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Variable Speed Motors - How do they work?

Knowledge is power (and motors need that!)






Efficiency Gains - Variable Speed Motors We may view the system as very inefficient. We’re locked into whatever efficiencies the motor can provide, given a somewhat variable amount of loading. If the motor happened to be an alternating current (AC) motor, typically, the following would be true:

  • 1. The more load on a motor, the more efficient that motor is.

  • 2. The higher the motor's horsepower (HP) rating, the higher the efficiency.

  • 3. The higher the operating speed, the more efficient the motor.

We will cover the physical makeup of AC and direct current (DC) motors in more detail. For now, we will use an AC motor to explain the effects of efficiency on the total system. As seen, efficiencies vary as noted above. If the conveyor motor happened to be 1 HP, we may expect to see only 70% efficiency, at 75-100% motor load. (% Efficiency = output power ÷ input power × 100). By strict definition, the 1-HP AC conveyor motor would be operating at a 30% loss at 75-100% motor load. --- AC drive and motor efficiencies at various speeds. --- an example of a 2-HP system. In this example, if we added a variable-speed AC drive, our efficiency of this constant torque (CT) system would be in the range of 80-90% when the conveyor is operated at 60% speed or higher. A conveyor is labeled a constant torque load and is indicated by a CT on the graphs. It should be noted that the AC drive is an efficient means of varying the speed of an AC motor. Its 5-10% losses are attributed to thermal losses because of the alternating current's switching of power devices several thousand times per second. Variable-speed output from a drive has a direct impact on the total system efficiency. A manufacturer can operate the production equipment at the most efficient speed and load point-if drive and motor efficiencies are known. --- AC drive and motor efficiencies: Efficiency (% of Rated Value) Motor Speed (% of Rated Speed) Motor Efficiency = cos phi = 0.83 (2 HP) VT drive CT System Drive and Motor Efficiency Process Changes / Improvements As previously indicated, in a fixed system there is no way to vary the speed of the conveyor. A fixed system won’t allow for changes in the process or production cycle. Some manufacturing circumstances may require a slow speed, others, a faster pace. The same conveyor system is used in processes such as baking. --- the same type of conveyor, with the addition of an industrial oven. Certain materials may require a longer baking cycle because of thickness. If a fixed-speed motor is used, only one type of material could be processed in this system. To stay competitive, many companies require flexibility in manufacturing. A variable-speed system is often necessary to change production cycle times and increase capacity. --- Industrial oven used in production: Production Industrial Oven Warehouse Fixed Speed Motor --- Manually controlled conveyor system: Production Industrial Oven Warehouse Fixed Speed Motor Operator Station System Coordination The system is typical of many manually operated processes. An operator turns on the system and turns it off for maintenance or at the completion of the production cycle. However, in an age of increased flexibility requirements, few processes are manually operated. Production cycles are constantly monitored by some type of computer system. Computer systems will automatically oversee the process and correct for load fluctuations, material density, and size requirements. In industrial processes, the use of PLCs (programmable logic controllers) is typical. Programmable logic controllers are beyond the scope of this guide, but will be addressed at various points. --- a conveyor system that is manually operated by a control station. Programmable logic controllers work effectively in place of the manually controlled operator station. Automatic control of the motor could there fore be accomplished, but only STOP and START control, in this case. Variable-speed drives would be effective in providing the flexibility and control needed by motors to meet almost any application requirements. --- Generic variable-speed drive system: Power Source (Supplies power to the drive); Controller Drive Motor Coupler Machine (Generates and sends a reference to the drive) (Controls the speed, torque and direction of the motor) (Changes one form of energy to rotating energy) (Connects the motor to the machine) (Device that performs the work)

Drive Principles of Operation At this point, we will look at a variable-speed drive system-from a generics standpoint. All drive systems, whether, electronic, mechanical, or fluid in nature. To understand a simple drive system, we will start at the end of the system and move backward. We will devote individual sections of this guide to each of the basic components listed. For now, the intent is to develop a basic understanding of a drive system. A foundation will be built, which will allow more complex concepts to be discussed in later sections. ​Machine The essence of any drive system is the application, or machine. This is the heart of the system, since it ultimately needs to perform the work. Consider the machine-the application. It could be a conveyor, a press, a pack aging machine, or literally hundreds of applications that operate at variable speed. Coupler The coupler is the device that connects the machine to the motor. Couplers come in all shapes and sizes. Its basic task is to make a solid connection between the motor and the machine. Couplers may accept one diameter of motor shaft and convert the output to another size shaft. In some cases, the coupler may actually be a device called a gearbox, which may include some type of speed-reducing or speed-increasing gears. Couplers could also be considered matching devices because of their ability to deliver power smoothly to the machine. To a certain extent, this device can also cushion shocks delivered by the motor to the machine. Motor This device changes one form of energy to rotating mechanical energy. It can be considered the prime mover because it takes power from the drive unit and translates it into motion. As we will see shortly, there are several types of motors using various forms of energy. In this guide, we will discuss mechanical, hydraulic, AC, and DC motors. The size of the motor usually dictates the amount of rotating motion it can generate from incoming power. We will see later that there are a few exceptions to this principle. Drive The drive can be considered the heart of the whole system. This section controls the speed, torque, direction, and resulting horsepower of the motor. The drive is very similar in nature to an automobile drive system. The transmission and drive shaft controls the speed, direction, and power delivered to the wheels. Much of this guide will be devoted to AC and DC drives. However, we will take a brief look at other types of drive systems that exist in industry. Power Source The drive must have a source of power to operate effectively. If the drive is electrical, it must have either single- or three-phase power available. The drive then accepts this power and modifies it to an output that is usable by the motor. If the drive is hydraulic, the power source could be considered the hydraulic-fluid reservoir, since it supplies the drive with the form of power it needs to accomplish the job. Controller The controller supplies a reference signal to the drive unit. Typically controllers are electronic and supply a small voltage or current signal to the drive. The larger the signal, the more power the drive generates, and the faster the motor rotates. In many cases, the controller is an automatic device such as a computer. The computer has the ability to take in signals from external devices such as switches or sensors. The controller then processes the signals, does calculations based on the sensor inputs, and generates a reference signal. This output reference signal is usually a speed signal to tell the drive how much power to generate. As we will see later, this is not always the case. The controller could generate an output signal to tell the drive how much power to generate in order to control motor torque or motor shaft position. The operator station can also be considered a controller. Instead of being an automatic device, the operator station provides a signal based on a manually operated switch or speed control set by a human operator. Types of Drives, Features and Principles In this section, we will briefly review the different types of variable-speed drives used in industry. For the most part, electronic AC and DC drives find their dominance in manufacturing and commercial HVAC applications of today. This brief look at drive technologies will assist you, should you encounter any of these types in the future. In addition, we will also review the benefits and limitations of each type. The types of drives we will consider are mechanical, hydraulic, and electrical/electronic (eddy current coupling, rotating DC, DC converters, and variable-frequency AC). --- Mechanical variable-speed drive Constant Speed AC Motor Coupler Variable Speed Output Variable Diameter of the Pulleys Will Dictate Output Speed Mechanical Mechanical variable-speed drives were probably the first type of drive to make their way into the industrial environment. ---A basic mechanical variable-speed drive. As seen, the mechanical drive operates on the principle of variable-pitch pulleys. The pulleys are usually spring-loaded and can expand or contract in diameter by means of a hand crank (shown on the left side of the constant speed AC motor). The mechanical drive still gets its power source from an AC power supply-usually three-phase AC. Three phase AC is then fed to the fixed-speed AC motor. The ability to vary the diameter of one or both pulleys gives this drive unit the ability to change its output speed. The principle of variable speed is exactly the same as the gears of a 15-speed bicycle. Shifting gears causes the chain to slip into a wider- or narrower-diameter sprocket. When that happens, a faster or slower speed is achieved with basically the same input power. Years ago, the benefits of this type of drive were low cost and the ability to easily service the unit. Many technicians liked to work on mechanical problems. The malfunction was rather obvious. However, the benefits of yesterday have turned into the limitations of today. Mechanical devices have a tendency to break down-requiring maintenance and downtime. The efficiency of the unit can range from 90% down to 50% or lower. This is due to the eventual slipping of the belt on the pulleys (sometimes called sheaves). Sometimes the speed range can be a limitation because of fixed diameter settings, a characteristic of the mechanics of the device. Size can also be a limitation. Typically floor-mounted, this device sometimes stood 3-5 feet tall for general applications. Size and weight could prohibit the use of this device in areas that would be required for mounting a drive. --- Hydraulic drive: Constant Speed AC Motor; Coupler; Pump Control Valve, Fluid, Reservoir, Hydraulic Motor, Variable Speed, Output Hydraulic Drives Hydraulic drives have been, and continue to be, the workhorse of many metals processing and manufacturing applications. The hydraulic motor's small size makes it ideal for situations where high power is needed in very tight locations. In fact, the hydraulic motor's size is 1/4-1/3 the size of an equivalent power electric motor. A hydraulic drive. A constant-speed AC motor operates a hydraulic pump. The pump builds up the necessary operating pressure in the system to allow the hydraulic motor to develop its rated power. The speed control comes from the control valve. This valve operates like a water faucet-the more the valve is open, the more fluid passes through the system, and the faster the speed of the hydraulic motor. Note that this system uses a coupler to connect the AC motor to the pump. The benefits of this type of drive system is the ability of the hydraulic motor to develop high torque (twisting motion of the shaft). In addition, it has a fairly simple control scheme (a valve), which operates at a wide speed range and has an extremely small size compare to most AC motors of the same power. However, this type of system has several major limitations. The most limiting factor of this system is the need for hydraulic hoses, fittings, and fluid. This system is inherently prone to leaks, leading to high maintenance costs. In addition, there is virtually no way to connect this system to an electronic controller. Automatic valve-type controls have been developed, but their use is limited in today's high-speed manufacturing environment. --- Eddy-current drive system Eddy-Current Drives Eddy-current drives have their roots in the heavy machinery part of industry. Grinding wheels are prime candidates for eddy-current drives. This system uses an AC-to-DC power-conversion process, which allows variable shaft speeds, depending on the amount of power converted. --- indicates a simple eddy-current drive system. As seen, an AC motor operates at a fixed speed. This causes the input drum to operate at the same speed. The function of the DC exciter is to convert AC power to DC power. This power is then fed to the coupling field. The coupling field generates a magnetic field based on how much DC power is being produced by the DC exciter. The more power produced, the more magnetic field is produced and the stronger the attraction of the coupling assembly to the input drum. How much power produced by the DC exciter is determined by the speed reference potentiometer (speed pot). The benefits of an eddy-current system include initial cost and the simple control method (usually 1 speed pot). In addition, this type of system can produce regulated torque because of its ability to fairly accurately control the DC exciter. However, several limitations dictate where and how this type of system is applied. Heat generation and power consumption are the major issues. For the coupling assembly to magnetically couple to the input drum, a large amount of power must be produced. When power is produced, heat is the by-product, and energy savings are not realized. Compared with other types of variable-speed drives, this type can be several times larger, thereby limiting the locations where it can be mounted. Size is also an issue when maintenance is required on the rotating machinery. Typically on-site repairs are required, which is more costly than shipping the unit back to the repair location. --- Rotating DC variable-speed drive: Constant Speed, AC Motor Coupler, Field Exciter, Motor Field Exciter Gen. Speed Reference (Pot) DC Motor Speed Reference (Pot) DC Generator, DC Motor Gen. Field Coils, Motor Field Coils Rotating DC Drives This system dates back to the mid 1940s. The system also gained the name M-G set, which stands for motor-generator set. As seen, that description is quite accurate. As seen, the variable-speed system is more complicated than an eddy-current system. The constant-speed AC motor causes the DC generator to produce DC power. The amount of power produced by the generator is dependent on the magnetic strength of the field exciter of the generator. The field exciter strength is determined by the position of the speed pot. As will be shown later, the DC motor requires two circuits in order to operate properly. In this case, the DC generator feeds power to the main circuit of the DC motor (called the armature). The DC motor also needs another circuit called the field. The field magnetism interacts with the magnetism in the main circuit (armature) to produce rotation of the motor shaft. The strength of the field magnetism depends on how much power is produced by the motor field exciter. The field exciter strength is determined by the position of the DC-motor speed pot. This system has several benefits. Years ago in the rotating machinery industry, this equipment was very traditional equipment. This system also had the ability to control speed accurately and had a wide speed range. It typically used motors and generator equipment that had a very large over load capacity, compared with modern-day motors. Today, a system of this type, however, would carry several limitations. Because of the need for three rotating units (AC motor, DC generator, and DC motor), this system is prone to maintenance issues. DC equipment uses devices called brushes, which transfer power from one circuit to the other. These devices need periodic replacement, meaning the machine needs to be shut down. This system is also larger than many of the other variable-speed units. In today's industrial environment, replacement parts are harder to find. The early units used a power conversion device called a vacuum tube (high-temperature electrical conduction), which is very difficult to acquire as a spare part. As to be expected, three rotating units increases the maintenance required on mechanical parts. --- Electronic DC drive; DC Drive 3 Phase AC Speed Reference (Pot) DC Motor Field Coils Motor Field Exciter Electronic Drives (DC) DC drives have been the backbone of industry, dating back to the 1940s. At that time, vacuum tubes provided the power conversion technology. Vacuum tubes led to solid-state devices in the 1960s. The power conversion device, called the silicon controlled rectifier (SCR), or thyristor, is now used in modern electronic DC drives. --- indicates the main components of a simple DC drive system. As seen, the DC drive is basically a simple power converter. It contains two separate power circuits, much like that of the rotating DC unit. Typically, three-phase AC power is fed to the drive unit. (Note: Some small horsepower DC drives will accept one-phase power.) The drive unit uses SCRs to convert AC power to DC power. The speed pot determines how much the SCRs will conduct power. The more the SCRs conduct power, the more magnetic field is generated in the main DC motor circuit, the armature. In a DC-drive system, there is always a separate magnetic circuit, called a field. The strength of the magnetic field is determined by the separate motor field exciter, or a permanent magnet. The motor field is usually kept at full strength, although in some cases, the field will be weakened to pro duce a higher-than-normal speed. The interaction between the motor armature and field produces the turning of the motor shaft. We will go into further detail on DC-drive technology later in this guide. There are some definite benefits to a variable-speed drive system of this type. This mature technology has been available for more than 60 years. Because electronic technology is used, a wide variety of control options are available. Monitors such as speed and load meters and operating data circuits can be connected to illustrate drive operation. A remote operator station, including an isolated speed reference and start/stop circuits, can also be connected to the drive. This type of remote control allows commands from distant locations in the building. The DC drive offers acceptable efficiency, when compared with other variable-speed technologies. In addition, DC drives offer a small size power unit and comparable low cost in relation to other electronic drive technologies. However, when comparing electronic DC-drive technology with AC technology, several limitations should be considered. Probably the largest issue with DC-drive systems is the need for maintenance on the DC motor. As indicated in the rotating DC-drive section, DC motors need routine maintenance on brushes and the commutator bars. Another issue that is critical to many manufacturing applications is the need for back-up capability. If the DC drive malfunctions, there is no way to provide motor operation, except through connection of another DC drive. In this day of efficient power usage, the DC drive's varying power factor must be considered when planning any installation. Total operational costs (maintenance, installation, and monthly operating costs) may be a limitation when comparing the DC system with the AC-drive system. --- Variable-speed AC drive 3 Phase AC Speed Reference (Pot) AC Motor AC Drive

Electronic Drives (AC) Basically, three types of AC drive technologies are currently available. Though each type differs in the way power is converted, the end result is the use of a variable-speed AC induction motor. All AC drives take AC input, convert it to DC, and change DC to a variable AC output, using a device called an inverter (i.e., inverts DC back to AC voltage). For purposes of this section, we will confine our discussion to a generic AC drive. --- a generic AC drive and its basic components. The basic objective involved in an AC drive is to change a fixed incoming line voltage (V) and frequency (Hz) to a variable voltage and frequency output. The output frequency will determine how fast the motor rotates. The combination of volts and Hertz will dictate the amount of torque the motor will generate. When we look closer at the principles involved, we find that the AC drive essentially changes AC power to DC power. The DC power is then filtered and changed back to AC power but in a variable voltage and frequency format. The front end section consists of diodes. Diodes change AC power to DC power. A filter circuit then cleans up the DC waveform and sends it to the output section. The output section then inverts the DC power back to AC. This is accomplished through a series of transistors. These are special transistors that only turn on or turn off. The sequence and length in which these transistors turn on will determine the drive output and ultimately the speed of the motor. With this type of variable-speed system, there are more benefits than limitations. When compared with DC drives, small-sized AC drives are equal to or lower in cost (5 HP or less). The efficiency of power conversion is comparable to that of DC drives. Also comparable is the ability to be con trolled remotely and to have various monitor devices connected. Because of modern transistor technology, the size of the AC drive is equal to or even smaller than that of an equal horsepower DC drive (125-150 HP or less). One major advantage of AC drives is the ability to operate an AC motor in bypass mode. This means that while the drive is not functioning, the motor can still be operating, essentially across line power. The motor will be operating at full speed because of the line power input. But the benefit would be that the system continues to operate with little or no downtime. There may be a few limitations when considering AC drive technology. With low horsepower units (above the 25- to 30-HP range), AC drives may carry a higher purchase price. However, the installation costs may be less because of less wiring (there is no separate field exciter). Some applications, such as printing and extrusion, lend themselves to DC technology. Comparable AC drives may need to be sized 1 or 2 HP frame sizes higher to accommodate the possible overload requirements. "Torque Control AC Drives" is devoted to flux vector and torque-con trolled AC drives. More detail is presented on the issue of overload, torque control, and AC/DC drive comparisons. Today's AC-drive technology can provide impressive response, filling the application needs that traditionally used DC drives. Review There are various types of variable-speed drive systems. There are many reasons to use variable-speed drives, but basically they fall into three categories: efficiency gains, process changes and improvements, and system coordination. For example, efficiency of AC motors can be quite high, which reduces the overall monthly cost of operating the system. Variable speed drives also allow for changes in the process, as well as process improvements. Some processes operate at less than full speed, so optimum product quality can be achieved. System coordination is a major factor in today's industrial environment. AC- and DC-drive systems are typically applied in a manufacturing process. Computers control the entire process, from infeed rate to output of the machine. Today's electronic drives offer easy connection to many types of automated equipment. A generic drive system includes the following components: machine, coupler, motor, drive, controller, and power source. No matter what type of system is discussed, these main components are involved. Various types of variable-speed drives are available in industry. The basic categories are mechanical, hydraulic, and electrical/electronic. Electronic drives can be further divided into the following categories: eddy current, rotating DC, DC converters, and variable-frequency AC. Each type of variable-speed drive system has its set of benefits and limitations. The trend today is moving away from mechanical and hydraulic types of variable-speed systems, and toward electronic systems. The reasons are again identified in the ability to control the process by computerized systems. This also allows for quick changes in the process to meet the rigorous demands of production schedules. QUIZ 1. What is a drive? 2. What are three reasons why variable-speed drives are used? 3. Name three factors that cause the efficiency of an AC motor to improve. 4. Coordination of variable-speed drive systems in industry are typically controlled by what type of device? 5. Name the basic parts and functions of a variable-speed drive system. 6. Name the categories of variable-speed drives and their principles of operation. 7. What are the two separate electrical circuits in a DC-drive system? 8. What three principles are involved in the operation of an AC drive? ​-- Answers -- 1. A drive is a device that controls the speed, torque, direction, and resulting horsepower of a system. 2. Efficiency gains, process changes and improvements, and system coordination. 3. Motor load, the horsepower rating, and the speed of operation. 4. A programmable logic controller (PLC) or other computer device. 5. Machine-device that performs the work; Coupler-connects the motor to the machine; Motor-changes one form of energy to rotating; Drive-controls the speed, torque, and direction of the motor; Controller-generates and sends a reference to the drive. Power Source-supplies power to operate the drive. 6. Mechanical, hydraulic, and electric/electronic: Mechanical-device that uses belts, chains, and pulleys to change the output speed through increasing and decreasing of pulley diameters. Hydraulic-device that uses fluid and a pump to operate a hydraulic motor. Control of motor speed is done through a valve. Electric/electronic-device that controls the speed of an electric motor by means of converting one form of energy to another. (An AC drive converts fixed power to a variable-frequency output. A DC drive converts AC power to variable-voltage DC power.) 7. The armature supply and the field supply. 8. Changing AC power to a fixed-voltage DC power, filtering the DC wave form, and inverting fixed DC voltage to a variable voltage and frequency AC output.



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