AC Motor WindingPrepared By:-Muhammad Atta ul Ghaffar(Sr.Electrical Engineer)CENTURY PAPER & BOARD MILLS LTD.
Basic Rewind Terminology,Formulas and Definitions Synchronous Speed.This term refers to the speed of the rotatingmagnetic field in the stator. The motor rotorwill never turn at synchronous speed due tovarious mechanical friction losses, windage,and built-in slip.The difference between synchronous speedand actual rotor speed is called slip and isexpressed in a percent of synchronousspeed.Synchronous speed must be known todetermine the number of poles and pole-phase groups (PPGs) in the stator windingand can be easily determined by use of thefollowing formula:
Basic FormulaN (speed) = 120 x F (frequency) / P(pole)Example: Synchronous speed of a 4pole motor is:N = 120 x 50 / 4N = 6000 / 4N = 1500 r/m
Phases As the name implies, three-phasemotor stator windings will consist ofthree separate groups of coils calledphases. Phases must be displacedfrom each other by 120 electricaldegrees. Phases must be electrically balanced(contain the same number of coils)and connected for the same numberof poles. Phases will always be designated asA, B, and C
Poles Poles refer to a coil or group of coils woundand connected to produce a unit of magneticpolarity. Poles are referred to as either north or south.The number of poles a stator is wound for willalways be an even number and refers to thetotal number of north and south poles. Forexample, a four-pole motor will have twonorth and two south poles alternately spacedaround the stator. The number of poles in the stator must beknown to determine the number of PPGs inthe motor and can be easily determined bythe formula below.
Poles….P = 120 x F / NExample: P = 120 x 50 / 1500P = 6000 / 1500P = 4
Poles….Alternate Pole Polarities for One Phase of a Four-Pole, Three-PhaseAC Motor
Active Coil Per Phase An active coil is a coil that is electricallyconnected in the stator circuit. Each phase must contain an equalnumber of active coils to ensureelectrical balance (phase current). In almost all cases, the total number ofcoils in the stator will be equal to thenumber of slots. Active coils per phase can always bedetermined by dividing the total numberof coils by three (number of phases).
Active Coil Per Phase Example:-Active coil per phase = total coil /phases= 36(total coils) / 3phases= 12 active coils perphaseIn this example the answer is a wholenumber, meaning all coils in thewinding will be active. In this case thewinding is known as an integral slot
Active Coil Per Phase Example:62 (total coils) / 3 phases = 20 2/3coils per phase In this example, the winding willrequire the use of two dummy coils tocompensate for the fractional number.
Dummy Coils Dummy coils are used to fill slots butare not electrically connected in thestator circuit. If only one dummy coil isrequired, it can be installed in any slot.If two dummy coils are required, theycan be installed in any slots 180mechanical degrees apart. Dummy coils will always be left opencircuited.
Pole-Phase Groups (PPGs) This term refers to the number of coilsconnected in series to form themagnetic pole of a phase. Each phasemust contain an equal number ofPPGs. Total PPGs can be determinedby multiplying the number of motorpoles by three (number of phases). Example: 4 poles x 3 phases = 12PPG
Coil Span or Pitch Coil span or pitch is defined as thenumber of slots separating the sidesof a coil including the slots in whichthe coil sides lie. The coil span or pitch is normallyreferred to by the individual numbersof the slots in which the coil sides arelaid during rewind, i.e., 1 and 6, 2 and7, 3 and 8, etc.
Types of Winding BASKET WINDING. Defined as a single layer winding, thebasket winding is also called the half-coil winding. This winding will have one coil sideper slot, with the total number of activecoils equal to one-half of the totalnumber of slots.
Distributed Winding This winding is defined as a doublelayer winding and is the most commontype of winding. It will have two coil sides per slot, withthe total number of active coils equalto the total number of slots
Types of Winding Connection WYE (STAR) CONNECTION The wye or star connection is definedas where the finishes of each phaseare connected together, and the startsare connected to the line leads. The diagram for this windingresembles an inverted letter Y. The wye can be connected in series orparallel configuration.
Delta Connection In the delta connection, the finish ofeach phase is connected to the startof the next phase. The diagram for this windingresembles the Greek letter delta. The winding can be connected inseries or parallel configurations.
Winding Burning Reasons Electrical Problems Winding short between conductor & coils Insulation to ground fault Air gap fault Rotor fault including casting voids andbroken rotor bars Improper matching of motor to load Loose Connections Electrical vibrations /Mechanicallooseness
Burning Reasons… Electrical Supply Problems Over or Under Voltage Voltage Unbalance VFD waveform Voltage Surge
Winding Burning Reasons Insulation FailureThe reasons of Insulation failure are:.OverloadExcessive Number of StartsContaminants/Ventilation failureVoltage Spikes from VFDsSystem DisturbancesExcessive Load Inertia