3Phase Motor cable calc.

05-12-2014 6:42pm

Bruthal wrote: »

Star starting draws 1/3 of delta starting while rotor is stopped I.e full starting current at instant of starting.

If motor start current is 6 times running, then star starting might be 2 times delta fla.

Overload will have some time delay. But if the rotor is slow to get up to speed, the overland can trip. Even if doesn't while still in star, it still can just after changing to delta.

As per my earlier post I would expect a certain amount of tweaking to be necessary. The overload information in the link I posted is well established, it will work.

05-12-2014 7:03pm

Another useful link for motors here. The calculated full load current aligns quite well with the one in the link. Always nice to be able to carry out a quick "sanity check".

Bruthal · 05-12-2014 8:25pm

2011 wrote: »

As per my earlier post I would expect a certain amount of tweaking to be necessary. The overload information in the link I posted is well established, it will work.

Its well established, and simple enough that it doesn't warrent having to prove anything.

But it was worth being pointed out by other posters about motors that are slow to get up to speed. One good one I seen myself was where a motor was supplied by too small a cable with a longish run. Exactly as said by others, was slow to get up to speed. Was tripping overload. Bigger cable needed.

Par1 · 05-12-2014 9:08pm

If the 55Kw is the rated power then thats the total power of the motor in my opinion (18.3Kw per phase). If the motor was rated at 165Kw then thats 55Kw per phase. So are we now saying motor rating at 165Kw??

Bruthal · 05-12-2014 9:56pm

Par1 wrote: »

If the 55Kw is the rated power then thats the total power of the motor in my opinion (18.3Kw per phase). If the motor was rated at 165Kw then thats 55Kw per phase. So are we now saying motor rating at 165Kw??

55kw motor. 93 amps per phase. With a 0.85 pf

Lets take your 18 amps per phase. 18kw at 400v single phase is 45 amps at parity.

If you connect 3 18kw loads in parallel to 2 phases, that will be 158 amps at pf 0.85

Now if you measure the current in the 2 phases supplying that, you get 158 amps per phase. Clearly 158 amps is flowing. You don't add the 2 phase currents together. Its a single flow.

Now connect the same loads to 3 phases in delta and its 93 amps per phase. Not 93 divided by 3.

Mississippi. · 05-12-2014 10:26pm

[QUOTE=2011;93318584

From memory overloads are set at 0.58 of the full load current.[/QUOTE]

Tuco 88
examine the star delta starter setup before you set the overloads. I have came across pre assembled star delta starters for motors approx this size where the overload is on the main contactor and not on the delta contactor. 97Amps on main contactor or 56 Amps on delta contactor.

Bruthal · 06-12-2014 9:18am

Another point to note about the 3 phase 55kw load in terms of being 3 18kw loads.

Forgetting pf. Taking 1.73 as root 3. Notice how the power in a 3 phase load is 400 x 1.73 x line current.

Its not simply 400 x 3 by line current. This is because in delta, the line current splits into 2 of the 3 windings. In star, the voltage is divided. It doesnt split into 2, since the phases at the other ends of both windings are 120 degrees out of step with each other compared to the phase in question. So its 1.73

Again its similar to the process in a single phase load. Power is 400 x line current.

Its not 400 x 2(line currents) x line current.

So if the 55kw 0.85 load was single phase, it would take 161 amps

If it was 3 18kw loads, they would take 54 amps each.

And in delta, the 55kw load takes 93 amps a phase.

Nothing higher being taken in the 3 phase connection there. Its not simply 3 times 93 amps. Its 1.73 times 93 amps which = the 161 amps if the 55kw load was connected single phase. Or as 3 18kw loads to single phase.

Used 1.7 as root 3 in previous post. Hence slight differences in currents.

Par1 · 06-12-2014 8:38pm

Bruthal wrote: »

Another point to note about the 3 phase 55kw load in terms of being 3 18kw loads.

Forgetting pf. Taking 1.73 as root 3. Notice how the power in a 3 phase load is 400 x 1.73 x line current.

Its not simply 400 x 3 by line current. This is because in delta, the line current splits into 2 of the 3 windings. In star, the voltage is divided. It doesnt split into 2, since the phases at the other ends of both windings are 120 degrees out of step with each other compared to the phase in question. So its 1.73

Again its similar to the process in a single phase load. Power is 400 x line current.

Its not 400 x 2(line currents) x line current.

So if the 55kw 0.85 load was single phase, it would take 161 amps

If it was 3 18kw loads, they would take 54 amps each.

And in delta, the 55kw load takes 93 amps a phase.

Nothing higher being taken in the 3 phase connection there. Its not simply 3 times 93 amps. Its 1.73 times 93 amps which = the 161 amps if the 55kw load was connected single phase. Or as 3 18kw loads to single phase.

Used 1.7 as root 3 in previous post. Hence slight differences in currents.

Your correct if there is 55Kw per phase but i personally doubt that very much. My opinion is (and as from OP) the motor is rated at 55Kw therefore the maximum load output from that motor is 55Kw. Again the rated ouput

If the motor is single phase then its 55Kw is drawn on one phase however it is three phase so its 55Kw is split over the 3 phases (a general advantage of choosing 3 phase) - the current is then calculated accordingly. The balancing of the load is assumed to be balanced or very close (most manufacturers recommend this) also it is highly recommended to have all three phase general loads balanced closley.

The motor is rated at 55Kw in the event that 55Kw is put onto one phase (however due to balancing this doesn't happen under normal conditions). A 55Kw motor draws the 55Kw from the 3 phases combined and not individually.

If this were the case lets take an example then. OK we request a 3 phase MIC of 250 Amps per phase from ESB (lets work in ampage for a minute and disregard Kw) Ok we now have 250 amps per phase so taking your advice we now size the conductor for 750 amps per phase like you are doing above....i think not my friend ! The motor can only draw 55Kw total which is why its rated at 55Kw (if it could draw more then the manufacturer would generally tell us)... end of story

Bruthal · 06-12-2014 9:09pm

Par1 wrote: »

Your correct if there is 55Kw per phase but i personally doubt that very much. My opinion is (and as from OP) the motor is rated at 55Kw therefore the maximum load output from that motor is 55Kw. Again the rated ouput

If the motor is single phase then its 55Kw is drawn on one phase however it is three phase so its 55Kw is split over the 3 phases (a general advantage of choosing 3 phase) - the current is then calculated accordingly. The balancing of the load is assumed to be balanced or very close (most manufacturers recommend this) also it is highly recommended to have all three phase general loads balanced closley.

The motor is rated at 55Kw in the event that 55Kw is put onto one phase (however due to balancing this doesn't happen under normal conditions). A 55Kw motor draws the 55Kw from the 3 phases combined and not individually.

If this were the case lets take an example then. OK we request a 3 phase MIC of 250 Amps per phase from ESB (lets work in ampage for a minute and disregard Kw) Ok we now have 250 amps per phase so taking your advice we now size the conductor for 750 amps per phase like you are doing above....i think not my friend ! The motor can only draw 55Kw total which is why its rated at 55Kw (if it could draw more then the manufacturer would generally tell us)... end of story

You still believe a 55kw 0.85 pf 3 phase load is taking 18 amps per phase?

If so, you are very lost there. And shouldnt use the phrase "end of story" until you show us how you got that 18 amps.

Nowhere have I made any suggestion or mention of anything other than a 55 kw 3 phase load. Although I would welcome you telling me where I do so...

55kw 0.85 power factor load will draw 93 amps per phase.
In single phase that will be 161 amps.

Par1 · 06-12-2014 9:28pm

Bruthal wrote: »

You still believe a 55kw 0.85 pf 3 phase load is taking 18 amps per phase?

If so, you are very lost there. And shouldnt use the phrase "end of story" until you show us how you got that 18 amps.

Nowhere have I made any suggestion or mention of anything other than a 55 kw 3 phase load. Although I would welcome you telling me where I do so...

55kw 0.85 power factor load will draw 93 amps per phase.
In single phase that will be 161 amps.

You believe the load is 55Kw per phase?...you can only get 93 amps per phase if there is 55Kw per phase..its simple ohms law my friend.

I corrected that typo in post 5 (i did actually type Amps earlier when i intended Kw). I am saying it is my belief that the motor is rated at 55Kw so if you have 3 phases its 55Kw divided by 3 to get Kw per phase. I do agree it does take 93 odd amps but not on each phase like you say

Bruthal · 06-12-2014 9:41pm

Par1 wrote: »

You believe the load is 55Kw per phase?...you can only get 93 amps per phase if there is 55Kw per phase..its simple ohms law my friend.

I corrected that typo in post 5 (i did actually type Amps earlier when i intended Kw). I am saying it is my belief that the motor is rated at 55Kw so if you have 3 phases its 55Kw divided by 3 to get Kw per phase. I do agree it does take 93 odd amps but not on each phase like you say

What does it take per phase then, and how does your answer equate to

P = V x I x 1.73 x pf... therefore...

I = P/ (V x 1.73 x pf)

Par1 · 06-12-2014 9:50pm

Bruthal wrote: »

What does it take per phase then, and how does your answer equate to

P = V x I x 1.73 x pf... therefore...

I = P/ (V x 1.73 x pf)

It takes your calculations i agree they are spot on...just not with the 55Kw per phase

Bruthal · 06-12-2014 9:55pm

Par1 wrote: »

It takes your calculations i agree they are spot on...just not with the 55Kw per phase

Can you tell me the phase current of the 55kw motor described in this thread. Any answer on that?

Not much point is saying other posters are incorrect if you cant show them your correct calculation of amps per phase.

And also, where have I made mention of a 165kw motor?? I never made any mention of any 55kw per phase motor. That wouldnt be a way I would describe any motor.

Par1 · 06-12-2014 9:58pm

So based on what you are saying then a 20 amp 3 phase isolator protected by a 20 amp 3 pole MCB the cable should be rated at 20+20+20 = 60 amps then cable sized for 60Amps?. So a typical 3 phase small 20 amp motor now needs probably a 16sq ( guess - without checking regs) feed to all auxiliary equipment....Hmmmmmmm ok that makes sense

Bruthal · 06-12-2014 10:06pm

Par1 wrote: »

So based on what you are saying then a 20 amp 3 phase isolator protected by a 20 amp 3 pole MCB the cable should be rated at 20+20+20 = 60 amps then cable sized for 60Amps?. So a typical 3 phase small 20 amp motor now needs probably a 16sq ( guess - without checking regs) feed to all auxiliary equipment....Hmmmmmmm ok that makes sense

What is the amps per phase of the 55kw motor in question here? Any answer????

Anyway, halfway down this page, look at motor delta diagram. Note the single winding current = 17.3 amps
Line current = 30 amps. Anything clicking yet?? As in the line current is 1.73 times the single winding current.......

So 93 amps line divided by 1.73 = 53 amps. Which amazingly, is the current a single phase 18.3333kw 400v 0.85pf load will take.

Its not the simple ohms law you suggested. But im beginning to think you are joking now.

Par1 · 06-12-2014 10:22pm

I cant quiet see where it says that 55Kw Rating is per phase like you assume and not total rated motor power (balanced equally over 3 phases). I have volume turned up and cannot hear any clicking yet...do i click something on that link then that information appears?

Bruthal · 06-12-2014 10:27pm

Par1 wrote: »

I cant quiet see where it says that 55Kw Rating is per phase like you assume and not total rated motor power (balanced equally over 3 phases).

Any answer to the question yet:pac:

Par1 · 06-12-2014 10:29pm

Yes basically you are of the opinion its 93 amps per phase i believe it is 93 amps balanced over 3 phases we will have to agree to disagree. The OP has enough info to make his own mind up.

Bruthal · 06-12-2014 10:33pm

Par1 wrote: »

Yes basically you are of the opinion its 93 amps per phase i believe it is 93 amps balanced over 3 phases we will have to agree to disagree. The OP has enough info to make his own mind up.

So what is the current on each phase?

Par1 · 06-12-2014 10:43pm

Bruthal wrote: »

So what is the current on each phase?

Nobody can tell my friend until the OP gets the name plate info as per his first post we do not have the relevant correct information as of yet as the nameplate had been damaged a while back so he is awaiting more info at some stage from the factory on specs.

As soon as that is known my friend then an exact calculation can be made and we wont have to worry about you or me guessing with information that is not apparent or may be inaccurate at his stage. Its like trying to guess how long is a piece of string....you go first then il guess as-well

Bruthal · 06-12-2014 10:49pm

Par1 wrote: »

Nobody can tell my friend until the OP gets the name plate info as per his first post we do not have the relevant correct information as of yet as the nameplate had been damaged a while back so he is awaiting more info at some stage from the factory on specs.

As soon as that is known my friend then an exact calculation can be made and we wont have to worry about you or me guessing with information that is not apparent or may be inaccurate at his stage. Its like trying to guess how long is a piece of string....you go first then il guess as-well

Well I can tell you for certain, the calculation shows 93 amps per phase, which equates to a phase current of 53 amps (current in each winding)

Now according to your good self, 93 amps is shared between all 3 phases.

So you say its 31 amps per phase?

Par1 · 06-12-2014 10:57pm

Bruthal wrote: »

Well I can tell you for certain, the calculation shows 93 amps per phase, which equates to a phase current of 53 amps (current in each winding)

Now according to your good self, 93 amps is shared between all 3 phases.

So you say its 31 amps per phase?

Oh right well if you can tell for certain values from a damaged nameplate (that have not even been posted yet!) then you must be physic...can you tell us all here the winning lotto numbers for Wednesday as-well?...Your a great man where have you been all along my friend

Bruthal · 06-12-2014 11:02pm

I think there is confusion here between phase and line currents.

When posters are saying its 93 amps per phase, they are not saying its 93 amps in each winding, but on each phase supplying the motor.

Where as technically, the phase current is the current in the winding.

Perhaps that is mystery solved...

Bruthal · 06-12-2014 11:08pm

Par1 wrote: »

Oh right well if you can tell for certain values from a damaged nameplate (that have not even been posted yet!) then you must be physic

No, just a poster.

can you tell us all here the winning lotto numbers for Wednesday as-well?

Yea. I will let you know on thursday.

Your a great man where have you been all along my friend

Right here. Not great though, just the average poster. I wouldnt be in your league of course.

Par1 · 06-12-2014 11:10pm

And the penny drops!!...Sound the horns we have a winner!!....Grease me up Mary!!

Bruthal · 06-12-2014 11:20pm

Par1 wrote: »

And the penny drops!!...Sound the horns we have a winner!!....Grease me up Mary!!

Im not so sure. It may have dropped on your end.

When posters said its 93 amps per phase, are you really saying you thought they meant 93 amps in each winding???

There is still your opinion that P = 400 x 1.73 x pf is the current over the 3 phases. Or at least thats what you seemed to be saying.

06-12-2014 11:29pm

Ok guys lets be nice.

It is quite easy to compare balanced 3 phase loads connected in star to 3 identical single phase loads. Balanced 3 phase loads connected in delta are a little more complex. Look at the diagrams and it may become more apparent.

arse..biscuits · 07-12-2014 7:54pm

If the electrical input is 55kW, and the pf is 0.85, and the connection is delta, then the line current is 55000/(1.732x400x0.85)=93.4A
That means that each of the 3 cables will have 93.4A flowing in them.
The current in each winding (which is called the phase current) will be 93.4/1.732=53.9A

There is some good info on this thread, and some not so good.

arse..biscuits · 07-12-2014 7:57pm

If the OP needs help cable sizing, just ask.

07-12-2014 9:26pm

arse..biscuits wrote: »

If the electrical input is 55kW, and the pf is 0.85, and the connection is delta, then the line current is 55000/(1.732x400x0.85)=93.4A
That means that each of the 3 cables will have 93.4A flowing in them.
The current in each winding (which is called the phase current) will be 93.4/1.732=53.9A

There is some good info on this thread, and some not so good.

Exactly, well put.

3Phase Motor cable calc.

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