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1 Induction Motor (Asynchronous Motor) ELECTRICAL MACHINES Compiled by Prof Mitali Ray

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2 Learning Outcomes At the end of the lecture, student should to: principle and the nature of 3 phase induction machines. –Understand the principle and the nature of 3 phase induction machines. most widely used machine in industry. –Perform an analysis on induction machines which is the most rugged and the most widely used machine in industry.

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3 Contents –Overview of Three-Phase Induction Motor –Construction –Principle of Operation –Equivalent Circuit Power Flow, Losses and Efficiency Torque-Speed Characteristics –Speed Control –Overview of Single-Phase Induction Motor

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4 Overview 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.

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5 Introduction General aspects A induction machine can be used as either a induction generator or a induction motor. Induction motors are popularly used in the industry Focus on three-phase induction motor Main features: cheap and low maintenance Main disadvantages: speed control is not easy

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7 Construction The three basic parts of an AC motor are the rotor, stator, and enclosure. The stator and the rotor are electrical circuits that perform as electromagnets.

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Squirrel Cage Rotor

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MZS FKEE, UMP 9 Construction (Stator construction) stationary electrical part of the motor.The stator is the stationary electrical part of the motor. several hundred thin laminationsThe stator core of a National Electrical Manufacturers Association (NEMA) motor is made up of several hundred thin laminations. stacked togetherhollow cylinder Coils of insulated wire are inserted into slots of the stator core.Stator laminations are stacked together forming a hollow cylinder. Coils of insulated wire are inserted into slots of the stator core. Each grouping of coilsElectromagnetism 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.

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10 Construction (Rotor construction) The rotor is the rotating part of the electromagnetic circuit. It can be found in two types: –Squirrel cage –Wound rotor However, the most common type of rotor is the “squirrel cage” rotor.

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11 Induction motor types: Squirrel cage type: Squirrel cage type: Rotor winding is composed of copper bars embedded in the rotor slots and shorted at both end by end rings Simple, low cost, robust, low maintenance Wound rotor type: Wound 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. Construction (Rotor construction)

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MZS FKEE, UMP 12 Construction (Rotor construction) Wound Rotor Squirrel-Cage Rotor /rotor winding Short circuits all rotor bars.

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13 Construction (Enclosure) 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.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. Stator Rotor Air gap The 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.The 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.

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14 Construction (Enclosure)

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15 Nameplate

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16 Rotating Magnetic Field connected flow in the windings inducedWhen 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. Synchronous Speed, n sThese flux will rotate at a speed called a Synchronous Speed, n s. The flux is called as Rotating magnetic Field Synchronous speed: speed of rotating flux Where; p = is the number of poles, and f = the frequency of supply

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aFcFc a’a’ c’c’ b’b’ b c a a’a’ c’c’ b’b’ b c a a’a’ c’c’ b’b’ b c a a’a’ c’c’ b’b’ b c FbFb FaFa F FbFb FcFc F FaFa F FbFb FcFc FcFc FbFb F Space angle ( ) in degrees F FaFa FcFc FbFb t = t 0 = t 4 t = t 1 t = t 2 t = t 3 t = t 0 = t 4 RMF (Rotating Magnetic Field)

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AC Machine Stator

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Axis of phase a a’a’ a’a’ FaFa Space angle (theta) in degrees t0t0 t 01 t 12 t2t2 a MMF Due to ‘a’ phase current

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1 Cycle Amp time t0t0 t1t1 t2t2 t3t3 t4t4 t 01 t 12 Currents in different phases of AC Machine

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Slip Ring Rotor The rotor contains windings similar to stator. The connections from rotor are brought out using slip rings that are rotating with the rotor and carbon brushes that are static.

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MZS FKEE, UMP 22 Slip and Rotor Speed 1.Slip s –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; n s = synchronous speed (rpm) n r = 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. s is not negligible –Note that : s is not negligible

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23 Slip and Rotor Speed Rotor SpeedRotor Speed –When the rotor move at rotor speed, n r (rps), the stator flux will circulate the rotor conductor at a speed of (n s -n r ) per second. Hence, the frequency of the rotor is written as: Where; s = slip f = supply frequency

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24 Principle of Operation Torque producing mechanismTorque producing mechanism connected flow in the windings When 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 E a 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.

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25 Direction of Rotor Rotates Q: How to change the direction of rotation? A: Change the phase sequence of the power supply.

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26 Conventional equivalent circuit Note: ●Never use three-phase equivalent circuit. Always use per- phase equivalent circuit. always bases on the Y connection regardless of the actual connection of the motor ●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 Equivalent Circuit of Induction Machines

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27 Step1 Rotor winding is openStep1 Rotor winding is open (The rotor will not rotate) Note: –the frequency of E 2 is the same as that of E 1 since the rotor is at standstill. At standstill s=1. Equivalent Circuit of Induction Machines f f

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28 Equivalent Circuit of Induction Machines

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29 Equivalent Circuit of Induction Machines Step2 Rotor winding is shortedStep2 Rotor winding is shorted (Under normal operating conditions, the rotor winding is shorted. The slip is s) Note: rotor is rotating –the frequency of E 2 is f r =sf because rotor is rotating. ffrfr

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30 Step3 Eliminate f 2Step3 Eliminate f 2 Keep the rotor current same: Equivalent Circuit of Induction Machines

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MZS FKEE, UMP 31 Step 4 Referred to the stator sideStep 4 Referred to the stator side Note: –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. –R c represents core loss, which is the core loss of stator side. Equivalent Circuit of Induction Machines

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MZS FKEE, UMP 32 IEEE recommended equivalent circuitIEEE recommended equivalent circuit Note: –R c is omitted. The core loss is lumped with the rotational loss. Equivalent Circuit of Induction Machines

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MZS FKEE, UMP 33 IEEE recommended equivalent circuitIEEE recommended equivalent circuit Note: can be separated into 2 PARTS Purpose :Purpose : –to obtain the developed mechanical Equivalent Circuit of Induction Machines I 1

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34 Analysis of Induction Machines For simplicity, let assume I s =I 1, I R =I 2 (s=stator, R=rotor) ZRZRZRZR ZmZmZmZm ZsZsZsZs V s1 I s1 I m1 I R1

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35 Analysis of Induction Machines ZRZRZRZR ZmZmZmZm ZsZsZsZs V s1 I s1 I m1 I R1 Note : 1hp =746Watt

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36 Power Flow Diagram P in (Motor) P in (Stator) P core loss (P c ) P air Gap (P ag ) P developed P mechanical P converted (P m ) P out, P o P stator copper loss, (P scu ) P rotor copper loss (P rcu ) P windage, friction, etc (P - Given) P in (Rotor)

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37 Power Flow Diagram Ratio:Ratio: P ag P rcu PmPm Ratio makes the analysis simpler to find the value of the particular power if we have another particular power. For example:

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38 Efficiency

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39 Torque-Equation Torque, can be derived from power equation in term of mechanical power or electrical power.

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40 Torque-Equation Note that, Mechanical torque can written in terms of circuit parameters. This is determined by using approximation methodNote that, Mechanical torque can written in terms of circuit parameters. This is determined by using approximation method Hence, Plot T m vs s TmTm nsns s max is the slip for T max to occur s=1 T st T max s max

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41 Torque-Equation

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42 Speed Control There are 3 types of speed control of 3 phase induction machines i.Varying rotor resistance ii.Varying supply voltage iii.Varying supply voltage and supply frequency

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43 Varying rotor resistance For wound rotor only Speed is decreasing Constant maximum torque The speed at which max torque occurs changes Disadvantages: –large speed regulation –Power loss in R ext – reduce the efficiency T n s ~n NL T n r1 n r2 n r3 n n r1 < n r2 < n r3 R1R1 R2R2 R3R3 R1< R2< R3R1< R2< R3

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44 Varying supply voltage Maximum torque changes The speed which at max torque occurs is constant (at max torque, X R =R R /s Relatively simple method – uses power electronics circuit for voltage controller Suitable for fan type load Disadvantages : –Large speed regulation since ~ n s T n s ~n NL T n r1 n r2 n r3 n n r1 > n r2 > n r3 V1V1 V2V2 V3V3 V 1 > V 2 > V 3 V decreasing

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45 V / fThe best method since supply voltage and supply frequency is varied to keep V / f constant Maintain speed regulation uses power electronics circuit for frequency and voltage controller Constant maximum torque Varying supply voltage and supply frequency T n NL1 T n r1 n r2 n r3 n f decreasing n NL2 n NL3

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