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What is a Variable Speed Drive Air Compressor?

Variable Speed Drive Air Compressor

The Basics of a Variable Speed Drive Air Compressor

Variable Speed Drive air compressors, also known as VSDs, are rotary screw compressors where the motor operates at different frequencies depending on the demand that is being placed on the compressor. These compressors alter the RPM of the motor in order to match the CFM that is required at the moment. When demand is low the compressor will work at a lower RPM and reduce the amount of energy that is being used and will use the full amount of energy and output when it is necessary. The reduction in RPM to match demand makes the variable-speed drive air compressor a much more cost-effective option when it comes to total power use in comparison to the traditional fixed-speed compressor. 

A fixed-speed compressor, in comparison to the VSD, would use much more energy over an extended period of time as it runs at a constant RPM when it is on. These may use less energy when it is not under load, but that can still be up to 70% power being used by the compressor in order to keep the motor idle until it is used again. For example, if there are two 150HP compressors and one is a traditional fixed speed compressor and the other is a variable speed drive air compressor, while at full load they will both use the same amount of power needed in order to meet the demand. 

If the demand is then lowered, the fixed speed will continue operating at the same level of energy consumption, but the VSD will lower its RPM down in order to meet the demand for the air compressor. Meanwhile, even though the demand is lower, the fixed speed will still be running at the same energy level as at full output. Even when demand is almost completely reduced the fixed-speed compressor will still operate at about 70% power while the variable-speed drive air compressor can even shut off completely and start back up when there is a demand.

The Advantages of a Variable Speed Drive 

Energy Efficiency

When it comes to a variable speed drive air compressor, a simple rule of thumb can be used to determine its energy efficiency: Energy consumption is a reflection of demand. The higher the demand on the compressor, the higher the energy that is used, and the lower the demand the less energy is being used as the RPM of the motor is being ramped up or down in order to match. 

This can reduce energy costs to over 50% due to the efficacy of the motors, making this change a worthy investment in the long run as those initial costs are almost erased by the energy savings. When it comes to the drivetrain of the motor on the VSD, it is directly connected to the screw elements which allows for the motor to be completely shut off when it is not used rather than having to sit in idling and draining power. 

When it is nearly completely shut off it vastly reduces the energy that is being consumed by the compressor as it has a minimal number of features functioning. This ability to partially shutdown and restart on demand is due in part to the “soft start” of these motors as the RPM just increases to the point that is needed, so as the motors ramp up the energy level does as well which reduces the energy spikes, unlike fixed speed which need large amounts of energy to start.

Increased System Life

The variable speed drive air compressor systems only operate at the level that is required of it at the time and therefore is not going at full power and output at all times which in turn is easier on the machinery in the long run as it does not get worked as much as a fixed speed would in a similar amount of time. 

The wear and tear on the machinery are also reduced by the efficacy of the starting and stopping processes of the machine as well due to the way in which it slows down, rather than coming to a hard stop when it is no longer under load. These characteristics of the variable speed drive air compressor are also beneficial to the motor speed and overall performance due to the optimization of these processes.

Potential Energy Incentives

Not only is the variable speed drive air compressor more energy efficient due to the way it matches demand, but power companies may also provide incentives to people that switch to a more energy efficient compressor, like the VSD. These compressors are beneficial to both parties as the energy consumption is reduced for the individual who is using the variable speed drive air compressor and the power companies benefit due to the reduced grid strain and decrease in energy spikes that can potentially cause power outages.

Additional Benefits of the Variable Speed Drive

The reduction in energy is one of the big reasons for individuals to switch to the VSD, but that is not the only reason that the switch is beneficial for consumers. One of the factors that really separates the variable speed drive air compressor from fixed speed compressors is the pressure control capabilities that it has. Most fixed speed compressors can have a standard deviation of +/- 10-30 PSIG and such high variability can cause inconsistencies across the plant, where if a Variable Speed was in place, they have a standard deviation of +/- 1.5 PSIG which allows for a much tighter pressure band and creates more consistent air pressure throughout the facility. 

Being able to work within such a tight pressure window allows for the system to operate at a lower delivery pressure as well in order to reach a consistent pressure. This consistency also allows for an overall increase in production due to being able to reach higher production levels and reduced mistakes due to inconsistent air pressure and an overall smoother operation. Not only does this create an overall smoother production, the VSD compressor also requires less frequent maintenance in comparison to its fixed-speed counterparts as it is not operating at full capacity all the time. The system undergoes a lot less “instant shock” and reduces and potentially eliminates water hammer due to the smooth acceleration and deceleration. 

The Short of the Benefits

  • Because the VSD compressor only runs when needed, it reduces energy costs—typically enough to pay for the additional investment in VSD technology.
  • A variable speed drive air compressor can start/stop under full system pressure. There is no need to unload, which saves both time and energy.
  • No time is lost to idling.
  • No blow-off losses occur in normal operations.
  • Power company penalties for peak current at startup are avoided.
  • Air system pressure is more consistent and also lower, minimizing leakage

Looking for More?

If you are looking for more information consider going to Warthog University

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