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Published byAngela Johns Modified over 4 years ago

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**EE20A - Electromechanical Energy Conversion Induction Machine**

Department of Electrical and Computer Engineering EE20A - Electromechanical Energy Conversion Induction Machine

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**Principle of Operation**

The stator coils, when energised, create a rotating magnetic field. Rotating magnetic field cuts through the rotor inducing a voltage in the rotor bars. This voltage creates its own magnetic field in the rotor. The rotor magnetic field will attempt to line up with the stator magnetic field. The stator magnetic field is rotating, the rotor magnetic field trying to line up with the stator magnetic field causes the rotor to rotate. The rotor magnetic field, never catches up, but follows slightly behind.

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Motor Analysis Slip is the difference between the speed of the stator magnetic field and the speed of the rotor SLIP,S, = (NS - N) / NS When motor is stationary, it behaves like a transformer At a given Speed, flux cutting rate is reduced => thereby reducing output voltage by a factor of the slip.

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**Per Phase Equivalent Circuit**

Analysis Per Phase Equivalent Circuit

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**Per Phase Equivalent Circuit**

Analysis Per Phase Equivalent Circuit

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**Per Phase Equivalent Circuit**

Analysis Per Phase Equivalent Circuit

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**Per Phase Equivalent Circuit**

Power per Phase Per Phase Equivalent Circuit Pag = I12Rr`/s Pcu = sPag Pmech_gross = (1-s)Pag Total Torque = (3Pmech_gross- PF&W)/wm

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Power per Phase Pag = Power across the air gap

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**P mech_gross = (1-s) Pag per phase**

Power per Phase P mech_gross = (1-s) Pag per phase

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**Pag : Pcu : Pmech = 1:s:(1-s)**

Power per Phase Pcu_losses_in_rotor Pmech_gross Pag : Pcu : Pmech = 1:s:(1-s)

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**Slip is variable and affects only rotor circuit**

Power per Phase Slip is variable and affects only rotor circuit Ignoring Stator values

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Power per Phase

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Torque Simple Algebraic manipulations yield

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Torque

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Torque Since the above calculations was derives as power per phase, then the total torque for all three phases would be three times the gross mechanical torque for each phase calculated above.

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Torque The maximum torque is obtained when:

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**Torque Characteristics**

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**Speed-Torque characteristics**

Modifications in the design of the squirrel-cage motors permit a certain amount of control of the starting current and torque characteristics. These designs have been categorised by NEMA Standards (MG1-1.16) into four main classifications: 1. Normal-torque, normal-starting current motors (Design A) 2. Normal-torque, low-starting current motors (Design B) 3. High-torque, low-starting-current, double-wound-rotor motors (Design C) 4. High-slip motors (Design D)

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**Design A Motor Hp range 0.5 – 500 hp.**

Starting current 6 to 10 times full-load current. Good running efficiency (87% - 89%). Good power factor (87% - 89%). Low rated slip (3 –5 %). Starting torque is about 150% of full load torque. Maximum torque is over 200% but less than 225% of full-load torque. • Typical applications – constant speed applications where high starting torque is not needed and high starting torque is tolerated.

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**Design B Motor Hp range – 0.5 to 500 hp**

Higher reactance than the Design A motor, obtained by means of deep, narrow rotor bars. The starting current is held to about 5 times the full-load current. This motor allows full-voltage starting. The starting torque, slip and efficiency are nearly the same as for the Design A motor. Power factor and maximum torque are little lower than class A, Design B is standard in 1 to 250 hp drip-proof motors and in totally enclosed, fan-cooled motors, up to approximately 100 hp. Typical applications – constant speed applications where high starting torque is not needed and high starting torque is tolerated. •Unsuitable for applications where there is a high load peak

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**Design C Motor Hp range – 3 to 200 hp**

This type of motor has a "double-layer" or double squirrel-cage winding. It combines high starting torque with low starting current. Two windings are applied to the rotor, an outer winding having high resistance and low reactance and an inner winding having low resistance and high reactance. Operation is such that the reactance of both windings decrease as rotor frequency decreases and speed increases. On starting a much larger induced currents flow in the outer winding than in the inner winding, because at low rotor speeds the inner-winding reactance is quite high.

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Design C Motor As the rotor speed increases, the reactance of the inner winding drops and combined with the low inner-winding resistance, permits the major portion of the rotor current to appear in the inner winding. Starting current about 5 times full load current. The starting torque is rather high (200% - 250%). Full-load torque is the same as that for both A and B designs. The maximum torque is lower than the starting torque, maximum torque ( %). •Typical applications – constant speed loads requiring fairly high starting torque and lower starting currents.

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Design D Motor •Produces a very high starting torque-approximately 275% of full-load torque. It has low starting current, High slip (7-16%), Low efficiency. Torque changes with load •Typical applications- used for high inertia loads The above classification is for squirrel cage induction motor

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**Wound Rotor Hp 0.5 to 5000hp Starting torque up to 300%**

Maximum torque 225 to 275% of full load torque Starting current may be as low as 1.5 times starting current Slip (3 - 50%) Power factor high •Typical applications – for high starting torque loads where very low starting current is required or where torque must be applied very gradually and where speed control is needed.

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**Current Effects on the Motor**

Induction motor current consists of reactive (magnetizing) and real (torque) components. The current component that produces torque (does useful work) is almost in phase with voltage, and has a high power factor close to 100% The magnetizing current would be purely inductive, except that the winding has some small resistance, and it lags the voltage by nearly 90°. The magnetizing current has a very low power factor, close to zero. The magnetic field is nearly constant from no load to full load and beyond, so the magnetizing portion of the total current is approximately the same for all loads. •The torque current increases as the load increases

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**Current Effects on the Motor**

At full load, the torque current is higher than the magnetizing current. For a typical motor, the power factor of the resulting current is between 85% and 90%. As the load is reduced, the torque current decreases, but the magnetizing current remains about the same so the resulting current has a lower power factor. •The smaller the load, the lower the load current and the lower the power factor. Low power factor at low loading occurs because the magnetizing remains approximately the same at no load as at full load

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**Methods to vary speed of the**

Induction Motor An induction motor is a constant-speed device. Its speed depends on the number of poles in the stator, assuming that the voltage and frequency of the supply to the motor remain constant. One method is to change the number of poles in the stator, for example, reconnecting a 4-pole winding so that it becomes a 2-pole winding will double the speed. This method can give specific alternate speeds but not gradual speed changes. •Another method is to vary the line voltage this method is not the best since torque is proportional to the square of the voltage, so reducing the line voltage rapidly reduces the available torque causing the motor to stall

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**Methods to vary speed of the**

Induction Motor •Sometimes it is desirable to have a high starting torque or to have a constant horsepower output over a given speed range. These and other modifications can be obtained by varying the ratio of voltage to frequency as required. Some controllers are designed to provide constant torque up to 60 Hz and constant hp above 60 Hz to provide higher speeds without overloading the motor. An excellent way to vary the speed of a squirrel-cage induction motor is to vary the frequency of the applied voltage. To maintain a constant torque, the ratio of voltage to frequency must be kept constant, so the voltage must be varied simultaneously with the frequency. Modern adjustable frequency controls perform this function. At constant torque, the horsepower output increases directly as the speed increases.

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**Per Phase Equivalent Circuit**

NO LOAD TEST Per Phase Equivalent Circuit

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**NO LOAD TEST n - ns = 0 ‘No load Speed Synchronous Speed’**

i.e. no power transfer which implies that Torque = 0 I1 = 0 & T = 0 Power Consumed = Core Losses + Friction & Windage Measure Vph , IIN and Wph ( Infinite Impedance ) since I1 = 0

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**NO LOAD TEST INL = I0 – jIm = INL ( cos NL - jsin NL )**

cos NL = Wph Vph INL Ro = Vph Xm = Vph I Im

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Lock Rotor Test

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**Lock Rotor Test In the Lock Rotor test, No Load Speed, n = 0**

Slip, s = ns – 0 = 1, s = 1 ns Then Rr Rr s Apply Voltage to Variac, VLR = (10% - 25% ) Vph Since INL<< I Then INL 0 Measure values VLR , ILR and WLR

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**Lock Rotor Test Zeq = VLR / ILR cos LR= WLR VLR ILR **

Zeq = Zeq {cos LR - jsin LR} = Zeq cos LR Zeq jsin LR Rs+ Rr Xs + Xr

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**Lock Rotor Test At Standstill Under d.c. conditions = 0 X= L**

R1 & R2 can be measured using an ohmmeter over two stator windings, which gives a value of Rs Rr = Zeq cos LR - Rs

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