Since the rotational speed of an induction motor is linearly dependent on the alternating current frequency of the electricity powering it, a device which changes the frequency of the electricity can be used to control the rotational speed of the motor. A variable frequency drive (VFD) is used for this purpose.

Several years ago, VFDs were expensive and unreliable. DC motors were most often used in applications requiring speed control - the speed of a DC motor is proportional to the voltage applied so speed control is a simple matter of adjusting the input voltage with an AC to DC converter. The only problem was that DC motors require more maintenance than AC motors due to the carbon brushes used in their operation. Advances in transistor manufacturing and computer processing power have reduced the cost and increased the reliability and functionality of VFDs, allowing for the use of lower maintenance AC induction motors in most variable speed applications.

The heart of a VFD's operation is AC to DC conversion followed by DC to AC conversion at a specified frequency. A simple rectifier circuit does the AC to DC conversion. Switching back to AC is a more complicated affair.

Using transistors to switch the DC power on and off quickly, a VFD can produce a lower effective voltage than what is available on the DC bus. For example, by repeatedly turning the power on for three one-thousandths of a second and off for one one-thousandth of a second, an output of 75% of the DC bus voltage is generated. This is called pulse width modulation. By varying the pulse width modulation over time, a VFD can approximate a sine wave output (approximate because the output is digital, so the curves are not smooth). The ability to generate a sine wave output at an arbitrarily specified frequency is what allows a VFD to control the speed of a motor.

The rate at which the pulse width modulation occurs is called the carrier frequency, which is generally in 2 kiloHertz to 20 kiloHertz range. The higher the carrier frequency, the smoother the output sine wave will be. This reduces strain on the motor but increases strain and lowers the efficiency of the VFD. Note that the 2 kHz to 20 kHz range is in the band audible to human hearing, so frequency drives make a high-pitched whine when operating. Higher frequencies are less noticeable.

One advantage of using a VFD that results from the AC to DC conversion is that it reduces the effect that motor has on a system's power factor. A motor, being a large inductive load, creates a lagging power factor in an electrical system. This must be compensated for with power factor correction capacitors or the electric company will impose fines as a penalty. A VFD prevents the rest of the power system from "seeing" the inductive load of the motor by isolating it behind a DC buss, so it has little effect on overall power factor.

The biggest drawback to VFDs is that the non-linear load of the AC to DC converter produces harmonics in the power system. Harmonics are high frequency noise in the electrical system which cause problems with power quality, since many devices such as transformers are designed to be operated with clean 60Hz power. Large VFDs or several small VFDs can produce severe harmonics which can overheat motors and transformers, cause efficiency losses in a power system, and make certain clocks (which detect the 60Hz of the incoming electric power to keep time) run fast. This can be prevented by installing filters on the feed side of the VFD.

Modern frequency drives do much more than adjust the speed of a motor. Many models are equipped with a wide range of functions and optional hardware.

Some examples of what is available:
• PID controllers - to control the motor based on an external signal, similar to a car's cruise control
• Feedback encoders - to report the exact speed of the motor for better control
• PLC communication cards - to provide automated control from another system
• DC injection braking - to stop the motor quickly by injecting DC current into the stator
• Torque control - to control the output power of the motor
• Acceleration control - to control the rate at which the motor changes speeds
• Soft starting - a combination of torque and acceleration control, used to prevent a strong motor from damaging a system when it starts
• Digital and analog inputs and outputs - to use the VFD as a self-contained control computer
• Zero speed control - to prevent the motor from turning, holding a machine in a stable position
• Error reporting - to keep a record of things like power quality and assist in troubleshooting
• Auto-tuning - to provide better control by analyzing how a motor reacts to the VFD outputs

If you are looking for VFDs in a catalog or guide of some sort and cannot find them in the motor control section, try looking under power supplies.

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