Calculation of inductive reactance, power dissipation & synchronous speed.

[Raven]

Hi, I am new to the forum, i would be greatful for help on the questions below. (including workings out).

1> Calculate the inductive reactance of a coil with inductance of 0.1H connected to a 50Hz supply.

2> An inductive load with 10ohms inductive reactance is connected to a 100V 50Hz AC supply. Calculate the power dissipated in the circuit (ignoring any conductor resistances).

3> The current in a coil changes from 5 Amperes to 2 Amperes in 50mS and induces a voltage of 30V into the coil. Calculate the valur of inductance in the coil.

4> Calculate the synchronous speed of the rotating magnetic field in a 4 pole synchronous motor.

5> R1=5ohms R2=10ohms 20V Supply.

Based on the above, calculate the power dissipated by the resistor 2 in the circuit.

describe what synchronus is?

 

[PoorFish]

Evening Raven

I am definitly no expert on all this but I will have a go and maybe someone can tell me if i have gone right or wrong here.

Question 1

Xl = 2πfl

Xl = 2π

 

[gullyboy_131]

1) 2 x n x 50 x 0.1 = 31.4 ohms

2) p= v2/R P=10000/10 P= 1Kw

3) E= dI/dt 3/50x10-3 = 60 L= v/E L= 30/60 L=0.5h

4) ns = f/p ns = 50/2 ns = 25rs = 1500rpm

5) P=V2/R P=400/10 P=40w

6) The speed of the rotating magnetic field

theres my go!Guiness Drink

 

[Andy™]

1) 2 x n x 50 x 0.1 = 31.4 ohms2) p= v2/R P=10000/10 P= 1Kw

3) E= dI/dt 3/50x10-3 = 60 L= v/E L= 30/60 L=0.5h

4) ns = f/p ns = 50/2 ns = 25rs = 1500rpm

5) P=V2/R P=400/10 P=40w

6) The speed of the rotating magnetic field

theres my go!Guiness Drink

Agree with all except 4 - he hasnt stated frequency input, so there is no way we can work out speed.

 

[gullyboy_131]

Agree with all except 4 - he hasnt stated frequency input, so there is no way we can work out speed.

i think living in the uk may be we should assume 50Hz,

 

[Andy™]

i think living in the uk may be we should assume 50Hz,

yes, but just to confuse, on exams they may state supply is 40 or 60Hz or something, to make sure you read the question and not assume UK standard

 

[gullyboy_131]

yes, but just to confuse, on exams they may state supply is 40 or 60Hz or something, to make sure you read the question and not assume UK standard

make you 100% right mate, thanks for the comments mate

 

[mikey-c]

Isn't the answer for Q.2 0 watts? It being reactive component only?

For question 5, are resistor in series or parallel?

Otherwise agree with all other answers.

Synchonous is when rotor speed matches rotational speed of magnetic field (?)

 

[gullyboy_131]

Isn't the answer for Q.2 0 watts? It being reactive component only?For question 5, are resistor in series or parallel?

Otherwise agree with all other answers.

Synchonous is when rotor speed matches rotational speed of magnetic field (?)

There will always be a difference between the rotating magnetic field and the rotor if they were to run at the same speed there would be no current induced so motor will not run ,the difference between them is known as slip %

 

[Raven]

Hello. Thank you for your help. I am in college now, but my bag is in the other room. The paperwork answer says 0w as mikey-c says.

but I can't remember the other answers. I will double check and post back later.

Thank You for your help.

 

[gullyboy_131]

Hello. Thank you for your help. I am in college now, but my bag is in the other room. The paperwork answer says 0w as mikey-c says.but I can't remember the other answers. I will double check and post back later.

Thank You for your help.

@mikey

cheers mikey i will remember Q.2 a silly one to get wrong in a exam!

you the man!

@raven

anymore questions we can help you with,this is good for revision

 

[a1]

Hello Raven,

May I trouble you to Select the "Thanks" button on the posts that you found useful please?

Many Thanks,

Admin.

 

[searley]

Calculate the synchronous speed of the rotating magnetic field in a 4 pole synchronous motor.

ns = f/p ns = 50/2 ns = 25rs = 1500rpm

how do i get from 25rms to 1500rpm ?

haveing a real blonde moment. thanks for looking lads

 

[Andy™]

Calculate the synchronous speed of the rotating magnetic field in a 4 pole synchronous motor.ns = f/p ns = 50/2 ns = 25rs = 1500rpm

how do i get from 25rms to 1500rpm ?

haveing a real blonde moment. thanks for looking lads

25 rev/second... x60 for minute

 

[searley]

is 50 just the power hertz because there is nothing in the question ?

 

[Andy™]

is 50 just the power hertz because there is nothing in the question ?

50 is the frequency, which you will need to know to work the answer, and you did say 50 in your question?

pay attention to the question on the exam too - it may not be 50Hz

 

[searley]

cheers andyc the question was already on this page and it did`nt mention 50 in the question. thanks anyway i gor there in the end.