Optimising the efficiency of VSD air compressors

Variable speed drive (VSD) technology is increasingly being deployed in many air compressor applications to cater for variable loads. As a result, developments for the motor/drive component and the compressor itself continue to improve. Sean Richardson, Product Manager with WEG Australia, and Andrew Dove, Product Development Manager with Sullair Australia, discuss the implications of the WEG motor/drive technology incorporated into Sullair’s Champion range of VSD air compressors.

Standard electrical motors are usually in one of two states - on or off. In these cases, the speed of the motor is fixed. Single-speed motors are typically drive fans, pumps, compressors and the like. The principal drawback of this type of on/off, fixed speed motor is that the speed may not be optimised to the demand of the application: it may be running too fast and wasting energy or too slow and struggling to perform, creating undue stress on components. The motor will always be in a state of compromise. Single-speed motors are most suited to applications when running at 100% capacity for fixed periods and then turned off when no longer required.

In many instances, an air compressor application may have varying demands, for example, changes in production shifts or ramping up or down of specific machines in a plant. In a typical facility there may only be a short time of the day during which the full 100% of air compressor capacity is required. For a fixed speed load on/load off motor, maximum efficiency only occurs when operating at 100% capacity - the remainder of the time these devices will consume energy unnecessarily. VSD compressors alleviate these issues by better aligning the power intake and energy demand to the load variances of the specific application.

Historically, traditional VSD compressors were perceived to have limited usage. Drive life was a concern if heat from the drive was not adequately dissipated. Low-speed operation was inefficient resulting in motor de-rating. Overall, drive life was not great and drive replacement costs were comparable to the replacement of the entire compressor. Motor life insulation issues compounded the problems for the drives. In some instances, VSD compressors could not easily be run from power generators due to problems with harmonic interference with the drive. The initial costs of a VSD system and the reduction in efficiency when used at full capacity were also key shortcomings.

When used appropriately, the system gains of variable speed drives outweigh any restrictions. The positive outcomes include reduced mechanical stress on the motor and consequently reduced maintenance and repair costs; greater control and automation allowing the operator to finetune response to specific application demands; and significant energy savings in varying load situations.

In recent years, government and industry have promoted the use of machinery with greater energy efficiency, particularly with respect to compressors. It is estimated that as much as 10% of the electrical power consumed by Australian industry is expended on compressed air. A compressor typically is the largest single user of energy on site and an estimated 86% of the cost of a compressor over its lifetime is spent on energy. The installation of a VSD compressor may increase energy efficiency by as much as 30-35% if applied in the right circumstances.

WEG designs and manufactures variable speed drives and motors and has developed several technologies including Optimal Flux Technology and the WISE Insulation System to overcome the deficiencies of traditional VSD technology. WEG has partnered locally with Sullair to develop the Champion range of VSD air compressors made here in Australia.

The key feature of WEG’s Optimal Flux Technology is the ability to maintain the torque at low speed by optimising the flux density of the motor. The WISE Insulation system is designed to protect motor windings and limit voltage pulse intervals and peaks typical of the VSD application.

Motor torque is a function of voltage, frequency and current. In order to maintain torque at low motor speeds, earlier versions of VSDs increased the current at lower motor output frequencies. The downside to this approach was a significant rise in operating temperature since fans were directly coupled to the motor shaft. As the speed of the motor slowed, the motor fan would slow, compromising the fan’s cooling efficiency. WEG has solved that problem by increasing the voltage/frequency ratio, thereby minimising the temperature rise associated with increased current. For every 10°C decrease in operating temperature, the motor life is doubled. WEG’s Optimal Flux Technology eliminates this issue making forced external cooling and ventilation requirements redundant.

WEG has developed the WISE Insulation System for all its industrial motors to insulate against high voltage spikes and ΔV/Δt rises associated with the use of VSDs. The technology is designed to reduce stresses on the motor windings thereby prolonging motor life.

The energy-saving benefits of the Champion VSD range are clear. Whenever air demand is less than the full capacity of the air compressor, the Champion VSD range achieves energy savings compared to traditional on/off load compressors. Given the significance of compressed air to industrial power bills, users have found that the Sullair Champion range of VSD compressors can save thousands of dollars per annum. Installing a VSD compressor may have higher up-front costs, but the energy consumption savings achievable may see a payback period of as little as six months. The benefits of the Optimal Flux Technology, WISE Insulation, variable speed and removal of external cooling requirements translates into a more energy-efficient and reliable compressor.

In order to benefit from the energy savings available from the VSD range, the size of the compressor needs to be chosen to match those requirements. Sullair Australia offers air audits to assess the overall air demand of an installation. From an air audit, air demand, load variation and usage levels are calculated to determine which compressor system is appropriate. An ill-suited VSD compressor will not give the benefits of reduced energy consumption if it is operating outside its efficiency range. The safest, surest way of selecting the most suitable compressor package is to commission an air audit.