What is a VFD, and How Does it Work?

If you've been around AC electric motors for a while, you've probably heard about Variable Frequency Drives (VFDs). You might be wondering what exactly they are and what kind of value they can offer to you.

The short answer is that VFDs are speed controllers for AC motors. Fitting one to your motor can provide you with substantial savings on power bills, improved performance, and protection of your valuable motor and equipment.

Let's take an in-depth look at exactly how VFDs work and how you can benefit from them.

AC Motor Speed Control

AC motors, especially induction-type motors, are the go-to workhorse motor for industrial applications. Cost-effective, reliable and long-lasting, they're found in many different settings, operating critical machinery in diverse conditions. Despite these advantages, one thing has traditionally been challenging to achieve: precise speed control. Unlike DC motors, an operator cannot control AC motor speed by adjusting the voltage. The frequency must be changed instead, which is more complex to achieve.

Various types of VFDs have emerged to solve this problem. In recent years, the development of advanced vector control type VFDs has provided operators with precise speed control rivalling the best DC motor drives. Let's take a look at how they work.

How Do VFDs Work?

By default, AC motors run at speeds determined by the line frequency (usually 60Hz) and speed control requires a means of changing that frequency. But, since the motor's torque is influenced by the ratio between the frequency and the voltage, a suitable speed controller must adjust them together. There are two distinct types of VFDs that achieve this in different ways.

Volts-Per-Hertz (V/Hz) VFD

The Volts-Per-Hertz (V/Hz) VFD is the simplest VFD type; it modifies the voltage and frequency. First, the rectifier converts the AC power to DC power. Second, the DC voltage is adjusted so that the voltage/frequency ratio will remain constant, typically using a pulse-width modulation (PWM) circuit. This DC pulse is intended to look like AC frequency and can be varied up or down. This way, the speed of the motor is adjusted with minimal effect on the torque.

This VFD design gives you reasonable speed control, down to a ratio of 20:1 of the motor's base speed. It has the unique advantage of enabling multiple electric motors to operate synchronously from the same control. However, because the voltage-frequency ratio required to maintain the same torque is not constant at lower speeds, precision falls dramatically. This makes V/Hz VFDs suitable only for applications requiring 'loose' speed adjustment near the base motor speed, such as pumps and fans.

To enable full speed control over the entire speed range, the Vector Control VFD was developed.

Vector Control VFD

A Vector Control VFD operates in a similar way to a V/Hz drive, except that it manages the relationship between the voltage and frequency in a much more precise way using a complex mathematical algorithm. This enables the voltage/frequency ratio to be managed dynamically as speed is reduced, maintaining consistent torque as the speed approaches zero.

Vector Control VFDs offer excellent speed control at speeds as low as 1000:1 of base speed and enable the motor to provide as much as 200% rated torque at start-up, giving your motor drive a far better response at low speeds. This makes them useful for precision applications such as textile manufacturing, CNC machines, and robotics – applications where DC motors have traditionally been the only suitable choice.

Encoders

Without an encoder or other feedback device, a VFD is considered an 'open-loop' system. Meaning you cannot guarantee your motor is turning at the desired speed since it only operates on the input power without feedback from the motor. To correct this, both VFD types can be made 'closed-loop' by fitting it with an encoder, which provides feedback on the actual motor speed and position.

With an encoder, a V/Hz VFD can achieve better speed control at lower speeds, but it will still reach a limit where speed control breaks down. By contrast, a Vector Control VFD fitted with an encoder is fully capable of speed control down to 0 rpm and can provide 200% of rated torque as holding torque. This makes it suitable for applications requiring control at all speeds.

Benefits of VFDs

Fitting your AC electric motor with the right VFD gives you a range of benefits that you may not have considered.

Performance and Efficiency

Without any controls, an AC motor operates at a speed determined by the line frequency. Even in constant-speed applications, this is rarely the exact speed you need. While most applications use (and require) a gearbox, it won't enable you to fine-tune things, especially if requirements or operating conditions change over time. A VFD allows you to manage the motor's speed, maximizing performance and efficiency.

Electric motors account for a substantial percentage of industrial electricity consumption. Considering this and the continual increase in regulations and legislation on energy efficiency, VFDs can provide the solution you need to keep your operation within efficiency guidelines. Additionally, you'll save on power bills and have the opportunity to take advantage of financial incentives and rebates.

Soft Starting

When an electric motor starts up under full power, it draws large inrush currents that cause the windings to heat up, risking damage to the motor. The sudden torque applied to the load can also damage bearings, gearboxes, and connected equipment. To prevent this, it's usually recommended to fit your electric motor with a soft starter. Still, VFDs can provide the same smooth acceleration and deceleration with the benefit of additional speed control.

Electrical Protection

Most VFDs come fitted with additional protective features to prevent damage to your motor. Thermal overload protection cuts power to your motor when it overheats, and voltage protection prevents damage in over- and under-voltage scenarios, both of which pose a risk to the motor windings and any connected electronics. Phase protection prevents a loss of phase or an incorrect phase sequence from damaging the motor; all of these typically come out-of-the-box with a VFD.

What Types of Motors Can I Use With A VFD?

Generally, VFDs are designed for 3-phase motors and cannot be used with single-phase motors as their winding design is not compatible. VFDs often provide the additional capability to convert a single-phase input to a 3-phase output, enabling your 3-phase motor to operate off a single-phase supply.

If you doubt whether your motor is compatible with a VFD, contact your supplier or manufacturer to double-check.

Summary

VFDs give you greater control over how your AC motor performs, enabling you to find opportunities to increase efficiency and performance and maximize productive quality. We offer a wide selection of VFDs for different application types and sizes so that you can always get the best out of your motor.

Ready to source a VFD? Find your motor control by following the steps in the below tutorial.