three phase motors

se849

Hi to all.Can somebody please tell me wat is the largest three phase motor a d.o.l. starter can run.i have a 5.5kw motor and am wondering will i need a star-delta starter.Also is there a formula i need to calculate the size of cable needed to supply this motor.many thanks

Stoner

yes there is a formula, you will need to know the length of cable run and the resistivity of the conductor in the cable you intend to use.

this will calculate the resistance

R=pL/A

R= resistance
p= resistivity of the cable you will use
L= is the CSA of the cable (for example 6.sq.mm ) this is a trial and error variable

Use to calculate R, the resistance,

Use this R to calculate the voltage drop using V=RxI where i is the starting current of the motor

se849

Thanks for that stoner.the motor is 5.5kw and am supplying 415v to it so will I still need a 4sq cable.thanks

2011

Unless it is a very long cable run a 4 sq. mm cable would be fine.

Can somebody please tell me wat is the largest three phase motor a d.o.l. starter can run

Good question, I am not sure but you are allowed to start a small motor such as yours witout a star delta starter.

hey_hey

Your not supposed to use direct on line for motors over 5kw....but i wont tell if you dont

2011

Your not supposed to use direct on line for motors over 5kw....but i wont tell if you dont

I think you are wrong, but I will check it out in work over the next few days.

Davy

hey_hey wrote: »

Your not supposed to use direct on line for motors over 5kw....but i wont tell if you dont

Probably 5hp rather then kw?

hey_hey

unless its changed since march maybe, but thats what we were told in tech.

se849

thanks guys,have to go and change my cable so,i originally ran a 5 x 2.5 cable.:(

2011

thanks guys,have to go and change my cable so,i originally ran a 5 x 2.5 cable

Perhaps not.

There are 2 main factors that you have to look at when sizing a cable:

1. The current that the cable will have to take
2. The volt drop on the cable (as Stoner pointed out)

By my calculation the current drawn by the motor will be 8.7 amps per phase (with a power factor of 0.88). A 2.5 sq. cable can take a continous current of 25 amps (table ET101, 52D installation method E ambient temp 30 DegC).

Now all you have to look at is the volt drop per amp per meter. You will find a table for this in the regulations. What is the length of run?? Is the motor being fed from the main board or a sub board??

Stoner

I stuck it into a software package yesterday, sorry for not getting back, I got 9.2 Amps back more or less equal to 2011. I agree that 2.5.sq.mm will do but when I thought about it first it sounded like a 4.sq.mm but I edited that out of my initial response.

Anyway best of luck with it, I think I used a 10M run in the calcs with ambient temp 30 degrees.

2011

when I thought about it first it sounded like a 4.sq.mm but I edited that out of my initial response.

It amy well be a 4 sq. if the run is long.


I arrived at 8.7 amps by using the formula below:

I line = Power / [(square root of 3) x (V line) x (Power factor)]

Where,
V line = 415 (volts)
Power factor = 0.88 (typical for a motor)
Power = 5500 (watts)
I line= ? (amps)

The software came up with nearly the same answer, it might just have used a slightly different line voltage or power factor.

I got my hands on the 4th edition of the regulations today, but cant find out how large a motor is allowed to start in DOL.
I would think that it is larger than 10kW

se849

thanks to all who replied.this was my first time to post and it truley has being very helpful to me.thanks again to all the legends:D:D:D

davelerave

Stoner wrote: »

I stuck it into a software package yesterday, sorry for not getting back, I got 9.2 Amps back more or less equal to 2011. I agree that 2.5.sq.mm will do but when I thought about it first it sounded like a 4.sq.mm but I edited that out of my initial response.

Anyway best of luck with it, I think I used a 10M run in the calcs with ambient temp 30 degrees.

what software are you using. thanks.

Stoner

Amtech,

They have good support here too, and they now own the estimation.co.uk products too and are kciking it out as a all in one package aimed at off the shelf customers.

Umiq88

7.5kW is the maximum allowed for a DOL starter above is star delta

Also can someone point me to a table for cable sizing of a forumla

I know the basic 1.5 will take around 15 2.5 will take 25A but is there a forumla I can use to be more accurate and is there a safety factor applied?

Just to note im not an electrician so dont be too worried for the stupid questions

2011

hey hey:

Your not supposed to use direct on line for motors over 5kw....but i wont tell if you dont

I must admit, much to my surprise you are sort of correct, more correct than me anyway!

I got my hands on a new copy (4th Edition ET101) of the regs to try to see how large a motor is allowed to be if started DOL. I had remembered connecting up some very large ones in the past. I could not find the answer so I asked an engineer that specialises in this sort of thing. He told me that the answer was not in the regs, but that as far as he knew the ESB had its own rules on this. As far as he knew the largest motor that the ESB would allow to be started in DOL was in the region of 5.5kW, unless the installation had its own transformer(s) (>38kV medium voltage supply). In a large project he had worked on recently some motors greater than 100kW were started in DOL!

2011

Just to note im not an electrician so dont be too worried for the stupid questions

There are no stupid questions.
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