A lot more than 50% of the compression and pumping systems above 3 MW require a variable-speed drive. Frequently medium-voltage Variable Speed Gearbox motors fed by frequency converters and parallel shaft gears are selected. Others are employing epicyclic gears with a hydrodynamic superimposition drive.
Taking advantage of the state of the art in electric motors and frequency converters, the driving system for the superimposition of the earth carrier is based on a Permanent Magnet Synchronous Superimposition (SI) motor and an Active Front-End Cycle Converter. Therefore – E means Electric.
That is a high-speed gear drive system incorporating parts that allow the use of a low-cost constant-speed (electric) driver for a variable-speed application.
More than 50% of the compression and pumping systems over 3 MW need a variable-speed drive. Often medium-voltage motors fed by frequency converters and parallel shaft gears are chosen. Others are using epicyclic gears with a hydrodynamic superimposition drive.
It started with hydraulic fluid couplings producing slippage with results not much better than the throttle.
The look of hydraulic clutches then improved and included a torque converter and a superimposed epicyclic drive.
Variable Frequency Drives (VFD) were developed and, at the same time, the cost of the shaft trains increased, and so did efficiency. The issue of THD (Total Harmonic grid Distortion) rose and kept VFD out of several applications.
Taking advantage of the state of the art in electrical motors and frequency converters, the driving program for the superimposition of the earth carrier is founded on a Long lasting Magnet Synchronous Superimposition (SI) electric motor and an Active Front-End Cycle Converter. So – E stands for Electric.
This is a high-speed gear drive system incorporating parts that allow the use of a low-cost constant-speed (electric) driver for a variable-speed application.
All of the above plots were taken by analyzing the current to the motor. In a normal situation, the vibration will be monitored and kept on a periodic basis. If the 360 Hz 
signal is present in the vibration spectrum, it isn't normally of any concern. If, however, additional unexplained frequencies appear in the vibration spectra, then it would be advisable to take a look at both the current waveform and the current spectrum. In the webpages that follow, some examples of problems discovered in DC drives and motors are presented.”
It started with hydraulic fluid couplings creating slippage with results very little much better than the throttle.
The design of hydraulic clutches then improved and included a torque converter and a superimposed epicyclic drive.
Adjustable Frequency Drives (VFD) were developed and, at the same time, the expense of the shaft trains increased, therefore did efficiency. The problem of THD (Total Harmonic grid Distortion) rose and kept VFD out of many applications.