Circuit breaker for a three phase motor

[JuniorSparky]

Hi guys,

The question may be too easy, but when I think more over something at the end I start to doubt what is right and what is wrong. So I was wondering how do we choose circuit breaker for a three phase motor (lets say 6 kW, 400V, pf=0,8?
We will need to calculate the current - I = P/ (sq.rt.3. x V x PF). So far so good. Now I'm getting a bit confused. This should give us the line current I = 10,83 A. But don't we need the phase current when we select circuit breaker for that circuit?

phase in star will be the same - 10,83 A
phase in delta will be - 6,25 A

Thank you

 

[telectrix]

the breaker will only be concerned with the line current. it don't care what the phase current is.

 

[Darkwood]

This kind of question is usually out of the scope of your training, so I ask why you ask first.

When picking the MCB type and rating of a motor your first step is to get the manufacturers tables for Kw rating and MCB that can cover it.. this is only for a standard inductive motor either DOL or Star/Delta with a normal start up ....

You are normally always taught to estimate everything for the max load first in a circuit but when circuits are highly inductive you need to know the required protective device that can handle the start up spike then work backwards.

A 5.5Kw motor (a standard rating) dol or star/delta would require a MCB (Hagar in my example) of 50A(b), or a 40A (c or d), this varies slightly with manufacturers but Im curious why you ask as this is as it advanced design.

 

[Marvo]

So I was wondering how do we choose circuit breaker for a three phase motor (lets say 6 kW, 400V, pf=0,8?
We will need to calculate the current - I = P/ (sq.rt.3. x V x PF).

Always work on the run current given on the plate on the motor itself. If a motor is rated at 6kw it means it can do 6 kilowatts of mechanical work and it would have been called an 8 Horsepower motor in days gone by. The kilowatt rating of a motor always refers to the mechanical output power not the electrical input power. To calculate the electrical kilowatt consumption from the 6 kw mechanical output power you'd need to factor in efficiency into your formula above.

As pointed out above an MCB is there to protect the installation wiring so you'd select the current rating of the MCB according to the usual wire size, installation method, length of circuit etc etc. You might use a C curve or even a B curve if it's required to accomodate the start current of the motor and obviously only if fault disconnect times can still be met.

To protect the motor against damage you install a thermal overload on the supply contactor or you install a manual motor starter which is basically the same thing in a different form factor.


Thermal Overload



Manual Motor Starters.

 

[JuniorSparky]

Thank you for the help. I just realized how far I am from motors. I'm about to do the AM2. We have to choose the circuit breaker for the circuits. I've read some info saying what we have to use, but I thought it can't be the same for everyone if we all have different scenarios. So I wanted to know how I can do that. But I guess it is the same as the MCB protects the cable.
The tutor might have been talking that we have to calculate the overload protection and I didn't even realized at the time it is different.

 

[Darkwood]

Thank you for the help. I just realized how far I am from motors. I'm about to do the AM2. We have to choose the circuit breaker for the circuits. I've read some info saying what we have to use, but I thought it can't be the same for everyone if we all have different scenarios. So I wanted to know how I can do that. But I guess it is the same as the MCB protects the cable.
The tutor might have been talking that we have to calculate the overload protection and I didn't even realized at the time it is different.

Either discuss with your tutor or check your notes, the issue with our experience and what you get taught is they are on totally different scope and getting the correct advice from us may not be what your tutor is looking for, he may be trying to evaluate you have taken in the scenario and situations he has taught you so my advice would be to paddle in the shallow end before you dive in the deep end.

Collectively me and Marvo will have about 50yrs work experience (I'm 21 ) which will be way beyond the field notes you are learning from so always best to 'in this case' talk to you tutor or class mates as the course and teaching are relevent to the answer you seek here... most times we can help but this is just one of those questions that can get very complicated very quickly ans that not the info' your course is asking for.

 

[JuniorSparky]

Thanks Marvo. I will get back to the tutor although I prefer to look for answers here. The tutors usually teach only what we have to study and if I want to know something more I am left in the dark (on a few occasions during my study I received answers like "This is out of the scope of your study"). I guess he will at least tell me what I need for particular question

 

[Darkwood]

Your enthusiasm on this is great, but on this particular subject its best to wait until you advance the basics as taking in too much in this area can confuse you as to what answers the tutor expects and that's the last thing we would want to happen, most other queries are usually easy enough to answer in full so please do keep the questions piling in, you just picked out an area that can get very complicated very quickly and as we said, this could contradict what your tutor is teaching you or looking for in your answers.

 

[Marvo]

Thanks Marvo. I will get back to the tutor although I prefer to look for answers here. The tutors usually teach only what we have to study and if I want to know something more I am left in the dark (on a few occasions during my study I received answers like "This is out of the scope of your study"). I guess he will at least tell me what I need for particular question

It's great to be inquisitive, it's how you learn but I can also see the tutor's view, there's a scope of work that must be covered and learned in a certain time frame so avoiding going off on tangents is probably the only way they can do this. By all means keep asking questions but some of the rabbit holes go quite deep (motor theory being one of them) so don't be tempted to go too far off the path, primarily stay on track with the course.

 

[JuniorSparky]

Thank you Darkwood and Marvo. I will get deeper into the subject when I get a bit deeper into the theory. I will stick to the simple answers at the moment then. At my stage I am familiar with the power equation so I guess this is what I have to use to find the current and choose overload protection.