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AC Three Phase Motor

Electrician Trade Theory, Module-3

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By Roni
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AC Three Phase Motor


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Q1: What is the formula to calculate the slip speed ($N_{slip}$) of 3 phase squirrel cage induction motor?

  • $N_{slip} = N_s - N_r$
  • $N_{slip} = N_r - N_s$
  • $N_{slip} = \frac{N_s - N_r}{N_s}$
  • $N_{slip} = \frac{N_s - N_r}{N_r}$

Q2: What is the type of control circuit?

control circuit diagram

  • Inching control
  • ON remote control
  • OFF remote control
  • Forward & reverse control

Q3: Which formula is used to calculate the total electrical degree in stator of an A.C motor?

  • Total electrical degree = 180 / No. of slots
  • Total electrical degree = 180 x No. of slots
  • Total electrical degree = 180 / No. of poles
  • Total electrical degree = $180^\circ \times \text{No. of poles}$

Q4: What is the name of the A.C motor starter?

AC motor starter diagram

  • DOL starter
  • Auto transformer starter
  • Semi automatic star delta starter
  • Fully automatic star delta starter

Q5: What is the formula to find synchronous speed of a A.C 3 phase induction motor?

  • Synchronous speed = $\frac{120F}{P}$
  • Synchronous speed = $\frac{120P}{F}$
  • Synchronous speed = $\frac{120}{PF}$
  • Synchronous speed = $\frac{PF}{120}$

Q6: What is the fuse rate to run a 10 HP three phase induction motor at full load?

  • 10 A
  • 15 A
  • 25 A
  • 30 A
Show Calculation $P = 10 \times 746 \text{ Watts}$ $P = 7460 \text{ Watts}$ Assuming standard values for a 3-phase system: * Voltage ($V$) = $415 \text{ V}$ * Power Factor ($\cos\phi$) $\approx 0.8$ * Efficiency ($\eta$) $\approx 0.85$ $I_{FL} = \frac{P}{\sqrt{3} \times V \times \cos\phi \times \eta}$ $I_{FL} = \frac{7460}{1.732 \times 415 \times 0.8 \times 0.85}$ $I_{FL} = \frac{7460}{488.7}$ $I_{FL} \approx 15.26 \text{ Amps}$ To handle the starting current (which is higher than full load current), the fuse rating is typically calculated as **1.5 times** the full load current. $\text{Fuse Rating} = I_{FL} \times 1.5$ $\text{Fuse Rating} = 15.26 \times 1.5$ $\text{Fuse Rating} \approx 22.89 \text{ Amps}$ The nearest standard fuse rating available above 22.89 A is **25 A**.

Q7: What is the name of the contact marked as 'X'?

contact marked X

  • Star contact
  • Delta contact
  • Auxiliary contact
  • Over load relay contact

Q8: What is the type of A.C motor stator winding?

AC motor stator winding

  • Single layer basket winding
  • Double layer basket winding
  • Involute coil winding
  • Diamond coil winding

Q9: Which formula is used to calculate percentage slip of an AC 3 phase Induction motor?

  • $\frac{N_s - N_r}{N_s} \times 100$
  • $\frac{N_r - N_s}{N_s} \times 100$
  • $\frac{N_s - N_r}{N_r} \times 100$
  • $\frac{N_r - N_s}{N_r} \times 100$

Q10: Which operation the control circuit is used?

control circuit operation

  • Remote control
  • Inching
  • Sequential control
  • Forward and reverse

Q11: What is the phase displacement between windings in 3 phase motor?

  • 90°
  • 120°
  • 180°
  • 360°

Q12: What is the name of the part marked as X?

motor part marked X

  • Shaft
  • Brushes
  • Bearings
  • Slip rings

Q13: What is the name of AC coil winding?

AC coil winding

  • Half coil winding
  • Whole coil winding
  • Single layer winding
  • Double layer winding

Q14: What is the name of the coil winding?

coil winding type

  • Concentric coil winding
  • Distributed coil winding
  • Mesh shaped coil winding
  • Diamond mesh shaped coil winding

Q15: Which speed is called as synchronous speed in 3 phase induction motor?

  • No load speed
  • Full load speed
  • Rotating magnetic field speed
  • Relative speed between stator and rotor

Q16: What is the name of the starter symbol?

starter symbol

  • D.O.L starter
  • Auto transformer starter
  • Automatic star/delta starter
  • Semi automatic star/delta starter

Q17: Name the part marked as 'X' of the winding machine?

winding machine part X

  • Mandrel
  • Wire feed
  • Wire guides
  • Spool carrier

Q18: What is the electrical degree of 6 pole stator of motor?

  • 360°
  • 720°
  • 1080°
  • 1440°
Show Calculation The total electrical degrees in a motor is calculated as: Total Electrical Degrees = $180^\circ \times \text{Number of Poles}$ * Number of poles ($P$) = $6$ $\text{Total Electrical Degrees} = 180^\circ \times 6$ $\text{Total Electrical Degrees} = 1080^\circ$

Q19: Calculate the number of coils per phase per pair of poles of 3 phase motor having 2 pole, 24 slots, 12 coils?

  • 1
  • 2
  • 3
  • 4
Show Calculation $N_{cpp} = \frac{\text{Total Number of Coils}}{\text{Number of Phases} \times \text{Number of Pole Pairs}}$ **Given:** * Total Number of Coils = $12$ * Number of Phases = $3$ * Number of Poles ($P$) = $2$ A pair consists of 2 poles (North and South). $\text{Pole Pairs} = \frac{P}{2} = \frac{2}{2} = 1$ Substitute the values into the main formula: $N_{cpp} = \frac{12}{3 \times 1}$ $N_{cpp} = \frac{12}{3}$ $N_{cpp} = 4$

Q20: What is the name of the starter symbol?

starter symbol diagram

  • Star delta starter
  • Rheostatic starter
  • Direct on-line starter
  • Autotransformer starter

Q21: What is the formula to calculate pitch factor?

  • Pitch Factor = Pole pitch / Winding pitch
  • Pitch Factor = Winding pitch / Pole pitch
  • Pitch Factor = Number of slots / Number of poles
  • Pitch Factor = Number of Poles / Number of Slots

Q22: How pole pitch is measured in terms of slots in AC winding?

  • Total electrical degree / Number of slots
  • Number of slots / Total electrical degree
  • No. of slots in the stator / No. of poles
  • No. of poles / No. of slots in the stator

Q23: What is the formula to calculate the mean circumference of the coil?

  • $L_m = L_{out} - L_{in} / 2$ cm
  • $L_m = (L_{in} + L_{out}) / 2$ cm
  • $L_m = 2 \times (L_{out} - L_{in})$ cm
  • $L_m = 2 \times (L_{in} + L_{out})$ cm

Q24: What is the synchronous speed of a A.C 3 phase induction motor having 6 poles at a frequency of 50 Hertz?

  • 800 rpm
  • 1000 rpm
  • 1200 rpm
  • 1440 rpm
Show Calculation The synchronous speed ($N_s$) of an AC induction motor is calculated as: $N_s = \frac{120 \times f}{P}$ Where: * $f$ = Frequency in Hertz * $P$ = Number of poles **Given:** * Frequency ($f$) = $50 \text{ Hz}$ * Number of poles ($P$) = $6$ **Calculation:** $N_s = \frac{120 \times 50}{6}$ $N_s = \frac{6000}{6}$ $N_s = 1000 \text{ rpm}$

Q25: Calculate the percentage slip in a 3 phase induction motor having 6 poles with a frequency of 50 Hertz rotating with actual speed of 960 rpm?

  • 2%
  • 3%
  • 4%
  • 5%
Show Calculation **Formula:** $N_s = \frac{120 \times f}{P}$ **Given:** * Frequency ($f$) = $50 \text{ Hz}$ * Poles ($P$) = $6$ **Calculation:** $N_s = \frac{120 \times 50}{6}$ $N_s = \frac{6000}{6}$ $N_s = 1000 \text{ rpm}$ **Formula:** $\text{ Slip} = \frac{N_s - N_r}{N_s} \times 100$ $\text{ Slip} = \frac{1000 - 960}{1000} \times 100$ $\text{ Slip} = \frac{40}{1000} \times 100$ $\text{ Slip} = 0.04 \times 100$ $\text{ Slip} = 4\$%

Q26: What is the rotor frequency of a 3 phase squirrel cage induction motor at the time of starting?

  • Equal to supply frequency
  • 3 times less than supply frequency
  • 3 times more than supply frequency
  • $\sqrt{3}$ times less than supply frequency

Q27: How the voltage is received in the rotor of induction motor?

  • Direct connection from stator
  • Due to back emf produced in stator
  • Direct connection to rotor from supply
  • By the transformer action of stator and rotor

Q28: Which method is applied to control the speed of 3 phase squirrel cage induction motor from its rotor side?

  • Cascade operation
  • Changing applied voltage
  • Changing applied frequency
  • Changing the number of poles

Q29: Which loss of 3 phase induction motor is determined by blocked rotor test?

  • Copper loss
  • Friction loss
  • Hysteresis loss
  • Eddy current loss

Q30: Why pre heating is necessary for motors before varnishing in rewinding process?

  • To dry the varnish quickly in winding
  • To easy flow of varnish in the winding
  • To increase the insulation resistance value
  • To drive out the moisture in between winding layers

Q31: Which type of test is conducted using internal growler in AC motor winding?

  • Ground test
  • Polarity test
  • Continuity test
  • Short circuit test

Q32: Which device is used to test stator winding short and open fault?

  • Tong Tester
  • Internal Growler
  • External Growler
  • Digital multimeter

Q33: What is the purpose of using thermal cutout in addition to fuse in A.C motor circuit?

  • Protect from heavy load
  • Protect against high voltage
  • Allow for continuous over loading
  • Protect against dead short circuit

Q34: Which type of motor is used to provide high starting torque at variable speed?

  • Universal motor
  • Permanent capacitor motor
  • 3 Phase slip ring induction motor
  • 3 Phase single squirrel cage induction motor

Q35: What is the relation between torque and slip in an A.C induction motor?

  • Slip increases torque decreases
  • Slip increases torque increases
  • Slip decreases torque increases
  • Slip decreases torque decreases

Q36: What is effect of A.C induction motor if rotor bar is in open circuit?

  • Vibration of shaft
  • Motor will not start
  • Runs in slow speed
  • Over heating of motor

Q37: Which type of wire is used for rewinding of A.C 3 phase motors?

  • Super enamelled copper wire
  • PVC covered copper winding wire
  • Single cotton covered copper wire
  • Double cotton covered copper wire

Q38: Which material is used as wedges in winding process?

  • Empire
  • Cotton
  • Bamboo
  • Terylene

Q39: Which test in winding is essential before giving supply?

  • Ground test
  • Polarity test
  • Open circuit test
  • Short circuit test

Q40: Why the rotor bars are mounted in a slightly skewed position in 3 phase motor?

  • Generate maximum flux
  • Reduce the stray losses
  • Maintain the rotor speed constant
  • Produce more uniform rotor field and torque

Q41: Which loss is determined by no load test of 3 phase induction motor?

  • Iron loss
  • Copper loss
  • Friction loss
  • Windage loss

Q42: Which method of speed control two variable speeds only obtained in 3 phase motor?

  • By rotor rheostat control
  • By changing applied frequency
  • By changing the applied voltage
  • By changing the number of stator poles

Q43: Why slip ring induction motor is fitted with wound rotor?

  • To reduce the slip
  • To control the speed
  • To reduce the losses
  • To get high starting and running torque

Q44: What is the function of timer in automatic star delta starter?

  • Trip at over load
  • Switch ON at pre set time
  • Change from star to delta
  • Switch OFF at pre set time

Q45: Which instrument is used to measure insulation resistance of a 3 phase induction motor?

  • Megger
  • Multimeter
  • Shunt type ohmmeter
  • Series type ohmmeter

Q46: Which test in winding is illustrated?

winding test illustrated

  • Polarity test
  • Ground test
  • Continuity test
  • Short circuit test

Q47: What is the starting current of an A.C 3 phase induction motor?

  • 1 to 2 times of full load current
  • 2 to 3 times of full load current
  • 4 to 5 times of full load current
  • 5 to 6 times of full load current

Q48: Which method is used to control the speed of 3 phase induction motor from stator side?

  • By cascade operation
  • By rotor rheostat control
  • By injecting emf in rotor circuit
  • By changing the applied frequency

Q49: What is the speed control method of 3 phase induction motor?

speed control method diagram

  • Cascade operation method
  • Rotor rheostat control method
  • Changing applied voltage method
  • Injecting emf in rotor circuit method

Q50: What are the two functional circuits incorporated with a three phase motor starter?

  • Open circuit and short circuit
  • Closed circuit and open circuit
  • Short circuit and closed circuit
  • Control circuit and power circuit

Q51: Which is the main property of leatheroid paper insulation?

  • Non moisturized material
  • Highly non-hygroscopic
  • Very good for class F insulation
  • Better ageing and dielectric strength

Q52: Which type of insulating material is selected for binding the coils and over hangs?

  • Cotton sleeves
  • Empire sleeves
  • Terylene thread
  • Fibre glass tape

Q53: Which insulation is used for cuffing in AC winding?

  • Fibre glass tape
  • Leatheroid paper
  • Empire fiber glass tape
  • Fabric based adhesive tape

Q54: What refers coil in AC winding?

  • Number of turns connected in series
  • Number of turns connected in parallel
  • Number of turns under two similar poles
  • Number of turns under two dissimilar poles

Q55: Which type of AC winding the number of coil/pole/phase is more than one at different pitches?

  • Involute coil winding
  • Diamond coil winding
  • Flat loop over lapped winding
  • Flat loop non-over lapped winding

Q56: Calculate the number of coils /phase/pole for a 3 phase double layer distributed winding for a motor having 36 slots, 36 coils and 4 poles.

  • 3 coils/phase/pole
  • 6 coils/phase/pole
  • 9 coils/phase/pole
  • 12 coils/phase/pole
Show Calculation **Formula:** $\text{Coils per Phase per Pole} =$ $\frac{\text{Total Number of Coils}}{\text{Number of Phases} \times \text{Number of Poles}}$ **Given:** * Total Number of Coils = $36$ * Number of Phases = $3$ * Number of Poles = $4$ **Calculation:** $\text{Coils per Phase per Pole} = \frac{36}{3 \times 4}$ $\text{Coils per Phase per Pole} = \frac{36}{12}$ $\text{Coils per Phase per Pole} = 3$

Q57: What is the type of rewinding process?

rewinding process type

  • Hand winding
  • Skein winding
  • Former winding
  • Machine winding

Q58: Which type of starter is used to start and run the 3 phase slip ring induction motor?

  • Direct on-line starter
  • Rotor rheostat starter
  • Auto transformer starter
  • Manual star-delta starter

Q59: What is the function of collar?

  • Provides insulation around field
  • Provides insulation for coil tapping
  • Helps tightening material for flange
  • Provides insulation for heat transfer from coil

Q60: Which type of winding wire is used to wind submersible pump motors?

  • PVC covered type
  • Terylene thread type
  • Super enamelled type
  • Double cotton covered type

Q61: What is the reason of long chord winding is avoided in AC motors?

  • Low efficiency
  • Low starting torque
  • More winding wire required
  • Less heat dissipation

Q62: Which type of winding has more space for cooling?

  • Between overhanging coils
  • Between overhanging coil and rotor
  • Between overhanging coils and yoke
  • Between overhanging coil and wedge

Q63: Where the panel boards are used?

  • Industrial motor drives
  • Domestic wiring circuits
  • 3 phase domestic wiring
  • Load distribution for AC & DC supply

Q64: Determine the torque in newton metres produced by a 7.5 HP squirrel cage motor rotating at 1440 rpm?

  • 21.63 Nm
  • 24.4 Nm
  • 33.05 Nm
  • 36.6 Nm
Show Calculation $1 \text{ HP} = 735.5 \text{ Watts}$ $P = 7.5 \times 735.5$ $P = 5516.25 \text{ Watts}$ $T = \frac{P \times 60}{2 \pi N}$ Where: * $P$ = Power in Watts * $N$ = Speed in RPM * $T$ = Torque in Newton Metres ($Nm$) $T = \frac{5516.25 \times 60}{2 \times 3.14159 \times 1440}$ $T = \frac{330975}{9047.78}$ $T \approx 36.58 \text{ Nm}$ (Rounding to one decimal place typically gives 36.6 Nm).

Q65: Which type of handle design of rotary switch is illustrated?

rotary switch handle design

  • Knob
  • Lever
  • Coin slot
  • Key operation

Q66: What is the purpose of using rotor resistance starter to start 3 phase slip ring induction motor?

  • Reduce rotor voltage
  • Reduce rotor current
  • Increase the torque
  • Reduce the power loss

Q67: Which method of speed control is only applicable for 3 phase slipring induction motor?

  • Cascade operation method
  • Rotor rheostat speed control
  • Changing the applied frequency method
  • Changing the number of stator poles method

Q68: What is the name of the winding?

winding type illustrated

  • Skew winding
  • Skein winding
  • Involute coil winding
  • Diamond coil winding

Q69: What is the name of 3 phase motor winding, if the coil pitch is less than pole pitch?

  • Full pitch winding
  • Whole coil winding
  • Long chorded winding
  • Short chorded winding

Q70: Which is the demerit of 3 phase concentric winding?

  • More space is required
  • A stepped former is required
  • More difficult to shape the coils uniformly
  • It is not easy to make the end connection

Q71: What is the name of the diagram used for 3phase motor winding?

motor winding diagram type

  • Ring diagram
  • Development diagram
  • Coil connection diagram
  • End connection diagram

Q72: Calculate the phase displacement in terms of slots for a 3 phase, 36 slots, 12 coils, 4 pole stator winding?

  • 3 slots
  • 4 slots
  • 6 slots
  • 8 slots
Show Calculation $\theta_{total} = 180^\circ \times \text{Number of Poles}$ $\theta_{total} = 180^\circ \times 4$ $\theta_{total} = 720^\circ$ $\theta_{slot} = \frac{\text{Total Electrical Degrees}}{\text{Number of Slots}}$ $\theta_{slot} = \frac{720^\circ}{36}$ $\theta_{slot} = 20^\circ \text{ per slot}$ For a 3-phase system, the displacement is always $120^\circ$ electrical. $\text{Phase Displacement (slots)} = \frac{120^\circ}{\theta_{slot}}$ $\text{Phase Displacement (slots)} = \frac{120^\circ}{20^\circ}$ $\text{Phase Displacement (slots)} = 6 \text{ slots}$

Q73: Which type of AC motor winding having the number of coil/pole/phase is more than one arranged in different slots?

  • Basket winding
  • Concentric winding
  • Distributed winding
  • Concentrated winding

Q74: Which type of testing of winding is illustrated?

testing of winding illustrated

  • Polarity test
  • Resistance test
  • Short circuit test
  • Voltage drop test

Q75: Why external resistance is included in the rotor circuit at starting through 3 phase slipring induction motor starter?

  • To get high running torque
  • To get high starting torque
  • To reduce the load current
  • To get increased speed at starting

Q76: What is the effect of motor, if the rotor windings in slipring induction motor is open circuited at starting?

  • Will not run
  • Runs at slow speed
  • Runs at very high speed
  • Runs but not able to pull load

Q77: What happens to a 3 phase induction motor if one phase fails during running?

  • Motor runs normally
  • Motor stop instantaneously
  • Motor runs slowly, finally it burns
  • Motor runs with irregular speed

Q78: What is the effect on 3 phase induction motor if one phase is cut-off during running with load?

  • Motor stops at once
  • Motor will run normally
  • Motor runs with humming noise with slow speed
  • Motor will run slow speed but winding will be burnt out shortly

Q79: What is the defect, if starter with single phasing preventer does not switch 'ON'?

  • Improper phase sequence
  • Fluctuations in line voltage
  • Loose contact in supply lines
  • Wrong terminal connections at motor

Q80: What is the defect in AC 3 phase induction motor runs at low speed if loaded?

  • Wrong motor connection
  • Wrong starter connection
  • Open circuit in rotor winding
  • Partially shorted stator winding

Q81: Which fault condition thermal overload relay protects A.C induction motor?

  • Short circuit
  • Open circuit
  • Over current
  • Under voltage

Q82: What happens to the rotor of a 3 phase induction motor if its speed attains to synchronous speed?

  • Rotor speed reduces
  • Rotor speed increases
  • Rotor speed remains same
  • Rotor bars get damaged

Q83: What is the effect of open circuit in rotor of an induction motor?

  • Motor does not start
  • Over heating in motor
  • Excess vibration of shaft
  • Motor runs with very low speed

Q84: What is the reason for frequent blowing of fuse after motor running some time?

  • Improper earthing
  • Over loading of motor
  • Heavy voltage fluctuation
  • Poor insulation in winding

Q85: What happens to a 3 phase induction motor, if one phase fails during starting?

  • Motor runs and stop immediately
  • Motor runs in slow speed continuously
  • Motor runs and draws more current
  • Motor continues to run with irregular speed

Q86: Which is the cause for the 3 phase motor starter with single phase preventer trips frequently?

  • Incorrect fuse ratings
  • Unbalanced line voltage
  • Incorrect settings of OLR
  • Improper phase sequence

Q87: What indication denotes the shorted coil defect in 3 phase motor stator winding while testing with internal growler by keeping hacksaw blade?

  • Hacksaw blade gets over heated
  • Rapid vibration of hacksaw blade
  • Hacksaw blade repels against the slots
  • Attracted by the winding turns on the slot
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2nd year, Trade Theory
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