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**Induction Motor (Asynchronous Motor)**

ELECTRICAL MACHINES Compiled by Prof Mitali Ray

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**Learning 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.

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**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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**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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**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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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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**Construction (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

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**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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**Construction (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 rings Simple, low cost, robust, low maintenance 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.

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**Construction (Rotor construction)**

Wound Rotor Squirrel-Cage Rotor /rotor winding Short circuits all rotor bars. MZS FKEE, UMP

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**Construction (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. 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.

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**Construction (Enclosure)**

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Nameplate

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**Rotating 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 Field Synchronous speed: speed of rotating flux Where; p = is the number of poles, and f = the frequency of supply

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**Space angle () in degrees**

RMF(Rotating Magnetic Field) a Fc -93 10 113 216 -1.5 -1 -0.5 0.5 1 1.5 a’ c’ b’ b c Fb Fa F Space angle () in degrees t = t0= t4 t = t1 t = t2 t = t3

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AC Machine Stator

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**MMF Due to ‘a’ phase current**

Axis of phase a a’ -90 -40 10 60 110 160 210 260 -1 -0.8 -0.6 -0.4 -0.2 0.2 0.4 0.6 0.8 1 Fa Space angle (theta) in degrees t0 t01 t12 t2 a

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**Currents in different phases of AC Machine**

1 Cycle Amp time t0 t1 t2 t3 t4 t01 t12

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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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**Slip 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 negligible MZS FKEE, UMP

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**Slip 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 = slip f = supply frequency

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**Principle of Operation**

Torque producing mechanism 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 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.

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**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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**Equivalent Circuit of Induction Machines**

Conventional equivalent circuit Note: 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

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**Equivalent 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

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**Equivalent Circuit of Induction Machines**

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**Equivalent 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. f fr

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**Equivalent Circuit of Induction Machines**

Step3 Eliminate f2 Keep the rotor current same:

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**Equivalent Circuit of Induction Machines**

Step 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. Rc represents core loss, which is the core loss of stator side. MZS FKEE, UMP

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**Equivalent Circuit of Induction Machines**

IEEE recommended equivalent circuit Note: Rc is omitted. The core loss is lumped with the rotational loss. MZS FKEE, UMP

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**Equivalent Circuit of Induction Machines**

IEEE recommended equivalent circuit Note: can be separated into 2 PARTS Purpose : to obtain the developed mechanical I 1 MZS FKEE, UMP

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**Analysis of Induction Machines**

For simplicity, let assume Is=I1 , IR=I2 (s=stator, R=rotor) ZR Zm Zs Vs1 Is1 Im1 IR1

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**Analysis of Induction Machines**

ZR Zm Zs Vs1 Is1 Im1 IR1 Note : 1hp =746Watt

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**Power Flow Diagram Pin (Motor) Pin (Stator) Pcore loss (Pc)**

Pair Gap (Pag) Pdeveloped Pmechanical Pconverted (Pm) Pout, Po Pstator copper loss, (Pscu) Protor copper loss (Prcu) Pwindage, friction, etc (P - Given) Pin (Rotor)

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**Power Flow Diagram Pag Prcu Pm**

Ratio: Pag Prcu Pm 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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Efficiency

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

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Torque-Equation Note that, Mechanical torque can written in terms of circuit parameters. This is determined by using approximation method Hence, Plot Tm vs s Tm ns smax is the slip for Tmax to occur s=1 Tst Tmax smax

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Torque-Equation

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Speed Control There are 3 types of speed control of 3 phase induction machines Varying rotor resistance Varying supply voltage Varying supply voltage and supply frequency

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**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 Rext – reduce the efficiency T ns~nNL nr1 nr2 nr3 n nr1< nr2< nr3 R1 R2 R3 R1< R2< R3

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**Varying supply voltage**

Maximum torque changes The speed which at max torque occurs is constant (at max torque, XR=RR/s Relatively simple method – uses power electronics circuit for voltage controller Suitable for fan type load Disadvantages : Large speed regulation since ~ ns T ns~nNL nr1 nr2 nr3 n nr1> nr2 > nr3 V1 V2 V3 V1> V2 > V3 V decreasing

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**Varying supply voltage and supply frequency**

The 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 T nNL1 nr1 nr2 nr3 n f decreasing nNL2 nNL3

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