3 phase fan motor.

[ncpitts]

Hi all, want to pick your collective brains.

I installed a supply to a 3 phase motor which is connected in star delta.

It is a 22kw motor .
I calculated that a cable of 6mm (4 core) by using KW x 1000 / 1.73 x 415 x 0.95 (pf)

This was installed and protected by a 60898 D MCB rated at 40A

However when the fan attempts to start the staring current gets to 88 amps (approx) and the breaker trips.

Any hints or advice welcome as motors are not my strong suit

Thanks

 

[gazj82]

Are you certain the motor is connected to the starter correctly?

 

[Darkwood]

If the motor is connected correctly your breaker is tripping on the change-over spike from star to delta you can't just put motors on MCB's without refering to manufacturers specs... its the first thing you do if you have to use a mcb.. its quite possible you require a larger mcb and may loose your ELI compliance as well as over -rate the cable although the latter may not be an issue depending on install.


Minimum size mcb required for 22kw is 63(d) or 50amp (HRC) ... this will mean your whole install could be incorrect... your actual question gives the impression you are not experienced in this type of install and you have not taken a design course.

MCB's must be selected in accordance with the motor starting DOL as the star/delta produce a high current spike at change over that is equal to that of DOL starting.

-Other options exist - not cheap fix though ...is this a big long run?
-You provide no info as to your Qualifications and Experience - are you competent and Qualified at other electrical levels?

There's alot of area's you can get this set-up wrong and as we have already established it seems you have got the supply set-up and upstream protection wrong, when you say 88amps i suspect your using a run of the mill clamp on ammeter and if so it will not have a good enough sampling rate to catch the inrush values.

As you have a fan on a 22kw motor it could be the case you have a extended starting duty this also could mean an mcb is not suitable for this set-up but this will all be down to your specific installation and everything from ducting length and design (if inline) can effect the choice of supply design.

Lastly 6mm on a 40amp (d) what is your ELI reading ... its going to need to be close to the incoming supply as 0.23ohms isn't a easy target?... If you don't meet this then the whole install is a hazard.

PS the mcb could even be tripping on the star arrangement as the breaker is well undersized so over sensitive to the normal start up currents.
-

 

[Clive P]

Hi,
As Darkwood says you need to look at your protection for starters ( no pun intended). I did a quick calc and your running current is over 40A and on an MCB, motors react a lot differently to other circuits and if I was you I would follow DW's advice.

Regards.

 

[Marvo]

It sounds like you're mistakenly trying to protect the motor with your MCB selection. Spec the thermal overload in the YD starter according to effective protection for the motor, spec the MCB according to the cable size and spec the cable to to accommodate the load.

I don't rate your chances of pulling this installation off on the cable size you've selected, as DW suggests I would have run a 63A circuit probably using a 16mm cable (length dependent) for this size of motor.

 

[Knobhead]

Not a lot going right here regarding the protection, current calculation and cable size.

Have a read of this

http://www.schneider-electric.hu/do...ntrol/asg-4-motor-starting-and-protection.pdf

 

[ncpitts]

Thanks guys.

To be clear , I am a 47 year old spark with 28 years experience 2360,2361,2380,2381,2385,2391 and 2400 (yes design) qualified so fairly competent .

However most (all) of my experience is absolutely nothing to do with motors , mainly lighting and larger heating systems.

The run on this is approx 30m and originally I was asked to put in a supply for 22kw over 3 phases (which I did and it was originally on a C curve)

The fitter at the mill has told me ( after the event ) that it is a fan motor and that he has tried a D breaker.

Which is why I have turned to you.

So my question is , loop impedance allowing , what cable and breaker size do you guys suggest. They do have a chap that repairs their motors and he suggested a 10mm with a D63

Thoughts Gents please

Btw thanks to all the replies so far

 

[Darkwood]

Ive given you the required mcb size that should cure your issue (provided there exists no other problems)... As you are an experienced Electrician albeit not with motors you can now do the standard calcs you know to find the correct cable size ... you have:-

Mcb rating 63amp
Motor FLC approx 40amp
Your distance and install methods 30m

Problem you now have is your ELI has to meet 0.14 and call me a skeptic but im not too sure your going to be able to achieve this so a total redesign using Motor rated BS88 50amp (gm) should see you only having to comply to 0.83ohms.

You need to take a measurement at the origin of the install then do your standard calcs to give you a cable size and predict your ELI reading.... to comply with ELI its sometimes the case you end up with a cable many orders larger than the ccc you would expect to need or additional earthing.

Im atad against doing all your calcs for you but I have guided you the right way ... if your still unsure either show us the calcs with your measured values at the origin or pass the job on... but regardless of it been a motor you now have a guide to fuse ratings so its just standard cable calcs now

 

[ncpitts]

Just a bit of closure,

The motor had a fault and is being taken for rewind our repair, the motor msn said he thinks a C63 will be sufficient.
However after taking your advice I have run a16mm 4 core, using the spare core as a cpc.

All fluffy now with a max Zs of 0.17. Ohms (Ze is 0.05)

Thanks for all your help and on the next one well ask before fitting !

 

[Lucien Nunes]

I'm glad you've got it sorted, but a point came to mind that you have / had a getout, if you were contracted to lay on a supply for 22kW of load. The 22kW rating of the motor refers to its shaft output (30hp in old money) and not the electrical input, so with <100% efficiency the input at full load will be greater than 22kW, By definition, a '22kW supply' is inadequate for a 22kW motor, even before you allow for power factor and starting. A motor supply should always be sized relative to the plated line current (with due allowance for starting) rather than the power, and if this was not quoted to you along with the starting duty, they could hardly expect you to spec the circuit correctly.

 

[telectrix]

as in previous posts, darkwood and lucien have improved my motor knowledge, which, i am sad to say, is a bit limited.

 

[Marvo]

Thanks for the follow-up NCPitts, it's always good to hear the final result and I'm glad you've got a conclusion in sight.

.....The 22kW rating of the motor refers to its shaft output (30hp in old money) and not the electrical input,.....

Correct and this very point also caused confusion in a similar motor thread recently.

 

[Darkwood]

Just a bit of closure,

The motor had a fault and is being taken for rewind our repair, the motor msn said he thinks a C63 will be sufficient.
However after taking your advice I have run a16mm 4 core, using the spare core as a cpc.

All fluffy now with a max Zs of 0.17. Ohms (Ze is 0.05)

Thanks for all your help and on the next one well ask before fitting !

Note for your future reference the 63(D) was a guide for a Hagar ND range and is not a rating that is the same across the board.. if you seek values for Merlin MCB's they specify minimum 50(D) but at this size motor a (c) time curve can be problematic but not always. My values are based on a 4pole Motor with a run up time no more than 10-seconds ... hopefully you have had an insight to the information required before you even start to design and the problems associated with MCB's covering larger motors as ELI compliance is your biggest barrier hence HRC fusing is more common place for your set-up.

Thanks for the finale to the thread its often the OP never bothers.... so again thx.

PS ...Its likely this set up could be seen as a fixed load circuit and the need to protect the supply cable with the MCB could be omitted thus a smaller cable may have been possible with a MCB over-rating it but this gets a little more complex to calculate as you effectively reduce the earthing csa thus increasing the ELI reading, it becomes a balancing act of reducing cost and complying and many times I've found it much cheaper to run additional earthing rather than have a larger cable... I left this out of the conversation as the cable sizes involved here are still relatively small and short runs so not worth the speculation of trying to safe big costs.

 

[ncpitts]

Cheers guys, nice to know you don't think I'm a complete dim wit,

Just to be clear is there a simple ish way to allow for starting ?

 

[Lucien Nunes]

To do it properly you need to know the method of starting, the inertia of the mechanical load, and the number of starts per hour. The more inertia for any given load power, the longer and/or heavier will be the starting current relative to running. A conveyor starting fully loaded every 30 secs wiill require much more generous provision than a hydraulic pump that starts relieved at the beginning of the shift and runs all day. DOL max current can be up to 7x FLC, i.e. the locked rotor current which the makers will quote. Y-D typically 1/3 of that.

 

[LeeH]

Really enjoyed reading this thread, I've picked up 2 good pointers for my reference.

So a thanks from me.