Selecting the right motor and drive combination can save energy and improve performance.
The venerable electric motor that was the muscle of the industrial revolution is becoming the smart muscle of the computer-controlled plant and commercial facility of the future. The advent of powerful, reliable electronic drives is keeping motors in the forefront of this technological evolution.
Electric motors have a tremendous impact on overall energy use. Between 30 to 40 percent of all fossil fuels burned are used to generate electricity, and two-thirds of that electricity is converted by motors into mechanical energy.
The Fundamentals of Energy Management series this month will focus on topics that will allow facility managers and engineers at commercial and industrial facilities to understand the basics of motors and drives. This information will help them to select and implement strategies with the goal of reducing motor and drive costs as well as decreasing downtime.
AC Induction Motors
AC induction motors are ideal for most industrial and commercial applications because of their simple construction and low number of parts, which reduce maintenance cost. Induction motors are frequently used for both constant-speed and adjustable speed drive (ASD) applications.
The two basic parts of an induction motor are the stationary stator located in the motor frame and the rotor that is free to rotate with the motor shaft. Today's motor design and construction are highly refined. For example, stator and rotor laminations have been designed to achieve maximum magnetic density with minimum core losses and heating. The basic simplicity of this design ensures high efficiency and makes them easily adaptable to a variety of shapes and enclosures.
A three-phase induction motor can best be understood by examining the three-phase voltage source that powers the motor. Three-phase currents flowing in the motor leads establish a rotating magnetic field in the stator coils. This magnetic field continuously pulsates across the air gap and into the rotor. As magnetic flux cuts across the rotor bars, a voltage is induced in them, much as a voltage is induced in the secondary winding of a transformer. Because the rotor bars are part of a closed circuit (including the end rings), a current begins to circulate in them. The rotor current in turn produces a magnetic field that interacts with the magnetic field of the stator. Since this field is rotating and magnetically interlocked with the rotor, the rotor is dragged around with the stator field.
When there is no mechanical load on the motor shaft (no-load condition), the rotor almost manages to keep up with the synchronous speed of the rotating magnetic field in the stator coils. Drag from bearing friction and air resistance prevents perfect synchronicity. As the load increases on the motor shaft, the actual speed of the rotor tends to fall further behind the speed of the rotating magnetic field in the stator. This difference in speed causes more magnetic lines to be cut, resulting in more torque being developed in the rotor and delivered to the shaft mechanical load. The rotor always turns at the exact speed necessary to produce the torque required to meet the load placed on the motor shaft at that moment in time. This is usually a dynamic situation, with the motor shaft speed constantly changing slightly to accommodate minor variations in load.
The rotor consists of copper or aluminum bars connected together at the ends with heavy rings. The construction is similar to that of a squirrel cage, a term often used to describe this type of ac induction motor.
The rotating magnetic field in the stator coils, in addition to inducing voltages in the rotor bars, also induces voltages in the stator and rotor cores. The voltages in these cores cause small currents, called eddy currents, to flow. The eddy currents serve no useful purpose and result in wasted power. To keep these currents to a minimum, the stator and rotor cores are made of thin steel discs called laminations. These laminations are coated with insulating varnish and then edge welded together to form a core. This type of core construction substantially reduces eddy current losses, but does not entirely eliminate them.
By varying the design of the basic squirrel-cage motor, almost any characteristic of speed, torque, and voltage can be controlled by the designer. To standardize motor features the National Electrical Manufacturers Association (NEMA) has established standards for a number of motor features.
The speed of an ac induction motor depends on the frequency of the supply voltage and the number of poles for which the motor is wound. The term poles refers to the manner in which the stator coils are connected to the three incoming power leads to create the desired rotating magnetic field. Motors are always wound with an even number of poles. The higher the input frequency, the faster the motor runs. The more poles a motor has, the slower it runs at a given input frequency. The synchronous speed of an ac induction motor is the speed at which the stator magnetic flux rotates around the stator core at the air gap. At 60 Hz the following synchronous speeds are obtained:
Number of poles RPM
Providing the motor is properly constructed, the output speed can be doubled for a given number of poles by running an ASD supplying the motor at an output frequency of 120 Hz.
The actual speed of an induction motor rotor and shaft is always somewhat less than its synchronous speed. The difference between the synchronous and actual speed is called slip. If the rotor rotated as fast as the stator magnetic field, the rotor conductor bars would appear to be standing still with respect to the rotating field. There would be no voltage induced in the rotor bars and no current would be set up to produce torque.
Induction motors are made with slip ranging from less than 5% up to 20%. A motor with a slip of 5% or less is known as a normal-slip motor. A normal-slip motor is sometimes referred to as a 'constant speed' motor because the speed changes very little from no-load to full-load conditions. A common four-pole motor with a synchronous speed of 1,800 rpm may have a no-load speed of 1,795 rpm and a full-load speed of 1,750 rpm. The rate-of-change of slip is approximately linear from 10% to 110% load, when all other factors such as temperature and voltage are held constant. Motors with slip over 5% are used for hard to start applications.
The direction of rotation of a poly-phase ac induction motor depends on the connection of the stator leads to the power lines. Interchanging any two input leads reverses rotation.
Torque and Horsepower
Torque and horsepower are two very important characteristics that determine the size of the motor for a particular application. Torque is the turning effort. For example, suppose a grinding wheel with a crank arm one-foot long takes a force of one pound to turn the wheel at steady rate. The torque required is one pound times one foot or one foot-pound. If the crank is turned twice as fast, the torque remains the same. Regardless of how fast the crank is turned, the torque is unchanged as long as the crank is turned at a steady speed.
Horsepower takes into account how fast the crank is turned. Turning the crank more rapidly takes more horsepower than turning the crank slowly. Horsepower is the rate of doing work. By definition, one horsepower equals 33,000 foot-pounds per minute. In other words, to lift a 33,000-pound load one foot in one minute would require one horsepower.
The discussion so far has only involved torque at a steady speed. More effort is required to start a load than to keep it going. An ac induction motor is built to supply the extra torque needed to overcome the inertia of starting a load. The speed-torque curve for a representative motor is shown in Figure 4 and illustrates in this example that the starting torque is 210% of rated-load torque.
Motor Losses and Loss Reduction Techniques
The only way to improve motor efficiency is to reduce motor losses. Since motor losses produce heat, reducing losses not only saves energy directly but can also reduce cooling load on a facility's air conditioning system.
Motor energy losses can be segregated into five major areas. Each area is influenced by the motor manufacturer's design and construction decisions. One design consideration, for example, is the size of the air gap between the rotor and the stator. Large air gaps tend to maximize efficiency at the expense of a lower power factor. Small air gaps slightly compromise efficiency while significantly improving power factor.
Motor losses may be grouped as fixed or variable losses. Fixed losses occur whenever the motor is energized and remain constant for any given voltage and speed. Variable losses increase with an increase in motor load. Core loss and friction windage losses are fixed. Variable losses include stator- and rotor-resistance losses and stray load losses.
The principal factors in energy-saving calculations are motor efficiency, run hours (at a certain load), and the cost of electricity. When a motor runs at nearly full load for many hours at a facility with high electrical costs, the higher resulting savings will indicate the use of a 'premium efficiency' unit. In some cases, the savings may be great enough to warrant taking a perfectly serviceable older motor off-line and upgrading to a new, premium-efficiency model. For applications with less than continuous use or at lower than full loading, upgrading a working motor will usually not make sense.
Some other application considerations: The full-load speed of high-efficiency motors is usually somewhat higher than standard efficiency models. When applied to centrifugal loads such as pumps and fans, the higher speed can translate to an increased horsepower requirement, and this can offset the anticipated energy savings. The higher output speed may also increase overall output by allowing a piece of machinery to finish its job faster. Results will vary with the application. There are energy efficient motor designs that can exhibit some unusual characteristics such as lower power factor and higher locked rotor amperage; these must be considered when choosing a motor.
Adjustable Speed Drive Systems
Commercial and industrial firms today use adjustable-speed drive (ASD) systems for a variety of applications. Most common of these include standard pumps, fans, and blowers. Newer applications include hoists and cranes, conveyors, machine tools, film lines, extruders, and textile-fiber spinning machines.
Many applications have unique demands and characteristics.
Drive vendors have responded to this demand by producing a variety of drives. The combination of the many types of drives available and the abundance of applications has made the selection of the optimum drive for a given application a challenge.
New generation ASDs have evolved with advancements in solid-state electronics. ASDs can now be applied to ac motors regardless of motor horsepower or location within a facility and can be used to drive almost all types of motorized equipment, from a small fan to the largest extruder or machine tool. Commercial and industrial facilities can expect to dramatically reduce both energy consumption and operating and maintenance costs while offering improved operating conditions by using new generation electronic ASDs. The latest generation of ASDs allows ac induction motors to be just as controllable and efficient as their dc counterparts were.
Historically a variety of terms have been used to describe a system that permits a mechanical load to be driven at user-selected speeds. These terms include, but are not limited to:
The term variable implies a change that may or may not be under the control of the user. Adjustable is the preferred term since this refers to a change directly under control of the user. The term frequency can only be applied to drives with an ac output, while the term speed is preferred since this includes both ac and dc drives. Thus, the term most commonly accepted is Adjustable-Speed Drive (ASD).
Basic ASD Components
Most ASD units consist of three basic parts. A rectifier that converts the fixed frequency ac input voltage to dc. An inverter that switches the rectified dc voltage to an adjustable frequency ac output voltage. (The inverter may also control output current flow, if desired.) The dc link connects the rectifier to the inverter. A set of controls directs the rectifier and inverter to produce the desired ac frequency and voltage to meet the needs of the ASD system at any moment in time.
The advantages of ASDs do not stop with saving energy and improving control. ASD technology can now be applied to manufacturing equipment previously considered too expensive or uneconomical. Such applications are often unique to a particular industry and its equipment, or even to a particular facility. Cost benefits, such as those obtained from improved quality, may be desirable for each application.