Energy-Saving and Precise Speed Control Outbalances the Harmonic Distortions in VFD

Key Takeaways

• Motor controls are an integral part of the production and manufacturing industry. Variable-frequency driven (VFD) motors are efficient, energy-saving, and offer precise control.

• The use of two-stage (AC-DC-AC) and single-stage (AC-AC) power electronic converters in VFD motors are viewed as non-linear loads connected to the power system and are a major source of harmonics that distort the voltages and currents.

• The PWM carrier frequency is significant in reducing the harmonics in the output currents and voltages of the VFDs. As the switching frequency increases, the smoothness of the voltage and current waveform increases, reducing the THD content of the motor control interface. The switching frequency of the VFDs is selected as 10 times the required fundamental frequency of the motor input control voltage.  

The air compressor shown above is driven by VFDs in order to save energy and maximize efficiency.

On a recent visit to a fertilizer industry, I noticed the engineers had fixed Variable Frequency Drives (VFDs) for all exhaust fans in the unit. When I asked an engineer why VFDs were being used for exhaust fans, they replied that VFDs save energy. As the engineering field becomes more aware of the impact of engineering on the environment, and the importance of sustainable design, VFDs become more in demand. The global industrialization trend has also contributed to the enormous increase in demand for VFDs in processing and manufacturing units. 

VFDs are power-electronic converters that supply input voltage and current to motors. The converters are controlled so as to supply variable voltage / variable frequency (VVVF) input to the motors for speed control. The shape of the current and voltage gets distorted in the process of achieving variable frequency or variable voltage. The harmonic distortions in VFD propagate from the drives to electrical systems connected in the same power line and affect the functioning of neighboring systems. They also tend to lower the power factor of the system. 

Advantages of VFDs

In industry, a majority of equipment is considered under the larger umbrella category of “electric motors.” The operation, service, and maintenance of these motors are of great importance when it comes to total energy consumption in the industry. Induction motors are the most frequently used motors for industrial applications. 

While these motors can operate well with AC utility supply, VFDs are used in motor based process systems for a variety of reasons:

1. Energy-saving: Consider the exhaust fans in the fertilizer industry, employed for removing carbon dioxide (CO2) from a chemical process. Depending on the quantity of raw materials given to the chemical process, the amount of fumes produced varies. When the fumes are less, it is unnecessary to run the exhaust fans at full speed. The speed of the fan is directly dependent on the volume of CO2 expelled from the process to the tank. The speed and torque control of the fan can save a lot of energy over a year, and these savings reflect on the annual profit of the plant. 

2. Production and manufacturing requirements: The various processes in the industry require motor operations such as hot rolling, screwing, mixing, pumping, material handling, conveyors, etc. Each process requires different motor operating speeds. If the speed is not precisely controlled, end-product quality can deteriorate. For instance, the movement of a conveyor belt carrying glassware is different from a conveyor belt carrying metal ores. According to the application of the motor—and the nature of the product—the motor speed control is inevitable. 

Variable Frequency Voltage Supply at the Expense of Harmonic Distortions in VFD

The AC motor speed control is realized by supplying variable frequency voltage to the stator. The synchronous speed (NS) of the motor is represented by the following equation, where f is the frequency of the input voltage and P is the number of poles: 

Since the number of poles of a machine is constant, the next best method to vary the speed of the motor is by supplying a voltage of variable frequency. 

Almost all industries rely on utility power for powering their manufacturing units. Some exceptions—like captive power plants and renewable power generation—exist. However, we go with the majority. The motors and pumps are connected to the fixed 50 or 60 Hz supply only, but a black box that we call the VFD is included between the utility and the motor input terminals. This VFD can be either of the following:

1. Rectifier and PWM inverter- The fixed frequency supply from the utility grid is rectified using controlled or uncontrolled rectifiers and fed into voltage source or current source inverters. These inverters are Pulse Width Modulated (PWM) to transform the DC voltage into an output AC voltage of required frequency and voltage. The AC motors are supplied by this voltage, and its speed varies with the frequency of the DC-AC converter (inverter) output. 

2. Cycloconverter- The controlled/uncontrolled rectifier-PWM inverter (AC-DC-AC) is a two-stage converter topology. The number of components and control is more comparable to a cycloconverter when the rectifiers are controlled type. The cycloconverter is a power-electronic converter that converts fixed voltage-fixed frequency AC voltage into variable voltage-variable frequency. The cycloconverter is an AC-AC converter with which proper PWM techniques give required frequency voltage to the motor input lines. Maybe a bonus tip here for the reader. Where is each type of controller likely to be found? By application, drive strength, etc?

In both of these VFDs, the supply voltage or current shape is disturbed in the mission controlled in a manner to achieve variable frequency. VFDs are non-linear loads supplying variable frequency voltage to the motor side and harmonics to the supply side. The switching operations in the inverters for changing the frequency makes the utility currents and voltages non-sinusoidal in nature. 

Depending on the type of converters and switching frequency, the order of harmonics varies.  The harmonics from the VFDs move towards the supply side due to low source impedance. In an interconnected power system, the harmonics from various sources reach the utility supply-side and increase the THD in the currents and voltages. 

As an example, taking a six pulse uncontrolled rectifier for testing, we can see that it produces 30% current harmonics with 5th and 7th being the dominant harmonic frequencies.  Apart from the main task of precise speed control, VFDs act as harmonic sources in the power system.

Influence of Switching Frequency on Harmonics

Even though VFDs help in employing a precise control of speed and torque in AC motors, the motor input current and voltages get distorted in shape. The non-sinusoidal nature of the voltages and currents introduce harmonics into the power system, and destroy the concept of “clean electricity.” 

In earlier days of power-electronic technology, it was a herculean task to achieve sinusoidal voltage output. The difficulty of switching solid-state switches limited the number of turn-ons and turn-offs (power pulses) in a cycle. Engineers compromised with square-wave output voltage in the initial days of power-electronic engineering. If you write To see the magnitude and harmonics of the control signal, the Fourier series for a  square wave with time period T, it will turn out as the following equation, where the magnitude of the square wave is V, and is the angular frequency corresponding to the time period T:  

By observing the Fourier series, we can conclude that the use of square wave inverter VFDs introduces odd harmonics—mainly 3rd, 5th, 7th—into the power system. 

As the output inverter voltage shape gets closer to pure sinusoidal, the harmonic content in the waveform decreases and thereby THD decreases as well. The switching frequency of the PWM inverter in VFD can reduce the THD content. With the advent of controller and driver technology, high switching frequency is achieved in VFDs. The high switching frequency causes frequent turn-on and turn-off of the power-electronic switches, thereby making current and voltage closer to sinusoidal. The high switching frequency increases the switching losses in the thyristors, MOSFETs, or IGBTs present in the VFDs—but with a harmonic-free or harmonic-less output voltage. 

The high switching frequency of the PWM inverter in VFDs comes with both pros and cons. The pros of the high carrier frequency PWM inverter overcomes the cons, such as switching losses and EMI noises. The increase in the carrier frequency not only improves the THD, but also enhances the motor efficiency and lifetime of the motor. 

When you design a VFD, in order to improve the motor efficiency and harmonics, the switching frequency should be selected as ten times the required fundamental frequency of the motor input voltage. When designed this way, the VFD makes the inverter module compact, as the inductor and capacitor size goes down with the increase in PWM frequency. If your design does not take into account harmonic generation, the reliability of VFD inverters will be reduced by the increased harmonics. Don't let harmonics overrule the accurate speed adjustments provided by VFDs.

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