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View Full Version : Correct calculation for Star Delta sizing - Contactor & Overload


flyers
May 4th, 2007, 09:32 PM
Hi guys,

I've only a lil` experience in motor sizing & calculation for contactor and overload. Hope you guys can show me the correct or proper calculation. My current calcluation as below, please correct me if I'm wrong.

For a 45kW motor:

Amp = 45000/(415*1.732*0.85)
= 74Amp

Main and Delta contactor & overload:
= 74/1.732
= 42.8Amp
With 10% spare:
= 48Amp

Star contactor and overload:
= 74/2
= 37Amp
With 10% spare:
= 41Amp

AB contactor:
1) 100-C60T10 x 2 pcs
2) 100-C43T10 x 2 pcs
3) 193-TCC45 (30 - 45) Amp

Question:

1) The overload should be sizing according to Main amp, 42.8Amp (48Amp with 10% spare) or the Star amp, 37Amp (41Amp with 10% spare)?

Thank you.

Lancie1
May 5th, 2007, 01:50 AM
I think you would need two overload relays, one for Delta contactor, and one for the Star contactor. Size each one according to your calculations and the local codes & laws.

Kidblue
May 5th, 2007, 02:00 AM
I think you would need two overload relays, one for Delta contactor, and one for the Star contactor

It's not the way i've always done it. Normally fit one overload (usually on the LINE or MAIN contactor) and normally rate at 0.58 of the FLC.

Alternatively, we fit one Thermal/Magnetic motor circuit breaker upstream of the 3 motor contactors rated for FLC.

Hope this helps.....

pouch
May 5th, 2007, 02:30 AM
Follow the link below to the Moeller wiring book chapter on Motor protection:

http://www.moeller.net/binary/schabu/sb0801g.pdf

Regards

Pouch

Gil47
May 5th, 2007, 02:55 AM
Kid blue is on the right track
if you divide by Sqrt of 3
or 1.732
or 1 over Sqrt 3 all come out as .58 or 58%
The voltage across the windings is line voltage 415 in delta
your phase voltage is 240 in star
because of the higher voltage in delta the current will also be higher
so the 1 overload in phase current is .58 of the line voltage
Thermal/Magnetic motor circuit breaker upstream of the 3 motor Contactors does remove the confusion involved in setting overloads
Note 3 contactors you show you are intending to have 4
The price you are considering paying for contactors timers and wiring could buy you a soft starter
We only use Star Delta now where long star up times in minutes occur such a some cenrifuges we havent wired one new in 20 years

504bloke
May 5th, 2007, 03:05 AM
It's not the way i've always done it. Normally fit one overload (usually on the LINE or MAIN contactor) and normally rate at 0.58 of the FLC.

Alternatively, we fit one Thermal/Magnetic motor circuit breaker upstream of the 3 motor contactors rated for FLC.

Hope this helps.....

Thats how we do it.

Lancie1
May 5th, 2007, 04:47 PM
Okay, I am WRONG again! Flyers confused me when he mentioned having two overloads: Main and Delta contactor & overload:
.
.
Star contactor and overload:
I see now there must be only one. Here in the US and back in the good old days, most motor starters had built-in attached overload relays, so I automatically assumed if there were two contactors then there must be two separate overloads. I forgot that the IEC and modern types have separate user-added overloads.

I am sorry, and I will not attempt to answer questions like this again. I am too old to cut the mustard, and my knowledge is as obsolete as the rest of me.

Thomas Sullens
May 5th, 2007, 10:19 PM
This (http://literature.rockwellautomation.com/idc/groups/literature/documents/wp/mot-wp003_-en-p.pdf) will help you.

JesperMP
May 6th, 2007, 03:19 AM
Flyers.

For a star-delta starter you have the possibility to place the overload protection in two positions, in the line or in the windings.

In the line is the same as just putting the overload before the motor as with a DOL starter. The rating must be the same as FLC.
The disadvantage is that if the overload is set to FLC, then it is really not protecting the motor while it is in delta (setting is x1.732 too high).

In the windings means that the overload is placed after the point where the wiring to the contactors are split into main and delta. The overload then allways measures the current inside the windings, and is thus allways correct. The setting must be x0.58 FLC (line current).
The disadvantage is that you must use separate shortcircuit and overload protections.

Also, I would use 0.80 for estimating cos phi x efficiency and not 0.85 for a 45 kW motor. Better to be safe than sorry.

In principle you dont have to add 10%, but I do normally take the next size contactor if the rating is too close to the actual load.

Estimated motor line FLC is then
45000W/(415V*1.732*0.80) = 79 A.

Main and delta contactor should be at least
FLC * 0.58 = 46 A.

Star contactor should be at least
FLC * 0.33 = 26 A.

That Moeller instruction that pouch linked to tells it all. I got that book as a gift once, and I look into it on occasions.

What is the application ?
Nowadays, the only place where we use star-delta is with (very) large fans. Everywhere else low-cost softstarters are taking over.

flyers
May 13th, 2007, 11:40 AM
Lancie1: Sorry for the confussion over the number of overloads. No you are not too old, just that I'm too fresh in this motor calculation thingy.

Kidblue: Yes, you are right, we normally put a FLC MCCB before the 3 main contactor and the overload will be located at the main contactor.

pouch: Thanks for your link, very good write up, I've no time to go thru them throughly, but just browse thru it. WIll read them when I have the time...great one!

Gil47/504bloke: Thx for your reply.

Thomas: Thx for your link too.

JesperMP: I do not know the STAR contactor calculation is divide by 3, i taught it is divided by 2. I'll check out the ebook linked. Thx.

flyers
May 13th, 2007, 08:58 PM
Costing, comparing contactors + overload with soft starter for 45KW motor:

Estimate (list price):

100-C60T10 x 2 pcs = USD250

100-C30T10 x 1 pc = USD60

193-TCC60 x 1 pc = USD70

100-MCA00 x 1 pc = USD12

TOTAL = USD(250+60+70+12) = USD392

While soft starter list price:

150-F85NBD (17A to 85A) x 1 pc = USD3140

For PowerFlex 400:

22C-D088A103 x 1 pc = USD 5430

It looks like soft starter is still a lot more expensive compare to contactor + overload. The only advantage I can see is space saving & less maintenance with using soft starter in this situation. The 45kW is to drive a shedder blade (to cut rubbish).

Or do I miss anything in the above calculation because some of you guy mention s/starter cost about the same as contactor + overload. Please highlight me. Thank you.

allscott
May 13th, 2007, 11:39 PM
Shop around some more for a softstart. I don't know if they are available in your area but theses guys; www.benshaw.com (http://www.benshaw.com/) will set you up for a lot less then that. Also look at Siemens, Cutler Hammer, Telemechanique....


Don't forget to price in the additional wiring that will be needed with a contactor/overload.

OkiePC
May 14th, 2007, 08:46 AM
Or do I miss anything in the above calculation because some of you guy mention s/starter cost about the same as contactor + overload. Please highlight me. Thank you.

No, I think you are right for a 60HP application. When you get up to around 150HP and larger, the soft starters will become more economical than the contactors.

You might want to check with RAM (http://www.ramusa.com/) industries. We have 3 of their DBS systems here that are outstanding. Their pricing is apx half what an AB soft starter costs, and they incorporate some really nice features like emergency DOL starting should the electronics fail...

JesperMP
May 14th, 2007, 12:24 PM
Hello Flyers.
Your prices for softstarters seems a bit high.
And the Powerflex is a VFD, not a softstarter.

We use the new Siemens 3RW40 range extensively now.
It is the low cost type, but with integral bypass contactor, so it makes for a very compact setup.
List price for a 200-460v 75kW (134A) 3RW4055-6BB44 is approx 1200 USD.
It is the smallest in this range with an integral bypass.

There is another lowcost range without the integral bypass 3RW30.
List price for a 200-460V 55kW (75A) 3RW3045-1AB14 is approx 750 USD.

If you add the extra panel space and wiring time, the softstarter is practically on par with an yD starter.
If your factor in that both contactors and softstarters are heavily discounted, then the panel space and installation cost weighs even more against yD.

Other manufacturers also have lowcost ranges.

allscott
May 14th, 2007, 08:06 PM
One last thing to add;

Are you sure you can't start the motor across the line? IF you are sure then disregard if you are not then we can debate that as well.

rsdoran
May 14th, 2007, 09:33 PM
Personally have no idea what y'all are saying.

The rule of thumb (theory etc) says a 60HP motor at 440-480 will be 1.5 amps per HP, that means it will be slightly higher at 415v, say 80 amps. I have no idea why that factor was divided by 2.

I do not see a specific reason to STAR start/Delta Run a 60hp motor overall, if it will run continuous for several hours at least.

If it is a cost evaluation I would not consider a star/delta or a softstart, just get a VFD, the ROI will be equvalent in most cases.

flyers
May 23rd, 2007, 06:37 AM
Hello Flyers.
Your prices for softstarters seems a bit high.
And the Powerflex is a VFD, not a softstarter.

We use the new Siemens 3RW40 range extensively now.
It is the low cost type, but with integral bypass contactor, so it makes for a very compact setup.
List price for a 200-460v 75kW (134A) 3RW4055-6BB44 is approx 1200 USD.
It is the smallest in this range with an integral bypass.

There is another lowcost range without the integral bypass 3RW30.
List price for a 200-460V 55kW (75A) 3RW3045-1AB14 is approx 750 USD.

If you add the extra panel space and wiring time, the softstarter is practically on par with an yD starter.
If your factor in that both contactors and softstarters are heavily discounted, then the panel space and installation cost weighs even more against yD.

Other manufacturers also have lowcost ranges.

Jesper,

I know PowerFlex is VFD, so it cost more, around USD5k++, while for Softstarter it's around USD 3k++.

USD3K++ still a lot more expensive than normal contactor. I'm comparing AB to AB products.

Thank you.