TABLE OF CONTENTS INTRODUCTION...3 ADVANTAGES...5 OPERATION CHARACTERISTICS...6 EXCITATION TYPES...8 CONSTRUCTIVE COMPONENTS...9 ACCESSORIES...

Transcription

1 SYNCHRONOUS MOTORS

2 TABLE OF CONTENTS Motores Síncronos INTRODUCTION...3 ADVANTAGES...5 OPERATION CHARACTERISTICS...6 EXCITATION TYPES...8 CONSTRUCTIVE COMPONENTS...9 ACCESSORIES...11 CONSTRUCTIVE CHARACTERISTICS...12 INSULATION SYSTEM...13 TESTS...13 SYNCHRONOUS MOTOR SELECTION...14 APPLICATIONS...15 SYNCHRONOUS MOTOR SPECIFICATION...16

3 INTRODUCTION The word SYNCHRONOUS is originated from Greek. The prefix SYN means with and CHRONOS means time. A synchronous motor literally operates in time with or in synchronism with the power supply system. Due to the fact the synchronous motors are fitted with special operating characteristics, industries are more and more using such motors. Included in the main reasons for the industries to specify SYNCHRONOUS MOTORS to drive a wide range of applications are the high efficiency and the fact they are suitable to operate as synchronous compensating machines for power supply factor correction. In addition to that, these motors also feature high torque, constant speed under load variation, along with low maintenance cost allowing major economical and operational advantages to end users. On point (1), figure 2 shows that field H1 is on maximum stage and that fields H2 and H3 are negative and with same value which is equal to half of H1. The 3 fields represented on figure 3 (upper part) take into consideration that the negative field is represented by an arrow pointed to the opposite direction in comparison to what would be normal. The resulting field (graphic sum) is shown on the bottom part of figure 3, position (1), having the same direction of phase 1 winding. Repeating the construction for points 2, 3, 4, 5 and 6 of figure 1, the resulting H field presents constant intensity. However, its direction will rotate until completing a turn at the end of the cycle. Cycle Operation Principle Stator and stator winding (armature) of Weg synchronous motors are identical to components of three phase induction motors. Identical to induction motors, the current that goes through the stator winding generates a rotating magnetic flow that circulates around the air gap. Figure 1 Time Stator rotating field - When the current goes through the coil, a magnetic field is generated which is based on coil axis and is proportional to the current value. Figure 1 shows a waveform of a balanced three phase system consisting of 3 sets of coils placed symmetrically on the area resulting in a 120º angle. Figure 2 represents a three phase motor winding. If the winding is powered by a three-phase system, currents I1, I2 and I3 will create at the same time their own magnetic fields H1, H2 e H3. These fields are spaced between them by a 120º angle as well. Besides that, as they are proportional to the respective currents, they will be de-phased in time, also between them by a 120º angle. The resulting H field, at each point, will be equal to the graphic sum of the 3 magnetic fields H1, H2 e H3 on that point. Figure 3 shows this graphic sum for 6 successive points. Figure 2 Figure 3 03

4 Synchronous Speed - The motor synchronous speed (rpm) is defined by the rotating field speed that depends on the motor pair of poles (p) and on the power supply frequency (f). The stator winding can consist of one or more pairs of poles that are distributed alternatively (one north and another south ) along the magnetic core outer side. The rotating field goes through a pair of poles (p) at each cycle. Considering the winding has poles or pair of poles, the field speed will then be: rpm = 60. f p The synchronous motor rotor is built with a number of poles corresponding to the stator winding number of poles. Under normal operation, there is no relative movement between rotor poles and stator magnetic flow, that is, they are in perfect synchronism. As a result, there is no induction of electric voltage into the rotor by the mutual flow and then there is no excitation originated from the AC power supply. Depending on the type of rotor used (cylindrical or salient pole), the pole coils can be built with insulated copper wire turns or with copper bars. The field excitation is done through a DC system. When going through the field winding, the poles are polarized magnetically becoming alternatively north pole and then south pole. The DC excitation can be applied to the field through the brushholders and slip rings, or through a brushless system and through electronic control (brushless). 04

5 ADVANTAGES Due to their special operating characteristics, synchronous motor applications usually result in economical and operational advantages to end users. Included in the economical advantages of using synchronous motors are: - High efficiency - Power factor correction In addition to that, there are other specific operational advantages of using synchronous motors as follows: - Special starting characteristics - Constant speed under load variation - Reduced maintenance cost High Efficiency Associated to the initial purchasing cost of a synchronous motor, further gains resulting from low operational cost should be also considered. On those cases, where just the efficiency aspect is taken into account when specifying a motor, a synchronous motor with PF=1.0 is usually the solution. When the reactive power (kvar) is not required and only the actual power (kw) is applicable, the current is minimized resulting in lower stator winding loss I²R. Once the required field current is the minimum applicable, there will be lower field winding loss I²R. Except for those cases where high torque is required, the low stator winding losses allow a synchronous motor with PF=1.0 to be designed in lower size if compared to a synchronous motor with PF=0.8 of equal power rating. Hence, synchronous motor efficiencies with PF=1.0 are normally higher than induction motor efficiencies of equal power rating. Power Factor Correction Electric power systems are based not only on the generated active power supply in kw, but also on the power factor on which it is generated Whenever the load power factor is below the specified values, the consumer may be subject to penalties. These penalties (fines) occur due to the fact that the low power factor results in increase of required reactive power (kvar) and, as a consequence, an increase of the power supply transmission and generation equipment capacity. On industries, inductive reactive load are predominant. These are usually low size or low speed induction motors. Such loads require considerable portion of reactive power(kvar) as magnetization current. Other than applying bank of capacitors to supply the power supply with reactive power, synchronous motors are normally used for such purpose. Power factor of synchronous motors can be easily controlled as they are fitted with an independent excitation source. This way, power factor can be increased without generating reactive power (motor with PF=1.0) or generate required reactive power (motor with PF=0.0). So depending on the application, a synchronous motor can supply the required power with substantial power reduction on the whole system. Comparison between efficiency levels of synchronous motors with PF= 0.8, PF=1.0 and induction motors. Efficiency (%) Induction Motor Power Rating (kw) 05

6 Special Starting Characteristics Constant Speed Large ball mills for iron mines, cement plants and compressors are few examples of applications that required high starting torque (150 to 200 % of the rated torque). Due to power supply system limitations, low starting currents (locked rotor) are usually required. A combination of high torque with low starting current can be better achieved with the application of synchronous motors without affecting operating characteristics. Starting current reduction can be usually achieved with a special design of stator and amortisseur winding. Starting the motor with reduced voltage is also an alternative applied so as to have the current reduced, although, with torque reduction. Independently of load variations and as long as the load is maintained within the motor pull-out torque limitation, the synchronous motor average speed is kept constant. This occurs due to the fact that the rotor poles remain locked in relation to the rotating magnetic field that is generated by the stator winding. Then, synchronous motor speed is kept constant either on overload variations or on voltage drop cases, in addition to following pull-out torque limitations. On certain applications such as on pulp and paper mills, the constant speed results in superior uniformity and quality of the supplied product. Reduce Maintenance Cost Since they do not require slip electric contacts for their operation, BRUSHLESS synchronous motors are not manufactured with brushes nor with slip rings. Hence, maintenance, inspection and cleaning on these components are not required. OPERATION CHARACTERISTICS TORQUES A synchronous motor must be always designed taking into account driven load characteristics, in addition to torque s and inertia. a) Starting torque It is the torque that the motor must supply to drive the standstill load resistant torque, that is, it is the load starting torque. b) Pull-in Torque It is the torque that the motor must supply to reach the correct speed, where the excitation field application will take the motor to the synchronism (pull-in torque). c) Pull-out Torque It is the torque that the motor must supply to keep the motor under synchronism in case of momentary overloads with reted excitation. 06 Inertia When driving high inertia loads, synchronous motors are designed in larger frame sizes so as to meet acceleration conditions. The time period the motor takes to accelerate causes amortisseur winding overheating. Therefore, this motor must be designed in such a way to meet the starting conditions. The correct load inertia definition, associated with motor and load torque analysis are quite important allowing this motor to meet starting and acceleration conditions. Starting The amortisseur winding, that operates as a squirrel cage of an induction motor, is intended to guarantee synchronous motor starting and acceleration. This way, starting and pull-in torque s vary with the square of the applied voltage, and the starting current is proportional to the applied voltage, exactly as on induction motors.

7 Starting characteristic curve of a synchronous motor at full voltage A synchronous motor starts exactly like an induction motor and then it accelerates the load up to the point where the motor torque becomes the same as the load resistant torque. Usually this point occurs with 95% of the synchronous speed or above that, and on this condition, the excitation voltage is applied to the motor, and the rotor synchronizes, that is, it will accelerate the combined rotor and motor inertia plus the load inertia up to precise synchronous speed. Driven load characteristics will determine acceleration and synchronism conditions. On high resistant torque loads, the amortisseur winding must make the motor and load torque accelerate at a time period higher than that for a shorter resistant torque. The proper amortisseur winding design requires precise knowledge of the load resistant torque. Based on the synchronous motor starting characteristic curve, starting torque decreases as it gets close to the synchronous speed. On load applications with resistant torque parabolic curve and considering that at 98% of the synchronous speed, the value of such torque is equal to the load rated torque, the motor is required to supply a torque equal or higher than the load torque on this point. If the specified motor torque, at 95% of the synchronous speed is equal to the load pull-out torque, this motor can not supply this torque at 98% of the synchronous speed and then such motor will not synchronize. This way, to ensure motor starting and synchronism, careful analysis of the starting torque curve along with load resistant torque curve must be carried out. Asynchronous Starting The main starting method applied on synchronous motor starting is the asynchronous starting through a squirrel cage with the short-circuited winding rotor or connected to a resistance, usually called starting resistance or discharge resistance. Through asynchronous starting, the rotor accelerates at a speed very close to the synchronous speed, with a slight slip in reference to the rotating field. On this point, a direct current is applied to the rotor winding and then taking the motor to synchronism. On brush-supplied machines, a field application relay is used, while on brushless motors a electronic control circuit is applied, which is installed attached to a rotating rectifier disc. This electronic circuit and the field application relay are intended to manage the synchronous motor starting sequence, since the rotor short circuits up to the field current application. 07

8 Starting Current On Brushless synchronous motor starting, the field winding is short-circuited through the elechome circuit. While motor remains on standstill, the field current frequency is initially equal to power supply frequency (60Hz for power supply of 60Hz) and reduces as the motor speed increases. When the excitation is switched-on, motor speed must be close to synchronism speed (around 95% of the synchronism speed), and the field current frequency will remain around 3Hz. The stator current also varies on the starting and then it becomes stable after motor synchronism. Stator current (Is) and rotor current (Ie) performance on asynchronous starting 1) Starting point Is Ie 3) Point when the field is switchedon and the motor synchronizes Stator Current Is 2) Rotor frequency decreases as the speed increases Stator Current Is Ie 4) Rotor and stator current stability Stator Current Is Ie Ie EXCITATION TYPES Synchronous motors require a DC power supply to power the field winding (rotor winding), which is usually done through slip rings and brushes (static exciter) or through a brushless rotating exciter. 1. Static exciter (with brushes) Synchronous motors supplied with static exciter are fitted with slip rings and brushes that allow current powering of the rotor poles through slip contacts. The DC power supply for the poles must come from an AC/DC converter and static controller Brushless exciter Synchronous motors with brushless excitation system are fitted with a rotating exciter, normally installed on the non-drive end of the motor. The exciter operates as an AC generator with the rotor attached to the motor shaft. The rotor is fitted with a three phase winding and the stator consists of alternating poles (north and south) and powered by an independent DC source. This three phase winding is connected to rectifier bridge. The current generated on the rotor is rectified and intended to power the motor field winding. The amplitude of such field current can be controlled through the rectifiers that power the exciter stator field. Synchronous motors with brushless excitation require low maintenance cost once they are not fitted with bushes. Since they are not built with slip electric contacts, avoiding sparking, synchronous brushless excitation motors are recommended for explosive atmosphere applications.

9 CONSTRUCTIVE COMPONENTS STATOR Lamination core - Consisting of silicon steel lamination of low losses, pressed, and the set is fastened through metallic bars or a bar-designed system. EXCITER It is intended to supply magnetizing current to the motor field winding. The brushless exciter consists of rotor, stator, rectifer bridge and discharge circuit. Tthe static exciter consists of slip ring and brushes and depends on an external source to power the motor field. Frame - It is mainly intended to support and protect the lamination core and stator winding. The frame can be constructed in horizontal or vertical mounting configurations and with degree of protection that meets application characteristics. It is manufactured with steel plates and welded with MIG welding resulting in a solid and rugged structural construction. The whole frame construction is duly treated for stress release caused by welding process. This construction results in an excellent structural piece so as to withstand mechanical strengths originated from eventual short-circuits and vibrations, and then making the motor suitable for the most severe applications. The frame inner part consists of bars for lamination core fastening to the winding. Usually the frame is based on a metallic rigid base (steel plate) and this part, in its turn, is based on a concrete foundation. The metallic base is fastened to the concrete base through studs. Wound stator - Consisting of static magnetic parts, the wound stator includes the silicon lamination core and the stator winding. The last one operates as AC power supply to generate the rotating magnetic field. ROTOR Depending on motor constructive characteristics and on the application, rotor can be built with cylindrical or salient poles. The rotating active parts include rotor ring, field winding and amortisseur winding. The field poles are magnetized through the exciter direct current or directly through slip rings and brushes; they gear themselves magnetically by the air gap and rotate in synchronism with the stator rotating field. The synchronous motor rotor fitted with salient poles consists of shaft, polar ring and poles. The poles are built with laminated steel plates that are fixed with steel bar and Salient pole rotor welded on the ends. The field coils are constructed with enameled copper wires or flat copper bars. 09

10 Once they have been wound and impregnated, poles are fixed to the shaft or to the polar ring with the application of bolts from top or bottom part of the pole, or connected through dov tail. The amortisseur winding is fitted in the poles and, depending on the motor design, it is built with copper bars or other material. After final assembly and impregnation, the complete rotor is balanced dynamically at 2 planes. The synchronous motor rotor of cylindrical poles consists of shaft, lamination core and pole winding. The winding is installed in the rotor slots forming the poles. Cylindrical pole rotor Shaft - The shafts are constructed of forged or laminated-steel and machined exactly as per specifications. The shaft end is usually cylindrical or flanged. Amortisseur winding - This is fitted in the slots placed at the polar shoes of the salient pole rotor or an external surface of the cylindrical pole rotor. Consisting of bars that go through the slots and are short-circuited at the ends and then forming a squirrel cage. The amortisseur winding operates on synchronous motor starting, along with ensuring speed stability under sudden load variations. 10 BEARINGS Based on the application, synchronous motors can be supplied with grease-lubricated ball or roller bearings or oil lubricated sleeve bearings. Sleeve bearings can be naturally lubricated (self lubricated) or with a forced lubrication system (independent lubrication system). Ball or roller bearings - Depending on the speed and thrusts they are submitted to, these grease lubricated bearings can be supplied either with ball or cylindrical rollers. On certain specific applications, special bearings can be also supplied. Naturally lubricated sleeve bearings - When the rotor turns, the lubricating oil is spread out by the internal oil ring and transferred directly to the shaft surface creating a layer of oil between the shaft and the bearing liner surface. The friction heating is dissipated just by radiation or convection. However, the ambient temperature must be informed when specifying a motor so as to ensure natural cooling. Forced Lubrication - The lubricating oil circulates around the bearing through an independent oil circulation system and, if required, it is cooled down through an independent hydraulic system. This system is required when the natural bearing lubrication coming from the internal oil ring is not enough due to the specific speed required or due to high friction losses.

11 ACCESSORIES Weg synchronous motors are supplied with standard accessories required for correct operation and monitoring of the main components. When specifying a motor, the end user must inform the required accessories that should be included in the design and motor manufacture. Accessories (supplied as standard) - Stator winding temperature detectors PT Bearing temperature detectors - Space heaters Bearing PT Special Accessories - Brake disc - Brake - Vibration detectors - Encoder - Frame lifting device Optional Accessories - Temperature detectors for air inlet and outlet - Water flow valve - Water flowmeter - Oil flowmeter - Oil flow sight - Water flow sight - Hydraulic unit for bearing lubrication - Oil injecting system under pressure for motor starting and stop(hydrostatic Jacking) - Oil thermometer (bearings) - Water thermometer (heat exchanger) - Air thermometer (Cooling) - Anchorage plate Thermometer 11

12 CONSTRUCTIVE CHARACTERISTICS CONSTRUCTION Weg synchronous motors are manufactured in B3, D5 or D6 mounting configurations and with grease lubricated ball or roller bearings or oil lubricated sleeve bearings. Synchronous motor Mounting configuration: B3 Enshield bearings Sleeve bearings can be mounted on pedestals attached to the endshields which make part of the motor. High speed motors are usually built with relatively long core length if compared to its diameter. While low speed motors are usually built with relatively short rotor core if compared to its diameter. COOLING SYSTEMS The cooling systems most commonly used are: - Open self-ventilated motors, Degree of Protection IP23; - Enclosed motors with air-air heat exchanger, Degree of Protection IP54 to IPW65; - Enclosed motors with air-water heat exchanger, Degree of Protection IP54 to IPW65. Synchronous motor Mounting configuration: D6 Pedestal bearings Besides the cooling methods mentioned above, motors can be supplied with forced ventilation, air inlet and outlet by ducts, and other cooling methods, always meeting installation environment and application characteristics. Open motors Enclosed motors Mounting Configuration: D6 Mounting Configuration: D6 Mounting configuration: B3 12 Mounting configuration: B3

13 INSULATION SYSTEM On High Voltage Motors - Coils are pre-fabricated with rectangular shape copper wire, coated with mica tape and epoxy resin impregnated. They are then heated up and cured resulting in high winding mechanical strength. This process is called polymerization and provides motor extended life time. Coils are fitted in the stator slots, insulated from the stator lamination core with class F (155 C) insulating material and fastened by fiber glass or magnetic wedges. The copper wires that form the coils are insulated with appropriate class H (180 C) enamel. Impregnation - Once coils have been inserted, slots closed, connections and coil heads tied, the wound stator is vacuum and pressure impregnated by applying class H solvent-free epoxy resin, ensuring excellent electric, mechanical properties to Weg insulation system, in addition to providing weathering resistance. Epoxy resins are ideal for impregnation once they offer, upon cure, superior resistance to weathering which is typical for environments where electric rotating machines operate. Considering they are 100% solid resins, that means, solvent free composition, they can ensure major homogeneity and prevent from occurring insulation bobbles after polymerization and final cure. The stator is then submitted to hi-pot test and short-circuit between turns: - Surge Test before and after the impregnation. TESTES Weg synchronous motors are tested in accordance with IEC34 Standard in its modern testing laboratory for low, medium and high voltage motors in output ratings up to 10,000kVA and voltage range up to 15,000V, with full computerized and high precision monitoring. Tests are grouped in three categories: routine, type and special tests. Routine tests are performed on all motors produced. Besides routine tests, type tests are normally performed randomly or under customer request. Special tests are performed only upon customer request. Routine Tests Visual inspection test Air gap checking and bearing tolerances Winding Ohmic resistance Insulation resistance. Temperature and space heater inspection Bearing and rotation direction marking Vibration checking No load test Short-circuit curve Hi-pot test Excitation system test Type tests Temperature rise test No load curve (V curve) Special tests Noise level test Instantaneous short-circuit Shaft voltage check Starting current Overspeed Loss and efficiency test Waveform measurement Polarization index Synchronous motor starting 13

14 SYNCHRONOUS MOTOR SELECTION Synchronous motors must be specified based on their application, that is, based on their service duty, resistant torque curve and inertia curve. The last two aspects are essential items for motor starting analysis, while service duty is important for correct thermal design. Power factor and excitation type are also important aspects to take into account for motor specification. Environment type defines the motor degree of protection. In reference to this aspect, load inertia will have a great influence on starting time and on the heat to be dissipated by the bars. Resistant torque and load inertia When specifying synchronous motors it is quite relevant to know driven load data. Resistant torque curve and load inertia have direct influence on motor starting characteristics. To drive high inertia loads, synchronous motors are built in larger frame sizes so as to meet acceleration conditions. Considering that a synchronous motor starts through its squirrel cage (same as on induction motors) and with rotor winding short-circuited (or closed in a resistance), the correct material used on dump bar (usually built with copper or copper alloys) and their geometry are essential to define motor starting characteristic curve. This curve must be always defined based on load resistant torque curve. Besides ensuring the starting through the squirrel cage generated torque, the dump bars must be also designed in such a way to allow heat dissipation generated during motor starting. Theoretically, it is not correct to say that a synchronous motor used on certain application (ex. pump), can be used on another different application (ex. exhauster). Service Duty The correct specification of a synchronous motor rated power must consider the motor service duty with overload frequency existing on such duty. Power factor Whenever power factor correction is required on a synchronous motor application, this required power factor must be specified previously. This means that a motor designed to operate with unit power factor can not supply the same rated output power under lower power factor. Environment characteristics The environment where the motor will be installed must be analyzed before specifying such motor. Environment type defines the Degree of Protection and motor cooling method. Explosive atmosphere application motors require brushless excitation. Ambient temperature and altitude considered when specifying a motor are 40ºC and 1000m above sea level. If motor operation environment presents values above those mentioned above, it is important to reconsider new data when specifying this motor. 14

15 APPLICATIONS Weg synchronous motors are manufactured specifically to meet every application requirements. They are used on all types of industry including: Mining (crushers, mills, conveyor belts and others) Steel plants (laminating machines, fans, pumps, compressors) Pulp and paper (extruders, chippers, debarkers, compressors, grinders) Variable speed Synchronous motors with variable speed are recommended for applications with high torque, low speed and wide speed adjusting range. Depending on load and environment characteristics, motor construction for such applications can be supplied with or without brushes,. Due to their higher efficiency level, reduced size and higher output rating capacity, synchronous motors can replace DC motors on high performance applications. Sewage systems (pumps) Chemical and Petrochemical (compressors, fans, exhausters) Cement (Crushers, mills, conveyor belts) Rubber (extruders, mills, mixers) Fixed speed Synchronous motor applications with fixed speed are recommended due to low operational cost once they offer high efficiency and can be used as synchronous compensating machines for power factor correction. Recommended motors for this application are those with brushless excitation. On several cases, a motor with lower torque values compared to standard values can be actually applied. This brings positive reduction on motor starting current, resulting in less electric system troubles on starting, along with reduction on mechanical thrusts resulting from motor winding. For a correct design and application of Weg synchronous motors, we recommend to supply complete application data. 15

16 SYNCHRONOUS MOTOR SPECIFICATION (CHECK LIST) Quantity: Application (driven machine): Output rating (kw): Voltage (V): Speed (rpm) Frequency (Hz): [60] Altitude (m): [1000] Amb. Temp. (ºC) [40] Power factor: [0.8 or 1.0] Service factor: [1.0] Mounting: [B3E] Installation: [inside or outside] Excitation: [brushless or with brushes] Excitation voltage (V) Starting: Full voltage [ ] Reduced voltage [ ] % Operating conditions: [ ] Continuous frequency and voltage [ ] Drive - from to Hz Bearings: [pedestal or on the endshield] Continuous or momentary thrust on bearings: Degree of Protection: [Open - IP23S or enclosed - IP55] Cooling: [air-air heat exchanger, air-water heat exchanger...] Starting: [1 hot/2 cold] Starting torque: [40%] Synchronization torque (pull-in) [30%] Synchronization pull-out torque (Pull out): [150%] Load inertia J (kgm2): (Supply torque curve x load speed) Rotation direction: [CW, CCW or both] Coupling (type): WEG supply [ ] Yes [ ] No Main motor dimensions Shaft height.. H: Total height: HD: Distance between feet holes(longitudinal).. B: Distance between feet holes (transversal).. A: Distance between feet hole and shaft shoulder.. C: Shaft - Diameter.. ØD: length.. E: Key diameter..ga: Key width..f: Main terminal box - Lead inlet: [bottom] Cable gland [yes no] Number of terminals [3 or 6] Accessory terminal box: [ Yes or no ] Accessories - [ ] Space heater Voltage (V) [ ] Winding temperature detectors [PT100 with 3 wires - 1 per phase] [ ] Bearing temperature detectors [PT100 with 3 wires - 1 per bearing Notes: 16

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