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Direct Online Starter

02/01/2013 6:52 AM

Hello Respected Members,

I am a fresh graduate, have limited to no design experience and have just started working for a private HVAC organization as an Electrical Engineer. Now I am still in the training phase of my job. My company is working on a new project and apart from the HVAC design and installing, testing commissioning etc they are also responsible for designing and carrying out the electrical works of the HVAC equipments. Like designing the mcc, mcp and cable laying etc. (Being done by my colleagues)

Now my supervisor/instructor as part of my training has given me the autoCAD drawings of the MCC and MCP design and has asked me to figure out how the engineers have come up with the values of the all the components such as MCCB's VFD's to be used, Direct online starters for the fans and etc.

I have managed to find out the VFD's and star delta delta calculations for electric and absorption water chiller pumps as well as how the circuit breakers have been sized. But I cant figure how they managed to get a value of 10KW for a DOL. All i have for that particular fan is that it has a 4hp motor and total CFM produced by it are 19890 and external static pressure of 0.6 Inches water gallon???

Can anyone please help me out here. Please :-(

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#1

Re: Direct Online Starter

02/01/2013 7:00 AM

Well, 4hp * 0.746kW/hp comes out at about 3kW, so is it that 10kW is a standardised minimum size DOL for this company, perhaps, and that a smaller overload gets fitted to match the motor?

[It's "inches water gauge", BTW.]

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#2
In reply to #1

Re: Direct Online Starter

02/01/2013 7:36 AM

Thanx for the reply. This is embarrassing, I'm sure my supervisor told me its "inches water gallon" I think he wanted to see if I will confirm it or not. Thanks for the correction. Can you please explain in detail what you meant. I'm confused. :-(

I mean if its 3KW then why a 10KW DOL. As for my VFD's they worked out fine as the calculated HP turned out to be 31.25HP corresponding to 23KW and they had selected 22KW VFD and that's fine in case of a VFD and from there i was able to get to MCCB size and stuff. But this 10KW is confusing...

Thank you

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#10
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Re: Direct Online Starter

02/02/2013 9:26 AM

Inches water gauge is the overpressure expressed as the height of a column of water. P=ρgh and all that good stuff - 1st year Physics O-level?

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#11
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Re: Direct Online Starter

02/02/2013 12:36 PM

In future original posts, include as much detail right from the start as you can, please.

As far as the 22 KW VFD being "fine" with a 23 KW load calculation (I suppose for 100% load), would you care to elaborate the reasons why 22 KW is "fine"? Is this because the spin-up current of the motor is less in the case of the VFD, and you will not likely be running 100% load very much of the duty cycle?

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#3

Re: Direct Online Starter

02/01/2013 9:19 AM

Interesting that you can graduate with an EE degree (presumably) and not know any of this, or even how to state your data correctly to get help. <sigh>

You have not stated the voltage you are working with and you have not indicated that the voltage chart you are selecting the DOL contactor from is the same as the above (hint, you may be looking at a 380V chart for contactors but you are using 230V, so someone else picked the contactor correctly because they used the proper chart). You cannot grasp the simple concept that PWSlack put forth, being that your employer might have standards that dictate a minimum size requirement for DOL contactors at 10kW, even if the motor is smaller. You seem to have an issue with understanding why someone would over size a cheap device like a contactor, which is perfectly acceptable, yet you fail to recognize that your VFD is SMALLER than your motor, which is unacceptable, and then you call it OK.

To give you the benefit of the doubt, maybe you are suffering from 1st year jitters, you are unable to slow your mind down long enough to see the details because you are too caught up in not looking bad. Step back, read ALL of the available data, do some research on the installation standards for the part of the world this equipment is going to, read the catalog information for the devices and the installation manuals and don't rush it.

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#4
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Re: Direct Online Starter

02/01/2013 9:37 AM

You've done it now.

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#5
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Re: Direct Online Starter

02/01/2013 9:43 AM

Yep, probably right... Better dig out my old asbestos long-johns.

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#6

Re: Direct Online Starter

02/02/2013 12:31 AM

In countries with weaker grids DOL is limited to about 5hp due to VD problem,but in countries with strong grids even 100hp may be allowed. Gothrough catalog of electrical products from famous companies like Telemecanique and some US companies you will find DOL ratings of each size of contactors.

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#7

Re: Direct Online Starter

02/02/2013 6:05 AM

If there are past records about projects similar to what you are doing, check what was done in the past.

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#8

Re: Direct Online Starter

02/02/2013 8:03 AM

The fan has Inertia problems at the starting time. Therefore, the inrush current on DOL starting mode will be higher than 3 times the full current rating of the motor, for few seconds (~5 to 7 sec). this will justify the 10 kW contactor (check the ratings chart from the maker...).

On the other hand, as they have mentioned to you (JRAEF...) you missed on the VFD size! it should be bigger than the motor rating.

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#9
In reply to #8

Re: Direct Online Starter

02/02/2013 9:13 AM

Although starting current is high in DOL starting,if starting torque required by the load is high,DOL could be used.

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#12

Re: Direct Online Starter

02/03/2013 7:00 PM

10KW seems about right for the smallest "standard" equipped starter in a MCC. Using a standard set up means just the protection needs to be altered for each motor.

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James Stewart (1); JRaef (2); LAA_Lucke (1); lyn (1); Munster (1); pnaban (2); PWSlack (2); Rookieengg (1); TonyS (1)

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