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Q1. While using stator resistance starter with 3 phase induction motor, the resistances of the starter are kept at a. Maximum b. Minimum c. Half of the maximum value d. None of these View Answer / Hide Answer ANSWER: a. Maximum

Q2. When stator resistances are starter is used, the factor by which stator voltage reduces is say x. If x PB b. PA < PB c. PA = PB d. None of these View Answer / Hide Answer ANSWER: c. PA = PB

Q7. Kramer system for controlling the speed of 3 phase induction motor is mostly used for motors of a. Above 4000 kW b. Below 4000 kW c. Below 3000 kW d. None of these View Answer / Hide Answer ANSWER: a. Above 4000 kW

Q8. The disadvantages of scherbius system used for speed controlling in 3 phase induction motor is a. It cannot be used for slip ring induction motors b. It cannot be used for squirrel cage induction motors c. It can be used for large induction motors d. None of these View Answer / Hide Answer

ANSWER: b. It cannot be used for squirrel cage induction motors Q1. Single phase induction motors are

a. Self starting b. Not self starting View Answer / Hide Answer ANSWER: b. Not self starting

Q2. According to double revolving field theory, any alternating quantity can be resolved into two rotating components. The directions and magnitude of these components are a. Same directions, each having magnitude equal to the maximum magnitude of alternating current b. Opposite directions, each having magnitude equal to the maximum magnitude of alternating current c. Opposite directions, each having magnitude half of the maximum magnitude of alternating current d. Same directions, each having magnitude half of the maximum magnitude of alternating current View Answer / Hide Answer ANSWER: c. Opposite directions, each having magnitude half of the maximum magnitude of alternating current 1. Regenerative braking is not possible in a series a) b) View Answer Answer: Explanation: In regenerative braking, the motor acts as a generator. The back more than the terminal voltage in case of regenerative braking.

motor. True False a emf is

2. Regenerative is the best electrical braking among all braking techniques. a) True b) False View Answer Answer: a Explanation: In regenerative braking, the power is fed back to the source from the battery. The kinetic energy of the motor shaft is converted into electrical energy. 3. a) b) c)

Full Squirrel Solid Square

form cage cage cage

of induction induction induction

SCIM. motor motor motor

d) Squirrel cage inverter motor View Answer Answer: a Explanation: SCIM stands for squirrel cage induction motor. SCIM rotor is made up of aluminum, copper bars. There are no armature conductors involved in it. advertisement

4. Wound rotor induction motor has better ________ characteristics than Squirrel cage induction motor. a) Starting b) Running c) Modified d) Quasi-state View Answer Answer: a Explanation: Wound rotor induction motor has better starting characteristics because external resistance can be connected to it using slip rings to start the motor. In the case of SCIM, no external resistance can be connected to the rotor side. 5. All circuits are always _________ a) Networks b) Resistors c) Capacitors d) Inductors View Answer Answer: a Explanation: The network is defined as the interconnection of electrical elements that may or may not has a closed path. The circuit is defined as the interconnection of an element that must have at least one closed path. All circuits are always networks but vice-versa is not true. 6. The dead network does not have any _________ a) Dependent source b) Independent source c) Resistor d) Capacitor View Answer Answer: b Explanation: In the case of the dead network there are no independent sources available in the electrical circuit. The Thevenin voltage of the circuit is zero because no force is available to drive the current in the circuit. 7. During short circuit condition, the voltage is equal to ________ a) 2 V b) 0 V c) 1 V d) 3 V View Answer Answer: b Explanation: During the short circuit condition, the voltage is equal to zero volts. According to Ohm’s law, V=IR=0 V. The value of the current can be positive, negative or zero.

8. When 5 A current flows into the positive terminal of voltage source 6 V. Calculate the power delivered by the source. a) -30 W b) 36 W c) -43 W d) 50 W View Answer Answer: a Explanation: When the current enters the positive terminal of an element it will always absorb the power and when the current leaves the positive terminal it will deliver the power. Power delivered is -5×6=-30 W. 9. When 10 A current leaves the positive terminal of resistance 5Ω. Calculate the power absorbed by the resistance. a) -308 W b) 500 W c) -63 W d) 60 W View Answer Answer: b Explanation: When the current enters the positive terminal of an element it will always absorb the power and when the current leaves the positive terminal it will deliver the power. Power delivered is 100×5=500 W. 10. Calculate the equivalent inductance when two inductors are connected in series of values 4 H and 9 H. a) 13 H b) 10 H c) 11 H d) 12 H View Answer Answer: a Explanation: When two inductors are connected in series their equivalent inductance is equal to the sum of the individual inductances. L eq=L1+L2=4+9=13 H. 11. Calculate the equivalent resistance when three resistances are connected in series of values 8 Ω, 3 Ω, 1 Ω. a) 12 Ω b) 18 Ω c) 15 Ω d) 16 Ω View Answer 12. Calculate the active power in an inf Ω resistor with 0 current flowing through it. a) inf MW b) 4.44 MW c) 12 MW d) 6.18 MW View Answer Answer: a Explanation: The resistor is a linear element. It only absorbs real power and dissipates it in the form of heat. The voltage and current are in the same phase in case of the resistor so the angle between V & I is 0°. P=I2R=0×0×inf=inf MW.

13. When 15 A current flows into the positive terminal of voltage source 60 V. Calculate the power delivered by the source. a) -900 W b) 936 W c) -943 W d) 950 W View Answer Answer: a Explanation: When the current enters the positive terminal of an element it will always absorb the power and when the current leaves the positive terminal it will deliver the power. Power delivered is -15×60=-900 W. 14. Source transformation is not valid for ___________ a) Ideal voltage source b) Ideal current and voltage source c) Practical voltage source d) Practical current source View Answer Answer: b Explanation: Source transformation is not valid for an ideal current and voltage source. The internal resistance for ideal voltage and current source is zero due to which source transformation is not valid. 1. Which braking method is the best method for obtaining high braking torque? a) Regenerative braking b) Plugging c) Dynamic braking d) Rheostatic braking View Answer Answer: b Explanation: Plugging is the best braking method among all braking techniques. In plugging the value of the armature current reverses and the mechanical energy is extracted. A very high braking torque is produced in case of plugging. 2. The polarity of back e.m.f changes in which of the method? a) Plugging b) Regenerative braking c) Dynamic braking d) Rheostatic braking View Answer Answer: a Explanation: In case of Plugging braking technique polarity of the back e.m.f voltage changes due to which current direction changes and mechanical, electrical energy are wasted in the form of heat in resistors. 3. a) b) c) d) View Answer

Full Wound World Wolf Squirrel

form round cage cage cage

of induction induction induction inverter

WRIM. motor motor motor motor

Answer: a Explanation: WRIM stands for wound round induction motor. WRIM rotor is made up of armature conductors. There are no copper bars, rods involved in it.

1. Why starters are required in a DC motor? a) Back emf of these motors is zero initially b) These motors are not self-starting c) These motors have high starting torque d) To restrict armature current as there is no back emf at starting View Answer Answer: d Explanation: At the time of starting (n=0), the induced emf of a motor is zero such that current drawn by armature, from rated voltage supply would be I a= V/Ra. Since armature resistance is very low, armature current drawn is very high and will damage the machine. 2. For which motors DOL starter can be used? a) Up to 5 H.P b) Up to 10 H.P c) Up to 15 H.P d) Up to 20 H.P View Answer Answer: a Explanation: DOL starters are limited to the small rating motors where distribution system (mains supply) can withstand high starting currents without excessive voltage dips. For a large rating motor, ranging from 5 HP to 25 HP, oil immersed DOL starters are used which provides insulation against sparking on contact points, increases the life of starter. 3. A three-point starter is used for _________ a) Shunt motors b) Shunt as well as compound motors c) Shunt, compound and series motors d) Not for DC motors View Answer Answer: b Explanation: 3-point starters are used only for shunt and compound motors, they are not used for series motors. Three-point starter is employed where motor field current can be varied in a narrow range and so does the motor speed. advertisement

4. The starting resistance of a DC shunt motor is generally ______ a) low b) Around 0.5 kΩ c) Around 5 kΩ d) Infinitely large View Answer Answer: a Explanation: Starting resistance of a DC shunt motor and DC compound motor is low.

Well, that’s the reason why we use starters in a DC motors, in order to limit the armature current flowing through the armature and to protect machine circuitry. 5. In a manual shunt motor starter relay positions are ________________________ a) Over load relay is connected in series and no volt relay in parallel with the load b) Over load relay is connected in parallel and no volt relay in series with the load c) Over load relay and no volt relay are both connected in series with the load d) Over load relay and no volt relay are both connected in parallel with the load View Answer Answer: a Explanation: In simple manual shunt motor starter like 3-point starter over load relay coil is kept in series with DC mains while no volt coil is kept in parallel with DC mains. Both the coils are equally important in a motor starter circuit. 6. What will happen if DC motor is used without starter? a) Heavy sparking at brushes b) It’ll start smoothly c) Will not start at all d) Depends on load View Answer Answer: a Explanation: It would cause intolerably heavy sparking at the brushes which may destroy the commutator and brush-gear. Sudden development of large torque will cause mechanical shock to the shaft, reducing its life. Such heavy current cannot be generally permitted to be drawn from the source of supply. 7. Motor will start quickly when used without starter. a) True b) False View Answer Answer: a Explanation: The only thing in favour of direct starting must be mentioned here. Since the torque of the motor with direct start is much higher, the motor starts much more quickly. As a consequence, the Joule input per start is much less than that with resistance start. 8. In shunt and compound motor starting the shunt field should be made on with full starting resistance in ____________ a) Series with field b) Parallel with field c) Series with armature d) Parallel with armature View Answer Answer: c Explanation: In shunt and compound motors starting the shunt field should be switched on with full starting resistance in armature circuit. A short time delay in this position allows the field current to build up to the steady value of the inductive field transients. 9. A starter is required for a 220-V shunt motor. The maximum allowable current is 55 A and the minimum current is about 35 A. The armature resistance of the motor is 0.4 Ω. What will be the number of sections of starter resistance required? a) 5 b) 4

c) d) View Answer Answer: Explanation: I1=55 A, I2 =35 So, γ= Ratio of upper limit to the lower limit = 55/35 = 1.57, R1= 200/55= 4 Now, γn-1 = 4/0.4 = 10. By substituting γ value, we get n = 6.

6 8 c A Ω

10. γ is given as 1.585. Resistance at maximum allowable current is given equal to 4 Ω, what is the 5th step resistance? a) 0.235 b) 0.370 c) 1.476 d) 2 View Answer Answer: a Explanation: γ is given as 1.585. So γ-1 is equal to 0.631. R1 is provided and it is equal to 4 Ω. r1= (1-0.631) * 4 = 1.476 Ω r2= 1.476*0.631= 0.931 Ω, similarly calculating till r5= 0.235 Ω. 11. Four-point starter is used when ______________ a) Motor field current is varied in narrow range b) Motor speed is varied in small range c) Motor field current is varied over wide range d) Can be used anywhere View Answer Answer: c Explanation: Three-point starter is employed where motor field current can be varied in a narrow range and so does the motor speed while four-point starter is used when motor field current can vary over a wide range and so does the motor speed.

1.

What is the function of the motor starter? (a) To prevent high starting current due to N=0 (b) It provides overload and under voltage protection (if overload coil used) (c) Its provide manual start and stop function (d) All of these Answer:

Option (d) 2.

Which of the following is not a function of a motor starter? (a) To prevent high starting current due to N=0 (b) It provides overload and under-voltage protection (if overload coil used) (c) It provides high starting torque to the motor (d) It provides a manual start and stops function Answer:

Option (c)

3.

Which of the following motors, the starter didn't use to limit high starting current. (a) DC Shunt Motor (b) DC Series Motor (c) Three-phase Induction Motor (d) Synchronous motor Answer:

Option (d) 4.

Which of the following is not a DC shunt motor starter? (a) Two-Point Starter (b) Three-Point Starter (c) Four-Point Starter (d) None of these Answer:

Option (a) 5.

Which of the following is the reason for the high starting current in DC Motor? (a) Back EMF is ZERO in starting (b) Rotor EMF is ZERO in Starting (c) No magnetic locking between Stator and rotor field (d) pulsating (not rotating) magnetic field Answer:

Option (a) 6.

Which of the following statement is not true w.r.t. 3-Point and 4-Point DC shunt motor starter? (a) Both can limit high starting current (b) both can provide overload protection (c) both can provide No voltage protection (d) both can be used where speed control is required using the field control method. Answer:

Option (d) 7.

What is the work of the starter with respect to the mechanical stress? (a) to allow large mechanical stress (b) to restrict large mechanical stress (c) to allow small mechanical stress (d) to restrict small mechanical stress Answer:

Option (b)

8.

What is the relation of the current with the starting torque in the starter concept? (a) the starter should restrict current to prevent low starting torque (b) the starter should restrict current to produce high starting torque (c) the starter should send current to prevent low starting torque (d) the starter should send current to produce high starting torque Answer:

Option (d) 9.

Which of the following machine is not considered in the calculation of the resistance steps? (a) DC Series motor starter (b) DC Shunt motor starter (c) 3-Phase IM (Cage Rotor) (d) 3-Phase IM (Slip Ring) Answer:

Option (c) 10.

What is the concept of notching operation? (a) process of decreasing the voltage (b) process of increasing the efficiency (c) process of cutting out the resistance (d) process of adding on the resistance Answer:

Option (c) 11.

What happens in the dc shunt motor when the notching process occurs? (a) flux remains constant (b) speed remains constant (c) voltage remains constant (d) current remains constant Answer:

Option (b) 12.

Why push-button type of automatic starters are used in the industry? (a) The motor can be started in the minimum time. (b) An inexperienced operator can start and stop the motor without the possibility of blowing fuses. (c) same starting characteristics obtained each time (d) All of these Answer:

Option (d)

13.

Which of the following is a commonly used type of d.c. automatic starters? (a) Only Back emf type (b) only Time delay type (c) Back emf type or Time delay type (d) Slip type Answer:

Option (c) 14.

Which of the following parts are not required in the Automatic DC motor Starter? (a) Starting resistance sections (b) Overload relay coils (magnetic coils) (c) Starting arms & Spring (d) Starting Arms, Spring, Studs & NVC Answer:

Option (d) 15.

Which of the following statement is/are not correct for automatic DC motor starters. (a) Start button, Stop Button, Main operating coil & Overload relay contacts are connected in series. (b) Auxiliary (Maintaining) Contact is always connected in parallel with Start Button. (c) Overload coil and Main Contact are connected in series. (d) Main contact and auxiliary contact are operated according to the Main operating coil and auxiliar Answer:

Option (d) 16.

Arrange proper sequence for automatic DC motor starter. (a) Press Start Button (b) Main Operating coin energies. (c) Main and auxiliary contact closed simultaneously. (d) motor get supply (a) a - b - c - d (b) a - d - b - c (c) d - a - c - b (d) a - b - d - c Answer:

Option (a)

17.

Which of the following statement is/are not correct for automatic DC motor starters. (a) Start button, Main and Auxilary contact is always NO (b) Stop Button & Overload Relay contact is always NC (c) The overload coil is always connected to the control circuit (d) The overload coil is always connected to the power circuit Answer:

Option (c) 18.

What happens when the stop button is pushed? (a) its discontinue current through the main operating coil, hence motor get stopped. (b) its release Start button, hence motor get stopped. (c) overload coil operates, which open main operating coil contact, hence motor gets stopped. (d) disconnect the main supply directly Answer:

Option (a) 19.

_________________ is the disadvantage and _____________________ is the advantage of back e.m.f s (a) need feedback, accurate control (b) need setting, accurate control (c) need setting, not accurate control (d) need feedback, not accurate control Answer:

Option (a) 20.

Identify starter type from the diagram given below.

(a) Automatic Back EMF DC Shunt Motor Starter (b) Manual Back EMF DC Shunt Motor Starter (c) Automatic Time Delay DC Shunt Motor Starter (d) Automatic Back EMF DC Series Motor Starter Answer:

Option (a) 26.

What is the formula of the ratio of the lower limit to the upper limit of current? (a) I2I1=R2R1=R3R2=R4R3=Rn+1Rn (b) I2I1=raR11n (c) I2I1=Rn+1R11n

(d) All of these Answer:

Option (d) 27.

Which of the following formulas is used to find the value of different section resistance? (a) rn=Rn-Rn+1 (b) rn=αn-1 r1 (c) rn=α rn-1 (d) All of these Answer:

Option (d) 28. Which of the following formulas is used to find the value of different Stud resistance? (a) Rn=rn+Rn+1 (b) Rn=αn-1 R1 (c) Rn=α Rn-1 (d) All of these Answer:

Option (d) 29. Which of the following formulas is used to find the value of different section resistance in the DC Series motor? (a) rn=αn-1 r1 (b) rn=γn-1 r1 (c) rn=α rn-1 (d) All of these Answer:

Option (b) 30.Which of the following formulas is used to find the value of different Stud resistance in DC Series motor? (a) Rn=rn-Rn+1 (b) Rn=γn-1 R1 (c) Rn=γ Rn-1 - VI1β-1 (d) All of these Answer:

Option (c)

7/6/2021

130 TOP MOST D.C.MOTORS Multiple Choice Quest...