Start time motor?

[RedSnake]

We are exchanging a supply unit with several hydraulic pumps.
We have 3 pumps supplied through the same fuses 250A they each have their own motor breaker.
Previously it was a 11 kW, 5.5kW and a 0.75 kW.
The 0.75 kW only starts occasionally and very rarely.
The other 2 start almost simultaneously.
What I realized was that the 5.5 kW motor has been changed to a 11kw motor so all of a sudden we have startcurrent at 335A.
22,5x8=180 A, 19,6x7,9=155A, 180+155=335A

I have the possibility to delay the start of one of the 11 kW pumps so I thought I would figure out the starttime of the motor.

Unfortunately I don't have all the figures for the pump that starts first the one I can't delay. :-(
It's a screw flexcore pump and have a max pressure of 40 bar but my guess it will never be more the 5-6 bar

The data for the other pump that I can delay is this.
It is a gear pump max pressure 45 bar.

The Load torque came from this formula

The only thing I ain't sure off is the η : Hydraulic-mechanical efficiency could find any number for it so I put it to 0,75 not sure if that is correct?

I did find this calculator I used..

But it feels the time is so short?

So I found another formula but then I got super long start times, seemed even more unreasonable, turned out he had missed some parentheses.... but there were some reference to ABB...

And there I found this formula

Which gave me approximately the same result as the calculator (ca 0.06 s).
Tst = ((0.0583 + 0.00289)/177.5)*157 =0.054122986 s

But can this be correct or am I missing something?



NAFO Sergeant Anna Gr 69th Sniffing Brigade

 

[iop95]

Time formula is correct.
Big questions about Tacc value and it's variation(maybe 5-15% decrease).
Motor voltage dip at start also.
I think real time start value is 30-50% longer (with some 5-10% motor voltage dip).

 

[RedSnake]

Well I did calculate the Tacc with this formula but here they call it Cacc?

And got the value 129,9762
And then the time becomes Tst = ((0.0583 + 0.00289)/129.9762)*157 =0.073912224s
Better?

NAFO Sergeant Anna Gr 69th Sniffing Brigade

 

[waross]

Are you getting minutes and seconds confused in the formula?
0.06 minutes seems reasonable. (3.6 seconds.)

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[iop95]

These formulas are approximate/simplified.
It assumes that the torque is constant in the 0 - rated speed range... but this is not true.
Time is in seconds.
Another major error can come from the voltage drop at the motor terminals.
And Tacc is net torque that develop acceleration not just motor torque.
Tacc = Tmotor - Tload
Need to know load torque (static + dynamic).

 

[zeusfaber]

Can you measure it? Stick a Hall clamp on one of the phases, connect it to a scope, flash the first pump up with the other two isolated and see how long it takes for the current to decay to something tolerable. I remember doing that to measure the starting current on a DOL-started grease pump and being surprised to see it reach steady-state within a couple of cycles of 60 Hz.

What does it sound like? If the start of a hydraulic pump sounds like a squeaky thump rather than a perceptible run-up, then you're looking at start times in the small-fraction-of-a second range.

A.

 

[RedSnake]

At the moment I can't do that since the motor and pump isn't here yet, it is 5 weeks to installation.
When this started we were buying a turnkey installation but due to incompetence on all sides, it ended up with me and my hydraulic coworking doing almost all the job and I am the one that ended up with doing the commissioning.
Which means that I am the last one on the chain standing there if things don't work as they should, and I hate surprises!

I asked the guy who the supplier hired to make the drawing changes about this too.
I have worked him since I started so I know he is good at what he does.
He made a FEBDOC calculation on the fuses with the motordata and came to the conclusion that I can drive both motor in total 355A for 4242 sec ca 70 minutes without blowing the fuses.
So I guess I am home free on that one.
However he also suggested I could measure the starting time on the 11kW we have installed right now, just for fun..
Hmm I will see if I will have any time over for fun..

NAFO Sergeant Anna Gr 69th Sniffing Brigade

 

[LittleInch]

DOL starters are pretty fast alright, especially at relatively small motor sizes.

The unknown is the load inertia / torque over time for the start when full of fluid.

Might just be straight line from zero to whatever full speed, full flow high pressure flow is.

So maybe 0.09secs?

In your OP why do you have tow different FLA for the two 11kW motors? (22.5 & 19.6?) Are they not identical?



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[RedSnake]

No one is a 1500 rpm 4-pol motor the other a 3000 rpm 2-pol motor.
Actually it should be 21A and 19,6A must have read the wrong motor number..

 

[electricpete]

These formulas are approximate/simplified.
It assumes that the torque is constant in the 0 - rated speed range... but this is not true

I agree. They generally work off of the average accelerating torque. What is actually relevant for starting time is the inverse of the average of the inverse of the accelerating torque. If that doesn't make sense, consider what happens when accelerating torque goes to zero at a certain speed, the average accelerating torque may still be positive and predict a succesful start, but that inverse of the accelerating torque goes to infinity and it's average goes to zero (correctly predicting stall).

If you have torque speed curve of pump (considering fluid conditions) and motor (considering voltage) along with driven inertia you can calculate everything. The less info available, the more of a swag.


Sometimes you have to split the difference. On the other side of the question: how much of a delay can the plant tolerate. If it is not critical to get the 2nd up and runinng quickly I'd give it a few seconds to make sure the other one can start successfully.