Motors Automation Energy Transmission & Distribution Coatings. Motors Specification of Electric Motors

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1 Motors Automation Enery Transmission & Distribution Coatins Motors Specification of Electric Motors --

2 Specification of Electric Motors 2 Specification of Electric Motors

3 WEG, which bean in 1961 as a small factory of electric motors, has become a leadin lobal supplier of electronic products for different sements. The search for excellence has resulted in the diversification of the business, addin to the electric motors products which provide from power eneration to more efficient means of use. This diversification has been a solid foundation for the rowth of the company which, for offerin more complete solutions, currently serves its customers in a dedicated manner. Even after more than 50 years of history and continued rowth, electric motors remain one of WEG s main products. Alined with the market, WEG develops its portfolio of products always thinkin about the special features of each application. In order to provide the basis for the success of WEG Motors, this simple and objective uide was created to help those who buy, sell and work with such equipment. It brins important information for the operation of various types of motors. Enjoy your readin. Specification of Electric Motors 3

4 Table of Contents 1. Fundamental Concepts Electric Motors Basic Concepts Torque Mechanical Enery & Power Electrical Enery & Power Apparent, Active and Reactive Power Power Factor Efficiency Torque Versus Power Ratio Sinle-Phase AC Systems Connection: Parallel and Series Star Connection Three-Phase AC System Delta Connection Three-Phase Induction Motor Workin Principle - Rotatin Field Synchronous Speed ( ns ) Slip ( s ) Rated Speed Insulation Materials and Insulation Systems Insulation Material Insulation System Thermal Classes Insulatin Materials in Insulation Systems WEG Insulation System Power Supply Characteristics Power Supply System Three-Phase System Sinle-Phase System Characteristics of the Electric Motor Power Supply Rated Voltae Multiple Rated Voltae Rated Frequency ( Hz ) Connection to Different Frequencies Voltae and Frequency Variation Tolerance Three-Phase Motor Startin Current Limitation D.O.L Startin Startin with Star-Delta Switch ( Y - Δ ) Compensatin Switch Comparin Star-Delta Starters and Series-Parallel Startin Electronic Start ( Soft-Starter ) Direction of Rotation of Three-Phase Induction Motors Acceleration Characteristics Torque Torque X Speed Curve Desins - Minimum Standardized Torque Values Characteristics of WEG Motors Load Inertia Acceleration Time Duty Cycles Locked Rotor Current Standardized Maximum Values Speed Reulation of Asynchronous Motors Chanin the Number of Poles Two Speed Motors with Independent Windins Dahlander Motors with Two or More Speeds Slip Variation Rotor Resistance Variation Stator Voltae Variation Frequency Inverters Brake Motor Brake Operation Connection Diaram Brake Coil Power Supply Brake Torque Air Gap Adjustment Operatin Characteristics Windin Heatin Up Motor Lifetime Insulation Classes Windin Temperature Rise Measurement Electric Motor Application Thermal Protection of Electric Motors Resistance Temperature Detector ( Pt-100 ) Thermistors ( PTC and NTC ) Bimetal Thermal Protectors - Thermostats Phenolic Thermal Protection System Service Duty Standardized Service Duties Duty Type Desination Rated Output Equivalent Power Ratins for Low Inertia Loads Service Factor ( SF ) Environment Characteristics Altitude Ambient Temperature Determinin Useful Motor Output at Different Temperature and Altitude Conditions Environment Aressive Environments Environments Containin Dusts and Fibers Explosive Atmospheres Deree of Protection Identification Codes Usual Derees of Protection Weather Protected Motors Space Heater Noise Levels Specification of Electric Motors

5 9. Explosive Atmosphere Hazardous Area Explosive Atmosphere Classification of Hazardous Areas Classes and Groups of the Hazardous Areas Protection by Enclosure Temperature Classes Equipment for Explosive Atmospheres Increased Safety Equipment Explosion-Proof Equipment Mountin Arranements Dimensions Standardized Type of Construction and Mountin Arranement Paintin Tropicalized Paintin Three-Phase Electric Motor Selection and Application Motor Type Selection for Different Loads WManet Drive System Application of Induction Motors with Variable Frequency Drives Normative Aspects Induction Machine Speed Variation by Frequency Inverter Characteristics of the Frequency Inverter Control Types Harmonics Inverter Influencin Motor Performance Environmental Information Packain Product Tests Variable Frequency Drive Motors Appendix International System of Units Unit Convertion Standards...66 Specification of Electric Motors 5

6 1. Fundamental Concepts 1.1 Electric Motors The electric motor is a machine capable of convertin electrical enery into mechanical enery. The induction motor is the most widely used type of motor, because it combines all the advantaes offered by the electrical enery such as low cost, easy of supply and distribution, clean handlin and simple controls - toether with those of simple construction and its reat versatility to be adapted to wide ranes of loads and improved efficiencies. The most common types of electric motors are: a ) Direct current motors These motors are quite expensive requirin a direct current source or a convertin device to convert normal alternatin current into direct current. They are capable of operatin with adjustable speeds over a wide rane and are perfectly suited for accurate and flexible speed control. Therefore, their use is restricted to special applications where these requirements compensate the much hiher installation and maintenance costs. b ) Alternatin current motors These are the most frequently used motors because electrical power is normally supplied as alternatin current. The most common types are: Synchronous motors: synchronous motors are three-phase AC motors which run at fixed speed, without slip, and are enerally applied for lare outputs ( due to their relatively hih costs in smaller frame sizes ). Induction motor: these motors enerally run at a constant speed which chanes slihtly when mechanical loads are applied to the motor shaft. Due to its simplicity, robustness and low cost, this type of motor is the most widely used and, in practical terms, is quite suitable for almost all types of machines. Currently it is possible to control the speed of induction motors by frequency inverters. Technolical Universe of Electric Motors SPLIT-PHASE START CAPACITOR SQUIRREL CASE PERMANENT CAPACITOR SHADED POLES ASYNCHRONOUS TWO-VALUE CAPACITOR WOUND ROTOR REPULSION SINGLE PHASE RELUCTANCE SYNCHRONOUS PERMANENT MAGNET AC MOTOR LINEAR INDUCTION PERMANENT MAGNET ASYNCHRONOUS SQUIRREL CASE WOUND ROTOR THREE PHASE PEMANENT MAGNET UNIVERSAL SYNCHRONOUS RELUCTANCE NON-SALIENT POLE SALIENT POLES SERIE EXCITATION INDEPENDENT EXCITATION DC MOTOR COMPOUND EXCITATION PERMANENT MAGNET PARALLEL EXCITATION This Classification Diaram shows the most widely used types of motors. Motors for specific use and with reduced application are not shown MANUFACTURED BY WEG Table Specification of Electric Motors

7 1.2 Basic Concepts For better understandin of the followin sections we will now review some principles of Physics concernin enery and forces Torque Torque, also known as moment of force, is the measure of enery required to rotate a shaft. Throuh practical experience we can note that for liftin a weiht similar to the one used in water wells ( see fi. 1.1 ). the required force F to be applied on the winch depends on the lenth E of the crank handle. The loner the crank handle the less force is required. By doublin the lenth E of the crank handle, the required force F is reduced by half. Fiure a shows that the bucket weihts 20 N while the diameter of the drum is 0.20 m, thus permittin the rope to transmit a force of 20 N on the drum s surface, i.e. at 0.10 m from the axis centre. In order to counterbalance this force, 10 N is must be applied on the crank handle if E has a lenth of 0.20 m. If E is twice as much, i.e m, force F becomes half, or 5 N. As you can see, to measure the enery required to make the shaft rotate, it is not sufficient to define the force applied but it is also necessary to indicate at what distance from the shaft center the force is applied. You must also inform at what distance from the shaft center the force is applied. The enery is measured by the torque. that is the result of F ( force ) x E ( distance ). F x E. In the iven example, the torque is: C = 20 N x 0.10 m = 10 N x 0.20 m = 5 N x 0.40 m = 2.0 Nm C = F. E ( N. m ) Therefore by usin an electric motor to lift a water bucket in 2.0 seconds, the required Power will be: F. d P mec = ( W ) t 490 P 1 = = 245 W 2.0 If we use a hiher power ratin motor, able to do this work in 1.3 seconds, the required power will be: 490 P 2 = = 377 W 1.3 The most common used unit in Brazil for measurin the mechanical power is HP ( horsepower ), equivalent to kw ( measurin unit used internationally for the same purpose ). Relationship between power units P ( kw ) = P ( cv ) P ( cv ) = P ( kw ) In this case the outputs of the above mentioned motors will be: P 1 = = cv P 2 = = cv For circular movements C = F. r ( N.m ) π. d. n v = ( m/s ) 60 Fiure Mechanical Enery & Power Power is a measure of how fast enery is applied or consumed. In the previous example, if the well is 24.5 m deep the work or enery ( W ) spent to lift the bucket from the bottom of the well up to the wellhead will always be the same: 20 N x 24.6 m = 490 Nm Note: the measurin unit for the mechanical enery. Nm, is the same that is used for torque - however the values are of different nature and therefore should not be confused. W = F. d ( N. m ) OBS.: 1 Nm = 1 J = Power x time = Watts x second Power expresses how quick the enery is applied, it is calculated by dividin the total enery or work by the time in which it is done. F. d P mec = ( cv ) 736. t Where: C = torque ( Nm ) F = force ( N ) r = pulley radius ( m ) v = anular speed ( m/s d = part diameter ( m ) n = speed ( rpm ) Electrical Enery & Power Althouh enery is always one and the same thin, it can be presented in several forms. By connectin a resistance to a voltae supply, an electric current will flow throuh the resistance that will be heated. The resistance absorbs enery, transformin it into heat which is also a form of enery. An electric motor absorbs electric enery from the power supply, transformin it into mechanical enery available at the end of the shaft. Specification of Electric Motors 7

8 DC Circuits The electric power on DC circuits can be obtained by the ratio amon voltae ( U ), current ( I ) and resistance ( R ) involved in such circuit, that is: P = U. I ( W ) or, U 2 P = ( W ) R or. P = R. I² ( W ) Where: U = voltae ( V ) I = current ( Amps ) R = resistance ( Ω ) P = averae Power ( W ) AC Circuits a ) Resistance In the case of resistances, the hiher the supply voltae, the hiher the current that results in faster heatin of the resistance. This means that the electric power will be hiher. The electric enery absorbed from the line, in case of resistance, is calculated by multiplyin the line voltae by the current, if the resistance ( load ) is sinle-phase. P = U f. I f ( W ) In a three-phase system, the power in each phase of the load is P f = U f x I f as it were an independent sinle-phase system. The total power is the sum of the power of the three-phases, i.e.: P = 3P f = 3. U f. I f Considerin that the three-phase system can be delta or star connected, we will have followin relationships: Star-connection: U = 3. U f e I = I f Delta-connection: U = U f e I = 3. I f Electric power is normally measured in watts ( W ) correspondin to 1 volt x 1 ampere or its multiple kilowatt ( kw ) = 1000 watts. This unit may also be used to measure the output of mechanical power. Electric enery is normally measured by kilowatt-hour ( kwh ) correspondin to the enery supplied by a power of 1 kw over a period of 1 hour ( this is the unit appearin on electricity bills ) Apparent, Active and Reactive Power Apparent power ( S ) It is the multiplication result of the voltae by the current ( S = U. I for sinle-phase systems and S = 3. U. I, for three-phase systems. This corresponds to the effective power which exists when there is no phase displacement of the current, i. e. for the resistive loads. Then, P S = ( VA ) Cos ϕ Evidently, for resistive loads, cos ϕ = 1, and the effective power can then be interpreted as apparent power. The measurin unit for apparent power is volt-ampere ( VA ) or its multiple, kilovolt-ampere ( kva ). Active power ( P ) It is the portion of apparent power that performs work, that is, the portion that is converted into enery. P = 3. U. I. cos ϕ ( W ) ou P = S. cos ϕ ( W ) Reactive power ( Q ) It is the portion of apparent power that does not perform work. It is only transferred and stored on passive elements ( capacitors and inductors ) of the circuit. Q = 3. U. I sen ϕ ( VAr ) ou Q = S. sen ϕ ( VAr ) Power trianle ϕ Thus, the total power for both connections will: P = 3. U. I ( W ) Note: this formula applies to resistive loads only, i.e. where there is no phase shift of the current. Fiure Power Trianle ( inductive load ) b ) Reactive loads For reactive loads, i.e. where there is phase shiftin in the case of induction motors, the phase shift must be taken into account and the formula then becomes P = 3. U. I. cos ϕ ( W ) Where: U = Line voltae I = Line current cos ϕ = Phase shift anle between voltae and current. 8 Specification of Electric Motors

9 Power Factor Power factor is indicated by cos ϕ, where ϕ is the anle of voltae displacement relatin to the current. It is the relationship between active ( P ) and the apparent power ( S ): ( fiure 1.2 ). P P ( kw ) cos ϕ = = S 3. U. I Then we can state that, Resistive load: cos ϕ = 1 Inductive load: cos ϕ ( delayed ) Capacitive load: cos ϕ ( advanced ) Note: the terms delayed and advanced refers to the current anle relatin to the voltae anle. A motor does not draw only active power, transformed after in mechanical power and heat ( losses ), but also absorbs reactive power needed for manetization, but that does not produce work. On the diaram of fiure 1.3, the vector P represents the active power and Q the reactive Power, which added results in the apparent power S. The electric motor plays a very important role in the industry, since it represents more than 60% of the enery consumption. Therefore, it is essential to apply motors with outputs and features well adapted to its function since the power factor chanes with motor load. Power factor correction The increase of power factor is made by the connection of a capacitive load, in eneral, a capacitor or a synchronous motor with overexcitation, in parallel with the load. For example: A three-phase electric motor, 100 HP ( 75 kw ), IV poles, runnin at 100% of the rated power, with oriinal Power Factor of 0.87 and efficiency of 93.5%. Now a reactive power should be determined to raise the power factor to Solution: Usin the table 1.2, on the intersection of 0.87 line with the column of 0.95, we et the value that multiplied by the motor absorbed power from the line in KW, ives the amount of reactive power necessary to increase the power factor from 0.87 to kvar = P ( HP ) x x F x 100% Eff. % = 100 x x x 100% 93.5% kvar = kvar Fiure The Power factor is determined measurin the input power, the voltae and the rated load Where; kvar = Three-phase power of the capacitor bank to be installed P( hp ) = Motor rated output F = Factor obtained in the Table 1.2 Eff. % = Motor efficiency Specification of Electric Motors 9

10 Oriinal Required Power Factor Power Factor Table Power factor correction 10 Specification of Electric Motors

11 Efficiency The efficiency defines how efficient is made the conversion of the line absorbed electric enery it into mechanical enery available at the shaft end. The efficiency defines how this transformation is made. By callin mechanical power available at the shaft end output ( P u ) and electric enery absorbed by the motor from the supply input ( P a ), the efficiency is the ratio between these two, i.e., P u ( W ) 736. P ( cv ) P ( kw ) η = = = P a ( W ) 3. U. I. cos ϕ 3. U. I. cos ϕ ou 736. P ( cv ) η% = U. I cos ϕ Torque Versus Power Ratio When mechanical enery is applied in the form of a rotatin movement, the developed output depends on the torque C and on the rotational speed n. The ratio is as follows: C ( kfm ). n ( rpm ) C ( Nm ). n ( rpm ) P ( cv ) = = C ( kfm ). n ( rpm ) C ( Nm ). n ( rpm ) P ( kw ) = = Inversely 716. P ( cv ) 974. P ( kw ) C ( kfm ) = = n ( rpm ) n ( rpm ) P ( cv ) P ( kw ) C ( Nm ) = = n ( rpm ) n ( rpm ) 1.3 Sinle-Phase AC Systems Alternatin current is distinuished by that voltae, which ( instead of bein a steady one, as for instance between the poles of a battery ) varies with time, alternately reversin its direction. In the sinle-phase systems, the alternatin voltae U ( Volts ) is enerated and applied between two wires to which the load absorbin current I ( amperes ) is connected - see Fi. 1.4a. cycle By representin the values U and I in a raph at successive instants, we obtain Fi b. Fi. 14b also shows some values which will be defined further on. It can be noted that the voltae and current waves are not in phase, i.e. they do not pass the zero point simultaneously, notwithstandin the fact that they are of the same frequency. This occurs with many types of electrical loads e.. electric motors ( reactive loads ). Frequency Is the number of time per second the voltae chanes its direction and returns to the initial condition. It is expressed in cycle per second or hertz, and is represented by Hz. Maximum voltae ( Umáx ) This is the peak value of the voltae, i.e. the instantaneous crest value achieved by the voltae durin one cycle ( one half of the cycle bein positive and the other half neative, this is reached twice per cycle ). Maximum current ( Imáx ) This is the peak of the current. Effective value of voltae and current ( U and I ) It is the value of the continuous voltae and current which enerate an output correspondin to that enerated by the alternated current. We can identify the effective value as: U max I max U = U máx / 2 e e I = I = I máx / For example: If we connect a resistance to an AC circuit ( cos ϕ = 1 ) with U máx = 311 V and I máx = A. the developed output power will be: U max I max P = U. I. COS ϕ = P = Watts = 2.2 kw Note: usually, when referrin to voltae and current, for example, 220 V or 10 A, without mentionin any other factor, we are referrin to voltae or current effective values, which are normally applied. Phase displacement ( ϕ ) Phase displacement means delay of the current wave with respect to the voltae wave ( see fi. 1.4 ). Instead of bein measured in time ( seconds ), this delay is usually measured in derees, correspondin to the fraction of a complete cycle, takin 1 cycle = 360º. However, phase displacement is usually expressed by the anle cosine ( see Item Power Factor ) Connection: Parallel and Series LOAD TIME cycle Fiure 1.4a Fiure 1.4b Fiure 1.5a Fiure 1.5b Specification of Electric Motors 11

12 Two equal loads can be connected, for example, to a sinlephase system, in two different ways: By makin a series connection ( fi. 1.5a ), where the total current flows throuh the two loads. In this case, the voltae across each load is the half of the circuit voltae. By makin a parallel connection ( fi. 1.5b ), where the voltae is applied across each load. In this case, the current in each load is half of the total circuit current. 1.4 Three-Phase AC System A three-phase system is formed by associatin three sinlephase voltae system, U 1, U 2 and U 3 which so the phase displacement between any two of them ch is 120º, which means, the delays of U 2 relatin to U 1, U 3 relatin to U 2, relatin to U 3, are equal to 120º ( considerin a complete cycle = 360º ). The system is balanced if the three voltaes have the same effective value, U 1 = U 2 = U 3, as shown in Fi. 16 Fiure. 1.7a - Connections Fiure 1.7b - Electrical diaram Cycle Fiure 1.7c - Phasorial diaram Line current ( I ) The current in any one of the three wires L 1, L 2 and L 3. Time Phase voltae and current ( U f and I f ) Is the voltae and current of each one of the considered sinle-phase systems. Fiure 1.6 By interconnectin the three sinle-phase systems and by eliminatin the unnecessary wires, we have a three-phase system: three balanced voltaes U 1, U 2 and U 3 the phases of which are reciprocally displaced by 120º and applied between the three wires of the system. There are two different ways of makin a connection, as shown in the followin diarams. In these diarams the voltae is usually shown by inclined arrows or rotatin vectors and maintainin between them the anle correspondin to the phase displacement ( 120º ), accordin to fiures 1.7a, b and c, e fiures 1.8a, b and c Delta Connection By connectin the three sinle-phase systems, as shown in fi.1.7a, b and c, we can eliminate the three wires, leavin only one at each connectin point. Thus three-phase system can be reduced to three-wires, L 1, L 2 and L 3. Line voltae ( U ) Is the rated voltae of the three-phase system applied between any two of these three wires L 1, L 2 and L 3. Lookin at the diaram in fi. 1.7b, one can see that: U = U f I = 3. I f = I f I = I f3 - I f1 ( Fiure 1.7c ) Example: Consider a balanced three-phase system with a rated voltae of 220 V. The measured line current is 10 amperes. By connectin a three-phase load to this system, composed of three equal loads connected in delta, what is the voltae across, and the current in each load? We have U f = U 1 = 220 V in each load. if I = I f. we have I f = I = = 5.77 A in each one of the load Star Connection By connectin one of the wires of each sinle-phase system to a common point, the three remainin wires will form a three-phase star system ( see fi. 1.8 ). Sometime the three-phase star system is made as a four wire or with the neutral wire system. The fourth wire is connected to the common point for the three-phases. 12 Specification of Electric Motors

13 The line voltae, or rated voltae of the three-phase system - and the line current - are defined in the same way as for delta-connections Fiure 1.8a - Connections Fiure 1.9 Fiure 1.8b - Electrical wirin diaram By analyzin the wirin diaram in Fi.1.8b, one can note that: I = I f U = 3. U f = U f U = U f1 - U f2 ( fiure 1.8c ) Fiure 1.8c - Phasor diaram Example: Consider a three-phase load composed of three equal loads. Each load is connected to a voltae of 220 V, absorbin 5.77 A. What is the rated voltae of the three-phase system feedin this load under normal conditions ( 220 and 5.77 A )? What is the line current? We have U f = 220 V ( rated voltae for each load ) U = = 380 V I = I f = 5.77 A 1.5 Three-Phase Induction Motor Fundamentally a three-phase induction motor consist of two parts: stator and rotor. Stator Consists of The frame ( 1 ) - is the supportin structure of the assembly; manufactured of iron, steel, die-cast aluminum, resistant to corrosion and with coolin fins. The lamination core ( 2 ) - constructed with manetic steel plates. The three-phase windin ( 8 ) - comprises three equal sets of coils, one se set for each phase, formin a balanced three-phase system when connected to a three-phase power supply. The rotor consists of: The shaft ( 7 ) - which transmits the mechanical output developed by the motor. The laminated manetic core ( 3 ) - the rotor laminations have the same characteristics of the stator laminations. Bars and short-circuit rins ( 12 ) - are aluminum die castins formed as one piece. Other components of the three-phase induction motor: End shields ( 4 ) Fan ( 5 ) Fan cover ( 6 ) Terminal box ( 9 ) Terminals ( 10 ) Bearins ( 11 ) This manual covers squirrel cae rotor motor where the rotor consists of a set of non-insulated bars that are interconnected by short-circuit rins. What characterizes an induction motor is the fact that only the stator is connected to the power supply. The rotor is not power supplied externally and the currents that flow throuh it are induced electromanetically by the stator from which comes the induction motor name Workin Principle - Rotatin Field When an electric current flows throuh a coil, a manetic field is enerated, the direction of which is alon the coil axis and proportional in value to the current. Fiure 1.10a Fiure 1.10b Specification of Electric Motors 13

14 a ) Fiure 1.10.a. shows a sinle-phase windin throuh which flow the current I, and the field H, enerated by the current. The windin is composed of one pair of poles, a North pole and a South pole, the effects of which are added to produce field H. The manetic flux passes throuh the rotor, across both poles and links up with itself by means of the stator core. When I is an alternatin current, field H is established in the same way, so that its value is represented at every instant, by the same chart shown in Fi.1.4b., also reversin its direction at every half cycle. The field H is pulsatin, its intensity varies proportionally to the current, always in the same direction - North-South. b ) Fiure 1.10b shows a three-phase windin consistin of three sinle-phase windins displaced 120º each other. If this windin is fed from a three-phase system, currents I 1, I 2 and I 3 will enerate their own manetic fields H 1, H 2 and H 3 in a similar way. The displacement between these fields is 120º; moreover, since they are proportional to the respective currents, the phase displacement in time between them will equally be 120º, which can be represented in a chart similar to Fi At any instant, the total resultin field H will be equal to the raphical sum of field H 1, H 2 and H 3. Fiure 1.11 shows this raphic sum for six successive steps Phasor diaram Phasor / vector Fiure 1.11 At instant ( 1 ), Fi shows that the field H 1 is at its maximum whereas fields H 2 and H 3 are neative and have the same value: 0.5. The resultin field ( raphic sum ) is shown in the upper part of Fi. 1.11( 1 ) and has the same direction of the windin of the phase 1. Repeatin this procedure for the instants 2, 3, 4, 5 and 6 of Fi. 1.6 we can see that the resultin field H presents a constant intensity, but its direction keeps rotatin to complete a whole turn at the end of a cycle. We can therefore conclude that a three-phase windin fed from three-phase currents enerates a rotatin field as if one sinle pair of poles was present, rotatin and fed with a constant current. This rotatin field, enerated by the threephase stator windin, induces certain voltaes into the rotor bars ( manetic flux lines o throuh the rotor bars ) which, bein short-circuited, enerate currents and, as a consequence, create a field on the rotor with reverse polarity if compared with the rotatin field polarity. Since opposite fields attract each other and considerin the stator field ( rotatin field ) is rotative, the rotor tends to follow the speed of this field. The result of this is that a motor torque is created in the rotor that makes it rotate and then drive the load Synchronous Speed ( ns ) The synchronous speed of the motor is defined by the rotation speed of the rotatin field which depends on the number of poles ( 2p ) of the motor and on the line frequency ( f ) in Hertz. The field makes a complete revolution at each cycle and f is the system frequency in cycles per second ( Hertz ). Windins may have more than one pair of poles which can be alternately distributed ( one North and one South ) alon the circumference of the manetic core. Since the rotatin field runs throuh one pair of poles at each cycle and the windin has poles or p pair of poles, the speed of the field is: 60. f 120. f n s = = ( rpm ) p 2 p Examples: a ) What is the sybchronous speed of a six-pole motor, 50 Hz? n s = = 1000 rpm 6 b ) A twelve-pole motor, 60 Hz? n s = = 600 rpm 12 It must be remembered that the number of poles of a motor must always be an even number in order to form pairs of poles. The table below shows the synchronous speed of the more common number of poles at 60 Hz and 50 Hz. Number of poles Synchronous speed per minute 60 Hertz 50 Hertz Table Synchronous speed For 2-pole motors, as in item 1.5.1, the field turns by one complete revolution at each cycle. Thus the electrical derees are equivalent to the mechanical derees. For motors with more than 2 poles, a smaller eometrical rvolution is realized by the field. For example: For a 6-pole motor, we will have, in a complete cycle, a field revolution of 360º x 2/6 = 120 eometrical derees. This is equivalent to 1/3 of the speed in 2 poles. We conclude, then, that: Geometrical derees = Mechanical derees x p 14 Specification of Electric Motors

15 Slip ( s ) If the motor runs at a speed different from the synchronous speed, i.e. differin from the speed of the rotatin field, the rotor windins cut the manetic force lines of the field and so, accordin to the electromanetism laws, induced currents will flow trhouh the rotor windins. The heavier the load the hiher must be the required torque to move it. To obtain a hiher torque, the speed difference must be reater so that induced current and enerated field become hiher. Therefore, as the load increases, the motor speed decreases. When the load is at zero ( motor at no-load ) the rotor practically rotates at its synchronous speed. The difference between motor speed ( n ) and synchronous speed ( ns ) is called slip ( s ), expressed as rpm or fraction of the synchronous speed or as a percentae of the synchronous speed: n s - n n s - n s ( rpm ) = n s - n ; s = ; s ( % ) =. 100 n s n s Therefore, for a iven slip s ( % ), the motor speed will be: s ( % ) n = n s. ( 1 - ) 100 Example: What is the slip of a 6-pole motor when the speed is 960 rpm? s ( % ) = s ( % ) = 4% Rated Speed Is the motor speed ( rpm ) operatin at rated power, at rated voltae and frequency. As described in item 1.5.3, it depends on the slip and on the synchronous speed. s % n = n s. ( 1 - ) rpm Insulation Materials and Insulation Systems Considerin that an induction motor is a simple desined and rued construction machine, its life time will exclusively depend on the quality level of the insulation materials. Motor insulation is affected by several factors includin moisture, vibration, corrosive environments and others. Amon all these factors, operatin temperature of the insulatin materials is the most critical. for the windin protection. When we refer to motor life time reduction, we do not refer specifically to excessively hih temperatures resultin in sudden insulation burn out. Insulation life time ( in terms of operatin temperature much below than the one affectin the insulation ) refers to permanent ain of the insulation material which becomes dry and loses its insulation properties. As a result, it will not withstand the voltae applied to it, thus causin short-circuit. If operatin temperature is kept below its limit, experiences have proved that the motor insulation can practically last for ever. Any increasin value above such limit will reduce insulation life time proportionally. Such limit of temperature is much lower that the temperature that causes insulation burn out and it depends on the type of used material. This limit of temperature refers to insulation hottest spot and not necessarily to the whole insulation. On the other hand, a sinle weak spot in the insulation is enouh to damae the windin completely. With increasin use of frequency inverters for the speed control of induction motors, other application criteria must also be considered for the preservation of the insulation system. For more details see Influence of the frequency inverter on the motor insulation Insulation Material The insulation material prevents, limits and directs the electric current flux. Althouh the insulatin material is primarily intended to block the current flux from a cable to round or to the lowest potential, it also serves to provide mechanical support, protect the cable from deradation caused by environment influences and to transfer the heat to the external environment. Based on system requirements, ases, liquids and solid materials are used to insulate electric equipment. Insulation systems affect the quality of the equipment, and type and quality of the insulation affect the cost, weiht, performance and its useful lifetime Insulation System A combination of two or more insulation materials applied to an electric equipment is desinated insulation system. This combination on an electric motor consists in manet wire, insulation of the slot, insulation of the slot closin, face to face insulation, varnish and/or imprenation resin, insulation of the connection leads and weldin insulation. Any material or component that is not in contact with the coil is not considered as part of the insulation system Thermal Classes Since the temperature of electro-mechanical products is basically the predominant factor for the ain of the insulation material and insulation system, certain basic thermal classifications are reconized and applied all over the world. The motor life time is reduced by half when subject 8% to 10 ºC in operation above the rated temperature of the class of insulatin material. To ensure a loner lifetime for the electric motor, the use of thermal sensors is recommended Specification of Electric Motors 15

16 Insulation materials and insulation system are classified based on the resistance to temperature for a lon period of time. The standards listed below refers to the classification of materials and insulation systems: Material Systems Material and System UL 746B UL 1446 IEC IEC UL 1561 / 1562 IEC IEEE 117 Table Standards for materials and insulation system The thermal classes defined for the materials and insulation systems are the followin: Temperature Class Temperature ( ºC ) IEC UL Y ( 90 ºC ) A ( 105 ºC ) E ( 120 ºC ) 120 ( E ) 130 B ( 130 ºC ) 130 ( B ) 155 F ( 155 ºC ) 155 ( F ) 180 H ( 180 ºC ) 180 ( H ) 200 N ( 200 ºC ) 200 ( N ) 220 R ( 220 ºC ) 220 ( R ) ( S ) above 240ºC above 240 ( ºC ) Table Thermal classes IEC - International Electrotechnical Commission - non-overnmental oranization for standards in the related electrical, electronic and technoloy areas UL - Underwriters Laboratories - American product certification body It is understood that the thermal class represents the maximum temperature that the electromechanical equipment can reach on its hottest spot when operatin at rated load without reducin its lifetime. The thermal classification of a material or system is based on a comparison with well-known reference systems or materials. However, for those cases where there is not any reference material, the thermal class can be obtained by exploitin the damae curve ( Arhenius Graphic ) for a certain time period ( IEC 216 specifies 20,000/hours ) WEG Insulation System In order to meet different market requirements and specific applications, associated to an excellent technical performance, nine insulation systems are used for WEG motors. The round enameled wire is one of the most important components used in the motor since the electric current flows throuh it and creates the manetic field required for motor operation. Durin the production process, the wires are submitted to mechanical traction efforts, flexion and abrasion electrical effects that also affect the wire insulatin material. Durin the operation, the thermal and electrical effects act on the wire insulation material. For this reasons, the wire requires an outstandin mechanical, thermal and electrical insulation resistance. The enamel used currently on the wire ensures such properties, where the mechanical property is assured by the outside enamel coat that resists to abrasion effects while insertin it into the stator slots. The internal enamel coat ensures hih dielectric resistance and the set provides thermal class 200 ºC to the wire ( UL File E ). This wire is used for all Class B, F and H motors. Smoke Extraction Motors are built with special wire for extremely hih temperatures. Films and laminated insulatin materials are intended to isolate thermally and electrically all motor windin parts. The thermal class is indicated on the motor nameplate. These films are aramid and polyester based films and also laminated films are applied to the followin areas: between the coils and the slot ( slot bottom film ) to insulate the lamination core ( round ) from the enameled wire coil; between phases: to isolate electrically one phase from the other phase Closin of the stator slot to insulate electrically that coil placed on the top of the stator and for mechanical purposes so as to keep the wires inside the stator slot Insulatin Materials in Insulation Systems The specification of a product within a certain thermal class does not mean that each insulatin material used has the same thermal capacity ( thermal class ). The temperature limit for an insulation system can not be directly related to the thermal capacity of the individual materials in this system. In a system the thermal performance of a material can be improved by protective characteristics of certain material used with this material. For example: a 155 ºC class material can have its performance improved when the set is imprenated with varnish for class H ( 180 ºC ). Fiure 1.12a - Wires and films used on the stator 16 Specification of Electric Motors

17 The imprenation varnishes and resins are mainly intended to maintain all enameled wire coil as a block with all stator components throuh alutination of such materials and to fill all voids inside the slot. This alutination avoids vibration and friction between the wires. Such friction could cause failures on the wire enamel, then resultin in a short-circuit. The alutination ( fillin of voids ) also helps the heat dissipation enerated by the wire and mainly, when motors are fed by frequency inverter, prevents/reduces the formation of partial dischares ( corona effect ) inside the motor. Two types of varnishes and two types of imprenation varnishes are currently used; all of them are polyester varnishes so as to meet motor construction and application requirements. Silicon resin is only used for special motors desined for very hih temperatures. Varnishes and resins usually improve thermal and electrical characteristics of the imprenated materials in such a way to classify these imprenated materials in hiher thermal class if compared to the same materials without imprenation. The varnishes are applied by the immersion imprenation process and then oven-dried. Solventless resins are applied by the continuous flow process. Fiure 1.12.c - Resin applied by continuous flow process The connection leads consist of elastomeric insulation materials that have the same thermal class as the motor. These materials are exclusively used to insulate electrically the lead from the external environment. They have hih electric resistance and proper flexibility to allow easy handlin durin manufacturin process, installation and motor maintenance. For certain applications, such as submersible pumps, the leads must be chemically resistant to the oil of the pump. The flexible pipes are intended to cover and insulate electrically the welded connections between the coils wires and the leads and the connections between wires. They are flexible to allow them to et shaped to weldin points and to the coil head tyin. Three types of pipes are currently used: Heat-shrink polyester tubin - Class of 130 ºC Polyester tube coated with acrylic resin - Class of 155 ºC Fiberlass tube coated with silicon rubber - Class of 180 ºC Fiure 1.12.b - Immersion imprenation process Specification of Electric Motors 17

18 2. Power Supply Characteristics 2.1 Power Supply System The power supply system can be sinle or three-phase. Sinle-phase system is mostly used in homes, commercial centers, farms, while three-phase system is used in industries. Both operate at 60 Hz or 50 Hz. b ) Sinle cable system with insolation transformer Besides requirin a transformer, this system has a few disadvantaes such as: Link power limitation to isolation transformer rated power; the roundin system of the isolation transformer must be reinforced. Lack of this will resuklt in absence of enery to the whole link Three-Phase System The three-phase voltaes mostly used in industries are: Low voltae: 220 V, 380 V and 440 V Hih voltae: V, V and V The star connected three-phase low voltae system consists of three phase leads ( L 1, L 2, L 3 ) and a neutral conductor ( N ). The last one is connected to the enerator star point or to the transformer secondary windin ( as shown in fiure Fiure 2.1 ). Power substation Fiure Sinle cable system with isolation transformer c ) Sinle wire earth return ( SWER ) system with partial neutral It is applied as a solution of the use of sinle wire earth return ( SWER ) system in reions with land ( soil ) of hih resistivity when it is difficult to et round resistance values of the transformer within the maximum desin limits. Fiure Three-phase system Sinle-Phase System Sinle phase motors are connected to two phases ( U L line voltae ) or to one phase and to neutral conductor ( U f phase voltae ). So the sinle-phase motor rated voltae must be equal to U L or U f system voltae. When several sinle-phase motors are connected to a three-phase system ( formed by 3 sinle-phase systems ), care must be taken in order to distribute them uniformly so as to avoid unbalance between phases. Sinle wire earth return ( SWER ) The sinle-phase earth return ( SWER ) is na electric system where the round lead operates as return lead for the load current. This is applied as solution for the use of sinle-phase motors from power supply not havin neutral available. Dependin on the available electric system and on the characteristics of the soil where it will be installed ( usually on farm power supply ), we have: a ) Sinle cable system The sinle wire earth return ( SWER ) system is considered the practical and economical option. However, it can be used only where the oriin substation outlet is star rounded. Power substation Power substation Fiure Sinle wire earth return system with partial neutral 3. Characteristics of the Electric Motor Power Supply 3.1 Rated Voltae This is the line voltae for which the motor has been desined Multiple Rated Voltae Motors are enerally supplied with sufficient terminals to enable alternative connections. This means that they can operate on at least two different voltaes. The main types of alternative terminal connections are: a ) Series-parallel connection The windin of each phase is divided into two equal parts ( halves ) ( please consider that the number of poles is always a multiple of two, so this type of connection is always possible ). By connectin the two halves in series, each half will have a voltae to the half rated phase voltae of the motor; By connectin the two halves in parallel, the motor can be supplied with a voltae equal to one half of the previous voltae, without affectin the voltae applied to each coil. ( refer to examples iven in fiures 3.1a and b ). Fiure Sinle cable system 18 Specification of Electric Motors

19 Fiure 3.1a - Series-parallel connection Y c ) Triple rated voltae The two previous alternative connection arranements can be obtained in one motor if the windin of each phase is divided into two halves enablin series-parallel connection. All terminals have to be accessible so that the three phases can be connected in star or delta. This means that there can be four alternatives for rated voltae: 1 ) Prallel-delta connection; 2 ) Star-parallel connection, bein the rated voltae equal to 3 x the first one; 3 ) Series-delta connection, i. e. the rated voltae bein twice the value of the first one; 4 ) Series-star connection, the rated voltae is equal to 3 x the third one. However as this voltae would be hiher the 690 V, it is only indicated as reference for star-delta connection. Example: 220/380/440( 760 ) V Note: 760 V ( only for startin ) This type of connection requires twelve terminals and Fi. 2.7 shows the normal numberin on the terminals as well as the connection diaram for the three rated voltaes. Fiure 3.1b - Series-parallel connection Δ This type of connection requires nine terminals on the motor. The most common dual voltae system is 220/440 V, i. e. the motor is parallel connected when supplied for 220 V, or alternatively, it is series connected when supplied for 440 V. Fi. 3.1a and 3.1b show normal terminal numberin, as well as connection diarams for this type of motor - both for star or delta connected motors. The same diarams apply to any other two voltaes, provided that one is the double of the other, e.. 230/460 V. b ) Star-delta connection Two ends of each phase windin are brouht out to terminals. By connectin the three-phases in delta, each phase receives total line voltae, e volts ( Fi. 3.2 ). By connectin the three-phases in star, the motor can be connected to a line voltae of 220 x 3 = 380 V. The windin voltae remains at 220 volts per phase. U f = U 3 Fiure Rated Frequency ( Hz ) This is the network frequency for which the motor has been desined Connection to Different Frequencies Three-phase motors wound for 50 Hz can also be connected to a 60 Hz network, a ) By connectin a 50 Hz motor, of the same voltae, to a 60 Hz network, the motor performance will be as follows: same output; same rated current; startin current decreases 17%; startin torque decreases 17%; breakdown torque decreases 17%; rated speed increases 20%. Note: please consider the required outputs for motors that drive machines with variable torque and speed. Fiure Star-delta connection Y - Δ This type of connection requires six terminals on the motor and is suitable for any dual voltae provided that the second voltae be equal to the first voltae multiplied by 3 ). Examples: 220/380 V - 380/660 V - 440/760 V In the example 440/760 V, the stated hiher voltae is used to indicate that the motor can be driven by star-delta switch. b ) If voltae chanes proportionally to frequency, the performance will be: motor output increase 20%; rated current is the same; startin current will be approximately the same; startin torque will be approximately the same; breakdown torque will be approximately the same; rated speed increases 20%. Specification of Electric Motors 19

20 3.3 Voltae and Frequency Variation Tolerance As per standard ABNT NBR ( 2008 ) and IEC , for induction motors, the combinations of voltae and frequency variations are classified as Zone A or Zone B ( fiure 3.4 ). Voltae Zone A 3.4 Three-Phase Motor Startin Current Limitation Whenever possible a squirrel cae three-phase motor should be started direct-on-line ( D.O.L. ) by means of contactors. It must be taken into account that for a certain motor the torque and current values are fixed, irrespective the load, for a constant voltae. In cases where the motor startin current is excessively hih, hamrful consequences may occur: a ) Hih voltae drop in the power supply system. Due to that, equipment connected to the system may be affected; b ) The protection system ( cables, contactors ) must be overdesined resultin in hiher cost; c ) Utilities reulations limitin the line voltae drop. Standard Features Frequency If D.O.L startin is not possible due to these problems, indirect connection system can be used so as to reduce startin current Star-delta switch Compensatin switch Series-parallel switch Electronic start ( Soft-Starter ) Zone B (external to Zone A) Fiure Limits of voltae and frequency variations under operation D.O.L Startin A motor must be capable of performin its main function continuously at Zone A, however it may not develop completely its performance characteristics at rated voltae and frequency ( see rated characteristics point in fiure 3.4.a ) showin few deviations. Temperature rises can be hiher than those at rated voltae and frequency. A motor must be capable of performin its main function at Zone B, however it may present hiher deviations than those of Zone A in reference to performance characteristics at rated voltae and frequency. Temperature rises can be hiher than those at rated voltae and frequency and probably hiher than those of Zone A. The extended operation at Zone B is not recommended. Source: ABNT NBR ( 2008 ) Fiure Command circuit - direct startin 20 Specification of Electric Motors