Presentation on theme: "LECTURE ON INDUCTION MACHINE"— Presentation transcript:
1 LECTURE ON INDUCTION MACHINE LECT. RAKESH KUMARDEPTT. OF EEBHSBIETLECTURE ON INDUCTION MACHINEBHSBIET LEHRAGAGA
2 The following points are to be explained in this lecture:- Induction Motor ConstructionBasic Induction Motor ConceptsThe Equivalent Circuit of an Induction Motor.Power and Torque in Induction Motor.Induction Motor Torque-Speed CharacteristicsVariations in Induction Motor Toque-Speed CharacteristicsStarting of Induction MotorsSpeed Control of Induction MotorSome important features of IMBHSBIET LEHRAGAGA
4 INTRODUCTION BHSBIET LEHRAGAGA Asynchronous machines are those machines which do not run at synchronous speed.Most AC motors are induction motors. Induction motors are favored due to their ruggedness and simplicity. In fact, 90% of industrial motors are induction motors.By induction motor, we mean that the stator windings induces a current which flow in the rotor conductors, like a transformer.BHSBIET LEHRAGAGA
5 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.
6 Cutaway view of a modern induction motor BHSBIET LEHRAGAGA
7 The induction machines may be classified in many ways. With rotary or linear motionThree phase supply or single-phase supplyWith wound or cage rotorBHSBIET LEHRAGAGA
8 ConstructionThe 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.
9 ConstructionAn induction motor is composed of a rotor, known as an armature, and a stator containing windings connected to a poly-phase energy source.BHSBIET LEHRAGAGA
10 Construction (Rotor construction) The rotor is the rotating part of the electromagnetic circuit.It can be found in two types:Squirrel cageWound rotorHowever, the most common type of rotor is the “squirrel cage” rotor.
11 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 ringsSimple, low cost, robust, low maintenanceWound 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.
12 The main parts of any IM are:- The stator slotted magnetic coreThe stator electric windingThe rotor slotted magnetic coreThe rotor electric windingThe rotor shaft with bearingsThe stator frame with bearingsThe cooling systemThe terminal boxBHSBIET LEHRAGAGA
18 Constructional features of squirrel cage and wound type motor Stator: The stator consists of stator frame, core and stator winding.Stator frame:- It is the outer body of the motor used to support stator core and windings and also to protect the inner parts of the machine. The frame may be die-cast or fabricated.BHSBIET LEHRAGAGA
19 The thickness varies from 0.35 mm to 0.7 mm. Stator core :- The stator core is assembled of high grade, low electrical loss, silicon steel punching.The thickness varies from 0.35 mm to 0.7 mm.The laminations are used to reduce eddy current loss.The laminations are slotted on the inner periphery and are insulated from each other. The insulated stator conductors are placed in these slots.BHSBIET LEHRAGAGA
20 Stator or field winding:- The stators conductors are connected to form a three phase winding. The three phases of the winding can be connected in either star or delta.BHSBIET LEHRAGAGA
21 The rotor conductor are placed in these slots. The rotor comprises of a cylindrical laminated iron core with slots on outer periphery.The rotor conductor are placed in these slots.The laminated cylindrical core is mounted directly on the shaft or a spider carried by the shaft.BHSBIET LEHRAGAGA
22 BHSBIET LEHRAGAGA There are basically 2 types of rotor construction. Squirrel Cage – No windings and no slip ringsWound rotor - It has 3 phase windings, usually Y connected, and the winding ends are connected via slip ringsBHSBIET LEHRAGAGA
23 SQUIRREL CAGE ROTOR:-In cage construction, rotor conductors, in the form of bars made of copper, or aluminium are placed in rotor slots parallel to the rotor shaft.The rotor bars are short circuited by end rings of same material at each end.The rotor slots are not parallel to the motor shaft but are skewed to reduce magnetic locking of stator and rotor and also to reduce humming noise while running.BHSBIET LEHRAGAGA
25 Squirrel cage rotor — a schematic BHSBIET LEHRAGAGA
26 Wound rotor:-The rotor is wound with an insulated winding similar to that of the stator.The rotor winding is always 3 phase winding. The winding may be star or delta connected, but star connections are usually preferred.The three terminals of star connections are brought outside the rotor and connected to three slip rings.The carbon brushes are pressed on the slip rings. External resistors can be inserted in series with the rotor winding for speed and starting torque control.BHSBIET LEHRAGAGA
28 Differences between squirrel cage IM and slip ring IM SIMPLE IN CONSTRUCTIONRELIABLE AND CHEAPMAINTENANCE FREEITS WINDING CAN ADJUST ITSELF TO ONLY NUMBER OF STATOR POLESSTARTING TORQUE CANNOT BE CONTROLLEDSLIP RING IMCOMPLEX IN CONSTRUCTIONHIGHER INITIAL COSTINCREASED MAINTENANCEBOTH THE STATOR AND ROTOR WINDINGS MUST BE WOUND FOR THE SAME NUMBER OF POLESSTARTING TORQUE CAN BE CONTROLLEDBHSBIET LEHRAGAGA
29 PRINCIPLE OF OPERATION Operation of 3-phase induction motors is based upon the application of Faraday’s Law and the Lorentz Force on a conductor.BHSBIET LEHRAGAGA
30 If a short circuited conductor is placed within a rotating magnetic field, an emf is induced in the conductor due to EMI.Due to this emf, current starts flowing in the conductor and sets up its own magnetic field.Due to the interaction of these two field, a torque is produced and conductor tends to moveBHSBIET LEHRAGAGA
31 How rotor rotates?A three phase IM stator have a three phase distributed winding.When we give supply to stator then a rotating magnetic field produces which rotates at synchronous speed.The rotating flux cuts the rotor conductors and emf produced in them.Because these conductors are short circuited so current is produced in conductors so rotor m.m.f is produced which produces synchronously rotating rotor poles.BHSBIET LEHRAGAGA
35 Final equivalent circuit of IM BHSBIET LEHRAGAGA
36 SLIPThe difference between the synchronous speed and rotor speed can be expressed as a percentage of synchronous speed, known as the slips = slip, Ns = synchronous speed (rpm),N = rotor speed (rpm)BHSBIET LEHRAGAGA
38 • At no-load, the slip is nearly zero (<0. 1%) • At no-load, the slip is nearly zero (<0.1%). • At full load, the slip for large motors rarely exceeds 0.5%. For small motors at full load, it rarely exceeds 5%. • The slip is 100% for locked rotor.BHSBIET LEHRAGAGA
39 Frequency induced in the rotor The frequency induced in the rotor depends on the slip:fR= frequency of voltage and current in the rotorf = frequency of the supply and stator fields = slipBHSBIET LEHRAGAGA
41 Harmonics in Induction Machines Time Harmonics:These affects torque and cause considerable heating in the machine and are hence a cause for concern.These harmonics are called time harmonics since they are generated by a source that varies non sinusoidally in time.BHSBIET LEHRAGAGA
42 Space Harmonics:The space harmonics, are a result of non- sinusoidal distribution of the coils in the machine and slotting.These have their effects on the speed, torque and current of the machine.BHSBIET LEHRAGAGA
43 Effects of harmonics on loaded machine BHSBIET LEHRAGAGA
45 Shaft power is the output power i.e. available at the shaft. AIR GAP POWER:-Air gap power is the power transferred from stator to rotor across the air gap.Pg = 3I22 (r2 / s)Shaft power :-Shaft power is the output power i.e. available at the shaft.Psh = Pg – rotor ohmic loss – friction and windage lossBHSBIET LEHRAGAGA
46 Torque from light load to full load conditions. OPERATING TORQUE:-Torque from light load to full load conditions.STARTING TORQUE:-Torque at start when slip = 1BREAKDOWN TORQUE:-Maximum torque that motor can develop. If motor is loaded beyond this torque, the motor will decelerate and come to stand still.BHSBIET LEHRAGAGA
48 Comments on the IM Torque Speed Curve Induced Torque is zero at synchronous speed.The graph is nearly linear between no load and full load (at near synchronous speeds).Max torque is known as pull out torque or breakdown torqueStarting torque is very large.Torque for a given slip value would change to the square of the applied voltage.If the rotor were driven faster than synchronous speed, the motor would then become a generator.BHSBIET LEHRAGAGA
49 Braking an Induction Motor(Plugging) Plugging is a braking action to bring the rotor to a quick stop. Plugging is obtained by interchanging any two stator leads.With this the phase sequence is reversed and the direction of rotating magnetic field becomes opposite to that of the rotor rotation.The electromagnetic torque now acts opposite to rotor rotation and produces braking actionBHSBIET LEHRAGAGA
50 Variations in Induction Motor Torque-Speed Characterictics BHSBIET LEHRAGAGA
51 Types of starters used in IM D.O.L StarterStar- delta starterAuto-transformer starterBHSBIET LEHRAGAGA
54 Speed control methods:- Pole changingStator voltage controlSupply frequency controlRotor resistance controlSlip energy recoveryBHSBIET LEHRAGAGA
55 Pole changing method:We know that ns=120f/p . By changing the value of p speed can be changed.By providing the stator with independent windings each wound for different member of poles. This results in two speed motor.The stator is provided with one winding or with two independent windings, but coils of each winding can be re-connected to produce a different number of poles in the ratio of 2:1.Two independent windings on the stator, each being designed to give different no. of poles in the ratio of 2:1 can give four different no. of poles in the ratio of 3:2:1.5:1 and thus, a four speed induction motor can be obtained.BHSBIET LEHRAGAGA
61 Stator voltage control:- The speed can be controlled by varying the supply voltage until the torque required by the load is developed at the desired speed.The torque developed is proportional to the square of the supply voltage and current is proportional to the voltage.Therefore, voltage is reduced to reduce speed for the same current, the torque developed by the motor is reduced.This method is suitable where load torque decreases with speed e.g. fan load.BHSBIET LEHRAGAGA
62 Speed-torque curves: voltage variation BHSBIET LEHRAGAGA
63 Variable frequency control: The variable frequency supply is obtained by the following converter.Voltage source inverterCurrent source inverterCycloconverterAn inverter converts a fixed voltage d.c. to a fixed ( or variable ) voltage a.c. with variable frequency.A Cycloconverter converts a fixed voltage and fixed frequency a.c. to a variable voltage and variable frequency a.c.BHSBIET LEHRAGAGA
64 Torque-speed curves with V/f constant BHSBIET LEHRAGAGA
65 Torque-speed curves with E/f held constant BHSBIET LEHRAGAGA
66 Rotor resistance control This method is applicable to slip ring induction motor only.The speed of motor can be controlled by connecting external resistance in the rotor circuit.The starting torque increases with increase in resistance, the pull out speed of the motor decreases but the maximum torque remains constant, the speed can be controlled from the rated speed to lower speed.BHSBIET LEHRAGAGA
68 Effect of changing rotor-circuit resistance(torque-slip curve) BHSBIET LEHRAGAGA
69 Slip energy recovery a scheme The basic principle of slip power recovery is to connect an external source of emf of slip frequency of the rotor circuit.This method is known as scheribus scheme.BHSBIET LEHRAGAGA
70 A portion of rotor a.c. power is converted into d.c by a diode bridge. The output of the rectifier is connected to the d.c. terminals of the inverter, which inverts this d.c. power to a.c. power and feeds it back to the a.c source. It provides the speed control synchronous speed.BHSBIET LEHRAGAGA
71 Some important features of IM BHSBIET LEHRAGAGA
72 Leakage reactanceLeakage reactance is the impedance due to the leakage flux.More the leakage reactance more magnetizing current is required to obtain working m.m.f .It also decreases the power factor and gives more losses in motor.BHSBIET LEHRAGAGA
73 COGGING:-When number of slots of rotor and stator are equal, then a magnetic interlocking takes place and motor does not start. It occurs during starting of motor.CRAWLING:-In this phenomenon motor starts to run stably at speed lower than rated speed due to presence of harmonics. It occurs during running of motor.BHSBIET LEHRAGAGA
74 LOSSES IN AN IM FIXED LOSSES- These loses are composed of Core loss Bearing friction lossBrush friction loss in slip ring IM onlyWindage lossVARIABLE LOSSES-These losses are composed ofStator ohmic lossRotor ohmic lossBrush contact loss only for slip ring IMStray load lossBHSBIET LEHRAGAGA
75 The relationship between the input electric power and the output mechanical power of this motor is shown : elowBHSBIET LEHRAGAGA
76 ROTATIONAL LOSSESThe higher the speed of an induction motor, the higher the friction, windage, and stray losses.On the other hand, the higher the speed of the motor (up to nSYNC), the lower its core losses.Therefore, these three categories of losses are sometimes lumped together and called rotational losses.BHSBIET LEHRAGAGA
77 Difference between SINGLE CAGE IM DOUBLE CAGE IM Low starting torque High operating slipLow operating efficiencyHigh starting torqueLow operating slipHigh operating efficiencyBHSBIET LEHRAGAGA
78 Induction generatorIf a polyphase IM is given a voltage and frequency for supply mains and rotate at speed higher than a synchronous speed by a prime mover, then rotor overtakes rotating magnetic field as a result emf and currents in rotor reverse their direction. This is called induction generator.BHSBIET LEHRAGAGA
79 Improving power factor of 3 phase IM 3-phase IM has an air gap between stator and rotor winding due to which motor needs high magnetizing current for the production of working magnetic flux.Power factor can be improved by following methods.Reducing the air gap between stator and rotor windingBy use of static capacitor across stator terminalFor wound motor, by use of auxiliary machinesBHSBIET LEHRAGAGA
80 Improving starting torque of a 3-phase IM Increasing rotor circuit resistanceBHSBIET LEHRAGAGA
81 Induction machine never reach synchronous speed. Why? BHSBIET LEHRAGAGA
82 SINGLE PHASE INDUCTION MACHINE PART-IISINGLE PHASE INDUCTION MACHINEBHSBIET LEHRAGAGA
83 Single Phase Induction Permanent-split capacitor motorOne way to solve the single phase problem is to build a 2-phase motor, deriving 2-phase power from single phase. This requires a motor with two windings spaced apart 90o electrical, fed with two phases of current displaced 90o in time. This is called a permanent-split capacitor motor in Figure
85 1-Phase Induction Motor This type of motor suffers increased current magnitude and backward time shift as the motor comes up to speed, with torque pulsations at full speed. The solution is to keep the capacitor (impedance) small to minimize losses. The losses are less than for a shaded pole motor.
86 1-Phase Induction Motor This motor configuration works well up to 1/4 horsepower (200watt), though, usually applied to smaller motors. The direction of the motor is easily reversed by switching the capacitor in series with the other winding. This type of motor can be adapted for use as a servo motor, described elsewhere is this chapter
87 Capacitor-start induction motor In Figure a larger capacitor may be used to start a single phase induction motor via the auxiliary winding if it is switched out by a centrifugal switch once the motor is up to speed. Moreover, the auxiliary winding may be many more turns of heavier wire than used in a resistance split-phase motor to mitigate excessive temperature rise. The result is that more starting torque is available for heavy loads like air conditioning compressors. This motor configuration works so well that it is available in multi-horsepower (multi-kilowatt) sizes.
88 Capacitor-run motor induction motor A variation of the capacitor-start motor Figure is to start the motor with a relatively large capacitor for high starting torque, but leave a smaller value capacitor in place after starting to improve running characteristics while not drawing excessive current. The additional complexity of the capacitor-run motor is justified for larger size motors.
90 Capacitor Start-Run Induction Motor A motor starting capacitor may be a double-anode non-polar electrolytic capacitor which could be two + to + (or - to -) series connected polarized electrolytic capacitors. Such AC rated electrolytic capacitors have such high losses that they can only be used for intermittent duty (1 second on, 60 seconds off) like motor starting. A capacitor for motor running must not be of electrolytic construction, but a lower loss polymer type.
91 Resistance split-phase motor induction motor If an auxiliary winding of much fewer turns of smaller wire is placed at 90o electrical to the main winding, it can start a single phase induction motor. With lower inductance and higher resistance, the current will experience less phase shift than the main winding. About 30o of phase difference may be obtained. This coil produces a moderate starting torque, which is disconnected by a centrifugal switch at 3/4 of synchronous speed. This simple (no capacitor) arrangement serves well for motors up to 1/3 horsepower (250 watts) driving easily started loads.
92 Wound rotor induction motors A wound rotor induction motor has a stator like the squirrel cage induction motor, but a rotor with insulated windings brought out via slip rings and brushes. However, no power is applied to the slip rings. Their sole purpose is to allow resistance to be placed in series with the rotor windings while starting.This resistance is shorted out once the motor is started to make the rotor look electrically like the squirrel cage counterpart.
93 Wound Rotor Induction M/C Why put resistance in series with the rotor? Squirrel cage induction motors draw 500% to over 1000% of full load current (FLC) during starting. While this is not a severe problem for small motors, it is for large (10's of kW) motors. Placing resistance in series with the rotor windings not only decreases start current, locked rotor current (LRC), but also increases the starting torque, locked rotor torque (LRT).
94 Wound Rotor Induction M/C Figure shows that by increasing the rotor resistance from R0 to R1 to R2, the breakdown torque peak is shifted left to zero speed. Note that this torque peak is much higher than the starting torque available with no rotor resistance (R0) Slip is proportional to rotor resistance, and pullout torque is proportional to slip. Thus, high torque is produced while starting.
96 Wound Rotor Induction M/C The resistance decreases the torque available at full running speed. But that resistance is shorted out by the time the rotor is started. A shorted rotor operates like a squirrel cage rotor. Heat generated during starting is mostly dissipated external to the motor in the starting resistance. The complication and maintenance associated with brushes and slip rings is a disadvantage of the wound rotor as compared to the simple squirrel cage rotor.
97 Wound Rotor InductionThis motor is suited for starting high inertial loads. A high starting resistance makes the high pull out torque available at zero speed. For comparison, a squirrel cage rotor only exhibits pull out (peak) torque at 80% of its' synchronous speed
98 Speed ControlMotor speed may be varied by putting variable resistance back into the rotor circuit. This reduces rotor current and speed. The high starting torque available at zero speed, the down shifted break down torque, is not available at high speed. See R2 curve at 90% Ns, Resistors R0R1R2R3 increase in value from zero. A higher resistance at R3 reduces the speed further. Speed regulation is poor with respect to changing torque loads. This speed control technique is only useful over a range of 50% to 100% of full speed. Speed control works well with variable speed loads like elevators and printing presses.
100 Shaded Pole Induction Motor Main windings and Shaded Pole winding at the Stator, while shaded pole is short circuited.
101 RELUCTANCE MOTORStatorSame as:Split phaseorCapacitor Start Motor
102 RELUCTANCE MOTOR Rotor Squirrel cage motor BUT, with Same as:Squirrel cage motorBUT, withUneven slots cut into laminations to form Salient polesUneven slots assist in startingRotor Slots generally ≠ Stator Slots
103 RELUCTANCE MOTOR Starting Motor becomes Synchronous As per induction motor with squirrel cage providing torqueCentrifugal switch operating as per normal(75%)As motor is lightly loaded slip speed is smallRotor salient poles become magnetised and lock with RMFMotor becomes Synchronous
104 RELUCTANCE MOTORIf rotor poles are a multiple of the stator polesMotor will operate at sub-multiples of synchronous speed
105 HYSTERESIS MOTORRotorOuter section made up of hardened steel
106 HYSTERESIS MOTOR Rotor Outer section made up of hardened steel This outer section supported on the shaft by a NON- MAGNETIC “Arbour”
107 HYSTERESIS MOTOR PROBLEM Rotor has a very high Hysteresis loss The rotor tends to become magnetisedA synchronous motor is bornPROBLEMSynchronous motors have ZERO START TOURQUE!A Shaded pole stator is used
108 UNIVERSAL MOTOR Fields are laminated for AC current Not the same as a series DC MotorFields are laminated for AC current
111 Limitation of single phase induction motor in reference to 3 phase induction motor. Following are the limitation of the of single phase induction motor :Single phase motor is not self starting .Single phase induction motor has low power factor as compared to three phase induction motor.For same rating single phase motor has big frame size.Single phase motor has lower efficiency.Single phase motor has higher core and copper losses.BHSBIET LEHRAGAGA
112 Why in split phase motor ,starting winding is connected through centrifugal switch? Centrifugal switch automatically disconnected the starting winding at about to 80% of synchronous speed. If centrifugal switch is not used then starting winding remains in circuit which gives noisy performance.BHSBIET LEHRAGAGA
113 Difference between single phase induction motor and three phase induction motor . Single phase induction motor are different from three phase induction motor in following aspects:Single phase motors are not self starting while three phase induction motors self starting.For same load torque, 1-φinduction motor requires more stator current and operates at a higher slip.For the same size ,1-φinduction motor output is less. It has higher temperature rise and lower efficiency as compared to three phase induction motor.BHSBIET LEHRAGAGA
114 What are the advantage of capacitor start capacitor run induction motor ? The advantage of capacitor start, capacitor run induction motor areImprovement of over load capacity of the motorHigher efficiencyHigher power factorQuieter operation of the motorBHSBIET LEHRAGAGA
115 Why starting winding is required in single phase induction motor? Single phase induction motor is not self starting . It provided with starting winding in addition to main winding to temporarily convert it into a two phase motor at the time of starting . The two currents produce a revolving flux and make the motor self starting.BHSBIET LEHRAGAGA
116 What is the role of shading coil in shaded pole motor? Due to the presence of shading coil in shaded pole motor ,the shifting of magnetic axis takes place when an alternating current is passed through field winding .the rotor starts rotating in the direction of this shift i.e. from unshaded part to the shaded part.BHSBIET LEHRAGAGA
117 In what direction will the rotor of shaded pole motor rotate? The rotor of a shaded pole motor will rotate in a direction from the unshaded part to the shaded part.BHSBIET LEHRAGAGA
118 How is the direction of rotation of a single phase induction motor reversed? The direction of rotation can be reversed by reversing the line connections of either the main winding or the starting winding .BHSBIET LEHRAGAGA
119 Why single phase capacitor type induction motor are superior in performance? The single phase capacitor type induction motors has relatively good starting torque, high power factor because the phase angle between the running winding current and the starting winding current is practically 90 electrical degree.BHSBIET LEHRAGAGA
120 Why power factor of single phase motor is lagging? The main reason of lagging power factor is high magnetizing current which is lagging in nature .single phase motor carries magnetizing current for both forward and backward fields. Therefore ,this magnetizing current is higher than 3 phase induction motor.BHSBIET LEHRAGAGA
121 What is the effect of increasing rotor resistance in a single induction motor ? The increase in rotor resistance of a single phase induction motor reduces its breakdown torque , lower the efficiency and increases the slip at which maximum torque occurs.BHSBIET LEHRAGAGA
122 A single phase induction motor is provided with a main winding and an auxiliary winding . Which of the two winding should be more resistance and why?The auxiliary winding should be more resistance to give higher starting torque . As this winding stays in the circuit at the time of starting only , so does not affect the copper losses in the machine.BHSBIET LEHRAGAGA
123 Repulsion motorStator of repulsion motor consist of single phase exciting winding and rotor have distributed d.c. winding.The brushes are short circuited on themselves and not connected to the supply circuit.Armature receives power from the stator by transformer action .BHSBIET LEHRAGAGA
124 Fig.(b),α=90 ̊Fig.(a) , θ=αFig.(c), α=180 ̊Schematic diagram shown in fig.(a). when angle α is 90̊ then there is no mutual induction between stator and rotor. So no torque will be produced as shown in fig(b) .When angle α=0 then magnetic axis of stator and rotor coincides so mutual induction between stator and rotor will be maximum as in fig(c).BHSBIET LEHRAGAGA
125 What are the two position of brushes at starting in which the repulsion motor does not develop the starting torque?The field axis and quadrature to field axis position of the brushes are such in which repulsion motor does not developed any starting torque.BHSBIET LEHRAGAGA
126 How speed variation is affected in repulsion motor? The speed variation is affected either by changing the position of brushes or by varying the impressed voltageBHSBIET LEHRAGAGA
127 What happens when the shaft torque of a repulsion motor with two stator windings increases? With the increases in torque on a repulsion motor stator current increases and power factor decreases.BHSBIET LEHRAGAGA
128 Why laminated yoke is used in an a.c series motor? Yoke , pole and armature cores of a.c series motors are laminated so as to reduce eddy current losses, and so efficiency is improved and heating is reduced.BHSBIET LEHRAGAGA
129 Why centrifugal switch is provided in repulsion start induction motor? The centrifugal switch is provided to short circuit all the commutator segments at about percent of synchronous speed. It is also used to lift the brushes from the commutator in some motors.BHSBIET LEHRAGAGA
130 Why is the normal full load slip of a single phase induction motor higher than that of a three phase induction motor?The normal full load slip of a single phase induction motor is higher than that of a three phase motor, owing to development of backward rotating field . The power is to be delivered to the backward field from the power converted into mechanical power by forward field.BHSBIET LEHRAGAGA
131 What are the advantage of using a capacitor start motor over a resistance start split phase motor? The capacitor start motor develops higher starting torque than does an equally rated resistance start split phase with a lower in rush current.BHSBIET LEHRAGAGA