Energy savings are an extremely important topic in virtually every segment of industry today. In general, the largest consumer of power in a converting line or machine is the drive system. As energy costs continue to increase and energy conservation becomes a greater priority, are there technologies or methods that can be implemented to reduce the energy consumption on converting machinery?
Before looking at the details and benefits of DC Common bus drive systems, let’s take a look at the typical standalone AC drive. The power design of today’s Pulse Width Modulated (PWM) AC drive is made up of three sections. The input section is the rectifier that converts single- or three-phase AC voltage into DC voltage. The DC link is the middle section that contains a capacitor bank to smooth and buffer the DC voltage. And third is the fast switching inverter section that pulses the DC voltage into a three-phase power signal suitable for an inverter duty-rated AC motor.
AC/AC Drive Systems
As shown in Figure 1, standard AC/AC drives are applied in a multi-axes coordinated drive system. Each individual drive is connected to the AC line via individual line components (fuses, reactors, contactors, and component wiring). Each drive section must deal with its regenerative power individually.
Let’s consider a drive system for a converting line with an unwind, pull roll master section, coater, laminator, and rewind. In this scenario, the machine sections that add tension to the web (unwind and laminator) must return their power to the drive, and in turn this energy is subsequently dissipated (wasted) by the regen resistors connected to the individual drives. In this case, 75A of current is wasted as heat.
In some cases, a Pseudo-Common DC bus is created with AC/AC drives that have an external bus connection by wiring the bus connections together. However, this application is problematic as the current carrying capability of these bus connections does not always match the drive power rating. Precautions also must be taken to prevent the smaller drives from charging the larger drives. In any case, the added components required to create a Pseudo-Common DC bus are costly and inefficient.
Common DC Bus Architecture
True common DC bus drive systems are far more efficient than the system composed of standalone AC/AC drives in several ways. When drive systems utilize a common DC bus design, a shared rectifier section is used to convert the AC power supply into a DC bus that is common to the parallel connected motor modules (inverters).
Power sharing is now permitted between each different drive section linked on the DC bus. When power sharing occurs on the DC bus between drives that are motoring and generating simultaneously, the drive system now uses less power from the rectifier as the generating drive sections can return their power to the DC bus to be shared by the motoring or consuming drive sections.
In the same example as above, the common DC bus system will use almost 75A less than the AC/AC drive system (Figure 2). Additionally, the line components (i.e. contactor, reactor, fuses, and rectifier) can be sized based on the maximum current draw of the system, not the summation of the individual motors. This also results in a more size-optimized and energy-efficient design as losses are realized in each individual line component and rectifier.
Active Front End Technology (AFE)
Active Front End (AFE) Infeed technology takes the DC common bus system to a level of additional energy savings. An AFE is an IGBT-based rectifier that regulates or controls the DC bus level, for both over and under voltage. This type of rectifier is suitable as a substitute or replacement for the Basic or regen SCR-based modules discussed above.
In addition to line regen capability, this functionality also allows the input voltage and current waveforms to the drive to be sinusoidal, prevents harmonics from being generated back to the line, and offers near unity power factor. Although the reduction in harmonics can be very important to plant operation, the main energy savings from the AFE comes from the improvement in power factor. AFE-controlled drives can have a .99+ power factor. In Figure 3, the effective line current in a diode bridge rectifier and AFE rectifier is detailed.
Power Factor Savings
Power factor is a measure of how effectively electrical power is being used. A high power factor (approaching unity) indicates efficient use of the electrical distribution system, while a low power factor indicates poor use of the system.
Power factor is the ratio of real power to apparent power. To determine power factor (PF), divide real power (kW) by apparent power (kVA). In a sinusoidal system, the result is also referred to as the cosine 0.
When a utility serves an industrial plant that has poor power factor, the utility must supply higher current levels to serve a given load. A utility is paid primarily on the basis of energy consumed and peak demand supplied. Without a power factor billing element, the utility would receive no more income from the second plant than from the first. As a means of compensation for the burden of supplying extra current, utilities typically establish a “power factor penalty” in their rate schedules. A minimum power factor value is established; usually 95 percent. When the customer’s power factor drops below the minimum value, the utility collects “low power factor” revenue.
Eliminating Mechanical Losses
There are two major areas in converting machinery where significant energy is lost through friction and mechanical inefficiency. The first is mechanical drive systems or gearboxes with high ratios. The second is on unwinds with mechanical tension control brakes (Figure 4).
High gear ratios are required when optimizing motor sizes when driving large-diameter rolls or on very low-speed web applications. Where planetary gearboxes are fairly efficient, high-ratio multi-stage worm gearboxes can easily have efficiencies under 60%. Low-speed applications and driven sections that previously looked to inefficient gearboxes are commonly becoming direct driven with torque motors and even conventional motors, thus eliminating the energy losses.
Driven Unwinds vs. Mechanical Brakes
Unwinds with mechanical brakes are an ideal place from which energy can be recovered. Mechanical brakes create web tension from friction, so the heat created in this process is energy that can be recovered. Pneumatic or electromechanical tension control brakes are commonly replaced with an AC drive system with line regenerative capability.
A driven unwind (Figure 5) must return the tension energy back to the AC line. In the past, regen DC drives have been successfully applied in these applications, but DC drive systems are no longer common and even during their prime, were very costly when compared to their mechanical counterparts. Early on in AC drive technology, the drives did not have the capability to regenerate the power back to the AC line and when applied as unwind brakes, required regen resistors to dissipate the tension energy. This was wasteful and costly.
Today’s AC drive systems now have the technology to regenerate the energy back to the AC line just as the DC drive did, but with added benefits to the user and machine designer alike. Sending the tension energy back to the line means power that once was wasted can now be retained, instead of the system producing heat and worn parts. Additionally, if the drive is equipped with AFE technology, it will return the energy with near unity power factor — something not possible for any DC drive system.
Drive Optimization (Mechatronics)
Paying attention to drive and motor sizes versus actual load requirements for the specific application and making sure that that coordinated drives are properly tuned is a point that will aid in energy savings. Oversized drive systems simply waste energy. The cost of energy waste is realized in the higher magnetizing current. An AC drive system’s magnetizing current can be nearly half of the full load current (FLA). Consider an example of a 100-hp AC drive system applied to an actual 30-hp load requirement. In this example, 40 amperes of line current is wasted. That relates to an energy savings =34A for a single drive.
Poorly tuned drives not only can affect machine performance and product quality, but waste significant energy. Drive systems that are tuned beyond the optimal can waste energy as they drive the current loop harder. The overactive current loop will waste energy as heat in the motor. As industry trends push the drive system’s performance, mechatronics can ensure higher performance without wasting energy. The main issues can arise from complex loads, compliance, lost motion, and machine resonances.
Applied mechatronics support can help to archive the required system performance without wasting energy and affecting machine life. Replacing outdated DC drive and motor systems with AC drive technology can offer energy savings from the improved energy efficiency of the AC system over its DC counterpart. In addition, savings from improved power factor can also be realized.
While the DC motor without regard to the drive is more efficient than an AC motor, the AC PWM drive is far superior to a DC SCR drive. When considering drive system efficiency, the AC drive system can offer an efficiency improvement in the range of ~3% when operating at near full load, where the DC drive efficiency is at its highest. Consider the example of single standalone drive systems, both at 100-hp, running at 90% load, 12 hours a day, 7 days a week. Just a single AC/AC drive replacement can provide over $1000 of energy savings per year.
Pump and Fan Losses
In applications such as flow control, energy savings can be acquired by adding an AC drive. The biggest potential for saving is offered by pumps, fans, and compressors that are still operated with mechanical throttles and valves. Converting to variable-speed drives can produce considerable economic benefits.
Changing the flow mechanically versus controlling the flow with an AC drive has many disadvantages. With mechanical flow control, the motor runs continuously at the speed required for the maximum delivery rate, which is rarely needed in practice. Additionally, throttles and valves lose energy and cause high temperatures and vibration levels that can have a negative impact on the drive and production operation.
Variable-speed drives with inverters offer a more economic alternative for a number of reasons. They can be controlled much more quickly and precisely. By adapting the flow rate directly to actual requirements, energy savings of up to 60% can be achieved, especially in energy-intensive applications. Consider the example in Figure 6 that compares flow control by a mechanical throttle to flow control by speed control for an overview of typical losses. In this example, the input power requirement of the driven fan or pump is only 56% of the input power requirement of the mechanical throttle example.
Drives and driven systems in converting lines are major energy consumers, but advances in technology continue to offer multiple avenues for reducing the total energy costs.
This article was written by William Gilbert, Industry Manager, Converting Solutions, at Siemens Industry, Motion Control Business, Norcross, GA. For more information, Click Here .