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Topic Title: Calculations for Designing Motor Supply
Topic Summary: Query
Created On: 15 March 2013 01:37 PM
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 15 March 2013 01:37 PM
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niallcampbell

Posts: 17
Joined: 25 July 2008

Hello folks,

I was wondering if anyone could help me with the following query.

Due to age/obsolescence I'm having to change an existing 7.5kW (14.5A FLC) 3phase motor out for a 15kW machine (27.8A FLC). However, I have concerns the supply cubicle may not be up to the job.

Can anyone point me in the direction of an online tool or tutorial for calculating supply requirements for motors?

Existing supply 3ph, 50Hz, 415V through 35A fuses and a 40A isolator.

Thanks for any advice, this is a new one on me and I have to admit I've not had enough exposure to design!

Ta.

Edit: I should point out that the machine is DOL not soft-start.
 15 March 2013 02:24 PM
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Dave69

Posts: 441
Joined: 16 July 2011

need more details really.
first of all why cant you get a direct replacement or what is wrong with the old motor?

As you state you are basically doubling the size of the motor it most likely the cabling to the motor will be too small, the contactor rating will be too small and certainly any over current protection will be too small.

15kW on DOL may cause you problems depending on your supply, star delta would help but better still a soft start.

is it a 400v motor. How long are the cable runs, etc. etc.?

And btw if your existing motor was backed up by 35A fuses you had a design fault to start with
 15 March 2013 04:22 PM
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niallcampbell

Posts: 17
Joined: 25 July 2008

Hi Dave,

Thanks - I've asked for further details to be supplied from site. Note that the motor has been running with this set up since late 1970's and is being replaced with a larger unit as the fan it drives is also being replaced and needs a larger motor to suit.

Without full nameplate details I'm working on a lot of assumptions right now i.e. HP/Efficiency/Power Factor etc.

Can you explain your reasoning behind the 35A fuses not being up to the job to start with? Of course again I'm assuming these are HRC fuses, but am waiting on full details. Bear in mind this machine has been running since late 70's with existing set up as I mentioned above!

I'm a young engineer so you'll have to forgive the questions! It annyos me when I can't work something out for myself so I thought I'd turn here.

I'm assuming you've taken FLC and multiplied it by a factor to give approximate fuse rating for protection?

Thanks,

NC.
 20 March 2013 10:58 AM
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niallcampbell

Posts: 17
Joined: 25 July 2008

For clarification, here are further details (hope image is AOK).
If anyone has any advice, or can point me in the right direction I'd be grateful. Currently digging out text books to read through again!

 30 March 2013 11:43 PM
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cwd65

Posts: 8
Joined: 16 March 2012

brook crompton motor circuit protection -direct on line start recommend
for flc 14a up to17a, use 40a standard hrc fuselink or motor rated hrc fuselink 32m40
for flc 23a up to 30a, use 63a standard hrc fuselink or motor rated hrc fuselink 32m63
note; dol starting current can be 4 to 8 times flc
star/delta starter would reduce the starting current by 66%
I agree soft starter probably best option
 15 November 2013 03:34 PM
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niallcampbell

Posts: 17
Joined: 25 July 2008

Hi folks,

Long time since I reviewed this topic - apologies.
Can you please advise where you obtained the information for selecting a 63A fuse?

I'm trying to learn and unfortunately this is getting the best of me. I'm ashamed to admit I need an idiots guide on this one!

I've found information on a site called Delta Rock, but rather than taking it as gospel, I'd like to know how the value is worked out. Call it morbid curiosity...

I'm happy with working out the FLC of the motor, I just can't understand where the recommended 63A fuse size comes from...

Many thanks for patience/explanation in advance!
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