Calculating motor start current

[Kayos]

Spending an extra (unnecessary?) £100k seems to me to make it worth getting the suppliers to sort it out, without any guesswork - and with a contracted degree of certainty.

The suppliers have done huge upgrade works in the last 10 years, changing from split phase to 3 phase and increasing the cables, sadly we are asking them to go far above that, hence the upgrade falling on us

 

[daleyd]

I bow to your liet detail. All I know is that my tiddly (and cheap) VFDs seem to tolerate several times the normal power for a few milliseconds. Note I did say ‘could’. My VFDs have a zero setting as one alternative for the speed ramp at start up, I think.

Top and bottom of this thread is RTFM, or take advice from the suppliers. We don’t have the manuals to read….

There are a few papers floating around in work that we use - below is an extract from one where modelling was done on various starting methods.

In my experience (with the caveat that you use a decent drive) even if you put a stupid setting in the ramp time (I would think that whoever is designing the system in question wouldn't be doing this) the drive knows what the ultimate current limits are and reduces voltage (and thus current) to suit. I've run some big >70 kw fans that don't reach full speed until the air is hot - at 20 deg c from a cold start they run into current limit and will sit at 40 odd hz until the air becomes less dense - the programmed current limit takes precedence not the ramp rate.

I totally agree that the manufacturer should provide this info, I can only pass on my experience - ideally in this case you want some real world data as it's such an expensive option to upgrade, especially if you use the worse case scenario of the drive absolute current limit and find the real world starting current is actually much less (Which is very likely)...

 

[daleyd]

if you use the worse case scenario of the drive absolute current limit and find the real world starting current is actually much less (Which is very likely)...

Yep, agree with that. It would certainly not be greater, that is for sure and definite.

As an aside, would you recommend spending an extra hundred grand without knowing all the relevant details?

No, exactly as I said above...

I totally agree that the manufacturer should provide this info,

 

[Agroshield]

There are a few papers floating around in work that we use...

Are you able to upload or link to a pdf of that please? If not, could you help me out and explain the acronyms WRIM, RVAT (?? auto-transformer) and RVSS (?? soft starter). Thanks.

 

[GRW]

I see a lot of VFD motors as the water board have lots of them, and we are required to supply a genset to start/run Works.
Typically you're looking at 1.3x to maybe 1.5xthe running current, so somewhere around 120A/Ph should be about the max.
Our issue is we have to cope with the THD and so our alternator calcs have to factor that into the sizing (I have a Modelling programme I can run in work which helps somewhat).
Off the top of my head I'd say we'd run that with a 100-130kva genset.... but off Mains you should get away with less than that.

 

[daleyd]

Are you able to upload or link to a pdf of that please? If not, could you help me out and explain the acronyms WRIM, RVAT (?? auto-transformer) and RVSS (?? soft starter). Thanks.

I'll see if it's on our website as free access and post a link up, I got the copy I took a snapshot of off our intranet.

WRIM is Wound Rotor Induction Machine Resistance Starting, the other 2 you are correct - Reduced Voltage Autotransformer and Reduced Voltage Soft Start - none of those methods are really applicable here but used as a comparison in the paper.

 

[Justme]

I know nothing about this but suggest you look at it from the other end.

How close to the £100k upgrade are your rough calcs?

Can you spec equipment that keeps you under the cut off?

Can you spec a smaller single unit to get it installed & then add the second delayed start unit?

 

[Kayos]

I know nothing about this but suggest you look at it from the other end.

How close to the £100k upgrade are your rough calcs?

Can you spec equipment that keeps you under the cut off?

Can you spec a smaller single unit to get it installed & then add the second delayed start unit?

If my rough calcs are correct, the £100k upgrade is needed and the project is finished.

I should have my Answers after the Monday visit, the DNO application will have to wait

 

[mike os]

The only relevant info is that provided by the supplier/ manufacturer, you can second guess all day.

The only answer is to insist they give you the peak startup current, cagey or not that is not proprietary information

 

[daleyd]

The only relevant info is that provided by the supplier/ manufacturer, you can second guess all day.

The only answer is to insist they give you the peak startup current, cagey or not that is not proprietary information

Absolutely this ^ without any proper info the best you can hope to have is some ball park estimates of a worst case scenario - but with so much hanging on this then I would suggest you need some actual data from the application. If it's such that the process can tolerate a reasonably gentle ramp on start up the max start up current may well not exceed the running current at all...

 

[daleyd]

Are you able to upload or link to a pdf of that please? If not, could you help me out and explain the acronyms WRIM, RVAT (?? auto-transformer) and RVSS (?? soft starter). Thanks.

Modeling of Motor Starting Methods in EMTP-ATP | Schneider Electric

The work described in this paper has been developed for the purposes of medium voltage motor starting analysis in customer projects...

www.se.com

 

[Anguz]

If its industrial, i guess at that size is is... and a vsd is being used, at work we would have to do a harmonic survey before considering a drive
As above ghe manufacturers of the drive would know what it would pull..
Whos drive are you using?

 

[Justme]

If my rough calcs are correct, the £100k upgrade is needed and the project is finished.

I should have my Answers after the Monday visit, the DNO application will have to wait

Sorry you misunderstood.

Are you ball park numbers so far over the line that even with better numbers or smaller kit you still have no chance or just over so with some planning / actual numbers you can get under the requirement?

Is the planned kit over spec'ed enough that you can down size slightly?

 

[knighty]

do you have the heat pump yet?

haven't worked on any co2 stuff myself so no personal experience, but with normal refrigeration you can add something like an unloader valve

I *think* they essentially use a couple of solenoid valves to separate the compressor from the system and then join the output to the input - essentially meaning there's no pressure difference across the compressor while it starts up... so the only load on the motor is the inertia of getting it up and spinning.... dropping the start up currently massively

I would expect at that kind of load you could keep the start up current lower than the max run current


40kw is about 50hp? I'm surprised they've specked a motor that big and not multiple smaller ones, you have more capacity control with 2x25hp compressors than 1x50hp (because you can turn one on and off)

 

[mtt.tr]

Ask the manufacturer they should be able to tell you what the expected peak current will be.

 

[Kayos]

Had a very exciting and interesting visit on Monday. Still amazes me what is hiding in industrial estates up and down the country

Some quick maths and a look at what we needed and a solution was formed, 3 stages of centrifugal compressors, pumps and fans all vfd controlled will allow us to keep the startup current well within limits and on another, bigger, project we are looking at cascading multiple heat pumps but all running as one unit, so we can do much more than a lead/lag setup

 

[GRW]

Had a very exciting and interesting visit on Monday. Still amazes me what is hiding in industrial estates up and down the country

Some quick maths and a look at what we needed and a solution was formed, 3 stages of centrifugal compressors, pumps and fans all vfd controlled will allow us to keep the startup current well within limits and on another, bigger, project we are looking at cascading multiple heat pumps but all running as one unit, so we can do much more than a lead/lag setup

Welsh Water do this a lot on their Works & Pumping Stations. They have Stage/Stagger start regimens in place to limit the Transient Inrush currents (some sites have several pretty meaty motors)

 

[mtt.tr]

Had a very exciting and interesting visit on Monday. Still amazes me what is hiding in industrial estates up and down the country

Some quick maths and a look at what we needed and a solution was formed, 3 stages of centrifugal compressors, pumps and fans all vfd controlled will allow us to keep the startup current well within limits and on another, bigger, project we are looking at cascading multiple heat pumps but all running as one unit, so we can do much more than a lead/lag setup

Surprised you are not looking at compressor style running systems. A firm I knew had about 8 smart controlled comps all about 75kw each

 

[Kayos]

Surprised you are not looking at compressor style running systems. A firm I knew had about 8 smart controlled comps all about 75kw each

We need to be careful, it has to be a manufacturered product, if we install compressors, pumps and evaporators as seperate items it becomes a bespoke install and needs inspecting, verification, and certificates. All at great expense.

A product is essentially type approved and as we are only modifying the control systems it is pre certified, granted they are all classed as notify before in terms of the DNO, but they have a UK certificate of compliance

 

[mtt.tr]

We need to be careful, it has to be a manufacturered product, if we install compressors, pumps and evaporators as seperate items it becomes a bespoke install and needs inspecting, verification, and certificates. All at great expense.

A product is essentially type approved and as we are only modifying the control systems it is pre certified, granted they are all classed as notify before in terms of the DNO, but they have a UK certificate of compliance

That's what I'm saying just the control system, comp industry has been doing all this already