AC Drive Systems and Control Methods

The AC drive and motor system has gained acceptance in the coordinated system environment because of the improvements in power semiconductor technology. In addition, high-speed process control, microprocessor, and communications improvements make the AC drive look like another node on the process network. Steel and aluminum processing, converting lines, and paper machines all require precise speed and torque control.

The basics of AC drives have already been covered. At this point, it would be helpful to review several AC applications and summarize the characteristics of each system.

Many drive applications use multiple motors to provide coordinated control. In certain applications, one section of the machine may operate faster than commanded speed. In cases like these, and where overhauling loads are possible, a common DC bus configuration is able to regenerate energy back to the DC bus. That energy is then used by another inverter section to power a different section of the system. --- a common DC bus configuration.

The supply section converts AC to DC through a fixed diode bridge rectifier. When horsepower is in the 1500- to 2000-HP range, the input converter section may be SCRs, to handle the high current. Reverse connected IGBT bridges may be used for full, four-quadrant regenerative braking.

This scheme also has the capability of operating in a very low harmonic mode.

Several manufacturers offer water-cooled units, which allow for increased sizes of drives, using diode bridge rectifiers, in smaller sizes than non-water-cooled. The braking chopper is part of a DB, IGBT sensing circuit that closes when a fast deceleration or stop is required. As in many cases with large horsepower, the controller unit is typically PLC mounted in a separate cabinet, along with other software and control devices.

---Coal classifier system -- indicates this system.

Coal Classifier

As an AC coordinated system component, a coal classifier uses devices found in many control systems. This system is found in coal-fired power utility plants, but the principles are the same whether filtering coal, cement, sand, or any other medium.

The classifier is part of a much larger, coordinated system. The output of the boiler /steam generator is dependent on the quality and purity of the coal powder that is burned as fuel.

Raw coal is loaded by conveyor into the hopper of the classifier. The classifier is operated by an AC motor and sifts through the coal particles, drop ping the small pieces into the coal pulverizer. The output of the pulverizer is a fine coal dust that burns cleanly and evenly, with the highest BTU output possible. The powder is then forced into the combustion chamber, where it’s used to fire the boiler and create steam for the turbine generator.

The classifier is operated as a closed-loop system. The plant operator controls the ultimate speed of the entire system. However, the classifier is part of that coordinated effort. A desired set point speed is entered at the operator console. The drive accepts that set point, and through PI control, looks at the actual speed feedback of the feed conveyor. The drive then makes speed corrections to power the classifier motor at the optimum speed. Too high of a speed would allow too many coal particles to enter the classifier, overloading the system. Too low of a speed would mean that few coal particles would enter the classifier and pulverizer. A lean burn would result in the combustion chamber, and BTU output would be reduced.

By means of PI control, this section of the system would operate at peak efficiency. The pulverizer unit would have a similar coordinated scheme, set up in the controller software.

HVAC Systems

AC variable-frequency drives (VFDs) are well known for their energy-saving capabilities. The savings can be quite substantial, as indicated in the next figures.

Assumptions:

• • Full rated flow = 178,000 CFM @ 3" of H2O
• • Fan/blower efficiency = 85%
• • Motor efficiency = 94%
• • Drive efficiency = 98%
• • Rated shaft power = 100 HP
• • Cost per KWH = $0.10

--- PI control using a VFD ( ABB Inc.): VFD Speed Feedback Transducer Signal Feed (Or directly into drive); Motor Supply

Fan Static Pressure Sensor directly into drive) (Transducer Signal Fed

---Fan energy use -- an energy-use comparison of variable-speed AC drive use versus outlet damper control.

---Fan efficiency improvement -- shows fan efficiency improvement using variable speed com pared with outlet damper control.

--- Annual savings for variable-speed fan -- the annual savings that a variable-speed fan can have compared with outlet damper control.

It’s clear that the amount of energy savings is substantial. The greatest savings are available when the fan is operated at 40-70% flow for the majority of the operating time. Savings can be also realized with VFDs versus inlet guide vanes. However, the highest savings will be realized using VFDs with a flow rate of only 30% for the majority of time. Even at that flow rate, a savings of less than $30,000 annually is seen, about the same as an outlet-damper system and at the same flow rate.

The system that enables the energy savings above.

In this example, the energy savings would come as a result of completely opening the outlet damper. The drive then operates as a closed-loop controller, responding to the static pressure feedback.

--- How a cooling-tower system operates. Cooling tower application ( Made by ABB Inc.) Fan motor speed based on Sump Temperature VFD Sump temperature sensor (Return) CWR (Supply) CWS

Cooling Towers

Another system that can realize substantial energy savings is a cooling tower.

The speed of the cooling tower fan(s) is controlled by the VFD. In traditional systems, the fan would operate at full speed 24 hours per day, unless cooling water was not required. With the VFD, operating in PI control, the set point temperature is converted to a voltage set point. The feedback from a temperature transducer allows the drive to calculate temperature error and respond with increased or decreased fan speed, or zero speed.

HVAC is a systems environment for AC drives. Seldom, if ever, are AC drives manually operated in office buildings, schools, or any other location where temperature or humidity is critical for daily operation. A typical VFD system connected to a building automation system.

--- Building automation system with VFD (ABB Inc.) DDC panel Fan Drive K Information VFD start/stop signal Fan or pump speed signal Fan or pump KW consumption Misc. Fault signals Additional information as Distributed digital control (DDC) provides the automated set points to the drives. The drive's on-board PI control provides motor speed appropriate to control the medium required. In return, the DDC systems can poll the drive for important operating information, such as start/stop, faults, KW consumption, and more.

Using the systems that are on the market today, it’s possible to connect to a variety of building automation systems, using almost any manufacturer's drive. However, comparisons should be made between vendors, to verify how much and what type of information can be obtained by the DDC sys tem. Some drives only allow start/stop and speed reference signals to be transmitted. A very few manufacturers will allow up to 60 parameters (points) to be viewed by the building automation system. In this age of timely information, the easier it’s to acquire operating information, the more efficient building operators can be.