JNE 314 Progress Check – Section # 3 AC Induction Motors Sub-section 3.1 – General – Multiple choice questions. Choose the correct answer. 1. The principles behind the AC Induction motor are: a) Electromagnetic Induction in rotor bars due to the rotating magnetic field b) Electromagnetic Induction in stator coils and the interaction in the current carrying rotor bars c) Faraday’s Law and Lenz’s Law on Electromagnetic Induction d) None of the above 2. The stator and the rotor of an AC Induction motor are laminated steel: a) To achieve better power: weight ratio b) To reduce Eddie currents c) To increase the induced currents d) None of the above 3. The synchronous speed of a 4-pole motor when connected to a 400V 50Hz AC 3- phase supply is: a) 1800 RPM b) 1750 RPM c) 25 rps d) There is not enough data to calculate the speed 4. A 12 pole AC Induction motor is connected to a 440V 60 Hz supply. What is the RPM when it is running with a 3 percent slip? a) 1164 RPM b) 582 RPM c) 10 rps d) 3492 RPM 5. Direct-on-line starting for an AC Induction motor is suitable: a) If the motor is small in size b) If the supply comes from a high capacity distribution system c) If the motor is star connected d) None of the above6. After a major overhaul of the motor, the main sea water pump of your ship is not developing satisfactory pressure. The engineers make the following suggestions. What can be the most likely reason? a) The pressure gauge must be changed b) Motor must be connected in Delta instead of Star c) Motor rotating direction must be changed d) Motor RPM is not correct as the voltage is 435V instead of 440V 7. We design some motors to run in Delta, because: a) Delta motors produce three times more torque than star b) Delta motors take only one third of the starting current c) Delta motors have better power weight ratio and run cooler d) Delta motor are efficient than star 8. Why do we use star-delta starters? a) In order to reduce starting phase voltage b) In order to reduce starting line current c) Both above d) None of the above 9. What happens if a motor designed to run in star is run in delta? a) Nothing adverse happens b) Motor will burn c) Motor will run with better torque d) None of the above 10. A dual voltage motor is designed to run in 220V 3-phase and 380V 3-phase. In order to get the same output from the motor on these two different voltages: a) It needs to be connected in delta for 220V and star for 380V b) It needs to be connected in star for 220V and delta for 380V c) It needs to be run on 380V for starting and 220V for running d) It needs to be run on 220V for starting and 380V for runningSub-section 3.2 – Motor Types, Operation and Protection 1. A wound rotor motor achieves better starting torque by a) Adding resistances to the rotor circuit which reduces the starting current b) Adding resistances to the rotor circuit which increases the power factor c) Adding resistances to the rotor circuit which reduces the power factor d) Adding resistances to the rotor circuit which reduces the voltage 2. When a load is added to an Induction motor: a) The rotor speed drops, which decreases the relative cutting resulting an increase of rotor current, which enable the stator to draw more current from supply b) The rotor speed drops, which increases the relative cutting resulting an increase of rotor current, which enable the stator to draw more current from supply c) The rotor speed drops, which decreases the relative cutting resulting a decrease of rotor current, which enable the stator to draw more current from supply d) The rotor speed drops, which increases the relative cutting resulting an decrease of rotor current, which enable the stator to draw more current from supply 3. Addition of resistance to the rotor circuit of a wound rotor motor a) Shift the locked rotor point in the torque-speed characteristic curve to the left b) Shift the locked rotor point in the torque-speed characteristic curve to the right c) Shift the full load torque point in the torque-speed characteristic curve to the left d) Shift the breakdown torque point in the torque-speed characteristic curve to the left 4. The thermal overload of a motor is set at, e) Maximum full load current f) 102% of the full load current g) 105% of the full load current h) 120% of the full load current 5. Normally fuses in a motor circuit: a) Protect the motor against overload b) Protect the motor against over current c) Protect the motor against over voltage d) Protect against short circuit6. When single phasing occurs in a motor shown below: a) Current in windings Ia and Ib will be subjected to overload b) Current in windings Ic will be subjected to overload c) Current in line L3 will be subjected to overload d) Current in line L2 will be subjected to overload 7. Speed of AC Induction motors can be controlled: a) directly by controlling the supply voltage b) Inversely by controlling the frequency c) Directly by controlling the pole pairs d) Inversely by controlling the pole pairs 8. Speed of AC synchronous motors is: a) Inversely proportional to the frequency b) Directly proportional to the frequency c) Directly proportional to the pole pairs d) Directly proportional to the supply voltage 9. which statement is correct? Synchronous motors: a) are similar to AC induction motors in construction of stator and rotor b) are similar to AC induction motor stator only c) can be started in star and run in delta d) take load similar to AC Induction motors 10. What is the purpose of the Amortisseur winding in a synchronous motor rotor? a) To act like a squirrel cage rotor at the start instead of using a pony motor b) To act like the DC poles when running c) To increase the rotor torque at the start.d) To assist in correcting the power factor at start 11. Synchronous motors can be used to improve the power factor of a distribution system by: a) Adding capacitive reactance to rotor circuit b) Adding inductive reactance to rotor circuit c) Reducing the stator voltage d) controlling the excitation to rotor circuit 12. When rotor excitation of a synchronous motor is gradually increased: a) Power factor increases gradually b) Power factor decreases gradually c) Power factor increases and then decreases gradually d) Power factor decreases and then increases gradually 13. The synchronous compensator of a shaft generator: a) Can be a synchronous motor b) Can be a synchronous alternator de-coupled from its prime mover c) Both the above d) None of the above 14. An AC 3-phase Induction motor, driving a ballast pump was observed to be running hot. What factor below cannot cause this situation? a) Overcurrent drawn by the motor b) Overvoltage c) Lack of cooling d) Single phasing 15. A 440V, 60 Hz, 4 pole brushless synchronous motor has to be IR tested. Assuming that all windings of the exciter and the main stator can be separated, how many IR readings need to be recorded? a) 8 b) 12 c) 14 d) 16Sub-section 3.3 – Motor operational aspects 1. What would be the best braking method for an engine room overhead crane electric motor? a) Regenerative followed by counter current b) Regenerative followed by DC injection c) Mechanical braking d) Capacitive braking 2. What would be the best braking method for an electric propulsion motor? a) Regenerative followed by counter current b) Regenerative followed by DC injection c) Mechanical braking d) Capacitive braking 3. A 3-phase electric motor does not start when the push button starter is activated. The voltmeter on the starter shows that the supply to the starter is uninterrupted. What cannot be a reason that the motor does not start? a) Motor starter control circuit fuses are burnt. b) Motor has an open circuit c) Motor has an earth fault d) Over current trip is not reset 4. What happens if a Star-Delta starter continues to run in Star? a) Nothing adverse will happen as starting current in Star is one third of Delta b) Motor will burn as phase current on load is quite higher than in delta c) Motor torque will be less as the windings are in Star d) Motor will trip on overcurrent. 5. Your ship is dry docked in a European port where the voltage available is 380 V instead of 440V you need. Frequency is same. What happens if you run motors under these conditions? a) Nothing adverse can happen b) Motors will take a higher current and will run hotter than normal c) Since the voltage is less motors will run cooler than normal. d) Motors will have less torque than normal