Electric motors and drives technical manual 2010 edition
2010 Edition Electric Motors & Drives Technical Manual Through the initiative of: International Copper Association – South East Asia Institute of Integrated Electrical Engineers of the Philippines, Inc PREFACE This publication deals primarily with small and medium-sized induction motors which are the most common type of alternating current motor They are internationally standardized and are efficiently manufactured in long production runs The combination of new materials and more sophisticated methods for calculation, design and production have made the modern three-phase induction motor a robust and reliable prime mover This publication was made possible through the initiative and support of the International Copper Association – South East Asia and administered, executed, and implemented by the Institute of Integrated Electrical Engineers of the Philippines All information and data contained in this publication is believed to be reliable, but all recommendations or suggestions are made without guarantee Furthermore, suggestions for use of material supplied shall not be construed as a recommendation or inducement to violate any law or infringe any patent iii iv Table of Contents Section Title Motor Specifications 1.1 Nameplate 1.2 Insulation Class 1.3 Enclosure Type 1.4 Temperature Class 1.5 Mounting 1.6 Manufacturer’s Identification Number 1.7 Terminal Markings 1.8 Motor Design 1.9 Types of Duty General Characteristics 2.1 System Nominal Voltage 2.2 Voltage 2.3 Power Factor 2.4 Efficiency 2.5 Speed 2.6 Vibration Characteristics and Balancing 2.7 Bearings 2.8 Torque Asynchronous Motor Starting Systems 3.1 Starting Methods 3.2 Single-phase Motor Starting Motor Protection and Coordination 4.1 Motors Protection 4.2 Protection Against Short Circuits 4.3 Protection Against Overload 4.4 Multifunction Relays 4.5 Motor Circuit Breakers Motor Starter Co-ordination 5.1 Concepts 5.2 Solutions 5.3 Motor Overload Protection 5.4 Terminology v Page 11 19 30 51 55 67 76 103 104 112 113 118 119 141 170 175 187 193 194 203 212 215 219 220 229 241 Motor Efficiency 6.1 Repair-Replace Decision Model 6.2 Premium Efficiency Motors Installation, Testing, and Maintenance 7.1 Installation and Maintenance 7.2 Description of Routine Tests 7.3 Recommended Winding Tests 7.4 Other Tests 7.5 Motor Starting Capabilities and Considerations 7.6 Maintenance and Reliability 7.7 Maintenance Programs 7.8 Machinery Condition Monitoring 7.9 Maintenance Planning vi 246 262 273 309 321 322 323 328 332 334 338 Motor Specifications 1.1 Nameplate Motor standards are established on a country by country basis Fortunately though, the standards can be grouped into two major categories: NEMA and IEC (and its derivatives) In North America, the National Electric Manufacturers Association (NEMA) sets motor standards, including what should go on the nameplate (NEMA Standard MG 1-10.40 "Nameplate Marking for Medium Single-Phase and Polyphase Induction Motors") In most of the rest of the world, the International Electrotechnical Commission (IEC) sets the standards Or at least many countries base their standards very closely on the IEC standards (for example, Germany's VDE 0530 standard and Great Britain's BS 2613 Standard closely parallel the IEC 34-1 standard) The NEMA and IEC standards are quite similar, although they sometimes use different terminology Thus, if one understands the IEC nameplate, it is fairly easy to understand a NEMA nameplate, and viceversa as shown in Fig 1.1A and B Fig 1.1A – Typical IEC Motor Nameplate Fig 1.1B – Typical NEMA Motor Nameplate The nameplate of a motor provides important information necessary for proper application For example, Fig 1.1C – AC Induction Motor nameplate shows a 30 horsepower (H.P.) three-phase (3 PH) AC Induction motor Fig 1.1C – AC Induction Motor Nameplate The following paragraphs explain some of the other nameplate information for this motor Voltage Source (VOLTS) and Full-load Current (AMPS) AC motors are designed to operate at standard voltages This motor is designed to be powered by a three-phase 460 V supply Its rated full-load current is 35.0 amps Base Speed (R.P.M.) and Frequency (HERTZ) Base speed is the speed, given in RPM, at which the motor develops rated horsepower at rated voltage and frequency Base speed is an indication of how fast the output shaft will turn the connected equipment when fully loaded This motor has a base speed of 1765 RPM at a rated frequency of 60 Hz Service Factor Service factor is a number that is multiplied by the rated horsepower of the motor to determine the horsepower at which the motor can be operated Therefore, a motor designed to operate at or below its nameplate horsepower rating has a service factor of 1.0 A 1.15 service factor motor can be operated 15% higher than its nameplate horsepower 1.2 Insulation Class NEMA NEMA defines motor insulation classes to describe the ability of motor insulation to handle heat The four insulation classes are A, B, F, and H All four classes identify the allowable temperature rise from an ambient temperature of 40° C (104° F) Classes B and F are the most commonly used Ambient temperature is the temperature of the surrounding air This is also the temperature of the motor windings before starting the motor, Although the motor torque available has been reduced, the load torque remains unchanged The result is a longer acceleration time If the reduced motor torque is equal to that of the load, leaving none available for acceleration, the motor and load will not accelerate beyond the speed point Further, if the motor torque is less than that of the load at initial startup, the rotor will not rotate That is, it will remain in a locked-rotor condition Figure below illustrates both of these conditions A major limiting factor for the starting capability of a motor is heating of the stator and rotor During acceleration, some of the electrical energy is used to drive the load, and the remainder is absorbed by the stator and rotor in the form of heat The primary source of this heating is I2R losses in the stator and rotor, which are much greater during acceleration than during normal operating conditions The starting current of a motor is frequently between and times rated current If we take the average value of this range, times rated current, the ratio of I2R at starting compared to running would 72 or 49, assuming the resistance remains unchanged Since the heating also increases winding resistance, it would be reasonable to expect at least 50 times normal heating during starting conditions 325 Fortunately, under normal stating conditions, the heating period is relatively short For example, NEMA MG1 Part 12.49 allows an acceleration time of up to 12 seconds for motors rate to 500hp (375kW) and rated 1kV or less During this period parts of the stator and rotor may reach temperatures in excess of their rated temperatures Conservative motor designers assume that all of the heat generated during staring is absorbed in the components that produce the heating – e.g., the stator and rotor Therefore these components heat very rapidly, and to relatively high temperatures However, since the duration of acceleration time is very short, it does not normally have a negative impact on motor life Upon attaining rated speed, the current and temperature drop to normal levels for the load conditions For motors larger than 500hp (375kW), or with loads with greater than normal inertia, the motor manufacturer should be consulted to determine the time limit for accelerating the load Motor starting capabilities are thermally limited by either the stator or the rotor If the stator is limiting factor, the motor is termed to be “statorlimited”; and if the rotor is the limiting factor, the motor is said to be “rotor-limiting.” In general, smaller motors, such as in NEMA frames, tend to be stator-limited; and larger motors well above NEMA frame size, tend to be rotor-limited According to the NEMA standards, there are three conditions that apply to the maximum inertia rating Applied voltage and frequency in accordance with 9.9 During the accelerating period, the connected load torque is equal to or less than a torque which varies as the square of the speed and is equal to 100 percent of rated-load torque at rated speed Two starts in succession (coasting to rest between starts) with the motor initially at the ambient temperature or one start with the motor initially at a temperature not exceeding its rated load operating temperature 326 If the starting conditions are other than those stated above, the motor manufacturer should be consulted When additional starts are required, it is recommended that none be made until all conditions affecting operation have been thoroughly investigated and the apparatus examined for evidence of excessive heating It should be recognized that the number of starts should be kept to a minimum since the life of the motor is affected by the number of starts [MG 1-12.54] Failure of motor components may be due to a number of stresses associated with acceleration In the rotor, the bars and end rings that make up the rotor cage are subject to high and cyclic (alternating) magnetic forces Heating of the rotor cage results in axial expansion of the bars and radial expansion of the end rings, creating stress in various sections of the bars and end rings Current tends to “crowd” the tops of the bars during starting; causing bending stresses as the top of the bars try to expand more than the bottoms This is depicted in the figure at the right As speed increases during acceleration to the thermal and other stresses already mentioned The mechanical and electrical forces also affect the stator windings The excessive starting current leads to rapid heating of the windings and consequently, rapid thermal expansion resulting in physical stress The torque forces associated with starting are many times normal, leading to 327 winding movement and possible motion between adjacent conductors, or between conductors and frame or core, which can result in a short circuit or ground fault Each acceleration period is a fatigue cycle, and the cumulative effects results in a finite life for the motor based on the number of starts However, there are no standards or guides for the minimum number of starts for a motor Returning to our opening statement, use caution with motor applications Do not assume that because a motor can drive a running load, it also has the capability to accelerate that load up to rated speed 7.6 Maintenance and Reliability Rotating Equipment Maintenance Problems The maintenance department of any industrial plant develops gradually, over a period of time The maintenance constitutes a very high percentage of the plant’s overall operating costs In North America it is not unusual to see companies spending up to 50% of their total operating budget on what is referred to as maintenance Performing planned and cost-effective maintenance on rotating equipment therefore is important, and more emphasis and planning towards having well managed, costeffective and reliable maintenance programs in place has to be considered in the company’s long range planning Maintenance problems are usually caused by: normal wear and tear, careless or untrained operations and maintenance personnel, improper lubrication or incorrect lubrication selection, and failure to make small repairs and adjustments which become catastrophic failures Additional maintenance problems are caused by: incorrect equipment or component design, excessive loads and speeds, incorrect alignment practices, excessive amounts of vibration, using low quality replacement parts such as bearings, seals and fasteners, and unwillingness by management to place meaningful priorities on the maintenance functions Note: Others causes of maintenance problems could be listed, but one thing is abundantly clear; rotating equipment maintenance costs money and failure to perform corrective maintenance on equipment eventually costs the company far more over the long term 328 Current research in North America indicated the following cost related statistics concerning rotating equipment maintenance and reliability: Over 25% of bearings used in rotating equipment fail prematurely due primarily to faulty installation and/or poor lubrication practices Over 70% of the failures experienced in plant and industrial hydraulic systems are caused by contaminated hydraulic fluids Inconsistent and poor lubrication procedures are one of the principles causes of machinery breakdown The average construction/contracting company spend between 25-45% of its operating budget on mobile equipment maintenance This value is for parts and labor, not for fuels, lube or depreciation In order to reduce these trends and high costs, management and employees must consider changing their view of what the maintenance function is to be Programs must develop which provide effective maintenance practices and are seen as “investment”, rather then costs Once this change in thinking takes place, and is put into practice with management and employee support, long-term maintenance expenditures can be reduced The recommended method for ensuring that reliable maintenance program works in a particular environment is to have the people in that environment develop the program Rather than just copy a maintenance program from someone else, develop it to respond to the in-plant equipment maintenance needs and ensure the program is satisfactory for the company’s objectives Note: Approximately one third of all maintenance dollars are wasted because the money is spent on “reaction” instead of “proactive” and “prevention” activities This is mostly due to inefficiencies in maintenance programs which probably the right things, but at the wrong time and often for the wrong reasons 329 Maintenance Program Objectives The primary objectives of any maintenance programs activities include: To ensure that the equipment operates safely and relatively trouble-free for long periods of time To maximize the availability of machinery and equipment necessary to meet the planned production and operational objectives To consistently maintain the plant equipment in order to minimize wear and premature deterioration To make the equipment reliable so it can be counted on to perform to set standards and conditions Maintenance Improvement and Reliability Program (MIRP) The following ten steps outline a plan when a company is considering developing an effective Maintenance Improvement and reliability Program (MIRP) Step 1: Begin by initiating a “total maintenance” approach Production and maintenance must collectively work together The maintenance department has to be viewed as being an integral part of the organization Step 2: Established a clear vision by having the employees and management identify the problems, then specify the goals and objectives that must be set in order to achieve success Step 3: Analyze the organization Will the organization, as a whole, support the type of improvements required? If not, consider changing the organizational structure and/or redesign the system to meet the identified needs Review the production and operational policies and procedures, as they may not be suited to the maintenance improvement and reliability program Step 4: Begin to develop an “action plan.” Identify what is going to be attempted, who is to be involve, what are the resources required, etc Action plans take on many different forms, but it is important that the 330 plan contains inputs drawn from the reviews and analysis rather than from complaints Step 5: Assess the condition of the equipment and facilities Be objective in the assessment Determine which equipment requires immediate attention Step 6: Select the appropriate maintenance program Is a computerized maintenance system needed? What technique will be employed, reactive, preventive or predictive maintenance? Determine the order maintenance activities will be carried out, first, then second, etc.? What type of reporting system will be used to track and record the data collected when measuring the performance of each piece of equipment? Step 7: Measure equipment condition When measuring for equipment condition which method(s) will be considered: vibration analysis; fluid analysis; non-destructive testing; or performance monitoring methods? Step 8: Prepare the maintenance personnel As the maintenance program activities and methods are implemented ensure that the maintenance personnel are trained to understand the program and why the activities and methods are performed Without this step no type of maintenance improvement and reliability program will succeed Step 9: Monitor equipment and machinery effectiveness to the detail the maintenance program requires Monitor for performance, reliability and quality Overtime, the recorded information can be used to evaluate the machinery and equipment condition and situation This is an on-going activity of any quality maintenance program Step 10: Initiate periodic reviews Equipment and machinery effectiveness is based on scheduled predictive and preventive maintenance activities The reviews of these activities may indicate common problems and trends which identify any design or operational changes required Include engineering, maintenance and production personnel in these periodic reviews Ensure that action plans develop from these review sessions, not just complaints 331 7.7 Maintenance Programs Methods of Maintenance There are three ways to perform maintenance: preventive, and reactive or “breakdown” maintenance Preventive Maintenance Programs: Normally involve the routine scheduling of maintenance activities The schedule is based on the past experience and the manufacturer’s recommendations The activities of the preventive maintenance program are usually based on periodic sampling and inspections An unskilled preventive maintenance team may leave the machine in a worse condition after the planned shutdown Sufficient spare parts and available maintenance personnel are allocated when overhauls and shutdowns are scheduled Often there continues to be “unplanned” losses, albeit, less than when no form of maintenance program is in place Many repairs are often initiated before components reach their maximum working life, which results in a great deal of unnecessary expense Predictive Maintenance Programs: This is a systematic method of monitoring the plant’s rotating equipment performance and is carried out on a regularly scheduled basis to determine the equipment condition Predictive maintenance utilizes information from past and current performance records to objectively predict mechanical problems Predictions based on the analysis of the information form the basis for corrective actions to be taken Note: Unlike breakdown maintenance and preventive maintenance, predictive maintenance is an active condition monitoring approach rather than a reaction or time based approach to maintenance To run efficiently in modern industries, the production machinery must operate near or at the design capacity with a minimum downtime The 332 specific purpose of a quality predictive maintenance program is to minimize unscheduled machinery failures, reduce maintenance costs and loss of production To accomplish these objectives a program is required which will: Regularly monitor the mechanical condition of all critical production equipment Identify outstanding problems From this program, the severity of each problem is quantified, and scheduled maintenance procedures are performed to prevent failures Predictive maintenance program evolved from preventive maintenance programs Preventive maintenance program for rotating equipment are generally based on periodic sampling and inspecting Most preventive maintenance programs have established schedules for periodic inspections of identified equipment which is critical to the operation Predictive maintenance programs reduce the frequency and severity of emergency repairs and can increase equipment life This system and the data gathered regarding performance and condition from the basis for predictive maintenance programs In a predictive maintenance program the specific maintenance tasks are based on actual need This approach reduces the failures and downtime as repairs are often now done, and maintenance intervals therefore, should be extended Reactive (Breakdown) Maintenance Programs: This type of maintenance program occurs by default if problems aren’t detected and corrected prior to absolute failure Typically reactive or breakdown maintenance is the most expensive of the three maintenance methods Reactive maintenance may, however, be justified for certain “non vital” machinery, or for machinery where lifetime and cost of failure does not justify a more planned approach to maintenance programs are 333 Predictive Maintenance Program Benefits Setting up an effective predictive maintenance program will provide many of the following benefits: improved operator safety reduced environmental hazards increased production increased machinery availability provide for scheduled rather than unscheduled downtime reduced risk of catastrophic failures minimize unnecessary repairs and repair time reduce spare part inventories improve product quality optimize maintenance department size better utilization of maintenance personnel 7.8 Machinery Condition Monitoring In previous machinery and equipment maintenance, a machine was often permitted to operate until compete failure occurred Actual machinery condition monitoring was quite simple, as there was no real sophisticated method for measuring machine condition, nor did management or the employees concern themselves with a more proactive approach to maintenance The maintenance plan was to periodically tear down and overhaul the machine as assurance against failure Four techniques were commonly used in the past to monitor the machinery condition and these techniques continue to be used, although each technique has come more sophisticated The techniques described “sense” the condition of the machine Any increases or decreases in temperature (touch and smell) Any increases or decreases in vibration (touch) Any change in noise or sound from the machine (listen) Any visual or observed changes and problems (sight) 334 Each technique helps in determining to what extent a mechanical fault exists and if it is progressing The corrective action is often based on “feel”, sound, or appearance Temperature Higher temperature often indicates that a bearing is acting abnormally High temperature can be detrimental to the bearing, the lubricant, and the shaft and seals This is evident when the machine has continued to operate for extended periods when the bearing or lubrication temperatures have been in excess of 260F (125C) Causes of high bearing and lubrication temperatures include insufficient or excessive lubrication, contaminated lubricants, overloading, bearing damage, faulty installation, insufficient bearing clearances, and improper or failed seals It is necessary to check the temperature of bearings periodically, both at the bearing itself and at other locations on the machine where there is high temperatures could be cause for concern Any significant change in temperature is usually a good indication that a problem exists, especially if the operating conditions of the machine have not been altered Bearing temperatures can be determined roughly by hand feel, or by routinely and accurately checked with a surface thermometer A permanently installed heat sensor may also be installed on or near critical parts of the machine Overheating is often first detected by smell resulting from hot plastics or oil Another method commonly used to “feel” the condition of a machine is to determine how much vibration exists at the machine By touching the bearing, high temperature and vibrations are felt The amount of vibration present is difficult to measure this way, but one may be able to compare the vibration felt today to how it felt yesterday, or several weeks ago 335 Vibration can be more accurately measured by using tools such as vibrations meters, analyzers, or monitors Use of a simple vibration meter The probe is placed on, or near the bearing and a vibration reading is given on the meter The amount of vibration measured is used to determine the severity of the vibration and the condition of the machine Listening One method used to identify irregularities on machinery and equipment is to listen for changes in sounds emitted from machines while operating under conditions of normal loads and speeds One can this by placing a screwdriver blade on the bearing housing and being safely positioned so the ear contacts the screwdriver handle The ear is listening to the internal sounds coming from the bearing Abnormal noises may be detected and traced to a specific component of the machine by experienced maintenance personnel More sophisticated methods are used to listen to bearings as well A stethoscope can be used to listen to the internal sounds of the bearing parts Microphones can be held over the machine or mounted at critical points to measure the sound amplitude being emitted Sound measurements can be used to determine the severity of the problem Sound and vibration are closely associated when determining irregularities in running machinery Grinding, squeaking and other irregular sounds can point to worn bearings The squeaking noise is often caused by inadequate lubrication Insufficient bearing clearances can make a metallic tone Indentations in the outer ring raceway will produce smooth, clear tones, and ring damage caused by shock loads or hammer blows lead to sounds varying in frequency according to the operating speed of the machine Intermittent noises probably indicate damage to certain spots on the rolling members Contamination in the bearing produces a rough grinding sound Damaged bearings produce irregular and loud noises Good bearings sound smoother, fewer irregular sounds, less grinding sounds, and more of a constant humming sound 336 Sight Maintenance personnel, as shown in illustration #3, can simply look at equipment to see if there is anything out of the ordinary happening Check for any apparent oil leaks or grease leaks around seal areas, or if any of the bearing housings are loose, cracked or improperly assembled Check the lubricant Discoloration or darkening of the oil is usually a good indication that the lubricant is either contaminated or worn out It is also very important to check whether or not there is sufficient lubricant Is the lubricant the proper one for the application? Check whether the air is free of obstructions Take a small sample of used oil and compare it with new oil If it is cloudy in appearance, water has more than likely mixed with it, therefore, the oil must be replaced Dark or thick oil is a sure sign of contamination or that the oil has started to carbonize Overheating may have caused this problem Corrective Maintenance Corrective maintenance work should be planned and scheduled, unless true emergencies unexpectedly arise Planning involves identifying all resources necessary to repair the machinery This identification of resources may include: tradesman man-hours worked available materials/replacement parts required special tools and equipment availability of contract personnel location of mechanical drawings supply of assembly/disassembly guidelines safety orientation installation and setting procedures sequence of tasks and time durations work schedules and shift rotations job cost estimates safety and environmental regulations and permits 337 arrangements for restrooms, lunchrooms, lockers, etc Note: The maintenance planner responsible for planning and scheduling the corrective maintenance work must have direct access to records which contain past maintenance history, information on equipment design, bill of materials, parts list, assembly/disassembly drawings, and current inventory status for specific parts and assemblies 7.9 Maintenance Planning There are three basic areas of planning administered by maintenance planners Long-Range Planning: These plans for maintenance requirements are allied with, and dependent on, long-range sales and production forecasts Planners work with management to outline what is needed in the way of decisions in order to reach certain goals in five to twenty years Short and Mid-Range Planning: These plans project from one to five years into the future Plans are developed under the direct supervision of the managers responsible for defined maintenance and production activities Maintenance program are involved in both short and midrange planning Immediate Planning: This type of maintenance planning may be referred to as “day-to-day” maintenance planning This type of planning is done on a pre-programmed routine and is carried out by the maintenance teams These plans are generated from the inspections, observations and performance measurements regularly performed as part of the predictive maintenance program These plans are primarily concerned with action oriented maintenance activities for today, tomorrow, and the following week 338 .. .2010 Edition Electric Motors & Drives Technical Manual Through the initiative of: International Copper Association – South East Asia Institute of Integrated Electrical Engineers... standards (for example, Germany''s VDE 0530 standard and Great Britain''s BS 2613 Standard closely parallel the IEC 34-1 standard) The NEMA and IEC standards are quite similar, although they sometimes... temperature class Slip-ring motors, commutator motors and brake motors can all be made in flameproof versions No external parts may cause sparks The motors must have both an internal and an external earthing