Variable-Speed Drives in PE Power

Variable-speed drives are the crucial component of motors that are workhorses for various industrial processes, from pumps and fans to conveyors and compressors. Therefore, preparing for Variable-Speed Drives in PE Power is mandatory per the NCEES® syllabus and guidelines for the PE Power exam. 

This study guide will cover all the technical aspects of variable-speed drives in PE Power, revealing how to cut energy consumption and boost efficiency using variable-speed drives. Let’s dive deep into the details.

What is a Variable Speed Drive (VSD)?

what is a variable speed drive

A Variable Speed Drive (VSD), also commonly known as a Variable Frequency Drive (VFD) or Adjustable Speed Drive (ASD), is an essential piece of equipment used to precisely control the speed and torque of electric motors in various industrial and commercial applications.

This device facilitates efficient energy management by adjusting the power supplied to the motor based on the specific requirements of its driving process.

One of the primary benefits of a VSD is its ability to control the speed and torque of the electric motor. By varying the frequency and voltage of the output AC power, the drive allows for seamless adjustment of motor performance to match the specific needs of the process.

This dynamic control optimises efficiency and prevents unnecessary energy consumption, ultimately leading to substantial energy savings.

Components of a Variable Speed Drive

Let’s have a quick look at the fundamental electronic components comprising VSDs.


The Variable Speed Drive starts its operation with a rectifier, which converts incoming alternating current (AC) power from the mains into direct current (DC). This rectification process is typically accomplished using diodes or thyristors. By transforming AC to DC, the drive ensures a stable and controllable power source.

DC Link and Capacitors

The DC power the rectifier generates is then directed to a DC link, which often contains capacitors. These capacitors play a pivotal role in smoothing the DC waveform, eliminating harmonics, and providing a stable power supply for subsequent stages. The capacitors store and release energy as needed, offering a buffer for power fluctuations.


The heart of the Variable Speed Drive is the inverter. This component converts the DC power from the capacitors to an adjustable output of alternating current (AC). The inverter achieves this transformation by utilizing insulated gate bipolar transistors (IGBTs) or other semiconductor devices.

It not only converts the DC power back to AC but also controls the frequency and voltage levels of the output, which can be precisely adjusted to meet the demands of the connected motor.

Understanding the Need for Variable-Speed Drives (VSDs)

To master adjustable-speed drives or variable-speed drives in PE Power, it is necessary to understand the vitality of electric motors and where these impressive drives cast their impact. Electric motors are pivotal in our daily lives and industrial operations, powering essential machinery that facilitates various business and leisure activities.

These motors rely on electricity to provide the torque and speed required for their tasks. Notably, the amount of electric energy consumed corresponds directly to the demanded torque and speed. 

In cases where these parameters deviate from the ideal values, mechanical controls are often employed to adjust or regulate the motor output. Unfortunately, this practice is inefficient, resulting in substantial material and energy wastage. Ideally, a motor’s speed should precisely match the process requirements to minimize energy wastage and promote optimal performance.

What Do Variable-Speed Drives Offer?

Variable-speed drives  (VSDs) or adjustable-speed drives solve this energy inefficiency problem by enabling precise control of either torque or speed. When VSDs are in torque control mode, the motor’s speed adapts to the load, ensuring optimal performance. Conversely, in speed control mode, the VSD modulates the torque to match the load’s requirements, enhancing efficiency.

*In the upcoming sections of this study guide, we will go into further detail about how VSDs achieve this goal.

Reasons to Invest in VSDs for Energy Efficiency

Investing in variable-speed drives  (VSDs) can significantly enhance the energy efficiency of various applications, including pump systems, fan operations, conveyors, and compressors. Four compelling reasons underscore the benefits of adopting VSDs:

Improved Operational Efficiency

Automation reduces the need for manual labour, resulting in saved man-hours and reduced labour costs.

Savings on the Electricity Bill

VSDs use only the energy necessary to run motors at partial loads, potentially leading to a remarkable up to 50% reduction in the energy consumption of the entire drive system.

Savings in Capital Expenditure

Immediate reductions in electrical consumption ensure a rapid financial return on investment in VSDs, particularly in pump and fan applications, sometimes within just months of installation.

Savings in Maintenance and Spare Part Costs

Using VSDs minimizes stress on mechanical equipment during startup and operation, prolonging the equipment’s lifespan and reducing maintenance and spare part expenses.

These tangible benefits of VSDs are not exclusive to large industrial operations but extend to smaller installations. Regardless of the scale, the energy and cost-saving potential achieved through low voltage AC variable speed drive control, compared to traditional methods like throttles and valves, can be calculated using our free EnergySave Calculator for compressors, fans, and pumps.

Case for Controlling Motors with Variable-Speed Drives

Approximately 70% of industrial electrical energy consumption is attributed to powering electric motors, making them indispensable workhorses in various industrial applications. Electric motors are extensively utilized in various industrial processes, including pumps for fluid movement, fans for air circulation, compressors, conveyors, and machinery relying on rotational force.

Energy Consumption Challenges

Electric motors represent a pivotal focal point for enhancing energy efficiency, reducing carbon dioxide (CO2) emissions, and identifying more energy-efficient solutions for clients. Within industrial contexts, certain applications like pumps, fans, and compressors are frequently operated at a constant speed, with flow rate and destination control achieved through manual valve adjustments in the pipelines.

Consequently, the startup process for such systems can be labour-intensive, potentially taking up to an hour for a single operator. The associated labour costs are compounded when personnel travel substantial distances to remote sites. In some instances, automation has been introduced, but the energy consumption remains suboptimal.

Energy-Efficient Alternative

However, there exists a more energy-efficient alternative. Implementing variable-speed drives  (VSDs) in these applications yields immediate energy consumption reductions while delivering long-term competitive advantages. Variable-speed drives or adjustable-speed drives can be applied to any electric motor process, offering significant monetary savings and operational benefits.

Practical Examples

For example, let’s consider a scenario where a pumping station is responsible for transporting fluids over varying distances. Maintaining a constant speed for the pump traditionally results in energy wastage during periods of low demand. By integrating a variable speed drive, the motor can adjust its speed to precisely match the required flow rate, significantly conserving energy and reducing operating costs.

Types of Variable-Speed Drives

These 5 standard variable-speed drives cater to various applications and offer unique advantages depending on the system’s requirements.

Variable Frequency Drives (VFDs)

Variable Frequency Drives, commonly known as VFDs or AC drives, are sophisticated electronic devices designed to control the speed and torque of AC motors. They operate by varying the frequency and voltage supplied to the motor. VFDs are instrumental in applications involving AC motors, such as pumps, fans, conveyors, and HVAC systems.

VFDs convert the incoming AC power to DC through rectification. They then produce a variable-frequency AC output by manipulating the DC supply. This precise control over motor speed allows VFDs to save energy by matching the motor’s speed with the varying demands of the process, resulting in lower energy consumption. Additionally, they minimize wear and tear on mechanical components.

Use-Cases: VFDs are commonly employed in applications involving AC motors. For instance, using a variable speed drive in pumps, fans, conveyors, and HVAC systems.

Beneficial Edge:

  • VFDs provide precise control over motor speed and torque.
  • They enable significant energy savings by matching motor speed with varying demand, reducing overall power consumption.
  • VFDs also extend the lifespan of mechanical components by reducing wear and tear.

Variable Voltage Drives (VVDs)

Variable Voltage Drives, or VVDs or voltage controllers, are specialized devices that control the voltage supplied to an AC motor. They are handy in HVAC systems, water pumps, and some industrial machines where constant-speed motors are prevalent.

VVDs work by varying the voltage supplied to the motor, regulating its speed and torque. These drives improve the efficiency and control of older, constant-speed motors, enhancing their performance and reducing energy consumption in compatible systems.

Use-Cases: In HVAC systems, water pumps, and some industrial machines, particularly those with older, constant-speed motors, using a variable-speed drive is a standard practice.

Beneficial Edge:

  • VVDs offer improved efficiency and control for older motors, enhancing their performance.
  • They can contribute to reduced energy consumption in compatible systems, leading to cost savings.

Direct Torque Control (DTC) Drives

Direct Torque Control (DTC) drives are advanced motor control systems that precisely and rapidly regulate torque and speed. These drives are commonly used in precision applications, including robotics and CNC machines, where quick and accurate torque adjustments are essential.

DTC drives achieve exceptional control through complex algorithms, motor current, and voltage measurements. They rapidly and accurately adjust motor parameters, resulting in precise torque management. As a result, they enhance productivity and minimize energy wastage.

Use-Cases: Using a variable speed drive like a DTC drive for precision applications like robotics and CNC machines is very common. They are ideal for operations that require quick and precise torque adjustments.

Beneficial Edge:

  • DTC drives provide fast and accurate motor control, enhancing productivity in high-precision applications.
  • Their precise torque management minimizes energy waste, optimizing efficiency.

Servo Drives

Servo drives are a specialized class of variable-speed drives designed for high-precision control and dynamic positioning applications. They are commonly employed in robotics, CNC machining, and automation systems.

Servo drives operate in conjunction with high-performance servo motors. These drives use encoder feedback to provide exceptional accuracy, speed, and torque control. They are ideal for applications where precise and dynamic positioning and motion control are paramount.

Use-Cases: The most common example is using a variable speed drive like a servo drive in robotics, CNC machining, and automation systems. They excel in applications demanding high precision, dynamic positioning and control.

Beneficial Edge:

  • Servo drives offer exceptional accuracy, speed, and torque control, making them ideal for applications where high-performance control is essential.

Pulse Width Modulation (PWM) Drives

Pulse Width Modulation (PWM) drives are cost-effective solutions for variable speed control in various applications. They work by modulating the width of voltage pulses to the motor.

PWM drives generate a variable voltage by rapidly switching the voltage on and off. This modulation technique provides an economical means of controlling motor speed. PWM drives are versatile and suitable for a broad spectrum of industries. Their ability to precisely modulate motor power results in enhanced energy efficiency and reduced energy consumption.

Use-Cases: PWM drives are cost-effective solutions for applications requiring variable speed control, such as conveyor belts and other motor-driven systems.

Beneficial Edge:

  • These drives are cost-efficient and widely applicable for motor speed control, making them suitable for various industries.
  • PWM drives enhance energy efficiency through precise modulation, reducing power consumption.

Working Principle of variable-speed drives (VSDs) in Motor Speed Regulation

The steps below will help you understand how VSDs control and regulate motor speed.

Step 1: Rectification of AC Power

rectification of ac power

The initial stage in operating a variable speed drive (VSD) involves converting incoming 3-phase AC power to DC power through rectification. This rectification follows the principles of diode rectifiers, which leverage the unidirectional conductance of diodes. 

*According to the laws of rectification, the diodes allow current to flow in a single direction, effectively changing the AC waveform to a unidirectional DC waveform. This DC power is then ready for further processing.

Step 2: DC Waveform Smoothing

dc waveform smoothing

The rectified DC power is subsequently passed through capacitors, a component of the VSD, where it undergoes smoothing and voltage regulation. 

*This step adheres to the principles of capacitive filtering and energy storage in compliance with the laws of capacitance and the voltage-current relationship.

Capacitors store energy as an electric field, effectively reducing the ripple and maintaining a stable, clean DC supply. This cleaned DC power is the basis for generating the variable AC power required to drive the motor.

Step 3: Generation of Variable AC Power

generation of variable ac power

The VSD calculates the motor’s precise voltage and current requirements in this phase. Now stabilized and regulated, the DC power is subjected to an inverter. 

*This inverter operates following the laws of semiconductor physics and pulse-width modulation (PWM) techniques. 

By leveraging power electronic devices like insulated gate bipolar transistors (IGBTs), the inverter transforms the DC power back into AC power with a variable frequency and voltage. The VSD ensures that the produced AC power aligns precisely with the motor’s requirements, following the principles of electromagnetic field theory and Faraday’s law of electromagnetic induction.

Step 4: Continuous Control and Optimization

continuous control and optimization

The core function of the variable speed drive lies in its ability to continuously calculate and adjust the frequency and voltage of the generated AC power. This dynamic regulation, known as vector control, ensures that the motor receives only the power—both in terms of speed and torque—that is precisely needed for the ongoing process. 

*The laws of control theory, particularly proportional-integral-derivative (PID) control, are instrumental in this phase. 

By carefully measuring motor performance and comparing it to the desired setpoints, the VSD makes real-time adjustments to maintain efficiency, minimize energy consumption, and safeguard optimal motor operation.    


Now you have a rich idea about Variable-Speed Drives in PE Power and how they offer efficient control for electric motors. If you’re gearing up for the PE Power exam preparation, you can find all the comprehensive resources at Study for FE, including the PE Power Practice Exam.

Study for FE is your go-to place for fast-paced and to-the-point PE power exam preparation. Connect with us today and give your exam preparation a most awaited boost.


Licensed Professional Engineer in Texas (PE), Florida (PE) and Ontario (P. Eng) with consulting experience in design, commissioning and plant engineering for clients in Energy, Mining and Infrastructure.