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Specifying An AC Drive Platform

By Darrow Hanesian

ndustrial processes and applications for electronic speed control of AC ery and facility automation applica- consume nearly half of the electrical induction motors can yield substantial tions. Gone are the days when an Ienergy produced in developed coun- returns in energy savings and productiv- inverter drive was a complex and tries today. Electrical drive systems ity over the product life-cycle. For these expensive investment. Water resistant, used for energy conversion are respon- reasons it is imperative to make dust protected inverter drives are as sible for a majority portion (approxi- informed choices when specifying easy to install and as simple to use as mately 63 percent) of the total amount motor and motor control equipment. basic mechanical speed control devices, of consumed energy. Even modest AC drives are among the most such as belts, , two speed gear- increases in the efficiency of drives used important devices deployed in machin- boxes and low reduction gearboxes. E-LINE V18,2_Layout 1 4/20/12 9:13 AM Page 65

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AC Drive Advantages speed control or the level of energy sav- induced magnetic field.The speed at An AC inverter drive can be installed ing that a drive can deliver. which the magnetic field rotates is virtually anywhere, without the need Also referred to as variable fre- known as synchronous speed and can be for a protective cabinet, it is not suscep- quency drives or VFDs, the latest determined by the following equation: tible to oil mist, atmospheric moisture developments in AC drive technology or low pressure water jets, it is also a have brought to market standard and n = 120 * f / N very clean alternative as it does not pro- custom products that blend affordabil- duce oil leaks, oil mist, dust or debris ity, compact size, and simplicity. When Where n = synchronous speed like gearboxes, belts and chains. In controlling the speed of an AC motor, 120 is an electrical constant many cases using an IP65 ingress pro- there are three main VFD platforms f = applied frequency tected drive is a lot less expensive than from which to choose, including: N = number of motor poles the purely mechanical alternatives; this Volts/Hertz (V/Hz), Open Loop Vector is particularly true of conventional and Closed Loop Vector. Reducing applied frequency to an heavy gearbox types providing limited While each platform offers distinct induction motor causes the magnetic speed variation but adding significantly advanced technological features and field to rotate at a slower speed. to the cost and the weight of a machine benefits, it can be confusing when trying or an installation. to specify which level of AC drive tech- Open Loop Vector Platform Addressing one of the most pressing nology is appropriate for a particular The design architecture of open loop issues in the competitive industrial mar- application. Following is an overview of vector drives is similar to V/Hz drives. ketplace, the use of an AC drive can the main benefits of each VFD platform. However from a hardware standpoint, substantially improve efficiency and the main difference is the addition of reduce energy usage by a huge margin, V/Hz platform current sensors, with the biggest overall particularly compared to clutches that V/Hz technology is the most econom- difference in firmware. Open loop vec- absorb energy for speed control and ical and easiest platform to apply. The tor (OLV) drives use sophisticated motor worm gearboxes that can be as little as drive controls applied voltage and fre- control algorithms that independently 60 percent efficient. There are also huge quency to an AC induction motor, with control magnetizing current and benefits to replacing soft start equip- the of an induction motor - producing current. The benefits of OLV ment that does not provide intelligent ically coupled to its through an drives are higher starting torque, more

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accelerating torque, better speed regula- L1 Three-phase AC Motor tion, improved torque production at low L2 Frequency M No position feedback inverter 3~ operating speeds, as well as control in L3 both speed and torque modes. Typical V/Hz and open loop vector connection diagram Closed Loop Vector Platform In a closed loop or CL vector drive, L1 there is typically a more sophisticated Three-phase AC Motor L2 Frequency M with rotor position feedback IntelligentIntelligent Parking Parking processor used for motor control as inverter 3~ well as a feedback device located at the L3 motor. The feedback device is usually Feedback system an encoder that monitors speed and LotLot Controller Controller position. By reporting this information Typical closed loop vector connection diagram back to the drive it is able to make adjustments to ensure accurate control of speed, torque and position. So as a result, the main benefits are better speed regulation, full torque production at 0 speed, basic positioning and elec- tronic gearing.

Common Applications In many applications, low price and simple speed control are required. With these requirements, the V/Hz drive is the best bet. It is the simplest to install and has the lowest price of the three technologies. In fact, V/Hz drives are frequently replacing older forms of motor control. Technologies such as mechanical variable speed drives, solid state starters and conventional motor starters are all susceptible to the price, performance and simplicity of V/Hz Figure 1 drives. Common applications for this type of technology include centrifugal pumps and fans, conveyors and mixers. With centrifugal loads, there is an added benefit of energy savings when using a variable frequency drive for control. To illustrate, there are a set of physical properties known as the affin- ity laws that govern centrifugal loads:

Q is proportional to n P is proportional to n2 Hp is proportional to n3

Where: Q is flow n is speed Hp is Horsepower

Figure 1 illustrates the benefit of Figure 2 using V/Hz drives to control flow instead of dampers, inlet vanes or throt- percent of the energy required to oper- speed regulation and high starting/accel- tling valves. A small drop in flow ate the same fan at 100 percent flow. erating torque, an open loop vector drive results in a large drop in power con- Certain applications require a little is appropriate. Figure 2 shows how an sumption. For example, a fan operating more capability than simple speed con- OLV drive dynamically responds to a at 80 percent flow consumes only 51 trol. In applications requiring tighter 100 percent step change in load, with E-LINE V18,2_Layout 1 4/20/12 9:14 AM Page 68

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versus synchronous servo motors. Common applications for CLV drives are lifts, hoists, incline/decline convey- ors and extruders of fragile material.

Conclusion Simplicity is another factor in select- ing the right technology for a specific application. It is difficult to justify using a closed loop vector drive in a simple speed control application. While the CLV drive is perfectly capable, it the motor operating at 50 Hz with no Additionally, CLV drives need feed- would be cost prohibitive and require load applied. back to close the speed and position an unnecessary level of expertise to OLV drives today have achieved a loops. Therefore a feedback device implement. Conversely, to apply a level of performance and simplicity (typically an encoder) is required on the V/Hz drive in an application requiring that frequently allow users to replace motor. This feedback allows the drive torque control would certainly yield older DC drives without affecting per- to continuously monitor speed and unsatisfactory results compared with an formance. Common applications bene- position, making corrections as needed. OLV drive. Each of these technologies fiting from OLV technology are extrud- And performance comes at a cost. More has its place in the industry today. The ers, filling machines, forming machines advanced microprocessors and feed- key is to weigh cost, performance and and presses. back devices for CLV applications are simplicity to determine the correct So when is it appropriate to select a expensive; however, CLV systems are drive for your requirements. CL vector drive over an OL vector migrating into areas once reserved for drive? Closed loop vector drives are high end servo technology and the sys- Darrow Hanesian is Global Product capable of controlling the motor speed tem price compared to servos is dramat- Manager for Lenze. down 0 Hz while producing “holding ically less, due to the better cost struc- torque.” ture of asynchronous induction motors Circle 58 on Reader Service Card

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