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 to match the CFM that is required at the moment. When demand is low, the compressor will work at a lower RPM, reduce the amount of energy being used, and use the full amount of energy and output when 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 than the traditional fixed-speed compressor.
A fixed-speed compressor would use much more energy over an extended period of time compared to a VSD. These may use less energy when it is not under load, but that can still be up to 70% of the power the compressor uses to keep the motor idle until it is used again. For example, if there are two 150HP compressors, 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 lowered, the fixed speed will continue operating at the same level of energy consumption. However, VSD will lower its RPM to meet the demand of 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.
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 used, and the lower the demand, the less energy is being used as the motor’s RPM is ramped up or down to match.
This can reduce energy costs to over 50% due to the efficacy of the motors. This change is a worthy investment in the long run as the energy savings almost erase those initial costs. 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 consumed by the compressor. This ability to partially shutdown and restart on demand is due in part to the “soft start” of these motors as the RPM 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.
The variable speed drive air compressor systems only operate at the required level at the time. Therefore, they are not always running at full power and output. This is easier on the machinery in the long run; it works half as much as a fixed speed would in a similar amount of time.
The efficacy of the starting and stopping processes also reduces the wear and tear on the machinery. The machine slows down rather than coming to a hard stop when it is no longer under load. The characteristics of variable speed drive air compressors also benefit the motor and overall performance.
The variable speed drive air compressor is more energy efficient because it matches demand. Power companies may also incentivize people to switch to a more energy-efficient compressor, like the VSD. These compressors benefit both parties as the energy consumption is reduced for the individual using the variable speed drive air compressor, and the power companies benefit from the reduced grid strain and decrease in energy spikes that can potentially cause power outages.
The energy reduction 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 its pressure control capabilities. 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.
Working within such a tight pressure window allows for a lower delivery pressure for consistent pressure. An overall increase in production due to being able to reach higher production levels, reducing mistakes due to inconsistent air pressure, and smoother overall operations. Production is smoother, and the VSD compressor requires less frequent maintenance. Not operating at full capacity all the time means less overall usage. The system undergoes a lot less “instant shock” and reduces and potentially eliminates water hammer due to the smooth acceleration and deceleration.
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