General principles of motor control
- 1. State the purpose and general principles of motor control. Discuss the differences between manual and automatic motor control. Discuss considerations when installing motors or control equipment. Discuss the basic functions of a control system. Discuss surge protection for control systems. The term "motor control" can have very broad meanings. It can mean anything from a simple toggle switch in tended to turn a motor on or off to an extremely complex system intended to control several motors, with literally hundreds of sensing devices that govern the operation of the circuit. The electrician working in industry should be able to install different types of motors and the controls necessary to control and protect them and also to troubleshoot systems when they fail.
- 2. Motor control was a simple problem when motors were used to drive a common line shaft to which several machines were connected. It was simply necessary to start and stop the motor a few times a day. However, with individual drive, the motor is now almost an integral part of the machine and it's necessary to de sign the motor controller to fit the needs of the machine to which it's connected. Motor control is a broad term that means any thing from a simple toggle switch to a complex system with components such as relays, timers, and switches. The common function of all controls, however, is to control the operation of an electric motor. As a result, when motor control equipment is selected and installed, many factors must be considered to ensure that the control will function properly for the motor and the machine for which it's selected.
- 12. When installing electric motors and equipment, several factors should be considered. When a machine is in stalled, the motor, machine, and controls are all inter related and must be considered as a unit. Some machines will have the motor or motors and control equipment mounted on the machine itself when it's delivered from the manufacturer, and the electrician's job in this case is generally to make a simple power connection to the machine. A machine of this type is shown in ill. 2. Other types of machines require separately mounted motors that are connected by belts, gears, or chains. Some machines also require the connection of pilot sensing devices such as photo switches, limit switches, pressure switches, and so on. Regardless of how easy or complex the connection is, several factors must be considered. ill. 1 Motor controlled by a simple toggle switch. ill. 2 This machine was delivered with self-contained motors and controls.
- 13. One of the main considerations when installing a machine is the power source. Does the machine re quire single-phase or three-phase power to operate? What is the horsepower of the motor or motors to be connected? What is the amount of in-rush current that can be expected when the motor starts? Will the motor require some type of reduced voltage starter to limit in-rush current? Is the existing power supply capable of handling the power requirement of the ma chine or will it be necessary to install a new power system? The availability of power can vary greatly from one area of the country to another. Power companies that supply power to heavily industrialized areas can generally permit larger motors to be started across-the line than companies that supply power to areas that have light industrial needs. In some areas, the power company may permit a motor of several thousand horse power to be started across-the-line, but in other areas the power company may require a reduced voltage starter for motors rated no more than one hundred horsepower.
- 14. When connecting motors, several factors should be considered, such as: horsepower, service factor (SF), marked temperature rise, voltage, full load current rating, and National Electrical Manufacturers Association (NEMA) Code letter. This information is found on the motor nameplate. The conductor size, fuse or circuit breaker size, and overload size are generally determined using the National Electrical Code (NEC) and /or local codes. It should be noted that local codes generally supersede the National Electrical Code and should be followed when they apply. Motor installation based on the NEC® will be covered in this text.
- 15. The type of motor best suited to operate a particular piece of equipment can be different for different types of machines. Machines that employ gears generally require a motor that can start at reduced speed and increase speed gradually. Wound rotor induction motors or squirrel cage motors controlled by variable frequency drives are generally excellent choices for this requirement. Machines that require a long starting period, such as machines that operate large inertia loads such as flywheels or centrifuges, require a motor with high starting torque and relatively low starting current. Squirrel cage motors with a type ‘A’ rotor or synchronous motors are a good choice for these types of loads. Synchronous motors have an advantage in that they can provide power factor correction for themselves or other inductive loads connected to the same power line. Squirrel cage motors controlled by variable frequency drives or direct current motors can be employed to power machines that require variable speed. Squirrel cage induction motors are used to power most of the machines throughout industry. These motors are rugged and have a proven record of service unsurpassed by any other type of power source.
- 16. The type of controller can vary depending on the requirements of the motor. Motor starters can be divided into two major classifications: NEMA (National Electrical Manufacturers Association) and IEC (International Electro technical Commission). NEMA is an American organization that rates electrical components. NEMA starter sizes range from 00 through 8. A NEMA size 00 starter is rated to control a 2 horsepower motor connected to a 460 volt three phase power supply. A size 8 starter will control a 900 horsepower motor connected to a 460 volt three phase power source. IEC starter sizes range from size A through size Z. Size A starters are rated to control a 3 horsepower motor connected to a 460 volt three phase source. Size Z starters are rated to control a 900 horsepower motor connected to a 460 volt source. It should be noted that the contact size for an IEC starter is smaller than for a NEMA starter of the same rating. It is common practice when using IEC starters to increase the listed size by one or two sizes to compensate for the difference in contact size.
- 17. Another consideration is the type of environment in which the motor and control system operates. Can the controls be housed in a general purpose enclosure similar to the one shown in ill. 3, or is the system subject to moisture or dust? Are the motor and controls to be operated in a hazardous area that requires explosion proof enclosures similar to that shown in ill. 4? Some locations may contain corrosive vapor or liquid, or extremes of temperature. All of these conditions should be considered when selecting motors and control components. Another type of starter commonly found in industry is the combination starter (ill. 5). The combination starter contains the disconnecting means, fuses or circuit breaker, starter, and control transformer. They may also have a set of push buttons or switches mounted on the front panel to control the motor. ill. 4 Explosion proof enclosure (NEMA 7). ill. 5 Combination motor starter with circuit breaker, disconnect switch, starter, and control transformer.
- 18. Another important consideration is the safety of the operator or persons that work around the machine. In 1970, the Occupational Safety and Health Act (OSHA) was established. In general, OSHA requires employers to provide an environment free of recognized hazards that are likely to cause serious injury. Another organization that exhibits much influence on the electrical field is Underwriters Laboratories (UL). Underwriters Laboratories was established by insurance companies in an effort to reduce the number of fires caused by electrical equipment. They test equipment to determine if it's safe under different conditions. Approved equipment is listed in an annual publication that's kept current with bimonthly supplements. Another previously mentioned organization is the National Electrical Code. The NEC is actually part of the National Fire Protection Association. They establish rules and specifications for the installation of electrical equipment. The National Electrical Code isn't a law unless it's made law by a local authority. Two other organizations that have great influence on control equipment are NEMA and IEC. Both of these organizations will be discussed later in the text.
- 19. When choosing a specific device for a particular application, it's important to remember that the motor, machine, and motor controller are interrelated and need to be considered as a package. In general, five basic factors influence the selection and installation of a controller.
- 20. Establish whether the service is direct (DC) or alternating current (AC). If AC, deter mine the frequency (hertz) and number of phases in addition to the voltage.
- 21. The motor should be matched to the electrical service and correctly sized for the ma chine load in horsepower rating (hp). Other considerations include motor speed and torque. To select proper protection for the motor, its full-load current rating (FLC), service factor (SF), time rating (duty), and other pertinent data-as shown on the motor nameplate-must be used. ill. 1 Five 2000 hp, 1800 rpm induction motors driving water pumps for a Texas oil/water operation. Pumps are used to force water into the ground and "float" oil upward. ill. 2 Horizontal 4000 hp synchronous motor driving a large centrifugal air compressor.
- 22. The fundamental tasks of a motor controller are to start and stop the motor and to protect the motor, machine, product, and operator. The controller may also be called upon to provide supplementary functions such as reversing, jogging or inching, plugging, or operating at several speeds or at reduced levels of current and motor torque (see Glossary). Section 430 of the National Electrical Code (NEC) provides requirements concerning the installation of motor circuits. This section is employed to deter mine the proper conductor size, overload size, and short circuit protection rating for motor installations. In some industries electrical engineers are responsible for determining the requirements for installing a motor or motors. In other industries the electrician is expected to perform this task.
- 23. Controller enclosures serve to provide safety protection for operating personnel by preventing accidental contact with live parts. In certain applications, the controller itself must be protected from a variety of environmental conditions, which might include: ++ Water, rain, snow, or sleet ++ Dirt or noncombustible dust ++ Cutting oils, coolants, or lubricants Both personnel and property require protection in environments made hazardous by the presence of explosive gases or combustible dusts.
- 24. Motor control equipment is designed to meet the provisions of the National Electrical Code (NEC). Also, local code requirements must be considered and met when installing motors and control de vices. Presently, code sections applying to motors, motor circuits, and controllers and industrial control devices are found in Article 430 on motors and motor controllers, Article 440 on air-conditioning and refrigeration equipment, and Article 500 on hazardous locations of the NEC. The 1970 Occupational Safety and Health Act (OSHA), as amended, requires that each employer furnish employment in an environment free from recognized hazards likely to cause serious harm. Standards established by the National Electrical Manufacturers Association (NEMA) assist users in the proper selection of control equipment. NEMA standards provide practical information concerning the construction, testing, performance, and manufacture of motor control devices such as starters, relays, and contactors. One of the organizations that actually tests for conformity to national codes and standards is Underwriters' Laboratories (UL). Equipment that's tested and approved by UL is listed in an annual publication, which is kept current by means of bimonthly supplements to reflect the latest additions and deletions. A UL listing does not mean that a product is approved by the NEC. It must be acceptable to the local authority having jurisdiction. .
- 25. Some of the complicated and precise automatic applications of electrical control are illustrated in ill. 3 and ills 1-7A and B. Factors to be considered when selecting and in stalling motor control components for use with particular machines or systems are described in the following paragraphs.
- 26. The motor may be started by connecting it directly across the source of voltage. Slow and gradual starting may be required, not only to protect the machine, but also to ensure that the line current inrush on starting isn't too great for the power company's system. Some driven machines may be damaged if they are started with a sudden turning effort. The frequency of starting a motor is another factor affecting the controller. A combination fused disconnect switch and motor starter is shown in ill. 4.
- 27. Most controllers allow motors to coast to a standstill. Some impose braking action when the machine must stop quickly. Quick stopping is a vital function of the controller for emergency stops. Controllers assist the stopping action by retarding centrifugal motion of machines and lowering operations of crane hoists.
- 28. Controllers are required to change the direction of rotation of machines automatically or at the command of an operator at a control station. The reversing action of a controller is a continual process in many industrial applications.
- 29. The maintaining of desired operational speeds and characteristics is a prime purpose and function of controllers. They protect motors, operators, machines, and materials while running. There are many different types of safety circuits and devices to protect people, equipment, and industrial production and processes against possible injury that may occur while the machines are running.
- 30. Some controllers can maintain very precise speeds for industrial processes. Other controllers can change the speeds of motors either in steps or gradually through a continuous range of speeds.
- 31. Many mechanical safeguards have been re placed or aided by electrical means of protection. Electrical control pilot devices in controllers pro vide a direct means of protecting machine operators from unsafe conditions.
- 32. Part of the operation of an automatic machine is to protect the machine itself and the manufactured or processed materials it handles. E.g., a certain machine control function may be the prevention of conveyor pileups. A machine control can reverse, stop, slow, or do whatever is necessary to protect the machine or processed materials.
- 33. Requirements Once properly installed and adjusted, motor starters will provide reliable operation of starting time, voltages, current, and torques for the benefit of the driven machine and the power system. The NEC, supplemented by local codes, governs the selection of the proper sizes of conductors, starting fuses, circuit breakers, and disconnect switches for specific system requirements.