LEC_#_01_Three_-_Phase_Induction__Motors.pptx

Induction motor

1. Three – Phase Induction Motors Production of rotating field and toque, reversing. Construction. Synchronous speed, slip and its effects on rotor frequency and voltage. ENG. NOORNABI SHAIKH

2. 3 – Phase Induction Motors Induction motor was invented by Nikola Tesla in 1888. the transfer of energy from stator to rotor is of electromagnetic induction. A rotating magnetic field is produced by a stator winding induces an alternating emf and current in the rotor. The resultant interaction of the induced rotor current magnetic field with stator rotating magnetic field produces motor torque. Thus, an induction motor is a “singly excited” motor (as opposed to a doubly excited synchronous motor). An induction motor is so called, because the driving force is produced by an electric current induced in a rotor due to its interaction with a magnetic field. Induction motor is the most commonly used type of ac motor due to its simple and rugged construction, minimal maintenance requirements, and good operating characteristics satisfy a wide variety of loads. Induction motor range in size from a few watts to about 40,000 hp. Small fractional – horsepower motors are single – phase and are used extensively for domestic appliances, such as refrigerators, washers, dryers, and blenders. Large induction motors (above 5 hp) are always designed for 3 – phase operation to achieve a constant torque and balanced network loading. 3 – phase induction motors are called “workhorse” of the industry.

3. 3 – Phase Induction Motor Action An elementary 3 – phase , 2 – pole induction motor is shown: The stator consists of three “blocks of iron” spaced 120 o apart. Y – connected coils wound around the iron blocks are energized form a 3 – phase ac system. Rotor consists of a laminated steel core containing conductors joined at the end to form a cage similar to squirrel cage; hence, the name squirrel – cage rotor. When stator windings energized from a 3 – phase supply, the currents in the coils reach their maximum values at different instants. As currents are displaced form each other by 120 electrical degrees, their respective flux contribution will also be displaced by 120 o , as shown. For example: At 0o degrees, phase A is maximum north pole, while phases B & C are weak south poles; At 60o phase C is strong north pole, while phases A & B weak north pole; At 120o phase B is strong north pole, while phases A & C are weak south poles, and so forth.

4. How a Rotating Magnetic Field produced in the Stator of 3 – Phase Induction Motor ??

5. Large arrows indicate the instantaneous direction of the resultant flux. Different angular positions assumed by the resultant flux vector show the plane of the flux to be revolving in a CCW direction. Flux generated by each coil is alternating but the combined flux contributions of the three staggered coils, carrying currents at appropriate sequential phase angles, produce a two – pole rotating flux. It is the rotating flux, not the alternating flux, that produces induction motor action. Rotating flux (rotating field) produced by 3 – phase currents in the stator coils, may be linked to the rotating field produced by a magnet sweeping around the motor, as shown : figure (a). The rotating magnetic field “cuts” the rotor bars (conductors) in its CCW sweep around the rotor. Speed of the rotating field is called the synchronous speed.

6. In accordance with Lenz’s law, the voltage, current, and flux generated by the relative motion between a conductor and a magnetic field will be in a direction to oppose the relative motion. To satisfy Lenz’s law, the conductors must develop a mechanical force or thrust in the same direction as the rotating flux (CCW). The direction of rotor – bar current that produces this CW flux is determined by the right – hand rule.

7. Reversal of Rotation Direction of rotation of an induction motor is dependent on the direction of rotation of the stator flux, which in turn is dependent on the phase sequence of the applied voltage. Interchanging any two of the three line – lead to a 3 – phase induction motor will reverse the phase sequence, thus reversing the rotation of the motor. Figure shows the phase sequence ABC causes CCW rotation of the magnetic filed. Likewise, phase sequence CBA will cause a CW rotation.

8. Induction Motor Construction Figure shows a cutaway view of a practical 3 – phase IM. Stator : the stator core assembly is made of thin laminations stamped from silicon – alloy sheet steel. [ silicon steel for magnetic material to minimize hysteresis loss ] laminations are coated with oxide or varnish to minimize eddy – current losses. Insulated coils are set in slots within the stator core. The overlapping coils are connected in series or parallel arrangements to form phase groups and the phase groups are connected Y or Δ . Series or parallel, Y or Δ are dictated by voltage and current requirements.

9. Rotor : Two basic types: Squirrel Cage Rotor 2. Wound Rotor Squirrel Cage Rotor: Small SC rotors , shown in fig. (a) use a slotted core of laminated steel into which molten aluminum is cast to form the conductors, end rings, and fan blades. Large SC rotors , shown in fig. (b), uses brass bars and brass end rings that are brazed together to form the squirrel cage. There is no insulation between the iron core and the conductors, and none is needed; the current induced in the rotor is contained within the circuit formed by the conductors and end rings. Skewing the rotor slots ; as shown in fig. (a), helps avoid crawling (locking in at sub-synchronous speed) and reduces vibrations.

10. Wound Rotor : use insulated coils that are set in slots and connected in a Y arrangement. The rotor circuit is completed through a set of slip rings, carbon brushes and a Y connected rheostat. Common lever is used to simultaneously adjust all three rheostat arms. Moving the rheostat to “zero - resistance” position, shorts the resistors and simulates a squirrel – cage motor. Rheostat is used to adjust starting torque and running speed. Air gap between stator and rotor is made quite small to minimize the reluctance. Each coil of stator spans a portion of the stator circumference equal to or slightly less than the pole pitch; the pole pitch is equal to the stator circumference divided by the number of stator poles, and it may be expressed in terms of stator slots or stator arc. If coil span (coil pitch) is equal to the pole pitch it is called a full pitch winding. But if the span is less than the full – pitch, it is called a fractional – pitch winding. Transfer of energy from stator to rotor, whether squirrel cage or wound rotor, is by means of electromagnetic induction and occurs in manner similar to that in transformer. As energy to do work is transferred electromagnetically across the air gap between the stator and rotor, the air gap is made quite small to minimize the reluctance.

11. Figure show full pitch (a) coil span for four – pole winding and figure (b) shows eight pole winding. The three black arcs represent the end view of the 3 – stator coils, each representing on phase. The angles are in mechanical degrees.

12. Difference Between WRIM & SCIM For slip ring, the rotor includes three phase winding similar to that of the stator winding. For squirrel cage, the rotor includes bars which are shorted at the ends with the help of end rings . 2 . For slip ring, high starting torque is accessible. For squirrel cage, only moderate starting torque is available and it cannot be controlled . 3. Only 5% of induction motors in industry use slip ring type. Squirrel cage type is most commonly used in industries by 95 %. 4. In slip ring, slip ring and brushes are present to add external resistance , while in squirrel cage type slip ring and brushes are absent . 5. For slip ring, frequent repair is needed for the easily damaged construction and brushes. For squirrel cage, the construction is robust and frequent maintenance is not necessary . 6. For slip ring, the rotor must be wound for the same number of poles as that of stator. For squirrel cage, the rotor regulates itself automatically for the same number of poles as that of stator . 7. Slip ring type is of low efficiency for the high loss of the rotor copper, while squirrel cage is of high efficiency for the low loss of the rotor copper . 8. Slip ring type is applied to lifts, hoists , cranes, elevators , while squirrel cage type is applied to fans , blowers, water pumps.

13. Synchronous Speed The speed of rotating flux, called synchronous speed, is directly proportional to the frequency of the supply voltage and inversely proportional to the number of pair of poles: expressed as Where: f s = frequency of supply n s = synchronous speed P = number of poles formed by the stator windings

14. Slip and its effects on rotor frequency and voltage The difference between the speed of the rotating flux and the speed of the rotor is called “slip speed”, and the ratio of slip speed to synchronous speed is called slip. Expressed in equation form: Where: n = slip speed (r/min) n s = synchronous speed (r/min) n r = rotor speed (r/min ) s = slip (pu) Slip depends on the mechanical load connected to the rotor shaft (assuming constant supply voltage and frequency). Increasing the shaft load decreases the rotor speed, thus increases the slip. If the rotor is blocked to prevent turning, n r = 0 , equation (3) reduces to:

15. Releasing the brake allows the rotor to accelerate. Slip decreases with acceleration and approaches to zero when all mechanical load is removed. If operating with no shaft load, windage and friction are sufficiently small, the very low relative motion between rotor and rotating stator flux may cause the rotor to become magnetized along an axis of minimum reluctance. At this rotor will lock in synchronism with the stator flux; at this slip will be zero, no induction motor torque will be developed, and motor will act as a reluctance motor. The application of a small shaft load will cause it to pull out of synchronism, however, and induction – motor action will again occur. Equation (3) for n r expresses the rotor speed in terms of slip:

16. Effect of Slip on Rotor Frequency: Frequency of the induced voltage in a rotor loop by a rotating magnetic field is given by: Where: f r = rotor frequency P = number of stator poles n = slip speed (rev/min) Substituting eq. (2) in equation (5)

17. Where: f BR = frequency of generated voltage in the blocked rotor. Substituting eq. (8) in eq. (7) results in the general expression for rotor frequency in terms of slip and blocked rotor frequency. Thus, At blocked rotor, also called locked rotor, there is no relative motion between rotor and stator. s = 1 , and the frequency of the generated voltage in the rotor is identical to the frequency of the supply stator voltage. That is

18. Effect of Slip on Rotor Voltage: Referring figure, the voltage generated in a rotor loop (formed by two rotor bars and the end connections) as it is swept by the rotating stator flux is Substituting equation (9) in the basic induced equation given as; AT blocked rotor, s = 1, then equation (10) becomes Substituting eq. (11) in eq. (10) Equation (12) is the general expression for the voltage induced in a rotor loop at any rotor speed, in terms of blocked – rotor voltage and slip.

19. PROBLEM: The frequency and induced voltage in the rotor of a certain six – pole wound rotor induction motor, whose shaft is blocked, are 50 Hz and 110 V, respectively. Determine the corresponding values when the rotor is running at 900 r/min. SOLUTION:

20. Development of Induced Torque in a n Induction Motor When 3 – phase voltages applied to the stator, 3 – phase set of stator currents developed. These currents produce a magnetic field B s , which is rotating in CCW direction. The speed of magnetic field’s rotation is given by: This rotating magnetic field B s , passes over the rotor bars and induces a voltage in them. The voltage induced in a given rotor bar is given by the equation: Where: v = velocity of the bar relative to the magnetic field B = magnetic flux density vector l = length of conductor in the magnetic field It is the relative motion of the rotor compared to the stator magnetic field that produces induced voltage in a rotor bar.

21. The velocity of the upper rotor bars relative to the magnetic field is to the right, so the induced voltage in the upper bars is out of the page, while the induced voltage in the lower bars is into the page. This results a current flow out of the upper bars and into the lower bars. Since rotor assembly is inductive, rotor current lags behind the rotor voltage (fig. b). This rotor current flow produces a rotor magnetic field B R . Induced torque in the machine is given by: Resulting torque is CCW, and so the resulting rotor accelerates in that direction. There is a finite limit to the motor’s speed. If the induction motor’s rotor is turning at synchronous speed, (n s = n m ) then rotor bars would be stationary relative to the magnetic field and no induced voltage. If e ind = 0, then there would be no rotor current and no rotor magnetic magnetic field. There would be no induced torque, and the rotor would slow down as a result of friction losses. An induction motor can speed up to near – synchronous speed, but it can never exactly reach synchronous speed. In normal operation both the rotor and stator magnetic fields B R and B S rotate together at synchronous speed n sync , while the rotor itself turns at a slower speed. [???] The rotor current produce s a ro t or magnetic field B R lagging 90° behind itself, and B R i nteracts with b net to produce a CCW to r que in the mach i n e.

22. TORQUE Any mechanical load applied to the motor shaft will introduce a Torque on the motor shaft. This torque is related to the motor output power and the rotor speed While the input to the induction motor is electrical power, its output is mechanical power and for that we should know some terms and quantities related to mechanical power Another unit used to measure mechanical power is the Horse Power It is used to refer to the mechanical output power of the motor Since we, as an electrical engineers, deal with watts as a unit to measure electrical power, there is a relation between horse power and watts h p = 746 watts HORSE POWER

23. PROBLEM # 02 A 400 – V, 8 – hp, 4 – pole, 50 Hz, Y – connected induction motor has a full – load slip of 5 percent. Determine: Synchronous speed of this motor Rotor speed at rated load Rotor frequency at the rated load Shaft torque at the rated load SOLUTION (a) Synchronous speed of the motor

24. (b) Rotor speed at rated load (c) Rotor frequency at rated load (d) Shaft torque at rated load

25. The shaft load torque in English units is given by equation: Where τ is in pound – feet, P is in hp, and n m is in revolution per minute

26. H.W PROBLEM A 3 – phase, 60 Hz, 25 hp, wye – connected induction motor operates at a shaft speed of almost 1800 rpm at no – load and 1650 rpm at full – load. Determine the following: The number of poles of the motor The per – unit and percent slip at full load The slip frequency of the motor The speed of the rotor field with respect to the rotor itself The speed of the rotor field with respect to the stator The speed of the rotor field with respect to the stator itself The output torque of the motor at the full load

27. Brief summary on both types differences: