7.5kw motor

[Besoeker]

its for a power unit driving hydralic pumps. The motor rating is correct. Im wondering wether i need to put i higher breaker in

The starting current for a typical cage induction motor is six to eight times FLC and will last for pretty much all of the run up time. Upwards of 80A in your case and possibly for a few seconds given that it isn't a high inertia load. Your overcurrent protection needs to reflect that.

 

[telectrix]

The metal links the OP mentions in the OP!

my mistook, thought i was looking for a diagram, but apparently he could not post one.

 

[Barry White]

Why do you think the 2.5 is to small?
We really need to see a rating plate.

/\ /\
what he says

without the rating plate we wont know anything you have wired it in delta which is "usually" the lower of the 2 voltages on the ratings plate theres a similar thread here http://www.electriciansforums.co.uk.../31498-supply-calculations-3-phase-motor.html
if it helps you showing the info needed to wire a motor. what motor protection have you fitted?

 

[Rockingit]

You've now posted this exact same thread on two (at least) places on the internet. Stop playing with things you clearly know little enough about and get someone who does know enough. There's enough of us around to at least save you the cost of a funeral.

 

[Darkwood]

Shurbunkin.... Normally Im happy to help but you are clearly dabbling in this with no experience, many questions need asking and its unlikely you'll know the answers.

-What brand is the mcb this is your first question?
-What mcb does the manufacturer recommend for DOL on a 7.5Kw motor (clue if its a hager they state either 63a(b), 50(c) or 50(d) but varies slightly from brand to brand)
-Is the circuit solely providing the motor or is it part of a larger system?
-Due to the 2.5mm been on a 50+ mcb can a fixed load clause be applied to the set up so you can over-rate the mcb to the ccc of the cable?
-If you can will your ELI still comply?
-What if any experience have you in Industrial motors and controls...If none walk away and don't take on work you are not competent in.

Other questions that effect circuit design -

Motor run up times
Incoming supply size
Frequency of start up also if a motor has inching option
Starting methods and stopping in some cases
Sorry I could go on here to cover all bases but as you have already said it for Hydraulics some of the questions need not be asked but still we need a lot of info that your not supplying.

PS ... give an insight as to why your little 16amp mcb is tripping in the scale of things and yes you may have to increase cable size to meet ELI even if the set-up is classed as fixed load and O/L protection of the cable is not necessary.... Can we assume the starter has O/L motor protection?

 

[telectrix]

sadly , i think the OP means to use the age-old system of trial-error-bang.

 

[rapparee]

Should current not be 10.8 amps per phase

 

[Darkwood]

Should current not be 10.8 amps per phase

Rough rule of thumb on a standard induction motor is double the Kw rating this give a figure that includes pfc, efficiency and losses .. so a 7.5Kw would be 15amp...

Just repaired one today that was 7.5 and it was a 14.4amp FLC but as I said its a rough rule of thumb.

 

[Besoeker]

Should current not be 10.8 amps per phase

No.
7.5kW means 7.5kW out so about 8.2kW in assuming about 92% efficiency.
Then you have power factor. Say 0.83.
So you arrive at 14A or thereabouts.

 

[telectrix]

Rough rule of thumb on a standard induction motor is double the Kw rating this give a figure that includes pfc, efficiency and losses .. so a 7.5Kw would be 15amp...

Just repaired one today that was 7.5 and it was a 14.4amp FLC but as I said its a rough rule of thumb.

i'd clarify that as with 3 phase motors. single phase is a much higher factor.

 

[Darkwood]

i'd clarify that as with 3 phase motors. single phase is a much higher factor.

Yes noted should has put that but had the theme of the thread in mind :dozey:

 

[rapparee]

thanks boys

 

[telectrix]

Yes noted should has put that but had the theme of the thread in mind :dozey:

i know that, but thought some readers might not. it's a bloody good rule of thumb, a bit pessimistic with the larger size of motors, but always good to look on the bright side.

https://www.youtube.com/watch?v=SJUhlRoBL8M

 

[Darkwood]

I use it onsite for a off the cuff guide its usually slightly over the top which isn't a prob when weighing up the cable and supply for a ball park figure but of course confirmation before actually ordering must be done.... with the larger ones Ive done this rule of thumb so many times I know roughly how much to drop the figure to be pretty accurate too... won many a challenge with work colleagues over the yrs and may free sarnies to boot

 

[Besoeker]

I use it onsite for a off the cuff guide its usually slightly over the top which isn't a prob when weighing up the cable and supply for a ball park figure but of course confirmation before actually ordering must be done.... with the larger ones Ive done this rule of thumb so many times I know roughly how much to drop the figure to be pretty accurate too... won many a challenge with work colleagues over the yrs and may free sarnies to boot

I have to provide guaranteed efficiency figures at the bid stage. And these are for variable speed drives at different duty points.

It's between a rock and a hard place. Bid too low and lose the job. Too high and fail performance tests and take a swingeing financial penalty. Half a percent wrong one way or the other is all it takes to lose the job or a hundred k. Ouch doesn't have enough syllables.

That led me to develop a program to model motor performance from basic nameplate data.
It has worked for me over the years. Given me sleepless nights too.

I tried to write the stuff in such a way that others in the company could use it. Not that they ever did. It was just too easy to get me to do the stuff - and carry the can if it went pear shaped. Thankfully that hasn't happened. At least not yet.

The rigmarole for the calculations doesn't lend itself to being posted on a forum but, just for a bit of entertainment, here's a couple of the subroutines I included to validate input data:


Sub Cretin()
Title = "Not a valid entry !"
Msg = "Some motor to have such an efficiency or power factor. Try again."
Style = vbCritical + vbDefaultButton2
R = MsgBox(Msg, Style, Title, Help, Ctxt)
End Sub


Sub Imbecile()
Title = "Improbable Value !"
Msg = "The current entered does not tie up with the Voltage and Power for the machine. Check the data. A calculated value is offered as a default. If in doubt use the suggested default value but in any case there is an opportunity now to enter new values."
Style = vbCritical + vbDefaultButton2
R = MsgBox(Msg, Style, Title, Help, Ctxt)
End Sub