(C&G 203 ?) Motor & Other Questions..

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1. Speed control of a 3 phase induction motor is achieved by varying the:

A Resistance in rotor winding

B Voltage applied to the stator

C Rotor winding inductance

D The number of stator windings

2. The purpose of capacitor in a capacitor start motor:

A Provide power factor correction

B Limit the starting current

C Provide an increased phase displacement

D Smooth the a.c. supply

3. When starting a 400v 3 phase cage rotor motor using star delta starter, the voltage across each stator phase is:

A 400v start 230v run

B 133v start 230v run

C 133v start 400v run

D 230v start 400v run

4. Cage rotor motors are considered to be best running at:

A Variable speed

B Constant speed

C High speed

D Low speed

5. A rotating field is produced in a 3 phase cage rotor motor when connected to:

A Rotor

B Stator

C Commutator

D Slip Rings

6. The thermal trip on a direct on line starter is to:

A Provide magnetic overload protection

B Provide thermal overload protection

C Provide remote starting

D Prevent motor over speed

7. A motor with a wound rotor has windings on:

A Rotor only

B Stator only

C Stator and rotor

D Commutator & field

8. Connections made to rotor windings on a 3 phase induction motor use:

A Mutual inductance effect rotor and stator

B Commutator and brushes

C Capacitor link to rotor windings

D Slip ring and brushes

9. A single loop of wire connected to slip rings rotated in a magnetic field. Voltage measured at slip rings will be:

A D.C.

B Unidirectional

C Intermittent

D A.C.

10. A capacitor start and run single phase induction motor runs as:

A Quad phase motor

B Single phase motor

C Two phase motor

D Three phase motor

11. A 400v 50Hz has a line current of 50A and 0.8 lagging, The power dissipated will be:

A 20KW

B 9.2KW

C 27.712KW

D 16KW

12. The reactance of a 1uf capacitor connected across a 50Hz supply is:

A 3.18 ?

B 31.8 ?

C 318 ?

D 3.18 ?

13. Cage rotor induction motor are considered to be:

A Variable speed machines

B Low speed

C high Speed

D Constant speed

14. Which of the following is suitable for starting a 3 phase rotor cage induction motor:

A Auto transformer and rotor resistance

B Rotor resistance and capacitor start

C Auto transformer and star delta

D Capacitor start and star delta

15. In a 2 pole generator the brushes on the commutator pick up the voltage generated which is:

A A.C.

B Unidirectional

C Intermittent A.C.

D Steady D.C.

16. A 3 phase 4 pole motor 50Hz induction motor has a slip of 4% the actual speed of the motor is:

A 1500prm

B 1440rpm

C 1400rpm

D 1359rpm

17. When a capacitor start capacitor run single phase induction motor reaches speed, a centrifugal switch:

A Removes all the capacitance connected to the field windings

B Reduces the number of field windings

C Leaves on the run windings connected

D Reduces the capacitance connected to the start winding

18. The rotor conductors of a 3 phase cage rotor inductance motor are constructed of a solid conductors:

A Shorted at each end of rings

B Connected to the supply by slip rings

C Connected to the supply by commutator

D Forming a star connection

19. The number of pair windings in a single phase capacitor start induction motor is:

A One

B Two

C Three

D Four

 

bobby

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Been going through my notes for these questions and im still having to take educated guesses at most of these questions can somone please Pray post the actual awnsers for us it would be a massive help thanks!

 

Admin1

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I am sure Specs Will oblige? Along with full workings out. Because he is da man. :D

 

seajayess

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Ok from what I can dig out from the back of the brain

1=D

2=C

3=D

4=B

5=B

6=B

7=A

8=D

9=D

10=B

11=D

12=3180 ohms think D

13=D

14=C

15=D

16=A forgot the 4%=B

17=D

18=A

19=B

Think I have them right but not 100% sure see what every body else think CJS

 

sarni31

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OK here goes ....

1 was looking for change in freq .. answer not there X( good start

2c increased? phase displacement

3d 2/3rds then full supply

4b

5b

6b

7c.. not sure about this one

8d

9d

10c two phase during start up?

11c sqrt3x400x50x0.8??

12d am going with d but kept getting 3.144ohms for some reason??

13d

14c

15a 2 pole geni so AC?

16b 50/2(2pairs)=25revs/secx60=1500revs/min-4%

17d

18a

19b

Guiness Drink

 

bobby

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Thanks for that both of your awnsers were mostly the same feel better now im armed with those awnsers in my head for this exam next week ;)

Edit:

I found another way of working out Q.16 not sure if this is an easier way or even the same way as sarni did it but here is my formula:

16. A 3 phase 4 pole motor 50Hz induction motor has a slip of 4% the actual speed of the motor is:

A 1500prm

B 1440rpm

C 1400rpm

D 1359rpm

3000/2 = 1500

1500-1440 <--- (picked 1 awnser randomly you may have to go through all 4 to get the correct percentage this is the forumlas downside)

= 60

60/1500 x 100 = 4% so I know the awnser is B.

 

sarni31

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if it works and will get you a correct answer then use it... ;)

good luck with the exam Guiness Drink

 

PoorFish

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15. In a 2 pole generator the brushes on the commutator pick up the voltage generated which is:

A A.C.

B Unidirectional

C Intermittent A.C.

D Steady D.C.

15a 2 pole geni so AC?
Would the commutator not make it DC?????????

 

sarni31

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not sure about this one ... you could be right :coat

yeah for AC it would be terminated by slip rings...

Guiness Drink

 

PoorFish

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sounds right to me

thanks for all other answersGuiness Drink

Im doing this exam next week an allPray

P

 

searley

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could any one show me how to get the formulas for these question asth ones on the post don`t work out.

11. A 400v 50Hz has a line current of 50A and 0.8 lagging, The power dissipated will be:

A 20KW

B 9.2KW

C 27.712KW

D 16KW

12. The reactance of a 1uf capacitor connected across a 50Hz supply is:

A 3.18 ?

B 31.8 ?

C 318 ?

D 3.18 ?

16. A 3 phase 4 pole motor 50Hz induction motor has a slip of 4% the actual speed of the motor is:

A 1500prm

B 1440rpm

C 1400rpm

D 1359rpm

thanks for the help

 

Jono Pashley

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11. Power = V X I X pf

Power = 400 x 50 x 0.8

Power = 16KW

Unless it specifically mentions three phase id be unsure of using the three phase calc.

12. using Xc Formula of 1 / 2 x crbvcurbvubruvbruvb x f x c

im getting 3183.1 ohms, which im unsure of so can someone else check that.

16. Sync Speed = Frequency / Pairs of Poles

Sync Speed = 50/2

Convert to rpm = sync speed x 60 = 1500

4% of 1500 is 60

1500-60 = 1440rpm

 

msb2003

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Agree with Jono for those, think no.12 may have a wrong value as a and d are the same!

the formula is Xc = 1/(2 x pi x f x c)

so its 1 / (2 x 3.14 x 50 x 0.000001) which is 3183 ohms

(0.000001 is 1 to the power minus 6, or micro)

think thats right!!

 

searley

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the questions where of this web site from welsh wizard.

the answer on the sheet is 3.18 but when i worked it out i got the same as you boys.his answer must be wrong.

 

searley

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number 11 must be wrong as somebody above has put down 27.712kw for his answer and me and other people get 16kw.

11. Power = V X I X pf

Power = 400 x 50 x 0.8

Power = 16KW

are am i missing something is this the wright way of working it out ?

 

mikey-c

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Power for 3 phase = √3 x I x V x pf

= 27.7 kW

But I'm not sure if this equation is covered on level 2, so you can argue for the answer being 16kW

 
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