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