How a VSD Compressor Can Save Your Business Money and Energy

Would you like to shave 30% off your compressed air energy bills? If your compressed air demand fluctuates, a variable speed drive (VSD) air compressor could reduce energy consumption for compressed air production by 30% or more–in some cases, up to 70%. How do you know if a VSD compressor is right for you? Learn the benefits of VSD, when to use a VSD vs. a fixed-speed compressor, and why you may want to consider using both.

Intec Group, LLC saved $18K on their annual energy bill by upgrading to a VSD compressor. Contact us to calculate your potential savings!

What Is a Variable Speed Drive Air Compressor?

A variable speed drive (VSD) controls the frequency supplied to the drive motor on the air compressor in response to real-time demand on the system. When demand is low, the motor will run at a reduced speed, requiring less energy to operate. When demand for compressed air is higher, the VSD will automatically increase the frequency of the air compressor motor to meet CFM demands.

A fixed-speed rotary screw air compressor, on the other hand, operates at the same RPM at all times. Instead of adjusting the motor speed, the fixed-speed air compressor adjusts the air inlet valve to let more or less air out depending on CFM requirements. Even when compressed air needs are low, the motor is always operating at the same speed, so it is always ready to meet the highest CFM it is rated for.

For example, if you have a 100 HP fixed-speed compressor with no load from the plant, it may still be consuming 30 to 70 percent of its full load energy. A VSD compressor, on the other hand, will be at nearly zero input energy! A 100 HP VSD compressor motor can ramp down to the equivalent of 20-35 HP (depending on the manufacturer) when demand is low. As demand increases, it can operate anywhere between its minimum and its maximum 100 HP. When operating at lower speeds (and when properly sized), a VSD compressor is much more efficient and uses less energy.

The Advantages of Variable Speed Drive Air Compressors

So, why choose or upgrade to a variable-speed drive air compressor? A VSD air compressor is able to adjust the amount of power used in response to CFM demand in real-time. This provides significant advantages, especially for companies whose compressed air needs vary over the course of a shift, day or season.

VSD Compressor Benefits

Advantage	Impact
Lower energy consumption when operating below peak demand	- Reduced energy costs for compressed air production (typically 30-70%, depending on demand variability)
Lower starting currents (“soft” start/stop)	- Eliminates large spikes in energy demand - Reduced stress on electrical components
Reduced stress on mechanical and electrical components	- Improved compressor reliability and longevity - Reduced downtime
Tighter pressure tolerances	- More consistent plant pressure - Greater overall system efficiency - Fewer pressure problems for tools and applications

Energy Savings

The biggest advantage of a VSD air compressor is energy savings. A fixed-speed compressor always draws the amount of power needed to meet the maximum compressed air demand. VSD will ramp down RPM, and therefore power consumption, when demand for compressed air is lower. Depending on the total volume of compressed air used in your facility and the amount of variability in your processes, this can add up to hundreds, thousands or even tens of thousands of dollars in energy savings each year. Energy savings are maximized for facilities with the greatest amount of variability in their processes. In some cases, operators can slash energy costs for compressed air systems by up to 70%!

Lower Starting Currents

Motors controlled by VSD have lower starting currents, eliminating the large spikes in demand seen when a fixed-speed motor turns on. When a fixed-speed motor first energizes, the inrush current can be as high as 600% of the operating current. The VSD converter uses a “soft” start/stop during operation, automatically controlling acceleration and deceleration levels to reduce amperage peaks. This, in turn, reduces stress on your internal power distribution. Power companies sometimes impose penalties for large amperage spikes, which VSD can avoid.

System Longevity

A VSD motor optimizes motor speeds and compressor performance. This reduces stress on mechanical and electrical components, prolonging the life of your system and minimizing downtime.

Consistent Plant Pressure

VSD air compressors provide more consistent pressure for plant operations. A fixed-speed compressor operates within fairly wide pressure bands due to the way airflow is controlled using the inlet valves. These bands may be as much as +/- 10-30 PSIG, meaning PSI for the system may vary by this much during the course of operation. A VSD compressor uses electronic controls and operates with much tighter tolerances, typically operating within a +/- 1.5 PSIG pressure band. This means much more consistent discharge pressure for all of the applications using compressed air in the facility. Tighter tolerances also mean that the compressor can operate at lower delivery pressures while still maintaining the required minimum working pressure for the system.

Typical Pressure Bands (Variation from Set Operating Pressure)
Fixed-Speed Compressor	Variable-Speed Compressor
+/- 10-30 PSIG	+/- 1.5 PSIG

Energy Incentives for VSD Compressors

Many power companies provide incentives for manufacturers to upgrade to energy-saving systems such as a VSD air compressor. Power companies incentivize these purchases because VSD reduces the overall load on the power grid and minimizes sudden spikes in demand that can overload the grid and cause an outage. These incentives can significantly offset the costs of upgrading to a new VSD air compressor.

Check your power company for incentives:

• ComEd (Chicago and Northern Illinois)
• Xcel Energy (Minnesota)
• DTE Energy (Michigan)
• Consumers Energy (Michigan)
• Focus on Energy (Wisconsin)
• JCP&L (Central New Jersey)

Don’t see your energy company? Contact us to find out about energy incentives in your area!

Choosing Between a Fixed-Speed vs. Variable-Speed Drive Air Compressor

Variable-speed drive air compressors have many advantages, but that doesn’t mean they are the right choice for every facility. The way you use compressed air, the amount of variability in your processes, and the characteristics of your facility will all impact the choice between a fixed speed and variable speed drive rotary screw air compressor.

Choose a Variable Speed Drive Compressor If…	Choose a Fixed-Speed Compressor If…
You experience fluctuating compressed air demands across shifts, weeks or seasons	You have little variability in demand and are always operating near the maximum CFM rating for your compressor
Your applications require tight pressure band tolerances	Pressure tolerances are not a big concern
The compressor will have consistent power and operate in reasonably clean, dry conditions	You have an inconsistent power supply or the compressor will operate in dirty, dusty or wet conditions
You have adequate capital, OR you qualify for energy incentives through your power company	You have limited working capital
You want to save energy and money for compressed air production	You do not anticipate significant energy and cost savings from switching to VSD

Remember: The more variability you have in compressed air demand, the greater your potential savings from switching to VSD.

When to Use a Variable Speed Air Compressor

The majority of compressed air users could benefit from upgrading to VSD. VSD air compressors provide the biggest energy savings for facilities with high degrees of variation in their CFM requirements. For example:

• Companies with multiple shifts that have different CFM needs (e.g., shifts 2 and 3 operate at lower demand than shift 1).
• Companies with high seasonal variability.
• Applications where demand fluctuates significantly over the course of a shift or between jobs (for example, a machine shop with manual tools whose usage varies depending on the jobs they take in).

VSD is also a good fit for applications that must operate within very tight operating pressure tolerances.

When to Use a Fixed-Speed Air Compressor

Fixed-speed compressors have lower initial costs, so they can be a good choice for companies with limited access to capital funds. Parts and maintenance are also generally less expensive for a fixed-speed compressor. However, it is important to remember that energy savings for a VSD, combined with incentives from your power company, may largely erase the cost difference between a fixed-speed and variable-speed drive air compressor over time.

A fixed-speed compressor may also be more efficient in scenarios where variations in CFM are minimal, and the system will always be operating near its maximum CFM rating. Fixed-speed air compressors are best for:

• Companies with very low variation in compressed air demand, such as robotic applications running on a 24/7/365 basis.
• Companies with inconsistent power supply. VSD compressors are highly sensitive to incoming power inconsistencies.
• Facilities with extremely dirty, dusty or wet ambient conditions. The electronics in VSD compressors are more sensitive to moisture and dirt than the components of a fixed-speed compressor. That being said, filtration can be added to the electronic cabinets to eliminate this concern, allowing these users to take advantage of the energy savings.

Using Both Fixed Speed and Variable Speed Drive Compressors Together

In many cases, the best option is to use both a fixed speed and a VSD compressor in tandem. Using multiple smaller air compressors instead of a single large compressor means that you can keep at least part of your facility up and running while one compressor is down for maintenance or repairs; the second compressor can typically meet about 50% of your maximum compressed air needs. Combining both types of compressors allows you to take advantage of the energy savings of VSD while saving some initial capital expenses with the fixed-speed compressor. See how it worked for SCM Group: A Three-Compressor System Maximizes Efficiency.

When operating both types of compressors in tandem, the VSD air compressor should have a larger capacity than the fixed speed. Here’s how it works: