3-Phase Queries

[Barry Dummigan]

A 3KW, 3-phase, 230V motor draws approximately 13Amps (if my calculations are correct).............
I imagine this is the rating for each individual phase?
What rating should I give my breaker? (Formula?)
If I take two phases for my control circuit, which formula could I use to fuse my circuit?

 

[Knobhead]

First question: are you in the UK?
Second question: where are you going to get 3Ph 230V from?

The motor will be dual rated for 415/230V or 380/220V. Have a look at the motor nameplate it will state two different voltages and currents.

The breaker is only to protect the cabling not the motor, the overloads do that. the control gear can be fed from the incoming feed via a small fuse fed from the incoming supply to the starter. The small current drawn by the control won’t make the slightest iota to the overall current.

As for your current calculation, I don’t know where you got that from.

3KW @ 380V .9Pf & 90%Ef = 5.57A

 

[Barry Dummigan]

Yes im in the uk..

Its a dual 415/230V
I reviewed my calculation and can see where I went wrong. :snore:

So if i multiply my ans = 5.5 by 2.5 (250% of normal operating current) = 13.75.. Would a 15Amp M.C.B. be sufficient, but more importantly appropiate for my circuit?

Thanks for the help BTW....

 

[telectrix]

without knowing exactly what the motor draws, it's hard to say on cable and MCB size/rating, but i would think whatever MCB you use, it will have to be a C type.

 

[Darkwood]

Most mcb manufacturers have mcb panels and control catalogue and in the rear of the book they will give you the rating of the motor, transformer etc with the need for your calcs Merlin for instance suggest a for a 3kw 4hp motor at 400v it give running current as 6.4amps and suggests a 20amp type(b) or (c) , or a 10amp type (d).
Regardless of how accurate your calcs are you still need to talk to technical or look it up with regards to the make of mcb you choose to use, as different manufacturers make mcb's with different tolerences at the same rating, but 9 out of 10 times the ratings will be similar across the different models.
This is one thing i think should be explained in colleges as just using pure calcs with regards to motors, transformers and discharge lighting can and often does end you up with incorrect mcbs and nuiscence tripping then you find yourself in an arkward position in that upgrading the circuit breaker may over-rate the cable you fitted.

 

[Darkwood]

Can you clarify if your using 3ph 400v or 200v, and if using 200v are you using some kind of inverter to achieve this voltage??? If using an inverter to control the motor then refer to the manual to find suggested mcb for your particular set-up as they have a controlled in-rush and can be used on smaller mcb's

 

[Knobhead]

Well the OP is still on line.

I must be getting senile where did 13.73A come from? You need to check your calculations. As I said before, the MCB protects the starting equipment and cabling only! Overloads protect the motor.

 

[Barry Dummigan]

Well the OP is still on line.

I must be getting senile where did 13.73A come from? You need to check your calculations. As I said before, the MCB protects the starting equipment and cabling only! Overloads protect the motor.

Sorry!!! Up to my eyes in assignment work here, all over the place..
I'm aware that an MCB protects starting equipment and cabling..

Lets try again....
A circuit breaker should be rated 250% of normal operating conditions 2.5*
So under normal running conditions my motor (in theory) draws 5.5A
16.5A (not 13.73)

Then I should downgrade to the nearest size of MCB, which would be 15A 3-pole MCB
to protect cabling and controls

 

[telectrix]

you should not downgrade as you put it. if your motor draws 16.5A then you need a 20A MCB. (In>Ib.) , and size your cable for 16.5A, i.e. 2.5mm should be ideal. (unless there are de-rating factors to be applied)

 

[Barry Dummigan]

you should not downgrade as you put it.

Sorry I havnt familiarised myself with proper electrical linguistics and terminology... Yet!!!
:smiley2:
Thankyou for your help, and others who posted!!!!

 

[Darkwood]

Be very careful Barry!!!! The advice we give on here can be real world which differs sometimes to calc and tables you have been shown, hence i said you could use any of the following:-

20Amp type (B)
20Amp type (C)
10Amp type (D)

These are the stated guides from the mcb manufacturers but if your doing homework here you must follow the method you've been shown to arrive at the answer, i would assume they taught you that type (C) is for motors and looking at your calcs then a 20Amp type (C) will be your answer.
This is why i dont agree with the teaching methods here, i believe they should give you actual manufacturers ref' tables so as to prepare you for realworld design. If you put 10amp (D) you would probably be marked as wrong but it is an option when reducing cable size costs as long as the Earth loop impedance isnt compromised.

 

[bhoywonder]

Please help what rating of mcb would be required to supply a 20Kva 400v to 110v transformer ?

 

[Knobhead]

I think you’ve hit the nail on the head DW.

Theory is great until your being beaten around the head by a foreman wielding a copy of the regs, wanting to know why the cable you’ve installed is twice the required size he’s just found in the tables.

Go in to real engineering and you may find the cable sizes are sized down on what you expected for a motor. They’re just not bothered about volt drop during start up!

As an apprentice I worked out a fan motor needed a 240mm cable, we put a 150mm cable in. When it came to commissioning I was told to “Press to press the button”. It started perfectly. Still not satisfied I tested the voltage at the motor during a start up, it fell to 345V. Nearly 100V drop from the starter to the motor but it started OK. The cable was rated to the FLC+115% of the motor, not some pie in the sky figure.