Presentation on theme: "Induction Motor (Asynchronous Motor)"— Presentation transcript:
1Induction Motor (Asynchronous Motor) ELECTRICAL MACHINESCompiled byProf Mitali Ray
2Learning Outcomes At the end of the lecture, student should to: Understand the principle and the nature of 3 phase induction machines.Perform an analysis on induction machines which is the most rugged and the most widely used machine in industry.
3Contents Overview of Three-Phase Induction Motor Construction Principle of OperationEquivalent CircuitPower Flow, Losses and EfficiencyTorque-Speed CharacteristicsSpeed ControlOverview of Single-Phase Induction Motor
4Overview of Three-Phase Induction Motor Induction motors are used worldwide in many residential, commercial, industrial, and utility applications.Induction Motors transform electrical energy into mechanical energy.It can be part of a pump or fan, or connected to some other form of mechanical equipment such as a winder, conveyor, or mixer.
5Introduction General aspects A induction machine can be used as either a induction generator or a induction motor.Induction motors are popularly used in the industryFocus on three-phase induction motorMain features: cheap and low maintenanceMain disadvantages: speed control is not easy
9Construction (Stator construction) The stator is the stationary electrical part of the motor.The stator core of a National Electrical Manufacturers Association (NEMA) motor is made up of several hundred thin laminations.Stator laminations are stacked together forming a hollow cylinder. Coils of insulated wire are inserted into slots of the stator core.Electromagnetism is the principle behind motor operation. Each grouping of coils, together with the steel core it surrounds, form an electromagnet. The stator windings are connected directly to the power source.MZS FKEE, UMP
10Construction (Rotor construction) The rotor is the rotating part of the electromagnetic circuit.It can be found in two types:Squirrel cageWound rotorHowever, the most common type of rotor is the “squirrel cage” rotor.
11Construction (Rotor construction) Induction motor types:Squirrel cage type:Rotor winding is composed of copper bars embedded in the rotor slots and shorted at both end by end ringsSimple, low cost, robust, low maintenanceWound rotor type:Rotor winding is wound by wires. The winding terminals can be connected to external circuits through slip rings and brushes.Easy to control speed, more expensive.
13Construction (Enclosure) The enclosure consists of a frame (or yoke) and two end brackets (or bearing housings). The stator is mounted inside the frame. The rotor fits inside the stator with a slight air gap separating it from the stator. There is NO direct physical connection between the rotor and the stator.StatorRotorAir gapThe enclosure also protects the electrical and operating parts of the motor from harmful effects of the environment in which the motor operates. Bearings, mounted on the shaft, support the rotor and allow it to turn. A fan, also mounted on the shaft, is used on the motor shown below for cooling.
16Rotating Magnetic Field When a 3 phase stator winding is connected to a 3 phase voltage supply, 3 phase current will flow in the windings, which also will induced 3 phase flux in the stator.These flux will rotate at a speed called a Synchronous Speed, ns. The flux is called as Rotating magnetic FieldSynchronous speed: speed of rotating fluxWhere; p = is the number of poles, andf = the frequency of supply
17Space angle () in degrees RMF(Rotating Magnetic Field)aFc-9310113216-1.5-1-0.50.511.5a’c’b’bcFbFaFSpace angle () in degreest = t0= t4t = t1t = t2t = t3
19MMF Due to ‘a’ phase current Axis of phase aa’-90-401060110160210260-1-0.8-0.6-0.4-0.20.20.40.60.81FaSpace angle (theta) in degreest0t01t12t2a
20Currents in different phases of AC Machine 1 CycleAmptimet0t1t2t3t4t01t12
21Slip Ring Rotor The rotor contains windings similar to stator. The connections from rotor are brought out using slip rings thatare rotating with the rotor and carbon brushes that are static.
22Slip and Rotor Speed Slip s Where; ns = synchronous speed (rpm) The rotor speed of an Induction machine is different from the speed of Rotating magnetic field. The % difference of the speed is called slip.Where; ns = synchronous speed (rpm)nr = mechanical speed of rotor (rpm)under normal operating conditions, s= 0.01 ~ 0.05, which is very small and the actual speed is very close to synchronous speed.Note that : s is not negligibleMZS FKEE, UMP
23Slip and Rotor Speed Rotor Speed When the rotor move at rotor speed, nr (rps), the stator flux will circulate the rotor conductor at a speed of (ns-nr) per second. Hence, the frequency of the rotor is written as:Where; s = slipf = supply frequency
24Principle of Operation Torque producing mechanismWhen a 3 phase stator winding is connected to a 3 phase voltage supply, 3 phase current will flow in the windings, hence the stator is energized.A rotating flux Φ is produced in the air gap. The flux Φ induces a voltage Ea in the rotor winding (like a transformer).The induced voltage produces rotor current, if rotor circuit is closed.The rotor current interacts with the flux Φ, producing torque. The rotor rotates in the direction of the rotating flux.
25Direction of Rotor Rotates Q: How to change the direction ofrotation?• A: Change the phase sequence of thepower supply.
26Equivalent Circuit of Induction Machines Conventional equivalent circuitNote:Never use three-phase equivalent circuit. Always use per- phase equivalent circuit.The equivalent circuit always bases on the Y connection regardless of the actual connection of the motor.Induction machine equivalent circuit is very similar to the single-phase equivalent circuit of transformer. It is composed of stator circuit and rotor circuit
27Equivalent Circuit of Induction Machines Step1 Rotor winding is open(The rotor will not rotate)Note:the frequency of E2 is the same as that of E1 since the rotor is at standstill. At standstill s=1.f
29Equivalent Circuit of Induction Machines Step2 Rotor winding is shorted(Under normal operating conditions, the rotor winding is shorted. The slip is s)Note:the frequency of E2 is fr=sf because rotor is rotating.ffr
30Equivalent Circuit of Induction Machines Step3 Eliminate f2Keep the rotor current same:
31Equivalent Circuit of Induction Machines Step 4 Referred to the stator sideNote:X’2 and R’2 will be given or measured. In practice, we do not have to calculate them from above equations.Always refer the rotor side parameters to stator side.Rc represents core loss, which is the core loss of stator side.MZS FKEE, UMP
32Equivalent Circuit of Induction Machines IEEE recommended equivalent circuitNote:Rc is omitted. The core loss is lumped with the rotational loss.MZS FKEE, UMP
33Equivalent Circuit of Induction Machines IEEE recommended equivalent circuitNote: can be separated into 2 PARTSPurpose :to obtain the developed mechanicalI1MZS FKEE, UMP
34Analysis of Induction Machines For simplicity, let assumeIs=I1 , IR=I2(s=stator, R=rotor)ZRZmZsVs1Is1Im1IR1
35Analysis of Induction Machines ZRZmZsVs1Is1Im1IR1Note : 1hp =746Watt
36Power Flow Diagram Pin (Motor) Pin (Stator) Pcore loss (Pc) Pair Gap (Pag)PdevelopedPmechanicalPconverted(Pm)Pout, PoPstator copper loss, (Pscu)Protor copper loss (Prcu)Pwindage, friction, etc (P - Given)Pin (Rotor)
37Power Flow Diagram Pag Prcu Pm Ratio:PagPrcuPmRatio makes the analysis simpler to find the value of the particular power if we haveanother particular power. For example:
39Torque-EquationTorque, can be derived from power equation in term of mechanical power or electrical power.
40Torque-EquationNote that, Mechanical torque can written in terms of circuit parameters. This is determined by using approximation methodHence, Plot Tm vs sTmnssmax is the slip for Tmax to occurs=1TstTmaxsmax
42Speed ControlThere are 3 types of speed control of 3 phase induction machinesVarying rotor resistanceVarying supply voltageVarying supply voltage and supply frequency
43Varying rotor resistance For wound rotor onlySpeed is decreasingConstant maximum torqueThe speed at which max torque occurs changesDisadvantages:large speed regulationPower loss in Rext – reduce the efficiencyTns~nNLnr1nr2nr3nnr1< nr2< nr3R1R2R3R1< R2< R3
44Varying supply voltage Maximum torque changesThe speed which at max torque occurs is constant (at max torque, XR=RR/sRelatively simple method – uses power electronics circuit for voltage controllerSuitable for fan type loadDisadvantages :Large speed regulation since ~ nsTns~nNLnr1nr2nr3nnr1> nr2 > nr3V1V2V3V1> V2 > V3V decreasing
45Varying supply voltage and supply frequency The best method since supply voltage and supply frequency is varied to keep V/f constantMaintain speed regulationuses power electronics circuit for frequency and voltage controllerConstant maximum torqueTnNL1nr1nr2nr3nf decreasingnNL2nNL3