Help on an exam question

[westos-electric]

Question
A capacitor of capacitive reactance 5ohm an inductor of 8ohm inductive reactance and resistor of 4ohm are connected in series to a 230v 50hz supply. The total impedance of the circuit will be?

5ohm

This is a question on a sample paper but i can't seem to work out how to do the working out

Can you help?

Cheers
Wayne

 

[pushrod]

Question
A capacitor of capacitive reactance 5ohm an inductor of 8ohm inductive reactance and resistor of 4ohm are connected in series to a 230v 50hz supply. The total impedance of the circuit will be?

5ohm

This is a question on a sample paper but i can't seem to work out how to do the working out

Can you help?

Cheers
Wayne

the capacitor and the inductance are producing opposite effects to each so you just want the difference between them.

look at your impedance triangle and you have sides of 4Ω (resistance) and 3Ω (reactance 8-5) you just need to find the hypotenuse

 

[westos-electric]

Hi

I've tried working this question out but getting something wrong.

Question

A 30kW 3 phase delta connected motor has a power factor of 0.86 lagging. calculate the line current

I thought it would be

30,000 / (400x0.86) but doesn't match the correct answer which is 50.35A

Could you confirm where i'm going wrong?

Thanks
Wayne

 

[Guest123]

You need also to divide by root 3 which is 1.732.

So....30,000 / (400 x 1.732 x 0.86) = 50.35A.

 

[westos-electric]

cheers knew i'd missed something!

 

[dichroic]

You may find this electrical maths pack useful.

Regards

Dichroic

 

[westos-electric]

Hi ive still had issues with this question

A small heating circuit draws 1.5kw of power and has an internal resistance of 0.5ohms what is the current drawn?

 

[Guest123]

This is an ohms law question........


Current (I) = square root of, power (P) / resistance (R)

= Sqrt of 1500 / 0.5 = 54.77A.

 

[westos-electric]

I've been reading about transformers and need help on this question

A transformer has an output of 12kva and a power factor of 0.9. the total losses in the transformer are 600w. What is the efficiency of the transformer

now i know / think the formula is output/ output power + losses

I'm not sure how this works out, can anyone help

cheers

 

[dichroic]

I've been reading about transformers and need help on this question

A transformer has an output of 12kva and a power factor of 0.9. the total losses in the transformer are 600w. What is the efficiency of the transformer

now i know / think the formula is output/ output power + losses

I'm not sure how this works out, can anyone help

cheers

The formula you require is:

input power = output power + losses

However you need to use:

Efficiency = output power / (output power + losses)

so: Output power = KVA x Power factor

12kva x 0.9 = 10.8kw

Efficiency = 10800 / (10800+600) = 0.947

0.947 x 100 = 94.7% efficient.

Regards

Dichroic