The Difference Between Variable Speed Drives and Variable Frequency Drives and How They Increase Energy Efficiency for US Industrial Manufacturers


U.S. industrial manufacturers rely on electric motors, which play a pivotal and costly role in production. Manufacturers can significantly reduce their energy consumption using electric motor speed controls.

Variable Speed Drives (VSDs) and Variable Frequency Drives (VFDs) are motor control systems that adjust the speed of electric motors. While some interchangeably use the terms, they are different from each other.

The Benefits of VSDs and VFDs

Both VSDs and VFDs decrease unnecessary power consumption and increase the energy efficiency of electric motors by manipulating the power the motors receive. However, when it comes to application, the two offer different methods.

VSD refers to alternating current (AC) drives or direct current (DC) drives, while VFD refers to only to AC drives. Moreover, VSDs vary a motor’s speed by changing the voltage, and VFDs vary a motor’s speed by changing the frequency.


VSD is a broad term encompassing various devices designed to control the speed of an electric motor, including other methods such as mechanical gearboxes and hydraulic drives. By varying a motor’s speed, VSDs can precisely control a motor’s speed control to operate conveyors, fans, and pumps more efficiently.


VFDs are solid-state electronic devices that change a motor’s speed while maintaining magnetic flux. The advantageous method of VFDs enables manufacturers to increase energy savings and expand equipment life.

VSDs Versus VFDs

VSDs and VFDs share many similarities and help electric motors become more efficient by lowering unnecessary energy consumption while saving manufacturers money — however, each has its strengths and weaknesses.

Torque in VSDs and VFDs

Torque plays a critical role in the operation of motors, and its significance varies based on the application. To address these diverse torque requirements, VSDs and VFDs come into play. VSDs are ideal for constant torque applications, as they can maintain a consistent torque level even when the motor’s speed changes, ensuring the equipment and users remain unharmed. VFDs are suited for variable torque applications, enabling adjustments to torque output per the motor’s speed to prevent machinery damage or operator injury.

Horsepower in VSDs and VFDs

The role of horsepower in VSDs and VFDs also varies depending on the application. VSDs maintain a linear relationship between horsepower and revolutions per minute (RPM), with higher RPMs resulting in increased horsepower and vice versa.VFDs are used when constant horsepower loads are necessary, as they generate higher torque at low speeds and lower torque at high speeds. In cases where continuous horsepower is not required, VFDs can adjust the horsepower relative to the speed through frequency changes.

Process Control in VSDs and VFDs

Regarding process control, VSDs control motor speed independently of torque, limiting their ability to affect system changes beyond speed adjustments. VFDs offer numerous advantages over VSDs in optimizing process control. These advantages include system optimization through data exchange, control of various components, enhanced communication capabilities, and integration with the Industrial Internet of Things (IIoT) for predictive maintenance and efficiency improvements.

Three types of VFDs

There are three types of VFDs, each with its advantages and limitations. Understanding the distinctions between these VFDs allow for informed selection based on specific application requirements. They include current source inverter (CSI), pulse width modulation (PWM), and voltage source inverter (VSI):

● CSIs regulate current and possess regenerative capabilities, making them reliable but less suitable for high dynamic response.
● PWMs are the most common, offering high-efficiency versatile ranges but may encounter heat issues.
● VSIs regulate DC voltage using a six-step output, suitable for voltage regulation but can cause power harmonic feedback and poor input power.

How VSDs and VFDs Increase Energy Efficiency for US Industrial Manufacturers

VSDs and VFDs are crucial in increasing energy efficiency for US industrial manufacturers. These advanced motor control technologies offer significant benefits by optimizing motor performance and reducing energy consumption in various industrial applications.

One of the primary ways VSDs and VFDs enhance energy efficiency is by allowing motors to operate at variable speeds. Traditionally, motors operated at a fixed speed, often leading to wasted energy. With VSDs and VFDs, the motor speed can be adjusted according to the application’s specific requirements.

The Energy Efficiency Advantages of VFDs

There is a distinction that lies in the efficiency and energy-saving capabilities of VSDs and VFDs. VSDs do not offer the same level of energy efficiency as VFDs, especially in applications with varying load demands. VFDs significantly reduce energy consumption by adjusting the motor’s speed to match the load requirements.

VFDs are also known for their advanced control features, including precise speed regulation, torque control, and soft starting and stopping of the motor.

When motors start at full voltage, they experience a surge in power demand, resulting in higher energy consumption and mechanical stress. VFDs gradually ramp up the motor’s speed, reducing the initial power spike and avoiding potential damage to the equipment. Similarly, during a shutdown, these drives decelerate the motor smoothly, preventing sudden power cut-offs and potential voltage dips.

These functionalities make VFDs suitable for applications where smooth acceleration and deceleration are crucial to prevent mechanical stress and wear on the equipment.

Energy Efficiency Incentives for VSDs and VFDs

Incentives and energy-saving initiatives from governments and utility companies also promote the adoption of VSDs and VFDs in the US industrial sector. Many regions offer financial incentives, tax credits, or rebates to industrial manufacturers that invest in energy-efficient technologies. These incentives and the long-term cost savings realized through energy efficiency encourage industrial manufacturers to upgrade their motor control systems and embrace VSDs and VFDs.

Contact APenergy Today

APenergy has 35 years of experience working with industrial manufacturers to identify areas of inefficiency and make data-driven decisions to reduce energy usage and costs. Utilizing utility incentives and rebate programs for VSDs and VFDs is one of many ways a customer can reduce the financial impact of purchasing new energy-efficient equipment.

APenergy can help companies set energy-saving goals, track progress, and achieve favorable ROIs through our incentive management service. Our expert team is ready to assist companies in becoming energy-efficient. For more information, contact the APenergy team, the leading energy efficiency consultants, at 740-862-4112 or message us at