3 Phase Motor Question ??

[Skwifta]

Hi and thanks in advance to anyone that can help clear this up for me. If Id got a 55Kw 3 Phase motor then worked out how many amps it draws using the following equation 1000 x 55 / (√3 x 0.8 x 415) = 95,645 amps. just wanted to know is this shared between the phases so it would be around 32 amps per phase, or is this 96 amps per phase
and what size mcb would be used ?? Thanks again..

 

[Marvo]

Yep, your 95A sounds about right. That will be 95A per phase.

You wouldn't normally attempt to protect the motor using an MCB, the MCB can be sized according to the cable that's supplying the circuit. Motor protection would be a separate issue.

 

[Skwifta]

that's that cleared up matey thanks loads !! think the star delta starter has a thermal cut out in it (it was supplied on manufacturers advice when given the motor spec), would this be the type of protection required ? and so am i right in saying in this situation you would be looking at using 70mm cable and also a 3 x 100 amp 3 Phase supply that is already drawing around 50-55 amps max per phase in normal day to day running would be insufficient ?

 

[davesparks]

I would be looking at calculating the size of cable required.

70mm sounds a bit big on the face of it, but then we don't know anything useful about it.

 

[sjhall]

70mm is way oversized, how far is motor from board ?
200A 3 phase supply will be fine for what you require taken into account that your only running on 55a per phase at present.

Note - 200a 3 phase supply = 200 per phase.
So no, the 100a 3p supply in at present won't cut it.

 

[Marvo]

If it's got a Y/D starter then make sure it has a thermal overload incorporated in it and set it up correctly to protect the motor. Where did you get the 70mm from? As Dave says calculate the size of cable required according to the usual installation method and length criteria. I don't see why the 100A supply wouldn't be fine TBH unless there's something you haven't told us.

 

[sjhall]

Marvo, he states in one of the posts that they are already drawing 55a ph at present ... But some of it is confusing to read tbh

 

[Marvo]

Maybe I misunderstood, I thought he was expecting the motor to draw 55A under load, maybe it's a second motor he's talking about. I just re-read both his posts and I'm still not sure....

 

[sjhall]

96a ph for the motor and factory already pulling 55a ph at present without motor installed ... I think :-0

 

[Darkwood]

70mm for the supply cannot be said to be correct or not ... we have zilch info regarding the start up duty of load, install methods length etc need I go on... I fitted 70mm to a lathe recently rated at only 33kw do to VD and install methods

The best advice with regards the nature of the question is for the OP to get an onsite overview by a competent person, this could be very costly to get wrong.

The OP is a trainee and if this is homework then his coursework will be his best reference as we dont know if this is a fielded question on his learning .....If this indeed a real job then first port of call is to ask whoever oversees his work .... this is a big step for even the most educated trainee and is out of the scope of most of the training levels as this goes into the electrical engineering too hence I query whether this is hypothetical regarding course work.

 

[Knobhead]

Just read through this.

My words of advise:
Get an electrician that knows what they are talking about.

So far we’ve had wild guesses and conjecture, not good in engineering.

Not that the OP will come back again.

 

[Skwifta]

The supply is is a 3 phase 100 amp per phase, each phase is drawing 50-55 amps in normal day to day running already without the said 55kw motor, the motor would be no more than 10 meter away from the main DB with the Y/D starter situated halfway between the main DB and the motor all cables would be run on tray.. The info plate on the motor says it draws 96 amps and the calculation in my original post clarifies that,
this is why i asked is this 96 amps per phase or if it would be divided between the 3, so i could then work out what size cable and circuit breaker would be used by referring to the relevant tables in BS7671, i myself would have assumed it would be somewhere round 25-35mm cable (swa) and would be fed from a 96 amp 3 pole type C breaker and i was also thinking that the supply was not big enough. this said i do only have limited knowledge i did the C+G 2351 and NVQ level 3 in electrical installation in 2003 along with what was then the 16th edition since then i have updated to the 17th edition + part P and completed a fire alarm and emergency lighting qualification, the person i work for has the same but has completed the 2391 inspection and testing so its not something either of us is capable of doing with our knowledge, i just wanted to find out for my own piece of mind if had worked it out right hence why im asking on this forum.... if ive not provided enough information i can try and find out a bit more, and can only give you my word that this work would not be attempted by me on the advice from this forum, i would always make sure id got relevant qualifications and referred to the relevant material in BS7671 before carrying out any work not just take someones advice from the internet...

while i was looking for info on the subject i came across this link :-

What is cable size for a 55kw 3 phase induction motor 10 metre run for cable 415v

Which is the reason 70mm was mentioned ??
Anyway thanks again for all your answers and if anyone does know the proper way to go about working it out im all ears or info on what course or qualification you would need to do this type of work even ??
Thanks again hope to hear back...

 

[davesparks]

Unless the existing DB you are connecting to is something like a new memshield3 board with the 27mm MCB ways that accept up to 125A MCBs then you won't get a big enough MCB in there.
Normal 18mm wide MCBs only go up to 63Amps.

 

[davesparks]

And that link you have given clearly states 70mmAL cable, so you would expect it to be quite a bit bigger than a copper cable.
This is due to aluminium being somewhat less conductive than copper.

 

[Darkwood]

There's much more to this than a current calc' ....

What is the motor for, whats it doing?
What is the starting duty of the motor?
Is the motor starting under load?
Is the motor the correct type to allow for star/delta starting or is it some other kind like a 2 speed set-up amongst many variants out there?
How many times a day will this motor be started?
Is their an inching option with the control.?
What starting method is been used for the motor?
If VSD controlled has the correct methods for limiting harmonics been undertaken?

Just to get us started with any one of these question could have implications on whether you are allowed to put this motor onto the incoming power supply?...

This kind of work does not really fall under the BS7671 well maybe the actual calculations yes once you know all the criteria but this is more an Electrical Engineering field and by the fact of the nature of the question and the fact you are probably unaware of any of the above questions needed to be asked then my advice is either get a competent person on board in this field and work along side them or walk away.

I have visions of service fuses and substation cutouts operating if you think this is just as simple a task as putting it on the existing set-up.

You will most likely require a larger supply and the forms to fill out will ask most of the questions ive just put to you and if you get them wrong their could be serious consequences as the DNO frown upon any load putting transients down the lines and upsetting other customers.

This is not a set-up you can resolve through a forum chat -too many crucial factors need to be known and any missed info could be vital and because this is not your field you will be unaware of what needs to be known.

Cable calcs is one of the last things to be worked out here ...first you need to know if wiring this to the supply is an option in the first place and this can take several weeks of liaising with the DNO.

Im interested in what this motor is doing if you could let us know?