3.18: AC Motors Flashcards
- Why are two-phase ac servomotors made long and narrow?
(a) to make it take less current from the supply
(b) to increase the starting torque and lower the inertia
(c) to reverse its direction of rotation.
(b) to increase the starting torque and lower the inertia
- A synchronous motor runs at
(a) Ns
(b) Nr
(c) Ns - slip
(c) Ns
There is no slip in a synchronous motor
- A stator rotating field applies to:
(a) only synchronous motors,
(b) only induction motors,
(c) both synchronous and induction motors
(c) both synchronous and induction motors
'’A synchronous electric motor is an AC electric motor in which, at steady state, the rotation of the shaft is synchronized with the frequency of the supply’’
- The direction of the induced emf in a conductor moving in a magnetic field is the:
(a) Left-hand generator rule
(b) Right-hand generator rule
(c) Left-hand rule for solenoids
(b) Right-hand generator rule
Ryan Giggs - Right hand rule for Generators which is said in answer
Lionel Messi - Left hand drive for motors
- The expression ‘slip’ applies to:
(a) only synchronous motors.
(b) only induction motors.
(c) both synchronous and induction motors.
(b) only induction motors.
Ns = Synchronous speed of magnetic induction field
Nrotor = like armature - part that is rotating by induction in centre
Difference between rotor speed and stator speed is known as slip
Note: Synchronous Machine operates at zero slip because both elements arent moving
- Which type of motor does not employ a squirrel cage rotor for normal running?
(a) Synchronous motor,
(b) Induction Motor
(c) Split phase induction motor.
(a) Synchronous motor -
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- The difference between field rotation speed and the speed of the rotor in an
induction motor is known as the:
(a) Phase speed
(b) Phase lag
(c) Slip speed
(c) Slip speed
- To improve the starting torque of an AC Inductive motor the:
(a) Rotor resistance is decreased.
(b) rotor power factor is increased,
(c) rotor inductance is increased.
(a) Rotor resistance is decreased.
- The A-Phase of a three phase power system is normally colour coded:
(a) Yellow
(b) Red
(c) Blue
(b) Red
Phase A (1)= Red
Phase B (2)= yellow
Phase C (3)= blue
RYB
- Which of the following statements is true about a three-phase squirrel-cage
induction motor?
(a) A three-phase supply is connected to the rotor
(b) It has no external connections to its rotor
(c) It has a high starting torque
(b) It has no external connections to its rotor
- Which of the following statements is true about a three-phase squirrel-cage
induction motor?
(a) the input is connected to the rotor
(b) there are no external connections to the rotor
(c) the input is connected to a set of slip rings.
(b) there are no external connections to the rotor
- Which of the following statements is true about a three-phase induction motor?
(a) The speed of rotation of the magnetic field is called the rotational speed
(b) A three-phase supply connected to the rotor produces a rotating magnetic field
(c) It is essentially a constant speed motor
(c) It is essentially a constant speed motor
- A 3-Phase motor is turning at its operating speed with a light load. If the start
winding opened, at this point in time, the motor would:
(a) Continue to run
(b) Increase speed
(c) Stop turning
(a) Continue to run
This is normal operation, very often the start winding is opened by a centrifugal switch when the motor reaches operational speed
- The phase voltage of a three-phase star-connected system is 100V. The line
voltage is :
(a) 173V
(b) 137V
(c) 200V
(a) 173V
Star formula:
Line voltage = phase voltage * 1.73
- A three-phase star-connected system has a line current of 40A. The phase
current is :
(a) 24A
(b) 40A
(c) 50A
(b) 40A
'’We know in the star connection, line current is the same as phase current’’
- In a three-phase brushless alternator, the output is taken from:
(a) The exciter stator
(b) The main generator rotor
(c) The main generator stator
(c) The main generator stator
Stator which is the armature
- A single phase is disconnected from a three phase a.c. motor while running. It will:
(a) stop immediately.
(b) continue to run but fail to start from a stationary condition.
(c) run at half speed.
(b) continue to run but fail to start from a stationary condition.
- In a synchronous motor the rotor is:
(a) excited by d.c.
(b) excited by a.c.
(c) self excited.
(a) excited by d.c.
Synchronous motors are constant speed motors
The rotor is energized with DC (it acts like a bar magnet /alternatively can be bar magnet)
- The difference between synchronous speed and rotor speed is expressed in:
(a) in revolutions per minute.
(b) in Hertz.
(c) as a percentage
(c) as a percentage
Synchronous speed is the speed of the magnetic field
Speed of rotor is the speed of squrriel cage piece
- Doubling the running time of an electrical machine would
(a) double the joules used
(b) double the watts used
(c) double the current used
(a) double the joules used
It is also the energy dissipated as heat when an electric current of one ampere passes through a resistance of one ohm for one second
- A capacitor in a single phase motor is to
(a) provide a phase shift
(b) prevent sparking at the switch
(c) provide smoothing
(a) provide a phase shift
495 A single phase is disconnected from a three phase A.C. motor while running, this
will result in
(a) A rapid increase in rotor speed
(b) It will cause the motor to run at 2/3 times normal speed
(c) The motor will not stop but will not restart once stopped
(c) The motor will not stop but will not restart once stopped
496 A three phase high powered induction motor fails to start, it only makes a loud,
unpleasant, low frequency noise when power is applied, the most likely cause is;
(a) Two of the stator windings have been switched
(b) One or more of the stator phases are open circuit
(c) The rotor windings have been reversed
(b) One or more of the stator phases are open circuit
497 In a 3-phase motor the field windings are
(a) Wound on salient poles
(b) Wound on shaded poles
(c) Wound so that they are electrically at 120 degrees from each other
(c) Wound so that they are electrically at 120 degrees from each other
498 The auxiliary (starter) winding of a single phase induction motor is normally
disconnected
(a) When the motor reaches normal running speed
(b) When the motor reaches approximately 80% of normal running speed
(c) Prior to applying the full supply voltage
(b) When the motor reaches approximately 80% of normal running speed
499 Decreasing the field current in an ac generator will
(a) Increase output frequency
(b) Decrease the output frequency
(c) Cause the output voltage to fall
(c) Cause the output voltage to fall
Why are two phase servomotors long and narrow?
Where will you find a stator rotating field?
Where does Slip occur?
What is the function of the squirrel cage?
How would you improve starting torque on AC inductive motor?
What is the purpose of thicker bars in a squirrel cage?
What colour are the phases 1 2 and 3 and what colour is neutral?
What is special about a 3-phase squirrel-cage induction motor?
What speed is a 3-phase induction motor?
What is the formula for phase vs line voltage star connection?
Star connection
Line to line voltage = phase voltage * 1.73
3-phase brushless alternator, where is output taken?
Synchronous motor = exited how?
Double running time = ?
What does a capacitor in a single phase motor do?
What is the reason if a 3 phase motor fails to start?
Decrease field current results in?
