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    E 1005 E 1005 Document Transcript

    • PDHengineer.com Course № E-1005 Induction and Synchronous Motor Fundamentals This document is the course text. You may review this material at your leisure before or after you purchase the course.  If you have not already purchased the course, you may do so now by returning to the course overview page located at:  http://www.pdhengineer.com/pages/E‐1005.htm (Please be sure to capitalize and use dash as shown above.)   Once the course has been purchased, you can easily return to the course overview, course document and quiz from PDHengineer’s My Account menu.  If you have any questions or concerns, remember you can contact us by using the Live Support Chat link located on any of our web pages, by email at administrator@PDHengineer.com or by telephone toll‐free at 1‐877‐PDHengineer.  Thank you for choosing PDHengineer.com.            © PDHengineer.com, a service mark of Decatur Professional Development, LLC.  E‐1005 C1
    • Induction and Synchronous Motor Fundamentals (1 PDH) PDHengineer.com Course No. E-1005IntroductionMotors are electromagnetic devices that are used to convert electrical energy intomechanical work. There are three classes of AC motors – synchronous motors, inductionmotors and series wound motors. The most common motor classes are synchronous andinduction.NEMA MG 1-2003 has the following definitions:An induction machine is an asynchronous machine that has a magnetic circuit interlinkedwith two electric circuits, or sets of circuits, rotating with respect to each other. Power istransferred from one circuit to another by electromagnetic induction.A synchronous machine is an alternating-current machine in which the average speed ofnormal operation is exactly proportional to the frequency of the system to which it isconnected.Synchronous motorsA synchronous motor is a synchronous machine used for a motor. A synchronous motorcannot start without being driven. They need a separate starting means.There are several types of synchronous motors. These include direct current excitedsynchronous motor (field poles are excited by direct current), a permanent magnetsynchronous motor (field excitation is provided by permanent magnets) and a reluctancesynchronous motor (starts as an induction motor, is normally provided with a squirrel-cage winding, but operates at synchronous speed).Synchronous motors have fixed stator windings electrically connected to the AC supplywith a separate source of excitation connected to a field winding on the rotating shaft. Athree-phase stator is similar to that of an induction motor. The rotating field has the samenumber of poles as the stator, and is supplied by an external source of DC. Magnetic fluxlinks the rotor and stator windings causing the motor to operate at synchronous speed. Asynchronous motor starts as an induction motor, until the rotor speed is near synchronousspeed where it is locked in step with the stator by application of a field excitation. Whenthe synchronous motor is operating at synchronous speed, it is possible to alter the powerfactor by varying the excitation supplied to the motor field. Page 1
    • An important advantage of a synchronous motor is that the motor power factor can becontrolled by adjusting the excitation of the rotating DC field. Unlike AC inductionmotors which run at a lagging power factor, a synchronous motor can run at unity or evenat a leading power factor. This will improve the overall electrical system power factor,voltage drop and also improve the voltage drop at the terminals of the motor.Synchronous motors can supply reactive power to counteract lagging power factor causeby inductive loads. As the DC field excitation is increased, the power factor (asmeasured at the motor terminals) becomes more leading. If the excitation is decreased,the power factor of the motor becomes more lagging.Refer to the above graph. The curves on the graph show the effect of excitation (fieldamps) on the stator and on the system power factor. There are separate V curves for No-Load and Full Load cases. A manufacturer may also have curves for other percentages offull load (25%, 50%, 75%). From this particular curve, to determine the field excitationthat will produce a unity power factor at full load: Go up the Y-axis to unity power factor(100%). Come across the X-axis to the peak of the Power Factor V curve for full loadoperation. Come back down the Y-axis from that point to determine the field amps. Inthis case, the field amps is just over 10 amps. Notice that at unity power factor, the statorfull load amps is at the minimum value. As the field amps increases above what isrequired for unity power factor, the motor becomes more leading. As the amps decreasebelow what is required for unity power factor, the motor becomes more lagging. In eithercase, the stator amps increases above that required for unity power factor.Synchronous motors can be classified as brush excitation or brushless excitation. Brushexcitation consists of cast-brass brushholders mounted on insulated steel rods andsupported from the bearing pedestal. The number of brushes for a particular size andrating depends on the field current. Sufficient brushes are supplied to limit the current Page 2
    • density to a low value. The output of a separate DC exciter is applied to the slip rings ofthe rotor. A brushless excitation system utilizes an integral exciter and rotating rectifierassembly that eliminates the need for brushes and slip rings.Synchronous motors are started using several reduced voltage methods. The mostcommon is starting across the line with full AC voltage to the windings. As the motorspeed increases, the discharge resistor provides the torque required for the motor to reachsynchronous speed. Once synchronous speed is reached, the starting resistor is switchedout of the field circuit and excitation can be applied to lock the stator and field poles insync. The DC excitation system is used to apply current to the field winding creating arotating electromagnet field that couples the rotor field to the rotating AC field in thearmature winding when the motor is operating at synchronous speed. If the North andSouth poles of the rotor and stator are aligned, the rotor will lock in step with the statorand the motor will synchronize. If the rotor poles are 180 degrees out of phase with thestator poles, but the motor is accelerating, it is likely that accelerating torque along withmagnetic attraction will combine to pull the rotor rapidly into pole alignment with thestator.Other starting methods include reduced voltage starting, such as using anautotransformer. Another starting approach is to switch out the starting resistor and applyDC excitation based upon the time after the motor AC supply power is applied. In thisapproach, the acceleration time of the motor needs to be known and the motor must beable to reach nearly synchronous speed without excitation.Some applications use a speed signal to apply DC excitation when the motor hasaccelerated to 90 – 95% of rated speed. The timing for switching out the starting resistorand applying DC excitation is monitored by electronics on the rotating field. Applicationof the field can be accomplished using solid-state devices instead of mechanical breakersor contactors.Once the motor’s field poles are in step with the stator frequency, two factors determinethe synchronous speed of the motor. The first is the frequency of the applied voltage, andthe second is the number of poles in the motor.Speed = Freq x 120 PolesSynchronous motor efficiencies are higher than those of induction motors. Their inrushcurrents are low. They can be designed with torque characteristics to meet therequirements of the driven load and available power supply. A synchronous motor’sspeed/torque characteristics are ideally suited for direct drive of large horsepower, lowrpm loads such as reciprocating compressors. Their precise speed regulation makes theman ideal choice for certain industrial processes. Page 3
    • Synchronous motors are used in the pulp, paper processing, water processing treatment,petrochemical and mining industries, to name a few. They are used for chippers,crushers, pumps, and compressor drives to name a few applications.Synchronous motors are designed to meet NEMA MG1-21.21. Noise tests are performedper IEEE 85 and performance tests per IEEE 115 for machine efficiency, temperaturerises, starting characteristics and other parameters.Induction motorsInduction motors are simple and rugged and relatively cheap to construct. They consistof a wound stator and a rotor assembly. They have fixed stator windings that areelectrically connected to an AC power source. Current is induced in the rotor circuit.The resulting magnetic field interacts with the stator field for the “induction” to occur.No separate power source is required to provide the rotor field. An induction motor canbe started and accelerated to steady state running conditions simply by applying ACpower to the fixed stator windings of the motor. They do not rely on brushes like a DCmotor does. Induction motors have a longer life than synchronous motors and arecommon for applications above 1 kW.There are a couple of types of induction motors – a squirrel-cage motor and a wound-rotor motor. A squirrel-cage motor is one where the secondary circuit consists of anumber of conducting bars that have their end pieces connected by metal rings or platesat each end. A wound-rotor motor in one where the secondary circuit has a polyphasewinding or coils whose terminals are either short circuited or closed through suitablecircuits.The rotor assembly of an induction motor, when looked at from the end, resembles asquirrel cage (or a hamster exerciser). Thus the name squirrel-cage motor refers to aninduction motor. The most common rotor type has cast aluminum conductors (bars) andshort-circuiting end rings. The position of the bars in relation to the surface of the rotor,the shape, cross sectional area and material of the bars determine the rotor characteristics.A bar with a large cross sectional area will exhibit a low resistance. A copper bar willhave a low resistance compared to a brass bar of equal proportions. The rotor design willdetermine the starting characteristics of the motor. The rotor turns when the movingmagnetic field induces a current in the shorted conductors.The stator of an induction motor is the outer body of the motor. This houses drivenwindings on an iron core. The standard stator has three windings for a three-phasedesign. A single-phase motor typically has two windings. The core of the stator is madeup of a stack of round pre-punched laminations pressed into a frame that is made ofaluminum or cast iron. Laminations are round with a round hole where the rotor ispositioned. The inner surface of the stator has slots or grooves where the windings arepositioned. The arrangement of the windings determines the number of poles that amotor has. A stator is like an electromagnet and has poles (north and south) in multiplesof two (2-pole, 4-pole, etc.). The voltage rating of the motor is determined by the number Page 4
    • of turns on the stator. The power rating of the motor is determined by the losses. Theseinclude copper loss, iron loss and the ability of the motor to dissipate the heat generatedby the losses. The design of the stator determines the rated speed of the motor as well asthe full load/full speed characteristics.The synchronous speed of the motor is the speed where the magnetic field rotates. It isdetermined by the number of poles in the stator and the frequency of the power supply. Itis the absolute upper limit of motor speed. There is no difference between the rotor speedand rotating field speed. This means no voltage is induced in the rotor bars and thereforeno torque is developed. When running, the rotor must rotate slower than the magneticfield, to cause the proper amount of rotor current to flow so that the torque that developsis able to overcome the winding and friction losses and therefore drive the load. Thisspeed difference is called slip.Most motors use the squirrel cage design. An alternate design, wound rotor, is used whenvariable speed is desired. Compared to squirrel cage rotors, wound rotors are expensiveand require more maintenance. A wound rotor motor has controllable speed and torque.Single-phase AC induction motors are typically used in devices requiring low torque likefans and other household appliances. A split-phase induction motor is used in largerhousehold appliances such as washers and dryers. These are designed to provide greaterstarting torque.Common termsLocked rotor torque – the minimum torque that the motor develops at rest for all angularpositions of the rotor at rated voltage and frequency.Locked rotor current – the steady state current from the line at rated voltage andfrequency with the rotor locked.Breakdown torque – the maximum torque that the motor develops at rated voltage andfrequency without an abrupt drop in speed.Pull up torque – the minimum torque developed during the period of acceleration fromrest to the speed that breakdown torque occurs.In order to perform useful work, the induction motor must be started from rest and boththe motor and load accelerated up to full speed. As the motor accelerates, the torque andthe current will alter with rotor speed if a constant voltage is maintained. The startingcurrent of a motor, with a fixed voltage will drop slowly as the motor accelerates and willbegin to fall significantly when the motor has reached between 80% and full speed. Thegeneral curve for an induction motor indicates a high current until the motor has almostreached full speed. The locked rotor current of a motor typically falls between 550% and750% of the full load current. Refer to the typical curve below. Page 5
    • An induction motor operates due to the torque developed by the interaction of the statorfield and the rotor field. These fields are due to currents which are resistive and reactive.The torque developed is dependent on the resistive current and is related to the I2R of therotor.Once the motor is up to speed, it operates at low slip at a speed determined by the numberof stator poles. The synchronous speed of a 4-pole machine operating at 60 hertz is 1800rpm.Enclosure types (protection) and cooling for induction and synchronousmotorsThere are several types of protection and cooling for motors. Only a few are listedbelow.Open MachineAn open machine means a machine that has no restriction to ventilation other than themechanical construction of the machine. There are several types of open machines,including the following: • A drip proof machine is a machine that is protected from drops of liquid or solid particles that could strike or enter the enclosure at any angle from 0 to 15 degrees downward from the vertical. • A splash proof machine is a machine that is protected from drops of liquid or solid particles that could strike or enter the enclosure at any angle not greater than 60 degrees downward from the vertical. • A guarded machine is a machine where all openings that have direct access to live metal or rotating parts are limited in size by the structural parts or by screens, baffles or other means to prevent accidental contact with hazardous parts. Page 6
    • • A semi-guarded machine is a machine in which part of the ventilating openings in the machine (usually the top half) are guarded, but others are left open.Weather Protected MachineThere are two types of weather protected machines: a Type 1 which is guarded with itsventilation constructed to minimize the entry of rain, snow and air-borne particles, orType 2 which is constructed similar to a Type 1, but its ventilation is constructed at theintake and discharge such that storms or high winds cannot blow directly into the electricparts of the machine itself.Totally Enclosed MachineA totally enclosed machine is enclosed to prevent the free exchange of air between theinside and outside of the case, but not airtight or dust tight. A totally enclosednonventilated machine is a machine cooled by free convection. A totally enclosed fan-cooled machine is equipped for cooling by fans integral to the machine but external to theenclosing parts. A totally enclosed water cooled machine is a machine cooled bycirculating water in contact with the machine parts. A water proof machine is generallyconstructed so that a stream of water from a hose will not enter the machine.Explosion Proof MachineAn explosion proof machine is a machine where the enclosure is designed to withstandthe explosion of a specified gas or vapor which may occur within it, and to prevent theignition of the gas or vapor surrounding the machine due to sparks, flashes or explosionsthat may occur within the machine casing.Insulation systemsThere are different insulating components used in the process of building a motor, suchas the enamel coating on the magnet wire and the insulation on the leads in the motorbox. Another important component is the dipping varnish which is used to seal scratchesthat may have occurred and binds the winding together so that it does not vibrate or chafewhen subjected to the magnetic force that exists in the motor. Insulation systems aredivided into classes based on the thermal aging and failure. Four classes are commonlyused in motors – A, B, F, and H. Refer also to IEEE Std. 1. The temperature classes areseparated by 25 degree C increments. The temperature capability of each class is definedas the maximum temperature at which the insulation can be operated to yield an averagelife of 20,000 hours.Class A - Rated 105 degrees CClass B - Rated 130 degrees CClass F - Rated 155 degrees CClass H - Rated 180 degrees C Page 7
    • The temperature rise of a motor is the change in temperature when it is being operated atfull load. For example, if a motor is located outside and the temperature is 80 degrees F,and is then started and operated at full load, the winding temperature would rise from 80degrees F to a higher temperature. This difference is the motor’s temperature rise.Nearly all electric motors are rated based on a starting temperature of 40 degrees Cambient temperature (104 degrees F). The temperature rise is added to the ambienttemperature.Example: Suppose a motor is designed with Class A insulation and a maximumtemperature rise of 55 degrees C. When operated in a 40 degree C ambient temperature,the total average winding temperature would be 95 degrees C (40 + 55 degrees C). ClassA insulation is rated for 105 degrees C temperature, and this difference of 10 degrees isused to handle “hot spots”. (The winding temperature is an average change of the entirewinding. Some of these spots may be hotter than others – called hot spots.)Suppose the same motor now is designed with Class B insulation. Class B insulation israted for 130 degrees C. The temperature difference is now 25 degrees C.Designing a motor with higher rated insulation allows for extra thermal capability tohandle higher than normal ambient temperatures, overloads, or extend motor life becausethe motor will be able to handle overheating. Overheating may be due to frequent starts,high or low voltages or voltage imbalances. By changing the insulation class, it isusually possible to increase the service factor of a motor from 1.0 to something higher,such as 1.15. The same change could also make the motor more suitable for operation inhigh elevations where the air is thinner and has less cooling effect. The service factor isthe ability to continuously run a motor at or above its nameplate full load horsepowerrating. A 1.15 service factor on a 100 hp motor allows it to be run continuously at 115%of its nameplate rating (or 115 hp).As a general rule, insulation life doubles for each 10 degrees of unused insulationtemperature capability. For instance, for a motor designed for 110 degrees C (includingthe ambient temperature, rise and “hot spot” allowance) with Class B insulation, therewould be a used capacity of 20 degrees C. This difference would raise the motorinsulation life from 20,000 to 80,000 hours. (20,000 hours = normal life; doubled for 10degrees = 40,000; doubled for another 10 degrees = 80,000 hours). This also applies if amotor isn’t loaded to its full capacity or if it’s operated at a lower than 40 degree Cambient temperature. Similarly, if motors operate above their rated temperature, then theinsulation life is halved for each 10 degrees of over temperature.Insulation life can also be affected, aside from temperature, by moisture, chemicals, oiland vibration to name a few other factors. Also, the classification of an insulation systemis based on the temperature rating of the lowest rated component used in the system.Therefore if one Class B component is used with Class F components, the entire systemmust be classified as Class B. Page 8
    • There are several NEMA specifications and IEEE specifications devoted to motor design,specification and installation. Refer to these for further information. Page 9