Flow control is often necessary for pumps and compressors working in an industrial application. Varying the speed of the pump/compressor is an excellent way to control the flow. To achieve this, the motor that drives the pumps or compressors uses variable frequency drives (VFD). VFD uses a 3-phase AC electric supply and changes the input voltage and frequency which in turns changes the speed of the compressor. For e.g. suppose, a motor works at its rated speed at a certain voltage and 50 Hz frequency. If the frequency is increased above 50 Hz, the motor will run faster than its rated speed, and if the frequency of the supply voltage is less than 50 Hz, the motor will run slower than its rated speed.

Some VFDs have an integral transformer and some have an external one, there are input filters and, in some cases, the VFDs are located at some distance from the motor because of harsh conditions. These all factors add to the losses in terms of core losses, eddy current losses, and heating losses. Figure-3 shows the losses for a motor with VFD.

Are there any other options that we can use instead of a VFD?

Yes, the other option is a hydrodynamic torque converter coupled with a planetary gear, popularly known by its registered trademark name Vorecon from Voith Turbo GmbH.

What is Vorecon variable speed planetary gear(VSPG)

Vorecon is a hydrodynamic variable speed planetary gear, for output of up to 50 MW and speeds exceeding 20000 rpm. In a drive train, It is used to control the speed of driven equipment such as pumps and compressors. It is a trademark product from Voith Turbo GmbH. In the drive train, the vorecon is located between the driver e.g a motor, and the driven equipment e.g a compressor, and pumps.

Main components and how vorecon works

The main components of a vorecon (VSPG) are A. torque converter, B. stationary planetary gear C. Rotating planetary gear D. Oil supply. These can be seen in the below image.

The input shaft is connected directly to the ring gear of the planetary gear. Thus, a large proportion of input power is transmitted loss-free to the ring gear of the planetary gear mechanically. Let us Assume this power is “A”.

A hydraulic torque converter is also coupled to the input shaft. The hydrodynamic torque converter consists of a pump wheel and a turbine wheel. There is no mechanical connection between the wheels. The power is transferred through a fluid. The power transfer from the pump wheel to the turbine wheel can be regulated through the liquid filled inside. The more liquid the more the power transfer and so on. the working of a hydraulic torque converter is shown in below video.

The pump wheel of the hydrodynamic torque converter is coupled with the input shaft and it branches off just a small proportion of the input power. This power from the pump wheel is transferred via the fluid to the turbine wheel and from the turbine wheel it is transferred to the planet carrier of the planetary gear. Let us assume this power is “B”.

The power from the ring gear “A” and the power from planet carrier “B” is added at planetary gear and the total power “A”+”B” is transferred to the sun gear and then from sun gear to the driven machine shaft.

The adjustable guide vanes control the flow of fluid through the hydraulic torque converter and as stated earlier this determines the speed of the turbine wheel or “B”. so, we can adjust the speed of the turbine wheel or power “B” as required. So the total power transmitted which is the speed of the driven machine also can be adjusted infinitely.

Below animation can be referred which shows its working in detail for a recent model Vorecon NX. though the arrangement is with planetary gear and a parallel shaft the basic working method as described in this article remains same.

Advantages of Vorecon variable speed planetary gear

(A). It combines the mechanical design with hydrodynamic power transmission which is wear-free. Thus, the service life is greatly enhanced (more than 3 times compared to VFDs per company claim) And the overall efficiency of the drive train equipped with vorecon is better (2% per company claim) than that of an electric variable speed drive.

(B). It is a single piece of equipment comprising a speed control system, gearbox, and oil supply. Thus, its footprint is also considerably (68% per company claims) less than an electric variable speed motor which may have to accommodate a transformer, electrical cabinets, cables, etc.

Disadvantages of Vorecon variable speed planetary gear

• At low speeds VSPG’s have lower efficiency compared to electric VFD’s which have high efficiency in wide speed range.
• Each driven machine needs its own fluid coupling.
• Very slow reaction time
• High torque at low speed is difficult 
• These can not be accomodated in a Gear less (high speed) solutions due to its inherent nature, where as a VFD can accommodate a gearless solution.

References:

above article is reproduced from the vorecon catalogue with minimal changes avaialbe at https://voith.com/corp-en/drives-transmissions/variable-speed-drives/vorecon.html