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Electric motors for turbo-compressors

article image A variable speed drive panel for a large VSD electric motor.

Traditionally, large compressors in modern plants have been driven by gas turbines. However, as Amin Almasi* writes, variable-speed electric-motor drives can be employed to do the job more economically.

The large compressors in modern plants demand drive powers in the region of 10 to 150 MW. Traditionally, they have been driven by gas turbine.
The “all electric” concept seeks to eliminate the gas turbine and use the variable-speed electric-motor drives. The main benefit from this would be the added productivity of the plant, from around 340 days for gas turbine drive systems, to a theoretical 365 days a year.

Advanced electric motors and modern compressors in the clean-gas service theoretically don’t need routine maintenance. Accounting for unscheduled outages of the compression units of 3 to 5 days per year, the compressor train still yields a minimum of 17 to 20 added production days.

This is important given modern, large plants have a production value of around $0.5 to 1.5 million per day.

Other benefits of electric motor driven compressors are derived from the better controllability of variable speed drives, and the unlimited number of (soft) starts. Electric motors are around 50 to 70 per cent of the initial cost compared to modern gas turbines with the same ratings.

Two-pole brushless synchronous motors in power ratings up to and beyond 100 MW have not been built in the past due to lack of demand. From the manufacturer’s prospective there is no reason not to build large two-pole synchronous electric motors.

Their design, materials, manufacturing methods, and analyses (electrical, mechanical and thermal studies) are identical to those of two-pole synchronous generators which are used in many power plants around the world in ratings up to 400 MW (and above).

Higher speed applications, variable speed requirements and new component designs for large electric motor applications may impose some changes. Sometimes, the qualification process for critical components is crucial and correspondingly extensive.

The detailed mechanical/electrical design reviews, the rotordynamics analysis, the torsional review, the control issues, the RAM (reliability, availability and maintainability) studies, and comprehensive network stability studies should be performed to satisfy both the operating company and the manufacturer. Independent design reviews can provide the necessary neutral assessment of the up-scaled electric motor technology.

The design of the associated variable frequency drives of the load commutated inverter type, on the other hand, amounts to a downsizing from existing high voltage DC technology and is less crucial.

Mechanical issues 

While economics say that electric motors are the best option, the units are not without their own mechanical issues.

For example, with large electric motors, the flexible rotor concept is generally used. In other words, the rotor runs super-critical (the first critical speed lies below the operating speed range). The rotor should be dynamically balanced.

The rotor design and construction are usually such that a subsequent field balancing would not be needed (whereas it is often possible). For large high-speed electric motors, the usual balancing methods based on rigid body balancing theories aren’t sufficient to create an adequate balancing condition for heavy elastic rotors (with relatively wide bearing spans).

A special task for electrical motors is the handling of thermal unbalances. Because of the use of various materials with different thermal expansion coefficients, combined with a non-uniform temperature distribution under a load condition, special care should be taken to achieve symmetrical mechanical and thermally insensitive design.

Dynamic studies and performance tests of high-speed large electric motors usually show high vibrations. Particularly high vibrations at bearings are reported (whether the bearing housing or the bearing locations of the shaft).

Even for some large electric motors, vibration velocities over 6 mm/s (more than three times of “1.8 mm/s”, the allowable limit by some electric motor codes) are measured. To clarify the source of these vibrations, a dynamic modal analysis should be used.

For large high-speed electric motors, if it’s impossible to reduce the vibration below standard limits, the achieved vibration level should still be below harmful fatigue levels. In such cases, the vibration and noise would be high but a failure will not be expected.

If such steps are followed electric motors are a viable option.

*Amin Almasi is a rotating machine consultant in Australia.

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