Motors and Drives Basics

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
2 3,600
4 1,800
6 1,200
8 900
10 720
12 600

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.



Motor Economics
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:
Variable-Speed Drive
Variable-Frequency Drive
Adjustable-Frequency Drive
Adjustable-Speed Drive

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.

ELECTRIC MOTOR CONTROLS Part 1

Once the proper motor is selected, understanding the many various control devices available and
their uses and limitations becomes an important part related to reliable operation and protection of the motor and the personnel using the motor.

Motor Control Topics
There are four major motor control topics or categories to consider. Each of these has several subcategories and sometimes the subcategories overlap to some extent. Certain pieces of motor control equipment can accomplish multiple functions from each of the topics or categories.

The four categories include:

1) Starting the Motor
Disconnecting Means
Across the Line Starting
Reduced Voltage Starting

2) Motor Protection
Overcurrent Protection
Overload Protection
Other Protection (voltage, phase, etc)
Environment

3) Stopping the Motor
Coasting
Electrical Braking
Mechanical Braking

4) Motor Operational Control
Speed Control
Reversing
Jogging
Sequence Control

• An understanding of each of these areas is necessary to effectively apply motor control principles and equipment to effectively operate and protect a motor.

MOTOR STARTING
All motors must have a control device to start and stop the motor called a “motor controller”.

Motor Controller
A motor controller is the actual device that energizes and de-energizes the circuit to the motor so
that it can start and stop.

• Motor controllers may include some or all of the following motor control functions: starting, stopping, over-current protection, overload protection, reversing, speed changing, jogging, plugging, sequence control, and pilot light indication.

Controllers range from simple to complex and can provide control for one motor, groups of motors, or auxiliary equipment such as brakes, clutches, solenoids, heaters, or other signals.

Motor Starter
The starting mechanism that energizes the circuit to an induction motor is called the “starter” and must supply the motor with sufficient current to provide adequate starting torque under worst case line voltage and load conditions when the motor is energized.

• There are several different types of equipment suitable for use as “motor starters” but only two types of starting methods for induction motors:
1. Across the Line Starting
2. Reduced Voltage Starting
Across the Line Starting of Motors
Across the Line starting connects the motor windings/terminals directly to the circuit voltage “across the line” for a “full voltage start”.

• This is the simplest method of starting a
motor. (And usually the least expensive).

• Motors connected across the line are capable
of drawing full in-rush current and
developing maximum starting torque to
accelerate the load to speed in the shortest
possible time.

• All NEMA induction motors up to 200
horsepower, and many larger ones, can withstand full voltage starts. (The electric
distribution system or processing operation may not though, even if the motor will).
Across the Line Starters

Figure 26. Manual Starter
There are two different types of common “across the line” starters including
1. Manual Motor Starters
2. Magnetic Motor Starters

Manual Motor Starters
A manual motor starter is package consisting of a horsepower rated switch with one set of contacts for each phase and corresponding thermal overload devices to provide motor overload protection.

• The main advantage of a manual motor starter is lower cost than a magnetic motor starter with equivalent motor protection but less motor control capability.

• Manual motor starters are often used for smaller motors - typically fractional horsepower
motors but the National Electrical Code allows their use up to 10 Horsepower.

• Since the switch contacts remain closed if power is removed from the circuit without
operating the switch, the motor restarts when power is reapplied which can be a safety
concern.

• They do not allow the use of remote control or auxiliary control equipment like a magnetic
starter does.
Magnetic Motor Starters
A magnetic motor starter is a package consisting of a contactor capable of opening and closing a set

Figure 27. Magnetic Starter
of contacts that energize and de-energize the circuit to the motor along with additional motor
overload protection equipment.

Magnetic starters are used with larger motors (required above 10 horsepower) or where
greater motor control is desired.
• The main element of the magnetic motor starter is the contactor, a set of contacts operated by
an electromagnetic coil.

Energizing the coil causes the contacts (A) to close allowing large currents to be
initiated and interrupted by a smaller voltage control signal.

The control voltage need not be the same as the motor supply voltage and is often low
voltage allowing start/stop controls to be located remotely from the power circuit.
• Closing the Start button contact energizes the contactor coil. An auxiliary contact on the
contactor is wired to seal in the coil circuit. The contactor de-energizes if the control circuit
is interrupted, the Stop button is operated, or if power is lost.
• The overload contacts are arranged so an overload trip on any phase will cause the contactor
to open and de-energize all phases.
Reduced Voltage Starting of Motors
Reduced Voltage Starting connects the motor windings/terminals at lower than normal line voltage
during the initial starting period to reduce the inrush current when the motor starts.

• Reduced voltage starting may be required when:
S The current in-rush form the motor starting adversely affects the voltage drop on the
electrical system.
S needed to reduce the mechanical “starting shock” on drive-lines and equipment when
the motor starts.
• Reducing the voltage reduces the current in-rush to the motor and also reduces the starting
torque available when the motor starts.
• All NEMA induction motors can will accept reduced voltage starting however it may notprovide enough starting torque in some situations to drive certain specific loads.
If the driven load or the power distribution system cannot accept a full voltage start, some type of reduced voltage or "soft" starting scheme must be used.
• Typical reduced voltage starter types include:
1. Solid State (Electronic) Starters
2. Primary Resistance Starters
3. Autotransformer Starters
4. Part Winding Starters
5. Wye-Delta Starters

Reduced voltage starters can only be used where low starting torque is acceptable or a means exists to remove the load from the motor or application before it is stopped.

What is Industrial Ethernet?

Industrial Ethernet Overview


The Ethernet network is a local-area network (LAN) protocol developed by Xerox Corporation in cooperation with DEC and Intel in 1976. Ethernet uses a bus or star topology, and supports data transfer rates of 10 Mbps (standard) or 100 Mbps (using the newer 100Base-T version).

The Ethernet specification served as the basis for the IEEE 802.3 standard, which specifies the physical and lower software layers. Ethernet uses the CSMA/CD access method to handle simultaneous demands. It is one of the most widely implemented LAN standards.


OSI Reference Model



Developed by International Standards Organization (ISO) and stands for Open Systems Interconnection (OSI).
It is designed to deal with connecting open systems to communicate with other systems.
It consists of seven layers: a complex structure is partitioned into a number of independent functional layers.
Each layer provides a set of services by performing some well-defined sets of functions. These services are provided by the layered-specific functional entities.
Services at a layer can only be accessed from the layer immediately above it.
Each layer uses only a well-defined set of services provided by the layer below.
Protocols operate between "peer" entities in the different end systems (peer-to-peer protocol rules)

Advantages:
More manageable -Layer N is smaller and built only on Layer (N-1).
Modularity - Different layers can be developed separately and each layer can be modified without affecting other layers as long as the interfaces with immediate layers are kept

Brief Description of model in Each Layer

Physical Layer
The physical layer is responsible for passing bits onto and receiving them from the communication channel.
This layer has no understanding of the meaning of the bits, but deals with the electrical and mechanical characteristics of the signals and signalling methods.

Data Link Layer
Data link layer is responsible for both Point-to-Point Network and Broadcast Network data transmission.
It hides characteristics of the physical layer (e.g. transmission hardware from the upper layers.
It is also responsible to convert transmitted bits into frames
It transmits the frames into an error free transmission line by adding error control and flow control.

Network Layer
Network layer is responsible for the controls of routers and subnets operation.
It also handles the formation and routing of packets from source to destination with congestion control.

Transport Layer
Transport layer is a kind of software protocol to control packets delivery, crash recovery and transmission reliability between sender and receiver.
Multiplexing between transport and network connections is possible.

Session Layer
Session layer provides dialogue control and token management.

Presentation Layer
When data is transmitted between different types of computer systems, the presentation layer negotiates and manages the way data is represented and encoded.
Essentially a 'null' layer in case where such transformations are unnecessary.

Application Layer
This top layer defines the language and syntax that programs use to communicate with other programs. For example, a program in a client workstation uses commands to request data from a program in the server.
Common functions at this layer are opening, closing, reading and writing files, transferring files and e-mail messages, executing remote jobs and obtaining directory information about network resources.

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