Compare system efficiency between Dynamatic® adjustable speed drives and Variable Frequency Drives
In the mid ‘80’s it became a common practice in the electrical equipment industry to replace Eddy Current Drives with variable frequency drives (VFD’s). Whether or not variable frequency drives were any more capable or reliable in the application usually did not enter into the discussion.
Most often the single justification for the VFD use is efficiency. VFD drive manufacturers claim their drives are 95% efficient and provide payback charts showing a rapid investment return.
The problem is the fact that is there is much more to the story. The untold story is about system efficiency, the difference between the power put into a process and the power that is delivered to a load. VFD drive manufacturers typically gloss over this. After thorough testing across multiple applications, the results achieved are very different than stated by the VFD Companies.
When you look at all the facts, Eddy Current comes out on top. Eddy Current systems actually have better system efficiency than VFDs above 82% of base speed.
Only Eddy Current Adjustable Speed Drives deliver TRUE efficiency
Eddy Current Drives utilize a DC magnetic field to link two members, one on the input shaft and one on the output shaft. Increasing the DC Current to the coil increases the coupling of the two members thus delivering more torque to the load. A tachometer is used to control the velocity and torque.
Eddy Current losses in efficiency are as follows:
- AC Motor – Equal to nameplate rating as motor is running across the line. This is true for both power factor and efficiency.
- DC Control – Typically 2% or less.
- Slip – Reduction in speed is dissipated in the drum and rotor (the coupled members). It reduces efficiency in proportion to reduction in speed.
The bottom line is that it is best to run an Eddy Current device at or near rated speed. Typically 82 – 100 % is recommended to optimize efficiency.
Variable Frequency Drives dramatically lower the overall system efficiency
VFD controls motor speed by varying the effective voltage and frequency applied to the stator of a standard AC Induction motor. We use the term effective because the applied voltage is actually a high frequency square wave, pulse width modulated (PWM) waveform that switches from bus voltage, typically 650VDC or more to 0V, thousands of times per second. This alters the effective base speed of motor, allowing variable speed operation. It also presents several negative effects both in harmonics and mechanically
A standard AC motor has a published efficiency and power factor. They are quite high, typically above 90%, but only for a sinusoidal excitation at rated frequency. On a VFD the losses are higher and the power factor is lower. These values are not widely known or published.
VFD losses in efficiency are as follows:
- IR losses – Heating is the largest loss caused by resistance to current flow in the motor winding and rotor bars. It is proportional to the square of the current flow.
- Eddy Current Losses – Losses caused by unintended current flow in the rotor and the stator. These are limited by laminations in the stator and rotor. They are proportional to current flow and increase with slip.
- Hysteresis losses – Heating created by reversing the magnetic polarity of the iron in the rotor and stator. This increases with slip.
All of the losses above become a larger percentage of output horsepower as speed is reduced.
In addition there are two other issues associated with VFD’s:
- Power Factor – VFD’s are advertised to present a power factor of near unity to the line. While true, the motor still operates with a lagging current. This is extra power that the utility would have to generate if the motor were across the line. Hence, utilities charge a power factor penalty as they only measure watts. With the VFD generating this extra current, the extra is retrieved from the line. Instead of a power factor penalty, you are forced to pay for the power up front. In addition, the power factor degrades dramatically with speed reduction, approaching 75% at half speed.
- Slip Losses – A little known fact is that an AC Induction motor is a magnetic clutch operating at a slip (against a rotating field). The slip increases under increased load, considerably more at low speeds. At a PWM equivalent base speed of 100 RPM the motor would operate at 50 RPM if its rated slip were 50 RPM (a 1750 RPM motor). Thus, torque boost (increase in voltage) is used to start under load. This slip is a loss that becomes a higher percentage of output as speed is reduced. If torque boost is used, the losses are higher still.
The bottom line is that an AC motor is quite efficient at rated speed and voltage, but the losses build as a percentage of output as speed is reduced. The power factor enters the efficiency equation and the control losses add in as well. These dramatically lower the system efficiency.