Why VFDs clearly stand out in terms of efficiency

In the first part, the author highlighted how VFD technology has become a common feature in compressors. In this article, he explains why the VFD can be advantageous despite being an expensive addition to the compressor system.

When varying the speed of a compressor, the user can ensure there is “on demand” air pressure, meaning that the pressure can be increased in the system at a rate depending on the speed of the motor. Since the drive varies the speed of the motor, the air compressor can pressurize at any rate desired by the user.

It is important to note, VFD technology provides energy efficiency superior to all other control technologies available today in air compressor applications.

The following money-saving results occur in all VFD situations:

The overall power consumption is reduced.

Power surges from the in-rush current during startup are reduced.

The system can deliver more constant pressure based on demand rather than simply filling up a storage tank device and run at 100 percent when required. 

VFD compressors can consume up to 35 percent less energy than fixed speed compressors sized for the same application.

VFD compressors vary their output – continually and automatically – to precisely match the demand for air.

Reducing the demand for power helps cut greenhouse gas emissions that contribute to global warming.

The VFD pays for itself – often in just months – through significantly lower energy costs.

VFDs are an expensive addition to the compressor system. For compressor applications that maintain a constant speed or those that are running at 100 percent when powered, VFD advantages may be mitigated by the cost of the VFD. 

Specific features and benefits of VFD drives 

PID control mode

Benefit:   Automatically regulate speed based on load conditions. The drive employs a built-in PID Controller (an external PID device is not required).

PID stands for Proportional Integral Derivative. It is fairly easy to explain how PID functions. In the VFD, the PID controller looks at the current value of an error, the integral of the error over a time interval and the rate of change of the error to determine how much of a correction to apply. The controller will continue to apply the correction, until a change is seen on the feedback. Depending on the update rate of the error calculation, which in turn may depend on how often the loop feedback is read, the corrective action can be adjusted in a matter of just a few milliseconds.

A PID controller’s job is to force the feedback to match the set point. Sometimes the error between the feedback and the set point is caused by a change in the set point, but in most applications, the set point is not adjusted very often. More than likely, the error in the loop will be caused by disturbances to the measured feedback. Many times, a PID regulator responds solely with proportional and integral controls. This is called a P-I regulator and is PID regulation without the ‘D’ or derivative function. A compressor is a good example of an application that does not require a ‘D’ derivative function when using PID regulation. 

In the example of a compressor, the set point is the desired pressure. The error is the difference between the desired pressure and the current pressure value in the tank. A pressure monitoring device in the compressor system sends regular pressure level signals to the VFD. Looking at the rate of change as the drop in pressure increases, the VFD’s PID regulator automatically adjusts to apply the appropriate correction. VFD parameters are programmable.

In the event of high performance requirements, the drive can be set to run the compressor often with a high rate of change, keeping the desired set point. If performance is lower, the drive can adjust to a lower rate of change to keep to the desired set point. As the VFD starts and stops the motor at the user’s prescribed speeds, the PID regulates the system. This constant regulation with feedback from the system and response from the VFD is called a PID loop. The main benefit for any PID loop is the idea that the user can set the PID regulator, set points, rates, and so on, and let the VFD control the system. Couple this regulation with preset speeds or encoder feedback regulation and the compressor uses significantly less energy. 

Cooling fan on/off

Benefit: Controlling the number of times the drive fan is switched on and off increases the lifespan of the cooling fan and reduces the need for maintenance.

VFDs can control the operation of their cooling fans. This allows for quieter running and increases fan operative life. If the drive is not running the motor, the fan can be switched off.  This can increase fan life because it is not running all of the time. Reducing the number of run time hours on a cooling fan limits the wear on mechanical components such as bearings.

Multiple control modes

Benefit: With vector control, the system can operate at a stable speed regardless of the load. A customer can run without an encoder operate open loop V/f     

In the compressor application, both V/f control and closed loop vector control are used. Vector controls use a feedback device called an encoder. The VFD uses this encoder signal to ensure that it operates the motor at the compressor controller’s desired speed. Open Loop Vector or sensor-less vector is a non-feedback version of vector control and can be used as well. 

V/f control is utilized often in compressors to fine tune the motor during operation. It is less expensive than flux vector control. If the compressor company standardizes on a particular type and brand of motor, drives can be programmed identically with the appropriate V/f settings to ensure proper operation. Removing the encoder also removes another electromechanical failure possibility to increase system MTBF (Mean Time Between Failure).

Energy saving control

Benefit: V/f pattern selection saves energy while operating with light load and low speeds.

Some VFDs can help maximize energy control savings with a dedicated energy savings function where the drive takes over the V/f operation of the drive. 

An “automatic energy savings” function is designed to be used whenever the rated torque is not required to maintain the set point air pressure. This function will regulate the output voltage to the motor to ensure the AC induction motor will always run at its rated slip frequency. Slip frequency is the difference between the shaft speed (rpm) and the synchronous speed. The synchronous speed depends on two factors, frequency of supplied voltage from the VFD to the motor and the number of the machine’s poles. The slip value is then optimized to the point where current and power is reduced. This function constantly changes and updates in order to maintain maximum energy savings. 

The user can manually improve the system by altering the V/f pattern in the drive. Different V/f patterns can be used at different points in the compressor operation. For example, if the pressure set point needs to be reached; but it is late at night and usage is low, the VFD turns the motor on to pressurize the tank. With utilization low, the V/f pattern can run the motor at a low-performance low-speed mode to save on energy. If this same compressor requires more pressure during higher air requirement portions of the day, a new higher performance V/f can be selected.  The VFD may have many preset patterns to choose from or may offer the user the option to generate custom V/f patterns. 

Restart after momentary power loss

Benefit: The motor continues running even after a two-second momentary power loss.

Some VFD’s can ride through a momentary power loss of up to two seconds depending on KVA rating of the drive. 

Continue to run when external frequency reference is lost

Benefit: The drive enables different responses to momentary frequency references that are lost.

VFDs can detect the loss of an analog frequency reference from an analog input. The frequency loss is detected when the frequency reference drops below 10 percent of the reference or below 5 percent of the maximum output frequency within 400ms. The drive allows two choices to be made; both by the user. 

· OPTION #1:  The drive will STOP the motor and shut down the compressor.

· OPTION #2:  The drive will continue operation at a frequency value set by the user, which becomes the new compressor set point. When the external frequency is restored, the operation is continued with the frequency reference. 

Avoiding mechanical resonance

Benefit: The drive skips over the frequency at which resonance occurs.

At times, mechanical resonances can start with machinery. For higher end stationary compressors used in factories, warehouses, and hospitals, mechanical buffers and cushioning are used at the site to avoid mechanical resonance issues. Some manufacturers design their drive software to help compensate for mechanical resonances when they occur. One such function is called ‘Jump Frequencies’. Up to three frequencies can be selected for the VFD to skip over during operation. A parameter is also available that will select a range of frequencies to skip over around the most critical frequency. For example, if a mechanical resonance is created at approximately 25.4 Hz, the user can select 25.4Hz to be skipped. The user can also select up to a + or – 20Hz dead band width range around 25Hz to avoid. Scaling is -+0.1Hz.

(The author is Industrial Drives OEM Sales Manager, Yaskawa America, Inc.)