Home > Behind the MVSD vs. VFD debate: Choosing the right drive for you (Part 2)

Behind the MVSD vs. VFD debate: Choosing the right drive for you (Part 2)

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Planetary gear MVSD 
The most common type of MVSD is a combination of a torque converter and revolving planetary gears. 
This MVSD comprises a torque converter and two sets of planetary gears:

  • A superposing planetary gear for the speed variation.
  • A fixed planetary gear for the reduction of the superposing speed.
The planetary gear is usually of the helical design for smooth operation and superior running properties. The hydrodynamic torque converter is a combination of the hydraulic pump impeller, the hydraulic turbine wheel and adjustable guide vanes. 
The hydraulic pump impeller is connected to the input shaft and the hydraulic turbine wheel to the output shaft. The fluid (oil) is accelerated in the hydraulic pump impeller and transmits the energy to the hydraulic turbine wheel. 
The fluid flows through the adjustable guide vanes, where the flow and the flow angle are adjusted. Based on the position of the guide vanes (ranging from the fully-closed to the fully-open), the torque and the speed of the turbine wheel can be adjusted. The actuator can open the guide vanes and accelerate the turbine wheel to properly control the torque/speed of the MVSD system. 
The variable-speed planetary-gear type MVSD requires oil for two oil circuits:


  • The power transmission oil system (hydraulic or working oil).
  • The lubrication oil system which can provide lubrication for the train (including the bearings).
The MVSD is based on the principle of power splitting. The majority of the power is directly driven through the input shaft to the revolving planetary gear (driving the annulus gear with a fixed speed). The output shaft is connected to the driven equipment. A small part of the power is driven through the torque converter, where the speed is adjusted and then superimposed in the planetary gear. 
All three components of the planetary gear (the annulus gear, the planet carrier and the sun gear) are moving in a superposing planetary gear system.  
A variable output speed is obtained by addition or subtraction of speed which is achieved by variation of the planet carrier speed. The turbine wheel of the torque converter is connected to the planet carrier of the superposing planetary gear via the fixed planetary gear. In the superimposing planetary gear the two power flows are combined and transmitted to the output shaft. 
Only the power branched-off through the torque converter (a portion of power) is subject to the hydrodynamic efficiency of the torque converter which could result in a reasonable overall efficiency at the full-load or the loads near the full-loads. 
For a lower speed range (a speed below the normal speed), the turbine wheel acts as a hydrodynamic brake (this is a reversal of the power flow). Because of this effect, the MVSD part-load efficiency is usually lower than the VFD. 
The minimum speed limit of the variable speed gear system (MVSD) could be around 55 to 70 percent of the maximum continuous speed. Some MVSD performance limitations could be the result of the circumferential speed of the annulus gear (driven by the driver). 
Generally with increasing the speed ratio, the possible transmitted power (the power rating) is decreasing. Some limitations are usually caused by the geometry of the revolving gear. 
The higher the gear ratio, the smaller the sun wheel and the smaller the number of planets because of the intersection.  
Making the right choice 
For low voltage electric motors almost always a VFD is preferred, as it is cheaper than an MVSD. 

MVSDs, such as hydraulic torque converters, are not recommended for large rotating machines (above 18 MW). MVSDs are special variable speed systems which should only be used in a right application, where there are technical, commercial or foot-print benefits. 
For the right application, an MVSD could probably be 5 to 20 percent cheaper than a VFD. 

The decision should always be based on the optimisation of the total purchase-operation costs. 


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