3Phase Motor cable calc.

Tuco88

I am looking for a little advise on this, its been a long time before I did any design work so I am a little unsure on the motor currents.

The setup is a 55KW 3Phase motor for a chilling process, The name place has been damaged a while back so we are awaiting more info at some stage from the factory on specs. It looks to be a PF of 0.85. The Run is 90m on ladder rack.

I worked out the Full load current to be = 55000/(400 x Squrt of 3 x 0.85) I got IL = 93.3 Amps, Is this amps per Phase?

excluding Eff and correction factors for now.

Does this go into the Mv/A/m as the Amps. I am worried because its a star delta motor, and if this is Star current I don't want it to be tripping out when running in Delta. ILine = IPhase in star?

Its SWA/XLPE multicore, was using method E on page 275 Regs.

Thanks for any info.

I am just looking for a ball park figure so I can plan out switch-gear and so forth.

Par1

You are correct with the calculation, i also make it 93.39 @ 400v but be careful because its 98.3 @ 380 so measure the line - line voltage first.

The 55 kw is over the 3 phases so its roughly 18.3A per phase at max torque so that should settle when running (delta) but expect it to pull that on startup (star) depending on weight/load etc

Dont forget star delta starting current is 3 x FLC so allow for that on fusing set-up (characteristics)

You will have volt drop starting (vs) and also running volt drop (vr)

I assume from above that your starter is located beside motor or very close if not that distance will have to be taken into account

Measure VL-VL as i said first then you can calculate current correctly

Ambient temperature will be a factor also so make sure you also get that whilst you are there

Best of luck with it

Mississippi.

Tuco88 wrote: »

I worked out the Full load current to be = 55000/(400 x Squrt of 3 x 0.85) I got IL = 93.3 Amps, Is this amps per Phase?

.

Yes that is amps per phase. Each phase will have a ballpark figure of 100 amps at F.L.C.

frankmul

Par1 wrote: »

The 55 kw is over the 3 phases so its roughly 18.3A per phase at max torque so that should settle when running (delta) but expect it to pull that on startup (star) depending on weight/load etc

Par1, could you explain this a bit more?

Par1

frankmul wrote: »

Par1, could you explain this a bit more?

55Kw/3 is 18.3Kw per phase...correct? I should of stated start-up torque as opposed to max torque but generally torque is at the higher end on start-up (unless additional weight is added further on of course)

Star Delta is usually referred to as 'Soft Starting' therefore it starts off in Star then when it gets close to normal running speed the motor connects to delta.

A major disadvantage to star - delta starting in my opinion is a reduction in starting torque. Therefore if a belt is unladen then torque is generally not an issue however add weight to the belt (for whatever reason) then more torque is needed especially on start-up. The reason for this is the phase voltage on the motor - delta connection is 380v (ish) whilst star is 220v (ish).

Star voltage VL = Sq root 3 x Vp
Delta voltage VL = Vp

Star current IL = Ip
Delta current lL = Sq root 3 x Ip


I have came across instances where a 'Stall' condition can exist (due to low torque) because the motor cannot accelerate because it does not have sufficient torque to overcome the load requirements. In this case the detection system set-up was actually able to detect the 'Stall' condition and trip the motor (avoiding damage).

I found that low torque can also increase Run up time which can be a factor also. I would always consider torque from the start because generally motors are not cheap so i avoid damaging them if i can

2011

Tuco88 wrote: »

Does this go into the Mv/A/m as the Amps.

mV per amp per meter is used to calculate the volt drop.

I am worried because its a star delta motor, and if this is Star current I don't want it to be tripping out when running in Delta.

Can you confirm that you are using a star delta starter?
The reason that I ask is that this is not used very much anymore.

If this has a star delta starter the loaction of the starter in relation to the motor is important beacuse both ends of the motor windings have to be wired to the starter (all 6 cores).

2011

Tuco88 wrote: »

The setup is a 55KW 3Phase motor

Be careful of this.
Does the motor have a mechanical output of 55kW at the shaft?
If it does then the electrical input will be about 10% more.
Contact the manufacturer for the efficiency and work backwards.

This motor selection guide link provides motor output in kW:


Do you know the make and model of the motor?
http://www.brookcrompton.com/pdf-files/20122E_Product Guide.pdf

Bruthal

Par1 wrote: »

You are correct with the calculation, i also make it 93.39 @ 400v but be careful because its 98.3 @ 380 so measure the line - line voltage first.

The 55 kw is over the 3 phases so its roughly 18.3A per phase

Its 93 amps per phase.

Par1 wrote: »

55Kw/3 is 18.3Kw per phase...correct?

It is 18kw per winding, but you have each line connecting into 2 windings in delta, not simply an 18kw winding connected to 2 phases.

Bruthal

2011 wrote: »

If this has a star delta starter the loaction of the starter in relation to the motor is important beacuse both ends of the motor windings have to be wired to the starter (all 6 cores).

Shouldnt make as much difference as would be expected if cable size is what you mean, as in delta, the 6 cores are doubled up compared to star. As in the 6 cores are in effect 2 sets of paralleled cables. In star, only 3 of the 6 cables are supplying the motor.

2011

Bruthal wrote: »

Shouldnt make as much difference as would be expected if cable size is what you mean, as in delta, the 6 cores are doubled up compared to star. As in the 6 cores are in effect 2 sets of paralleled cables. In star, only 3 of the 6 cables are supplying the motor.

Well the supply cable is 90m so the starter could be anything up to 90m from the motor. Therefore up to 180m of 3 core SWA may be required.

Bruthal

2011 wrote: »

Well the supply cable is 90m so the starter could be anything up to 90m from the motor. Therefore up to 180m of 3 core SWA may be required.

O yea you were talking about having the starter beside it to save cable.

2011

Bruthal wrote: »

O yea you were talking about having the starter beside it to save cable.

I'm just saying that the starter location is very important. Perhaps a cable to a starter local to the motor is an option. I don't know the arrangement. One way or the other the cabling will be expensive.

Bruthal

2011 wrote: »

I'm just saying that the starter location is very important. Perhaps a cable to a starter local to the motor is an option. I don't know the arrangement. One way or the other the cabling will be expensive.

Yea the closer the starter is to the motor, the less cable needed for the power side of things for sure.

Tuco88

Hi lads, thanks for all the Info and the fast respond. Its getting to be a bit painful now ha. The control panel will be between no more than 10m from the Motor its final location is to be decided tomorrow. Its 400v Line Voltage on the button, I have a feeling I might be looking at two runs of 35mm^2? I did not hear a word on the motor info yet there chasing down the previous owner of the plant id say its a waste of time.

55kw is down on the Input from what I can make out, PF looks to be 0.85, the made good use of a forklift when moving the motor to make life very hard for me. Its green in color id back money its of English origin. We tested all the winding and its in good shape.I am a still worried on the Delta current tho. I have 93 Amps FLC (lets say for now) its sure to say with all the correction factors and so on it will be greater. Do I take the 93 Amps for my Mv/A/M cable calc ? or do I need to look at Delta current of (I)L Sqrt 3 so 161Amps? Should I be sizing a cable for this or is this just Protection characteristics to be taking into consideration?

My understanding is than it will start on (soft start Phase voltage) Star 1/3 (I)L then by the use of a timer, change over to Delta receiving full Amps/High Torque output.

Its driving a big feck off cooling Fan by belt, It was driving this up til about 4 years ago (there is now a new switch room and plant layout)

What id do to go back to something with a VSD again!

Thanks again.

2011

Tuco88 wrote: »

The control panel will be between no more than 10m from the Motor

Does this contain the star delta starter ?
Or is it just the control station ?
Is it 55kW when connected in delta?
If so then the delta current is 93 amps under full load conditions.

Do I take the 93 Amps for my Mv/A/M cable calc ?

Yes, for the cable feeding the starter. After the starter it will be ½ of this per cable because two cables will be feeding the delta load.


By the way this is an example of a soft starter. It is very different from a star delta starter.

Par1

Is the 55Kw the rated power of the motor?.

Tuco88

Yes 55 kw is what it's rated. It's going to be mains out to the control panel and two legs to the motor. Am I correct in saying the the overload is to go under the main contactor of the "delta loop" set to around 55% of the f.l.c was my idea.

2011

Tuco88 wrote: »

Yes 55 kw is what it's rated.

For a delta connection or is it 55 for a star connection?
Your supply voltage is fixed so each configuration will provide a different power output.

If star provided the same output there would be no point in switching to delta.

Am I correct in saying the the overload is to go under the main contactor of the "delta loop" set to around 55% of the f.l.c was my idea.

From memory overloads are set at 0.58 of the full load current.

Hoagy

The overload is normally fitted below the delta contactor.
You will often find that they have done the 55% calc for you if it's a purpose made ASD starter, so the scale may already be FLC.
With a fan of that size there will be a long run up time in star and if a phase drops the starter won't protect it, so I'd be fitting a phase failure relay for peace of mind.

Hoagy

2011 wrote: »

For a delta connection or is it 55 for a star connection?
Your supply voltage is fixed so each configuration will provide a different power output.

If star provided the same output there would be no point in switching to delta.




From memory overloads are set at 0.58 of the full load current.

The motor rating is always for a delta connection. Running it in star just limits the currents while it's spinning up.

2011

Hoagy wrote: »

The motor rating is always for a delta connection.

Always good to check.
I have seen plenty of motors rated for the same output value in star & delta but for 2 different line voltages (for example 400 volts in star and 690 volts in delta).

2011

A very good link here. This states that the overloads should be set to 0.58 of full load current.

Hoagy

2011 wrote: »

A very good link here. This states that the overloads should be set to 0.58 of full load current.

Good link.

I take your point about the overload, also I see they are fitting it under the line contactor.
The difficulty I've had with that is tripping just after a long run-up.

Hoagy

double post.

2011

Hoagy wrote: »

also I see they are fitting it under the line contactor.
The difficulty I've had with that is tripping just after a long run-up.

What do you mean?

Hoagy

2011 wrote: »

What do you mean?

If the overload is fitted under the line contactor then it carries line current during the star period even though it is set for 0.58 of FLC.

If the run up is long enough it may trip while still in star, but more often it may trip just after the changeover into delta.

The simplest solution is fit the overload to the delta contactor so it doesn't carry the star current. but there is no protection during the star period.

I suppose the ideal solution is to have two overload relays, one on the line contactor set to FLC and one on the delta contactor set to 0.58 of FLC.

If there's room.

2011

Hoagy wrote: »

If the overload is fitted under the line contactor then it carries line current during the star period even though it is set for 0.58 of FLC.

If the run up is long enough it may trip while still in star, but more often it may trip just after the changeover into delta.

Due to the reduced current drawn in star configuration I don't see why the delta overload would trip. From the above link:

For normal star-delta starting, the controlgear must be sized in accordance with the following currents:
• Main contactor K1M 0.58 Ie
• Delta contactor K2M 0.58 Ie
• Star contactor K3M 0.34 Ie

I would expect to have to adjust overload settings during commissioning. The above are guidelines to ensure that overloads with the correct ranges are selected. A grip on ammeter would be invaluable. I think that overlad settings would be revised downwards as motors are generally oversized.

frankmul

Reduced starting current but it still is 2-3 times the delta running current.

2011

frankmul wrote: »

Reduced starting current but it still is 2-3 times the delta running current.

Properly selected overloads and fuses are designed to take account of starting currents.

frankmul

2011 wrote: »

Properly selected overloads and fuses are designed to take account of starting currents.

I agree, standard overloads can cause an issue on long run up period.

Bruthal

2011 wrote: »

Due to the reduced current drawn in star configuration I don't see why the delta overload would trip. From the above link:.

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.

2011

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.

2011

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

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

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

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.

[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

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

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

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

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

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

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

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

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

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

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

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

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

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

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