A 5kVA motor operates from a 240V 50Hz supply at a lagging p.f. of 0.6
and is in parallel with a heating and lighting load of 3kW. What is the
overall p.f. and what capacitance is required to improve this to 0.95
lagging? What is the apparent power before and after correction? What is
the total current flow before and after correction?

On Sun, 3 Jan 2016 13:56:29 -0000 (UTC), Cursitor Doom
wrote:

A 5kVA motor operates from a 240V 50Hz supply at a lagging p.f. of 0.6
and is in parallel with a heating and lighting load of 3kW. What is the
overall p.f. and what capacitance is required to improve this to 0.95
lagging?

I have no idea but wouldn't someone have to know the impedance of all
the stuff to be able to answer that? Or is it that you can calculate
that from the power factor (size and direction)?

Like, if the heating load was purely resistive then I don't believe
that would play a part in the calculations. I believe motors and
lighting ballasts are typically inductive and hence why you would use
capacitors to help improve the pf.

What is the apparent power before and after correction? What is
the total current flow before and after correction?

All this sounds like one of those 'If a train is traveling at 60 mph
in a Westerly direction and a car at 30mph in an Easterly, what time
did they set off?' (But I'm probably wrong). ;-)

Just from a practical pov. Would one normally try to correct the pf of
each load so if they are switched in and out the pf would still stay
the best it can be?

Cheers, T i m

On Sun, 03 Jan 2016 16:02:39 +0000, T i m wrote:

On Sun, 3 Jan 2016 13:56:29 -0000 (UTC), Cursitor Doom
wrote:

A 5kVA motor operates from a 240V 50Hz supply at a lagging p.f. of 0.6
and is in parallel with a heating and lighting load of 3kW. What is the
overall p.f. and what capacitance is required to improve this to 0.95
lagging?

I have no idea but wouldn't someone have to know the impedance of all
the stuff to be able to answer that? Or is it that you can calculate
that from the power factor (size and direction)?

You don't need the impedances; everything you need to know to solve this
is provided in the question.


Like, if the heating load was purely resistive then I don't believe that
would play a part in the calculations.

If they were irrelevant to the problem, they wouldn't have been included!

On 03/01/2016 17:08, Cursitor Doom wrote:
On Sun, 03 Jan 2016 16:02:39 +0000, T i m wrote:

On Sun, 3 Jan 2016 13:56:29 -0000 (UTC), Cursitor Doom
wrote:

A 5kVA motor operates from a 240V 50Hz supply at a lagging p.f. of 0.6
and is in parallel with a heating and lighting load of 3kW. What is the
overall p.f. and what capacitance is required to improve this to 0.95
lagging?

I have no idea but wouldn't someone have to know the impedance of all
the stuff to be able to answer that? Or is it that you can calculate
that from the power factor (size and direction)?

You don't need the impedances; everything you need to know to solve this
is provided in the question.


Like, if the heating load was purely resistive then I don't believe that
would play a part in the calculations.

If they were irrelevant to the problem, they wouldn't have been included!



So, this is a puzzle really, rather than a technical question where you
are actually looking for technical help. Do other NG users think as I
do, that this should really be signalled more clearly in the header?

On Sun, 03 Jan 2016 20:04:29 +0000, newshound wrote:

So, this is a puzzle really, rather than a technical question where you
are actually looking for technical help. Do other NG users think as I
do, that this should really be signalled more clearly in the header?

It's an exam question, really so I take your point. But it'll be the last
one I'll post, anyway. (no doubt that'll be a relief for a lot of people
here) ;-)

On 03/01/2016 21:33, Cursitor Doom wrote:
On Sun, 03 Jan 2016 20:04:29 +0000, newshound wrote:

So, this is a puzzle really, rather than a technical question where you
are actually looking for technical help. Do other NG users think as I
do, that this should really be signalled more clearly in the header?

It's an exam question, really so I take your point. But it'll be the last
one I'll post, anyway. (no doubt that'll be a relief for a lot of people
here) ;-)

Shame this will be the last one, perhaps the header could mention puzzle
or exam question. If anyone is really bothered they can easily create a
message filter that traps the words "puzzle" of "exam".

It'll only be a relief for those few who think they know so much, but in
reality have forgotten it all.

Does the heater change resistance as it heats up?
Brian

--
From the Sofa of Brian Gaff Reply address is active
Remember, if you don't like where I post
or what I say, you don't have to
read my posts! :-)
"Cursitor Doom" wrote in message
...
On Sun, 03 Jan 2016 16:02:39 +0000, T i m wrote:

On Sun, 3 Jan 2016 13:56:29 -0000 (UTC), Cursitor Doom
wrote:

A 5kVA motor operates from a 240V 50Hz supply at a lagging p.f. of 0.6
and is in parallel with a heating and lighting load of 3kW. What is the
overall p.f. and what capacitance is required to improve this to 0.95
lagging?

I have no idea but wouldn't someone have to know the impedance of all
the stuff to be able to answer that? Or is it that you can calculate
that from the power factor (size and direction)?

You don't need the impedances; everything you need to know to solve this
is provided in the question.


Like, if the heating load was purely resistive then I don't believe that
would play a part in the calculations.

If they were irrelevant to the problem, they wouldn't have been included!

Scuse me I'm just off to dig half a hole.
Brian

--
From the Sofa of Brian Gaff Reply address is active
Remember, if you don't like where I post
or what I say, you don't have to
read my posts! :-)
"T i m" wrote in message
...
On Sun, 3 Jan 2016 13:56:29 -0000 (UTC), Cursitor Doom
wrote:

A 5kVA motor operates from a 240V 50Hz supply at a lagging p.f. of 0.6
and is in parallel with a heating and lighting load of 3kW. What is the
overall p.f. and what capacitance is required to improve this to 0.95
lagging?

I have no idea but wouldn't someone have to know the impedance of all
the stuff to be able to answer that? Or is it that you can calculate
that from the power factor (size and direction)?

Like, if the heating load was purely resistive then I don't believe
that would play a part in the calculations. I believe motors and
lighting ballasts are typically inductive and hence why you would use
capacitors to help improve the pf.

What is the apparent power before and after correction? What is
the total current flow before and after correction?

All this sounds like one of those 'If a train is traveling at 60 mph
in a Westerly direction and a car at 30mph in an Easterly, what time
did they set off?' (But I'm probably wrong). ;-)

Just from a practical pov. Would one normally try to correct the pf of
each load so if they are switched in and out the pf would still stay
the best it can be?

Cheers, T i m

On 03/01/2016 13:56, Cursitor Doom wrote:
A 5kVA motor operates from a 240V 50Hz supply at a lagging p.f. of 0.6
and is in parallel with a heating and lighting load of 3kW. What is the
overall p.f. and what capacitance is required to improve this to 0.95
lagging? What is the apparent power before and after correction? What is
the total current flow before and after correction?

Hmm, would that be a constant pf independent of motor load? :-)

It's unusual to specify a motor in terms of kVA and assume that this is
the load it takes from the supply.

Real part of current = 0.6 x 5,000 / 240 = 12.5A
Reactive part = sqrt( (5,000/240)^2 - 12.5^2) ) = 16.67A

3kW load current = 3,000 / 240 = 12.5A

Therefore total real load is 25A.
Complex current we would get if pf is 0.95 = 25 / 0.95 = 26.3A,
Reactive current allowed at pf 0.95 = sqrt( 26.3^2 - 25^2 ) = 8.22A

A correction capacitor must therefore supply leading current of 16.67 -
8.22 = 8.45A

Xc = 1/wC = 240 / 8.45 ; C = 8.45 / (2 x pi x 50 * 240) = 112uF

Am I far out?

On Sun, 03 Jan 2016 21:55:24 +0000, Fredxxx wrote:

On 03/01/2016 13:56, Cursitor Doom wrote:
A 5kVA motor operates from a 240V 50Hz supply at a lagging p.f. of 0.6
and is in parallel with a heating and lighting load of 3kW. What is the
overall p.f. and what capacitance is required to improve this to 0.95
lagging? What is the apparent power before and after correction? What
is the total current flow before and after correction?

Hmm, would that be a constant pf independent of motor load? :-)

It's unusual to specify a motor in terms of kVA and assume that this is
the load it takes from the supply.

Real part of current = 0.6 x 5,000 / 240 = 12.5A Reactive part = sqrt(
(5,000/240)^2 - 12.5^2) ) = 16.67A

3kW load current = 3,000 / 240 = 12.5A

Therefore total real load is 25A.
Complex current we would get if pf is 0.95 = 25 / 0.95 = 26.3A,
Reactive current allowed at pf 0.95 = sqrt( 26.3^2 - 25^2 ) = 8.22A

A correction capacitor must therefore supply leading current of 16.67 -
8.22 = 8.45A

Xc = 1/wC = 240 / 8.45 ; C = 8.45 / (2 x pi x 50 * 240) = 112uF

Am I far out?

I'm just about to hop into bed for the night, but I'll set your mind at
rest - you got the cap value near enough bang on; the rest I'll have to
check tomorrow.
Good point about the phrasing of the question, btw., hadn't struck me
until you mentioned it.
Nighty Night!

On 03/01/2016 21:55, Fredxxx wrote:
On 03/01/2016 13:56, Cursitor Doom wrote:
A 5kVA motor operates from a 240V 50Hz supply at a lagging p.f. of 0.6
and is in parallel with a heating and lighting load of 3kW. What is the
overall p.f. and what capacitance is required to improve this to 0.95
lagging? What is the apparent power before and after correction? What is
the total current flow before and after correction?

Hmm, would that be a constant pf independent of motor load? :-)

It's unusual to specify a motor in terms of kVA and assume that this is
the load it takes from the supply.

Real part of current = 0.6 x 5,000 / 240 = 12.5A
Reactive part = sqrt( (5,000/240)^2 - 12.5^2) ) = 16.67A

3kW load current = 3,000 / 240 = 12.5A

Therefore total real load is 25A.
Complex current we would get if pf is 0.95 = 25 / 0.95 = 26.3A,
Reactive current allowed at pf 0.95 = sqrt( 26.3^2 - 25^2 ) = 8.22A

A correction capacitor must therefore supply leading current of 16.67 -
8.22 = 8.45A

Xc = 1/wC = 240 / 8.45 ; C = 8.45 / (2 x pi x 50 * 240) = 112uF

Am I far out?

You might be if it were mine, I like to use heat pumps for heating so
they are reactive loads and not constant either. Of course you would
correct the PF for each one.

On 03/01/16 13:56, Cursitor Doom wrote:
A 5kVA motor operates from a 240V 50Hz supply at a lagging p.f. of 0.6
and is in parallel with a heating and lighting load of 3kW. What is the
overall p.f. and what capacitance is required to improve this to 0.95
lagging? What is the apparent power before and after correction? What is
the total current flow before and after correction?


Probably silly question.

A friend of mine who is not that great at his current job is thinking of
retraining as an electrician. Would this question be typical in basic
installation qualifications or something more advanced.

If basic, then (gulp) I fear for him ...

--
Adrian C

On 03/01/2016 21:58, Adrian Caspersz wrote:
On 03/01/16 13:56, Cursitor Doom wrote:
A 5kVA motor operates from a 240V 50Hz supply at a lagging p.f. of 0.6
and is in parallel with a heating and lighting load of 3kW. What is the
overall p.f. and what capacitance is required to improve this to 0.95
lagging? What is the apparent power before and after correction? What is
the total current flow before and after correction?


Probably silly question.

A friend of mine who is not that great at his current job is thinking of
retraining as an electrician. Would this question be typical in basic
installation qualifications or something more advanced.

If basic, then (gulp) I fear for him ...

Most electricians don't have to worry about motors and power factor.
However, the 17th edition regulations and the corresponding exam covers
a very wide subject range. It's not a trivial subject and he'll have to
spend a lot of time learning.

On 03/01/2016 21:58, Adrian Caspersz wrote:
On 03/01/16 13:56, Cursitor Doom wrote:
A 5kVA motor operates from a 240V 50Hz supply at a lagging p.f. of 0.6
and is in parallel with a heating and lighting load of 3kW. What is the
overall p.f. and what capacitance is required to improve this to 0.95
lagging? What is the apparent power before and after correction? What is
the total current flow before and after correction?


Probably silly question.

A friend of mine who is not that great at his current job is thinking of
retraining as an electrician. Would this question be typical in basic
installation qualifications or something more advanced.

If basic, then (gulp) I fear for him ...


If he is doing industrial design it might be.

If he is doing houses there isn't much chance he will need to know.

I can well remember a visit at the place I worked back in the 60s from a
techy from the Leccy board as then was complaining that whatever we were
doing was not liked by the local sub station. What we were doing was running
a hundred or so tvs with autotransformers and half wave rectifiers on a soak
test rack.
I think we had to have it split b between mains phases to get it sorted
out.
Now being the simple person I am, I cannot see why this would be any
different to 100 houses watching Coronation street, but there we go.
Brian

--
From the Sofa of Brian Gaff Reply address is active
Remember, if you don't like where I post
or what I say, you don't have to
read my posts! :-)
"dennis@home" wrote in message
web.com...
On 03/01/2016 21:58, Adrian Caspersz wrote:
On 03/01/16 13:56, Cursitor Doom wrote:
A 5kVA motor operates from a 240V 50Hz supply at a lagging p.f. of 0.6
and is in parallel with a heating and lighting load of 3kW. What is the
overall p.f. and what capacitance is required to improve this to 0.95
lagging? What is the apparent power before and after correction? What is
the total current flow before and after correction?


Probably silly question.

A friend of mine who is not that great at his current job is thinking of
retraining as an electrician. Would this question be typical in basic
installation qualifications or something more advanced.

If basic, then (gulp) I fear for him ...


If he is doing industrial design it might be.

If he is doing houses there isn't much chance he will need to know.

On 04/01/2016 07:58, Brian-Gaff wrote:
I can well remember a visit at the place I worked back in the 60s from a
techy from the Leccy board as then was complaining that whatever we were
doing was not liked by the local sub station. What we were doing was running
a hundred or so tvs with autotransformers and half wave rectifiers on a soak
test rack.
I think we had to have it split b between mains phases to get it sorted
out.
Now being the simple person I am, I cannot see why this would be any
different to 100 houses watching Coronation street, but there we go.
Brian

I certainly recall TV sets of the type you describe, though often the
heaters were supplied by one half wave rectifier, and the electronics by
the other "half" to try and equalise the current taken on +ve and -ve
voltage excursions.

There are two reasons, both have an effect on the substation, one is the
harmonic nature of the waveform with even harmonics, and the other was
the net DC current supplied back to the substation.

A DC current would bias the magnetising flux to become non-symetrical
and the magnetising current would increase depending how close the flux
level got to saturation.

In article ,
Fredxxx wrote:
On 04/01/2016 07:58, Brian-Gaff wrote:
I can well remember a visit at the place I worked back in the 60s from a
techy from the Leccy board as then was complaining that whatever we were
doing was not liked by the local sub station. What we were doing was running
a hundred or so tvs with autotransformers and half wave rectifiers on a soak
test rack.
I think we had to have it split b between mains phases to get it sorted
out.
Now being the simple person I am, I cannot see why this would be any
different to 100 houses watching Coronation street, but there we go.
Brian

I certainly recall TV sets of the type you describe, though often the
heaters were supplied by one half wave rectifier, and the electronics by
the other "half" to try and equalise the current taken on +ve and -ve
voltage excursions.

I din't think anyone bothered to rectify heater supplies. They were either
6.3v ac or fed in a long series chain.

There are two reasons, both have an effect on the substation, one is the
harmonic nature of the waveform with even harmonics, and the other was
the net DC current supplied back to the substation.

A DC current would bias the magnetising flux to become non-symetrical
and the magnetising current would increase depending how close the flux
level got to saturation.

--
Please note new email address:

The mind boggles as to what you are building....
Brian

--
From the Sofa of Brian Gaff Reply address is active
Remember, if you don't like where I post
or what I say, you don't have to
read my posts! :-)
"Cursitor Doom" wrote in message
...
A 5kVA motor operates from a 240V 50Hz supply at a lagging p.f. of 0.6
and is in parallel with a heating and lighting load of 3kW. What is the
overall p.f. and what capacitance is required to improve this to 0.95
lagging? What is the apparent power before and after correction? What is
the total current flow before and after correction?

Brian-Gaff wrote

The mind boggles as to what you are building....

He isnt building anything, just posing problems
that are interesting to some to solve.

"Cursitor Doom" wrote in message
...
A 5kVA motor operates from a 240V 50Hz supply at a lagging p.f. of 0.6
and is in parallel with a heating and lighting load of 3kW. What is the
overall p.f. and what capacitance is required to improve this to 0.95
lagging? What is the apparent power before and after correction? What is
the total current flow before and after correction?