Integrated VSD compressors
January 1st 2004

There has been a spate of claims supporting the concept of retrofitting VSD drives to standard fixed speed compressors. Paul ONeill of Atlas Copco Compressors argues the case for integrated VSD and warns that any short term gain from retrofitting could prove to be an expensive short cut when it comes to equipment reliability

There are strong reservations within the compressor industry concerning the reliability of unmatched, bolt-on additions to standard compressor units. It is argued that simply adding a frequency converter to a standard compressor and regulation system is inviting problems. These issues range from the potentially damaging effect of resonant frequency bands and electromagnetic interference, to the most common experience of all, premature motor failure due to improper application of a frequency converter to a standard drive motor, rather than one specifically developed for VSD duty.

There is genuine concern in relation to retrofit advocates offering to guarantee performance criteria when it is doubtful whether some claimants can be as confident with questions of equipment reliability and the subsequent cost penalties of equipment failure.

The retrofit disadvantages

Retrofitting fixed speed machines in the field means simply adding on the regulation of the VSD system. The standard regulation system of the fixed speed machine is still maintained, typically load/ no load. The standard drive system is maintained, either gearbox or belt drive, with consequent transmission losses. These losses are, of course, eliminated in direct driven VSD compressors. When the retrofitted fixed speed machine slows down to its minimum speed, say 50%, its regulation system will default back to load/ no load. This means the pressure must rise to its unload pressure setting. This results in two disadvantages: the load to unload pressure band is typically 0.5 to 1 bar. This means air networks that experience great fluctuations in demand cannot have the tight regulation band of 0.1 bar that is often claimed. Also, when the unit unloads, the separator vessel will bleed down and the compressor will run unloaded, consuming approximately 25% of full load power. When the machine transitions to full load again it may take some time for the separator vessel to re-pressurise, that will effect the stability of the system pressure and the compressor's ability to maintain a good tight band.

Here follows some of the pros and cons of the retrofit/ integral VSD arguments. They take into account the optional solutions: standard induction motor with separate converter, standard induction motor with integrated converter, and the fully integrated compressor with its drive and motor designed for VSD application.

1) Standard induction motor with retrofit converter. The perceived advantages are the simplicity of bolting on an inverter package to an existing compressor installation and the lower capital costs involved.

Set against these apparent benefits are a number of factors. A particular problem is stray bearing currents, induced by switching frequency in the inverter, that find a path to ground via the bearing. These currents machine the bearing, resulting in potential premature failure.

Another cause is improper bearing lubrication. Bearing lubrication is affected by bearing currents, heat and speed range. In a standard motor, bearings are not designed to operate at any other than nominal full load and so in varying speed applications they fail to lubricate themselves properly.

Insulation breakdown is another contributory cause of premature failure. Frequency switching can cause voltage spikes up to three times the nominal voltage. Wave reflections in the leads create vibrations within the windings and cables, resulting in a motor short.

Standard induction motors have a reduced turndown capacity. They cannot develop the consistent torque or cooling characteristics throughout the entire speed range. So more heat is generated, leading to bearing problems.

Then, higher installation costs must be a consideration - a larger workspace footprint with a separate box requiring physical installation, wiring and programming. Questions of EMC compliance, ownership of equipment warranty and installer capability and experience must also be considered

It is estimated that the combined capital and installation cost of a bolt-on VSD inverter for an average, middle range 75kW compressor, including electrical connections, amounts in our experience to approximately more than 50% of the capital cost of the original compressor.

2) Standard induction motor with integrated converter. The singular benefit of this configuration is one of lower manufacturing costs. Set against this advantage are all the foregoing disadvantages of standard induction motor compatibility, as well as lower performance capability, questionable reliability and shorter productive lifetime.

3) The fully integrated VSD solution.

The motors for VSD operation should be specially designed for the application. The motor manufacturer optimises the design of the motor, focusing primarily on the magnetic flux of the motor system as well as the rotor length and rotor slot design to ensure that the motor develops the right amount of torque throughout the speed range.

Motor stator winding turns are also optimised to reduce stresses on the windings that can occur in standard induction motors during acceleration. The specific design of the stator and rotor together reduces the amount of stray losses that occur to reduce motor heating and the damaging effects the can occur to the motors insulation system. At the same time, the rotor and fan are designed to minimise windage losses and noise and to provide effective cooling in all operational conditions.

20% turndown

In the case of the Atlas Copco VSD compressors, for example, the machines do not need to unload and blow down the pressure from the separator vessel, as do many competitive compressors because of the outstanding torque producing capability of the motor at low speeds. The integrated VSD machine will simply ramp down as the demand on the system is lowered until a minimum speed is reached, 20% of maximum speed, and then the motor simply stops.

No unloaded running power consumption, no requirement to blow down the separator vessel, no inefficient transition period from full load power to no load power. This means lower overall energy consumption and the ability to maintain a narrower overall pressure band that allows the overall system pressure to be reduced, offering further energy savings.

It is essential that the entire compressor package is EMC compliant, not just the drive or the motor as is frequently the case. This compliance should be independently verified through the use of external audits of the design during the development phase.

Range and experience counts

When considering the relative advantages of integral versus retrofit VSD, a prospective purchaser should apply the following criteria to the system on offer.

Simply, does it offer the benefits of:

Motors and drives supplied from the same manufacturer and specifically designed for the application

Superior but simple compressor regulation system

20% turndown capability

No unloaded power consumption

Maintained sump pressure for stable air net pressure

Narrowest pressure band in the industry

Fully engineered and packaged compressors from the factory

Single power input connection

Guaranteed EMC compliance

The smallest footprint VSD compressors available

A single source of advice and problem solving

At the end of the day the right choice of VSD system will be self-evident.

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