commutations within one fundamental period. This results in a commutation to occur every 60 degrees.

Ideally, the pulses are sequentially timed for firing so that the timed average integral of the drive yields a perfect sinusoidal waveform to the motor. This voltage is then applied to the motor in a simulated sinewave pattern and delivered in very short pulses in rapid succession, sometimes as high as 20,000 pulses per second.

Referring to Figure 1b, the inverter output consists of a series of rectangular pulses with a fixed height and adjustable width. There are three sets of pulses, a wide set in the middle and a narrow set at the beginning and end of both positive and negative portions of the AC cycle.

The sum of the areas of the pulses equals the effective voltage of a true AC sinewave. If the portions of the pulses above or below the true AC sinewave were deleted and then used to fill in the blank spaces under the curve, they would almost perfectly match. It is in this manner that a VFD controls the output voltage to the motor. The sum of the width of the pulses and the blank spaces between them determines the frequency of the wave seen by the motor. If the pulse was continuous and without any blank spaces, the frequency would still be correct, but the output voltage would be much greater than that of a true AC sinewave.

The IGBT is the electrical current component now used to generate the voltage pulse although silicon-controlled rectifiers (SCRs) can also work. In the future, injection-enhanced gate transistors (IEGTs) will be alternatively used to perform this task.

In the long term, memristors will most likely become the component of choice for this specific task. Memristors are the fourth type of passive circuit element, linking the electric charge and magnetic flux together. They have been hypothesized to be able to operate for more than 30 years but were not fabricated until 2008 by Hewlett Packard. The manufacturer hopes to eventually use these devices as a passive transistor, reducing their heat generation compared to other types of volatile memory logic.

The present method used to produce this waveform uses triangular and sinewaves through a comparator to provide a voltage pulse output whenever the sinewave’s value is greater than the associated triangle wave.

An embedded microprocessor governs the overall operation of the VFD controller. Basic programming and functional control of the microprocessor is provided using proprietary software with user-modifiable programming of the display, variable, and function block parameters permitted through a keypad or external software to control, protect, and monitor the VFD, motor, and driven equipment.

VFDs and Harmonics Control

Figure 3a. Volts per hertz (V/f) VFD control method.

Although a six-pulse rectifier is the most robust and cost-effective solution currently available in the VFD industry, the downside to this configuration is the current draw at each diode bridge is not uniform, and when the supply is not perfectly balanced, the drive will produce harmonics and possibly result in premature failure of the impacted bridge.

Harmonic control and abatement methods are covered in IEEE Standard 514-2022 “Harmonic Control in Electric Power Systems” and will be discussed in more detail in future columns.

While harmonics may not be a problem for smaller, low voltage motors, it can be a concern for larger motors, especially when the amount of total load on the VFD increases. Many manufacturers have created new techniques using autotransformers to achieve the required phase balance and displacements. However, the addition of this extra component along with the extra rectifiers and converters adds more cost,
increases the size of the units, and generates more heat.

Due to these disadvantages, it is often more beneficial to only use six-pulse drives for smaller low voltage motors, generally in the range of 100 horsepower or less and switch to either 18-pulse drives or the more modern active front end (AFE) drives for larger low voltage motors.

It is no longer recommended to use 12-pulse drives as they cost approximately the same and have less benefits than an 18-pulse drive, plus many manufacturers no longer make them. Many manufacturers now make 24-, 30-, and 36-pulse drives, with some producing up to 48-, 54-, and 72-pulse drives, but these are usually applied to much larger medium voltage motors (300 HP up to 10,000 HP).

Figure 3b. Vector control VFD control method.

Simply speaking, the higher the pulse configuration input, the lower the total harmonic distortion (THD) the drive will impress on the power supply. This means a 36-pulse drive is generally superior to a 24-pulse drive. For example, although they are more complicated than lower pulse drives, using a 24- or 36-pulse, medium-voltage drive enables phase shifting cancellation of harmonics, producing higher order harmonics that tend to have fewer negative impacts on the power system.

There are several methods available to lower or negate the impact of harmonics on a VFD and electrical system. For a more complete explanation and discussion on harmonics, go to the Engineering Your Business columns in the August 2023 and September 2023 issues.

VFD Control Methods

There are four open- or closed-loop VFD systems described in Table 1 used to control VFD-driven motors with two basic control methods. The first and most common type of VFD control is a scalar method referred to as volts per hertz (V/Hz) and also known as volts per frequency (V/f).

V/Hz control assumes a linear relationship between frequency and voltage and does not account for the magnetic flux or the load of the motor. Scalar control is suitable for applications that do not require precise speed or torque control such as fans, pumps, and conveyors as it maintains a constant ratio between the voltage (V) and frequency (Hz) (Figure 3a).

As AC motors are designed for a magnetic field (flux) of constant strength, the magnetic field strength is proportional to the ratio of applied voltage (V) to the frequency (Hz). V/f control avoids this variation in the magnetic field strength by varying the voltage along with the frequency to maintain a constant V/Hz ratio.

It is the easiest and simplest VFD control method and is often used specifically due to its simplicity and how little motor data the drive needs to operate. It generally operates in an open-loop configuration with closed-loop often used to provide a feedback signal control parameter (process variable).

Tuning the VFD to the motor is not required, but it is still recommended. V/f control is often used when there is a demand or operation which could exceed 1000 Hz, so it is often employed in submersible pumps and other long cable offset applications.

V/f is the only control method that lets several motors operate from a single VFD. In such cases, all motors start and stop at the same time and follow the same speed reference.

But V/f does have limitations. For example, there is no guarantee the motor-shaft of any specific unit is rotating. Additionally, the motor’s starting torque is limited to 150% of its output at 3 Hz, but the limited starting torque is more than enough for most variable torque (i.e., pumping) applications. In fact, just about every variable torque pump application uses V/f control.

Speed regulation is typically 2% to 3% of maximum frequency and speed response is rated at 3 Hz. Speed response is defined as how well the VFD responds to a change in reference frequency. An increase in speed response results in quicker motor responses when the reference frequency changes.

Control methods also have speed control ranges (expressed as ratios). A V/f’s speed control range is 1:40. Multiplying this ratio by the maximum frequency determines the VFD’s minimum motor’s running speed. For example, with a 60 Hz maximum frequency and 1:40 speed control range, a drive using V/f control can theoretically control a motor down to 1.5 Hz. This translates to 90 RPM for a 3600 RPM (two-pole) nominal speed motor, far below the actual minimum allowable
speed.

The second method, vector control and also called field-oriented control (FOC), (Figure 3b), is a VFD control method in which the stator currents of a 3Ǿ AC motor are identified as two orthogonal components that can be visualized with a vector.

Vector control is a more advanced and complex method of controlling VFDs as it uses two parameters, the frequency and phase angle, to adjust the voltage and current applied to the motor. Vector control separates the magnetic flux and the torque components of the current and controls them independently.

Vector control can compensate for the non-linearities and losses of the motor and can provide fast and accurate speed and torque control, even at low speeds. The control system of the drive calculates the corresponding current component references from the flux and torque references provided by the drive’s speed control.

Typically, proportional-integral (PI) controllers are used to keep the measured current components at their appropriate reference values. Vector control methods offer significant benefits over scalar V/f methods in some applications. For example:

  • Stability for load and setpoint changes
  • Higher efficiency and power factor with lower slip and losses than V/f method
  • Can be used with or without an encoder (i.e., sensorless operation possible)
  • Open-loop vector control enables the motor to produce high torque at low speeds and closed-loop vector control allows a motor to produce up to 200% of its rated torque at zero speed. Closed-loop vector control also provides accurate torque and speed control for many industrial and pumping applications (refer to Table 1).
  • Short rise times for setpoint changes leading to better control behavior
  • Short settling times for load changes resulting in better response to disturbances
  • Acceleration and braking are possible with maximum settable torque
  • Motor protection due to variable torque limitation in motor and regenerative mode
  • Drive and braking torque controlled independently of the speed
  • Maximum breakaway torque possible at zero speed.

______________________________________________

This concludes the introduction along with the concepts and applications of variable frequency drives. This past month also coincides with my 50th anniversary in the water well and pump industry, as I started on June 10, 1974. I have witnessed great changes and improvements in our industry over these intervening years; many will be discussed next year.

Next month, we will continue this current topic and discuss potential pitfalls and design concerns related to variable speed and frequency drives.

Until then, work safe and smart.

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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