Chapter 2 Poly phase Induction Motor Prepared by:- Jaydeepsingh Chauhan Electrical Department SVBIT Gandhinagar

induction motor 2 Induction Motor Most popular motor today in the low and medium horsepower range Very robust in construction Speed easily controllable using V/f or Field Oriented Controllers Have replaced DC Motors in areas where traditional DC Motors cannot be used such as mining or explosive environments Of two types depending on motor construction: Squirrel Cage or Slip Ring Only Disadvantage: Most of them run with a lagging power factor

induction motor 3 Stator Squirrel Cage Rotor Slip ring Rotor Cut section Of Induction Motor Construction of Induction Motor

induction motor 4 AC Machine Stator ‘a’ phase axis ‘b’ phase axis ‘c’ phase axis 120 0

induction motor 5 Squirrel Cage Rotor

induction motor 6 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.

induction motor 7 1 Cycle Amp time t0t0 t1t1 t2t2 t3t3 t4t4 t 01 t 12 Currents in different phases of AC Machine

induction motor 8 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)

induction motor 9 Torque Production in an Induction Motor In a conventional DC machine field is stationary and the current carrying conductors rotate. We can obtain similar results if we make field structure rotating and current carrying conductor stationary. In an induction motor the conventional 3-phase winding sets up the rotating magnetic field(RMF) and the rotor carries the current carrying conductors. An EMF and hence current is induced in the rotor due to the speed difference between the RMF and the rotor, similar to that in a DC motor. This current produces a torque such that the speed difference between the RMF and rotor is reduced.

induction motor 10 Slip in Induction Motor However, this speed difference cannot become zero because that would stop generation of the torque producing current itself. The parameter slip ‘s’ is a measure of this relative speed difference where n s,  s,f 1 are the speeds of the RMF in RPM,rad./sec and supply frequency respectively n,  are the speeds of the motor in RPM and rad./sec respectively The angular slip frequency and the slip frequency at which voltage is induced in the rotor is given by 

Starting Method for Induction Motors Therefore, 3-phase induction motors employ a starting method not to provide a starting torque at the rotor, but because of the following reasons; 1) Reduce heavy starting currents and prevent motor from overheating. 2) Provide overload and no-voltage protection. Direct On-Line Starter (DOL) Star-Delta Starter Auto Transformer Starter Rotor Impedance Starter Power Electronics Starter

Direct On-Line Starter (DOL)

Star-Delta Starter

Auto Transformer Starter

Rotor Impedance Starter

Single Phase Induction Motor A single phase induction motor is not self starting but requires some starting means. If the stator winding is connected to single – phase a.c. supply, the stator winding produces a magnetic field that pulsates in strength in a sinusoidal manner. The field polarity reverses after each half cycle but the field does not rotate.

Double – Field Revolving Theory Torque – slip characteristic of 1- phase induction motor

Starting of Single Phase Induction Motors Split – phase Induction Motor

Capacitor – Start Motor

Permanent – Split Capacitor Motor

Capacitor - Start Capacitor - Run

Shaded Pole Induction Motor

Construction of Circle Diagram Conduct No load test and blocked rotor test on the induction motor and find out the per phase values of no load current I0, short circuit current ISC and the corresponding phase angles Ö0 and ÖSC. Also find short circuit current ISN corresponding to normal supply voltage. With this data, the circle diagram can be drawn as follows. With suitable scale, raw vector OA with length corresponding to Io at an angle Ö0 from the vertical axis. Draw a horizontal line AB. Draw OS equal to Isn at an angle Φ SC and join AS. Draw the perpendicular bisector to AS to meet the horizontal line AB at C. With C as centre, draw a portion of circle passing through A and S. This forms the circle diagram which is the locus of the input current. From point S, draw a vertical line SL to meet the line AB. Divide SL at point K so that SK : KL = rotor resistance : stator resistance. For a given operating point P, draw a vertical line PEFGD as shown. then PE = output power, EF = rotor copper loss, FG = stator copper loss, GD = constant loss (iron loss + mechanical loss). To find the operating points corresponding to maximum power and maximum torque, draw tangents to the circle diagram parallel to the output line and torque line respectively. The points at which these tangents touch the circle are respectively the maximum power point and maximum torque point.

Efficiency line 1. The output line AS is extended backwards to meet the X-axis at O ′. 2. From any convenient point on the extended output line, draw a horizontal line QT so as to meet the vertical from O ′. Divide the line QT into 100 equal parts. 3. To find the efficiency corresponding to any operating point P, draw a line from O ′ to the efficiency line through P to meet the efficiency line at T1. Now QT1 is the efficiency. Slip Line 1. Draw line QR parallel to the torque line, meeting the vertical through A at R. Divide RQ into 100 equal parts. 2. To find the slip corresponding to any operating point P, draw a line from A to the slip line through P to meet the slip line at R1. Now RR1 is the slip Power Factor Curve 1. Draw a quadrant of a circle with O as centre and any convenient radius. Divide OCm into 100 equal parts. 2. To find power factor corresponding to P, extend the line OP to meet the power factor curve at C ′. Draw a horizontal line C ′ C1 to meet the vertical axis at C1. Now OC1 represents power factor.