Behind the MVSD vs. VFD debate

The choice boils down to whether it is low or high voltage motors in use.

The mechanical variable-speed drives (MVSD), particularly the hydrodynamic torque converters, could dampen pulsations generated by the connected machines (such as the synchronous motor produced pulsation torques).  

It is particularly true for large dynamic torques in transient situations such as start-up or the short-circuit transient excitations. This mechanical "soft" start and the dampening effects could provide a positive impact on a rotating machine train.  

Sometimes, an MVSD could be a cost effective and compact alternative to an electric VFD (variable-frequency drive) system.  

However, the behaviour of an MVSD system is very difficult to properly model or predict.

The MVSD system is obviously one of the biggest unknowns in the rotating machine industry.   

The examples of available mechanical drive options are:   

· The MVSD speed-control and speed-increase for high output speeds. This is useful in relatively high power applications, say up to 18 MW.   

· The geared variable-speed coupling. This is cheaper than the option above, however delivers lower efficiency at part-loads, usually less than 10 MW.  

· The variable-speed coupl­ing. This offers speed control, no speed increase, is relatively cheap and relatively less efficient.  

For example, in a case study for a middle size compressor train, dynamic torques are reduced by a MVSD from around 210 percent of the nominal torque (at the electric motor driven shaft) down to around 95 percent of the nominal torque at the compressor-to-MVSD coupling. A short-circuit (transient) torque is also dampened through the MVSD from an excitation above 380 percent down to around 155 percent of the nominal torque (on the compressor shaft). 

Because of the design of the MVSD packages there are some special (and unique) features in the alignment procedures for this equipment that should be understood prior to the order placement.

An MVSD could also need some special requirements in the commissioning works.

The MVSD systems (particularly the hydrodynamic torque converters) require a large amount of oil for the operation. The special oil skid provided by the MVSD manufacturer is usually an integral part of MVSD system base-plate/package. The main oil pump is most often a shaft driven pump.  

This pump should be sized properly to handle all different opera­tional situations (an ample margin and a proper match with the oil requirement cases). 

There should also be a full-sized, second start-up (and stand-by) oil pump.  Sometimes, a rundown tank or a (third) emergency DC electric motor driven backup oil pump system could be required.

A combined oil system with other equipment in the train is a compact option, which requires an excellent coordination between the MVSD manufacturer and the rotating machine vendor(s).  

It may not be a popular option for some operators (since usually a manufacturer-standard combined oil skid is offered), but if implemented properly it could result in a considerable saving. 

Complex gear system  

Another feature is the planetary gear of a MVSD (which is usually a very complex gear system).

The gear system itself is a compact unit and the drive and driven shafts are usually on the same planes (horizontally and vertically).  

However, the planetary gear unit (that is usually used) can offer some complex behaviour. An MVSD, because of its complex nature, is a nonlinear system and should be linearised for the modelling (such as the torsional vibration study, and other studies). 

The efficiency at a part-load is relatively low. MVSD options are manufactured by a few vendors.

Sometimes, very limited options for some components (such as the instruments, bearings, or mechanical parts) are used by the MVSD manufacturer, which means some deviations on the project specifications. 

Usually, a relatively long list of deviations should be accepted for an MVSD system. MVSD systems are very special and complex mechanical systems (with many manufacturer standard components), which need a large amount of oil and special oil system accessories.

There could be some design, commercial and operational advantages for an MVSD compared to other options. Less space is required for an MVSD compared to a VFD system.  

An MVSD does not generate harmonic pulsations (a problem of some VFD systems) and it offers some vital mechanical dampening effects to some disturbances.  

However, for an MVSD, some unknowns are expected, special commissioning (and alignment) procedures are required, and there are very limited options available to the user. Considering all these factors, the reliability of an MVSD cannot be higher than a certain level. 

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.

he 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 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.   

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.  

Successful examples of MVSD systems usually include special revamp/renovation projects. 

Giving an indication of the cost difference is not easy and this can change with every application (depending also on the VFD details/ manufacturer). For the right application, an MVSD could probably be 5 to 20 percent cheaper than a VFD.  

It is difficult to give a general comparison between a VFD and an MVSD, as it depends very much on the application (speed, power, operating characteristics and foot-print). 

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

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