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Eddy Current vs. VFD Technology


TEST-3-speedometer-1063350_1280 CroppedThere is more than one way to change the speed of your electric AC motor, and some of the most commonly used methods are eddy current drives and variable frequency drives (VFDs).

The Need for Adjusting Speed

Not all electric motor applications need to run at motor's standard speed. An application may need more torque (which requires a reduction in speed) or more additional speed (which in turns means a reduction in torque). A motors' speed really should match what the application needs, and when they do not, there are significant losses in efficiency. When you consider some experts' estimate that 65% of industrial power consumption comes from electric motors, you realize how much of an impact this can have on your overall plant efficiency and energy consumption.

However, if a motor is not a perfect match to the loading needs of the application, the speed and torque can be adjusted through the use of system devices (valves, dampers, etc.). That approach, however, is not ideal and still leads to wasted power and inefficiency -- which is something you normally try to avoid. An alternate, more effective solution is the use of eddy current drives or variable frequency drives (VFDs).

Eddy Current Drives

Eddy current drives go by several names including magnetic clutch, magnetic drive, and eddy current clutch. These drives are variable speed drives that are installed between a constant speed electric motor and the equipment it is driving. The technology behind them has been around for many years and they are still used in many industries and applications.

The eddy current drive uses a DC magnetic field to link the input and output shaft. The strength of the field is dependent on direct current applied to coils on the drive's rotor. As the current is increased, so is the strength of the magnetic field. This results in reduced speed and increased torque at the output shaft.

Variable Frequency Drives

VFDs go by several names as well, including adjustable speed drive, variable speed drive (VSD), microdrive, AC drive, adjustable frequency drive, and inverter.

Unlike eddy current drives which are placed between a motor and a load, VFDs are placed between the electrical supply and the motor. They adjust the speed and torque of the motor by modifying the frequency and voltage that it receives as input, a more direct approach seen with eddy current drives. Essentially, a VFD regulates the power received by the motor.

Within the VFD, the AC power for the electrical supply is rectified into DC power. Capacitors within the drive receive the DC power and smooth out the waveform. From there, the DC power enters an inverter that changes it back into AC at a frequency and voltage that adjusts the motor speed to the desired level.

Eddy Current Drives vs VFDs

Both of these drives make it possible to use your motors more efficiently, reduce your overall energy costs, and have a positive impact on your maintenance and repair costs.

VFDs allow you match the speed of the motor to the load requirements, which eddy current drives do not, and help prevent damage with unexpected voltage issues such as voltage spikes, over voltage, and under voltage. Eddy current drives, on the other hand, often claim higher efficiency gains compared to VFDs, especially at higher levels of speed reduction. Eddy current drives do not offer the same level of control as VFDs do, nor do they provide variable torque. 

Finally, VFDs are used in all industries and can work for just about any application, while eddy current drives are based on an older technology but are still used in many applications.


Modifying the speed/torque of an electric motor to what the application demand is a more efficient approach to using your motors. Both VFDs and eddy current drives have their pros and cons, but if you have any questions about which one is right for you, contact us here at HECO .

 Justin Hatfield - 269-377-7359


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