MECHANICAL VARIABLE SPEED DRIVES

COMPARING A MECHANICAL TO AN ELECTRICAL

VSD FOR USE IN TURBOMACHINERY

By Amin Almasi

Mechanical variable speed drives (mVSD), particularly a hydrodynamic torque converter or fluid coupling, can soften the transmitted load and damp the load peaks and load pulsations generated by connected devices, such as electric motors. An mVSD can also soften large dynamic torques in transient situations, such as a start-up or short circuits.

There have been many designs for mVSDs in industrial applications over the years. Before the advances in electrical VSDs (eVSD) systems, a wide range of mVSD systems were designed and used for various services and applications. Some have left the market; others were modified for use in specific applications.

In addition, new mVSD designs have been introduced as not all applications can be served by eVSDs. Examples of available mechanical drive options are:

1. MVSD speed control and speed increase for high-speed output; this is used for medium and large machinery

2. Geared variable-speed coupling: Cheaper than option 1, but lower efficiency at part-loads might be expected

3. Variable-speed coupling: Speed control with no speed increase; relatively cheap, but less efficient.

For example, in a medium-sized, electric-motor-driven machinery train, transient torques are reduced from around 210% of the nominal torque for an electric-motor-driven shaft down to around 105% of the nominal torque with a driven machinery-to-mVSD coupling. A short-circuit (transient) torque is also damped through the mVSD from an excitation above 380% down to around 155% of the nominal torque on the driven machinery shaft.

mVSD vs eVSD

There can sometimes be design, commercial, and operational advantages for an mVSD compared to other options, such as an eVSD in specific applications. A smaller footprint is usually required for an mVSD.

An mVSD does not generate harmonic pulsations, which can be a problem in some Variable Frequency Drive systems. An mVSD also offers vital mechanical damping and softening effects to counter disturbances.

For an mVSD, though, some unknowns are expected: special commissioning and alignment procedures are required; and there are limited options available to the user. But based on experience, the reliability and availability of properly designed and applied mVSDs has been satisfactory.

Like any complex mechanical system there are limits. The reliability of an mVSD cannot be higher than a certain level, in spite some manufacturer claims. As a rough indication, the run time before an unexpected shutdown could be around one or two years for some mVSDs. Unexpected shutdowns are mainly related to bearings and oil systems.

A reference check is important for mVSDs. Make sure the same model of mVSD has already been used in the intended type of turbomachinery and that it is operating successfully over the long term in similar applications.

The best range for mVSD applications is in medium-sized applications of 500 kW to 4 MW. For small power ratings and low-voltage electric motors an electrical VFD is almost always preferred as it is cheaper than an mVSD.

The use of mVSDs as hydraulic torque converters is not recommended for large turbomachinery above 8 MW. There may be a handful of successful applications of that size, but this range is dominated by other technologies such as large eVSDs. Therefore, the limit of mVSDs, depending on the application, might lie somewhere between 5 MW and 8 MW.

Overall, mVSDs are special variable speed systems which should only be used in certain applications. They should provide definite technical, commercial, or footprint benefits. In those systems where there are constrains on space, weight, or budget, as well as in some special revamp or renovation projects, mVSDs may provide obvious value.

As mentioned earlier, they are cheaper, though the exact percentage varies depending on how the mVSD is being used. There are also differences between the rates charged by manufacturers. But generally speaking, an mVSD will be anywhere from 5% to 25% cheaper than an electrical VSD.

Any comparison between an mVSD and an eVSD should always consider the details of the application. The speed, power, operating characteristics, footprint, and other factors have to be carefully considered. Any final decision concerning the appropriate variable speed technology should always be based on purchase costs plus estimates operational costs.