Understanding Variable Frequency Drives and Appropriate Applications

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1 Understanding Variable Frequency Drives and Appropriate Applications Steve Bruno Product Marketing Manager Oil Injected Screw Compressors 30kW -90kW Atlas Copco Compressors LLC What is a Variable Frequency Drive (VFD)? A Variable Frequency Drive (VFD) is a type of controller that drives an electric motor by varying the frequency and voltage supplied to that electric motor. A VFD can have many different names but is most commonly referred to as a variable speed drive (VSD), frequency converter or an inverter. The drive s output frequency is directly related to the motor s speed; the higher the frequency, the higher the speed that the motor will turn. If an application does not require an electric motor to run at full speed, the VFD can be used to decrease the frequency and voltage to meet the requirements of the electric motor s load. As the application s motor speed requirements change, the VFD can simply increase or decrease the motor speed to meet the speed requirement. How does a Variable Frequency Drive work? At the most basic level, the VFD rectifies the supplied AC voltage and stores the resulting DC voltage in its capacitor bank. This DC voltage is then rectified to the desired voltage and frequency to spin the motor at the desired speed and torque required by the process (compressor, fan, etc.). The first stage of a Variable Frequency AC Drive is the rectifier, which is made up of a group of gated diodes or silicon rectifiers (SCRs). These diodes are typically in a group of six and may vary from six to 24 diodes total. Diodes only allow current to flow in one direction, similar to check valves used in plumbing systems. For example, whenever A-phase voltage (voltage is similar to pressure in plumbing systems) is more positive than B- or C-phase voltages, the diode will open and allow current to flow. When B-phase becomes more positive than A-phase, the B-phase diode will open and the A-phase diode will close. The same is true for the 3 diodes on the negative side of the bus. Thus, we get six current pulses as each diode opens and closes. For the common 480V RMS power system, the AC ripple will vary between 580V and 680V. The second stage of a VFD is the capacitor bank which acts to remove this ripple and create a smooth DC voltage. The capacitor banks acts similarly to a reservoir in a plumbing system and is designed to minimize fluctations, creating a smooth DC voltage. The diode bridge converts AC-to-DC and is sometimes referred to as a converter. The converter that converts the DC back to AC is also a converter, but to distinguish it from the diode converter, it is usually referred to as an inverter. It has become common in the industry to refer to any DC-to-AC converter as an inverter.

2 When one of the top switches in the inverter closes, the phase of the motor connects to the positive DC bus and the voltage on that phase becomes positive. When one of the bottom switches in the converter closes, that phase is connected to the negative DC bus and becomes negative. Thus, any phase on the motor can become positive or negative and can generate any frequency. A VFD outputs the rectangular waves form, not the sinusoidal outputs shown in blue in the figure above. It is important to note that the types of motors used in VFD applications can run properly with the square waveform. However, not every motor is designed this way, so it is important to ensure that the motor is suited for a VFD application. To reduce the motor frequency to 30 Hz, one can simply switch the inverter output transistors more slowly. But, if the frequency is reduced to 30 Hz, then the voltage must also be reduced to 240V in order to maintain the V/Hz ratio. How can voltage be reduced if the only voltage available is 650 VDC? Pulse Width Modulation or PWM Continuing the plumbing analogy, imagine controlling the pressure in a water line by turning the valve on and off at a high rate of speed. While this would not be practical for plumbing systems, it works very well for VFDs. Notice that during the first half cycle, the voltage is ON half the time and OFF half the time. Thus, the average voltage is half of 480V or 240V. By pulsing the output, any average voltage on the output of the VFD can be achieved. Benefits of Variable Frequency Drives 1. Reduce energy consumption and energy costs. If an application does not need to be run at full speed, then energy costs can be reduced by controlling the motor with a variable frequency drive, which is one of its primary benefits. VFDs allow you to match the speed of the motor-

3 driven equipment to the load requirement. There is no other method of AC electric motor control that allows you to accomplish this task. Electric motor systems are responsible for more than 65 percent of the power consumption in the industry today. Optimizing motor control systems by installing or upgrading to VFDs can reduce energy consumption in a typical facility by as much as 70 percent. Additionally, the utilization of VFDs improves product quality and reduces production costs. When combining energy efficiency tax incentives and utility rebates, a typical return on investment for a VFD installation can be as little as six months. 2. Increase production through tighter process control. By operating motors at the most efficient speed for the application, fewer mistakes will occur resulting in higher production levels and higher revenues. Jerks on conveyor and belt start-up are also eliminated, allowing high throughput. 3. Extend equipment life and reduce maintenance. Equipment lasts longer and has less downtime due to maintenance when it s controlled by VFDs, which ensure optimal motor application speed. Because of the VFDs optimal control of the motor s frequency and voltage, the VFD will offer better protection for the motor from issues such as electro-thermal overloads, phase protection, under voltage, overvoltage, and more. VFDs also allow the motor and driven load to start smoothly, which eliminates the instant shock of across-the-line starting as well as belt, gear and bearing wear. VFDs also help reduce and/or eliminate water hammer because of smooth acceleration and deceleration cycles. Rotary Screw Air Compressor Variable Speed Drives Drives are used in applications to speed up and slow down an electric motor so that the output of the machine can match the demand of the system. Depending on the process, a variable speed drive will not only help a customer save money on the electric bill by running the process just enough to meet the demand, but it can also adjust the process to provide just-in-time delivery of products to the other parts of the plant. In compressed air systems, holding the pressure supplied to the plant at a steady lower pressure set point rather than using a fixed speed compressor s pressure band will reduce the amount

4 of air lost in leaks. The majority of VSDs are used to control pumps or fans and to save energy by running the pump or fan at the proper speed to match the demand. Because the torque and power curves of these applications follow what is referred to as a variable torque curve, tremendous energy savings can be realized by using a VSD. Figure 1 illustrates a centrifugal pump application where the speed of the motor is slowed to 80 percent of full load RPM, resulting in energy usage (BHP) that is 51.2 percent of full load energy usage. This exponential energy savings makes purchasing a drive worth the initial investment in this type of application. It is important to note that the majority of variable speed drives sold are for variable torque applications due to the amount of savings that can be realized when using a VSD in this application. In the compressed air industry, variable torque loads are found on centrifugal air compressors and linear torque curves are found on positive displacement rotary screw air compressors. As seen in the chart above, the rotary screw compressors have a flat torque curve along the full RPM range, which is much different than the torque curve of the centrifugal compressors. If a drive designed for a variable torque application is used in a linear torque application, the drive will not be able to provide sufficient torque in the lower RPM range. Misapplying a drive for any given application will shorten the life of the motor and the drive in the compressor. In 2014, Atlas Copco s GA37-90kW VSD series of compressors started using the first drive designed specifically for oil-injected rotary screw air compressors. Until the launch of the Atlas Copco Neos drive, oil-injected screw compressors used drives originally designed for other applications. Why not use a VFD on every rotary screw compressor? VFDs are not suitable for every rotary screw air compressor application. Because the voltage has to be sent through a drive before reaching the motor, VFD compressors are not as efficient as fixed speed air compressors at 100 percent load. In a fixed speed compressor, you simply have a starter sending power to the motor, whereas a VFD compressor has some energy losses in the form of heat. Fixed speed compressors are ideally suited for a base load that works in conjunction with a VFD compressor which adjusts to the varying demand of the air system.

5 Air compressors are often installed in dirty, dusty, hot and wet environments. Many drives are not originally designed with air compressors in mind and thus are not suitable for these environments. Dust and dirt can be ingested by the drive s cooling fans and cause the drive to eventually overheat and fail. Fluctuating power supplies causing voltage imbalances or induction welders which induce electromagnetic noise onto the plant s power grid are also the enemy of most compressor drives. These drives can fail and potentially damage the compressor s motor resulting in costly repair bills and extended downtime for the plant s air system. How is the Atlas Copco Neos drive different? Drawing on years of experience with variable speed drive technology, the Atlas Copco Neos is the world s first drive specifically designed for rotary screw air compressors and their unique torque demand. The Neos is designed to handle fluctuating power grids and noise found in industrial factories without faulting out or needing to be reset. It is IP5x-rated, meaning dust will not affect operation of the equipment, allowing compressors to be installed in dusty environments. The Neos is also rated for outdoor installation under a lean-to and for temperatures up to 122F. Atlas Copco designed the Neos drive specifically for air compressors and the environments where these compressors are installed. The readers of Plant Engineering magazine know how valuable a reliably designed air compressor is to the continuous operation of their plants, and in 2015 the Neos drive was awarded the Plant Engineering Product of the Year Silver Award in the Compressed Air category. # # #