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What are the benefits of using PWM to control the output voltage and frequency of a VFD?

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If you work with variable frequency drives (VFDs), you probably know that they can adjust the speed and torque of an electric motor by varying the output voltage and frequency. But how do they do that? And what are the benefits of using a specific technique called pulse width modulation (PWM) to control the output waveform? In this article, you will learn what PWM is, how it works, and why it is useful for VFD applications.

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Ian Brough

Area Sales Manager @ Banner Engineering | Automation I Data collection and IIOT solutions
Tahir Mehmood Awan

Technical Sales & Operations Engineer
Xavier Montesdeoca

Field Service Specialist at Schlumberger

1 What is PWM?

PWM is a method of creating a variable output voltage from a fixed input voltage by switching it on and off rapidly. The ratio of the on time to the off time is called the duty cycle, and it determines the average output voltage. For example, if the input voltage is 10 V and the duty cycle is 50%, the output voltage will be 5 V on average. By changing the duty cycle, you can create any output voltage between 0 and 10 V.

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2 How does PWM work in a VFD?

A VFD uses PWM to create a sinusoidal output waveform from a direct current (DC) input. It does this by using an inverter, which is a device that converts DC to alternating current (AC) by switching transistors or other electronic components. The inverter can control the frequency and amplitude of the output AC by varying the switching frequency and duty cycle of the PWM. For example, if the switching frequency is 10 kHz and the duty cycle is 50%, the output frequency will be 5 kHz and the output voltage will be half of the DC input voltage.

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Dominik Kuhnle

Sales Professional - Motion Control
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The frequency inverter consists on the input side of a rectifier, which is powered by a three-phase network with constant voltage and frequency. This rectifier generates a constant direct voltage, also referred to as intermediate circuit voltage, which is smoothed by intermediate circuit capacitors. The resulting intermediate circuit voltage is converted by the inverter on the output side using IGBT technology. This creates a three-phase system with variable voltage and frequency. By adjusting voltage and frequency, the speed of the connected three-phase motor can be continuously and almost losslessly changed.

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3 Why is PWM useful for VFDs?

PWM offers several advantages for VFD applications, such as precise and smooth control of the motor speed and torque, reduced harmonic distortion and noise in the output waveform, reduced stress and heat on the motor and VFD components, and the use of smaller and cheaper filters and capacitors. These advantages improve power quality and efficiency, extend the lifespan of components, reduce maintenance costs, and reduce the size and cost of the VFD.

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Ian Brough

Area Sales Manager @ Banner Engineering | Automation I Data collection and IIOT solutions
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In my experience PWM is an easy way to reduce problems due to electrical noise and poor grounding. It is a good alternative to w expensive analog circuitry.

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4 What are the challenges of using PWM in VFDs?

PWM can be challenging to use in VFDs, however. Common issues include electromagnetic interference (EMI), overvoltage spikes or ringing in the output waveform, and higher switching frequencies. To prevent EMI, proper shielding and grounding is required. Snubbers or surge suppressors can help to avoid overvoltage spikes or ringing. To minimize switching losses and heat generation, efficient cooling and heat dissipation is necessary.

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Dominik Kuhnle

Sales Professional - Motion Control
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High-frequency disturbances primarily arise from the rapid switching of IGBTs in the motor-side inverter, causing steep voltage edges. Each edge generates pulse-shaped disturbance currents through parasitic capacitances. These currents flow through motor lines and windings to the ground and must return to the inverter. On this return path, the disturbance current influences the earth and network impedance, consisting of transformer impedance and parasitic capacitances of the grid supply line. The resulting voltage drops can affect the grid voltage and other connected devices. It is crucial to take appropriate measures to minimize these disturbances and ensure system reliability.

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5 How to optimize PWM for VFDs?

When optimizing PWM for VFD applications, you need to consider the motor type and characteristics, such as the power rating, speed range, load profile, and efficiency. Additionally, take into account the VFD type and features, including the input voltage, output voltage and frequency range, switching frequency, modulation scheme, and protection functions. You should also consider the system requirements and constraints like power quality, EMI, noise, temperature, and space. With the right parameters and components for your PWM-based VFD, you can ensure optimal performance and reliability for your motor control system.

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Tahir Mehmood Awan

Technical Sales & Operations Engineer
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How can we configure VFD for Tubewell To be energized by Solar having HIgh DC Sting Voltages.? What are the parameters need to be set Or either VFDs have the same functions for all AC/DC motors or not? Or there different types of VFDs being used in market?

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Dominik Kuhnle

Sales Professional - Motion Control
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Manufacturers of frequency converters typically provide suitable optimization methods. For instance, standing or rotating measurements are recommended for asynchronous motors. In the realm of synchronous motors, speed controller optimization methods are employed today, applying specific noise signals to the system to calculate the control path. It is crucial to emphasize that blanket statements are not possible, as optimization always depends on the specific application.

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6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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Xavier Montesdeoca

Field Service Specialist at Schlumberger
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We should be aware that the VSD using the PWM, will produce high THD when the out put frequency is far away of the input frequency, instead the THD will be low if the output frequency is close of the VSD input frequency.

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Dominik Kuhnle

Sales Professional - Motion Control
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Die Frequenz ist entscheidend: so wird durch die Reduktion der Pulsfrequenz die Stromwelligkeit erh?ht. Dies kann bei kleinem Tr?gheitsmoment eine Vergr??erung der Drehmomentwelligkeit an der Motorwelle zur Folge haben und auch zu einer Erh?hung der Motorger?usche führen. Ein weiterer Punkt ist das eintretende Strom-Derating bei hoher Pulsfrequenz

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What are the benefits of using PWM to control the output voltage and frequency of a VFD?

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1 What is PWM?

2 How does PWM work in a VFD?

3 Why is PWM useful for VFDs?

4 What are the challenges of using PWM in VFDs?

5 How to optimize PWM for VFDs?

6 Here’s what else to consider

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