CR4 - The Engineer's Place for News and Discussion®

Previous in Forum: dear friends   Next in Forum: What is Fire Rate kwh/m from Busway?
Close

Comments Format:






Close

Subscribe to Discussion:

CR4 allows you to "subscribe" to a discussion
so that you can be notified of new comments to
the discussion via email.

Close

Rating Vote:







29 comments
Active Contributor

Join Date: Mar 2009
Posts: 16
Good Answers: 1

how to select contactor for star delta starter??

12/20/2009 9:56 AM

What should be the % ratting of three contactors used for star delta starter. Let say if we have to start an induction motor of capacity 100KW, 440V what should be the Ratting of all three contactors. should it be the same. As far as i knw star contactor ratting lesser as starting torque is reduced to 1/3rd. but what about the other two contactors..?

Register to Reply
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
2
Guru
United States - Member - New Member Engineering Fields - Power Engineering - New Member

Join Date: Sep 2006
Location: California, USA, where the Godless live next door to God.
Posts: 3832
Good Answers: 568
#1

Re: how to select contactor for star delta starter??

12/20/2009 5:46 PM

58% of the FLC. You also must mechanically interlock the Star contactor with the Delta Contactor (K2 or KM2 or Q2 or M2etc.), so be careful because some contactor mfrs do not allow mechanical interlocking of disimilar size contactors, so all 3 would end up the same size.

__________________
** All I every really wanted to be, was... A LUMBERJACK!.**
Register to Reply Good Answer (Score 2)
Guru
India - Member - New Member

Join Date: Sep 2009
Location: Bangalore, India
Posts: 2710
Good Answers: 157
#2
In reply to #1

Re: how to select contactor for star delta starter??

12/20/2009 11:34 PM

Amplifying a bit this which is a GA ...

In addition to the mechanical interlock, it is also advisable to provide a "transition delay' of 50-100 milliseconds between star contactor drop and delat contactor pickup. Many good manufacturers provide a complete solution .. here is an example from ABB ..

Main and Delta contactors :A 110

Star Contactor : A 95

Relay (within Delta circuit) :TA 110 DU

Timer with 50ms transition delay : TE5S

__________________
Nothing worthwhile can ever be taught, it can only be learnt.
Register to Reply
Anonymous Poster
#6
In reply to #1

Re: how to select contactor for star delta starter??

12/21/2009 7:28 AM

Very good response from all... intresting.. but i beg to differ regards mech interlocking...

The contactors are actual electrically interlocked using NO and NC contacts with delay timer . there is no need to mechanically interlock.. mechanical interlock between contactors is useful when 2 contctrs are supplied from different sources and where only 1 contactor operates at a time.. this logic is true even for 2 ACB / 2 MCCB interlocking...

Nowadays electrical interlocking is preferred since it is semi-automatic..also PLC logic can be used for interlocking..

Register to Reply
Guru
India - Member - New Member

Join Date: Sep 2009
Location: Bangalore, India
Posts: 2710
Good Answers: 157
#7
In reply to #6

Re: how to select contactor for star delta starter??

12/21/2009 7:41 AM

i respectfully submit that Jraef is right in recommending the mechanical interlock, and so do i.

Electrical interlocking is done through small auxiliary contacts which are usually common across the entire range of a manufacturer. Their travel from open to close can be as small as 3 mm. While a large contactor may have a travel of 10mm. i have actually seen a welded 150A contactor whose NO aux contact had opened, indicating a false OFF condition.

So, my respectful submission is, in the interest of safety, please use mechanical interlocks between Star and Delta contactors in S/D starters and between F and R contactors in reversing starters.

__________________
Nothing worthwhile can ever be taught, it can only be learnt.
Register to Reply
Commentator
Engineering Fields - Electrical Engineering - New Member India - Member - New Member

Join Date: Dec 2009
Location: Bangalore,India
Posts: 80
Good Answers: 1
#11
In reply to #7

Re: how to select contactor for star delta starter??

12/23/2009 12:38 AM

Sir,

I have a doubt. What type of mechanical interlock should be used between star and delta contactors. if the transition from star to delta need to take place how the mechanical interlock will function. if the starter is operated from PLC/ DCS and if there are several star delta/ F&R starters which need to operate in a particular sequence as in process plants.. whether will not the mech interlock cause a hindrance.. please help me on this doubt

Register to Reply
Guru
India - Member - New Member

Join Date: Sep 2009
Location: Bangalore, India
Posts: 2710
Good Answers: 157
#12
In reply to #11

Re: how to select contactor for star delta starter??

12/23/2009 12:54 AM

Mechanical interlocks are designed along with the contactor itself as an accessory. Each manufacturer will have his own design. Good manufacturers design them keeping in mind that two different sizes of contactors may have to be interlocked in Star/delta and Autotransformer starters.

The purpose is to prevent, under any circumstances, two contactors being ON simultaneously. Definitely required for reversing,star/delta and autotransformer starters. With one contactor ON, it makes it impossible to switch On the other one .. even by mechanically pushing it !

i see this as a safety mandate, and i am certain that any well-thought-out scheme using PLCs or SCADA or any system will not be affected in any way by the mechanical interlocking.

You might do well to visit the websites of ABB, Schneider, Eaton and such major manufacturers to see the design of the mech interlock.

__________________
Nothing worthwhile can ever be taught, it can only be learnt.
Register to Reply Score 1 for Good Answer
Commentator
Engineering Fields - Electrical Engineering - New Member India - Member - New Member

Join Date: Dec 2009
Location: Bangalore,India
Posts: 80
Good Answers: 1
#14
In reply to #12

Re: how to select contactor for star delta starter??

12/23/2009 1:01 AM

Thank you so much Mr. Sridhar for your valuable suggestion

Register to Reply
Commentator
Engineering Fields - Electrical Engineering - New Member India - Member - New Member

Join Date: Dec 2009
Location: Bangalore,India
Posts: 80
Good Answers: 1
#3

Re: how to select contactor for star delta starter??

12/21/2009 1:13 AM

Dear Friend, The electrical companies manufacturing starters have Type-2 coordination chart. You should tell what back up protection is required, whether it is MCCB or MPCB or Switch fuse unit. the rating of contactors will be different for different back up protection units. Yes you are right, the current rating of star contactor is less compared to Main and delta contactors. main and delta will have same ratings. please contact ABB, Siemens or schneider for your doubt

Register to Reply
Power-User

Join Date: Nov 2009
Location: Nnewi,Anambra,Nigeria
Posts: 141
#4

Re: how to select contactor for star delta starter??

12/21/2009 3:54 AM

P=active power. P=√3VIcosφ ; I=P/√3Vcosφ

Your I value should be yr contactors rating.Though in some cases lower contactor rating is used for the star contactor as the star current is reduced by one third.But to avoid other problems of using different contactors sizing by some experts and manufacturers,it is better and safer to use thesame size for all three.

I have experienced several changes of smaller sized contactor(star contactor) used for star-delta start of an induction motor untill i bought same size as that of line and delta,and then the problem was over.Though that starter was not built by me,but i corrected the problem.

It is stated even in textbooks that we use 58% of motor full load current for yr motor overload.

Patrick Whowha

__________________
Patrick Whowha
Register to Reply
Participant

Join Date: May 2012
Posts: 1
#29
In reply to #4

Re: how to select contactor for star delta starter??

05/25/2012 5:20 AM

Dear Patrick,

Good day. I have a star delta motor 15kw 400vac 30A. I need to use star delta starter for this motor so can you please suggest the contactor current rating for star ,delta and Main. Also please suggested me the capacity of thermal over load relay.

can I use main and delata 0.58*motor current (30)

star 0.33*motor current (30)

what is the formula to find thermal relay?

Looking for your best suggestions!!

Ranjith

Register to Reply
Power-User

Join Date: Oct 2009
Location: Istanbul. European side
Posts: 151
Good Answers: 4
#5

Re: how to select contactor for star delta starter??

12/21/2009 4:16 AM

Star contactor can be selected smaller than the others, providing the star contactor pulls first before the main contactor. Then no current flows when thids contactor pulls.

Register to Reply
Commentator
Engineering Fields - Electrical Engineering - New Member India - Member - New Member

Join Date: Dec 2009
Location: Bangalore,India
Posts: 80
Good Answers: 1
#16
In reply to #5

Re: how to select contactor for star delta starter??

12/24/2009 12:57 AM

good idea Mr. Nezihozfirat... I think if we select Star contactor 1 size lower in terms of electrical current compared to main and delta contactors ,the magnetic moment of inertia is less... so the tension on the rod and spring connected to moving contacts is less.... hence both the contacts are separated faster compared to delta...

however i feel that, as Mr. Sridhar told, that this problem is more pronounced for higher rating and for motors which are heavy duty.. it also depends on the overload and duty cycle of motors and the load application..

Register to Reply
Power-User

Join Date: Oct 2009
Location: Istanbul. European side
Posts: 151
Good Answers: 4
#17
In reply to #16

Re: how to select contactor for star delta starter??

12/24/2009 1:49 AM

The method of selecting the Star contactor 1 size smaller than the others is being applied by many well known switchgear manufacturers since 30 years. I think this method saves money to the manufacturer. I have seen this in the panels of Siemens, Atlas Copco, Johnson Control and most other manufacturers. This method works succesfully.

The other point of discussion, is interlocking.

Mechanical Interlocking is very common at the generator sets changeover panels, of medium sizes, which are using contactors. However in star-delta system, I personally did not see the common use.

Electrical interlocking is absolutely available in any star delta system. Star and Delta contactors to pull at the same time is electrically restricted.

The cutouts in the event of contact welding is ensured by magnetic breakers. (Of course the magnetic release setting must be correctly adjusted.)

Above are the common industrial uses to comply the minimum safety standards. Promoted ideas, with non expensive solutions are always welcome.

Kindest Regards

Register to Reply Score 1 for Good Answer
Anonymous Poster
#18
In reply to #17

Re: how to select contactor for star delta starter??

12/24/2009 2:16 AM

Dear Sir. you are right.. the actual selection of starter system(whether DOL or Star delta) for a motor is decided by type testing the system for withstanding a pre- decided short circuit level and for motor duty applications. The type -2 coordination tests are conducted for short circuit levels- say 16kA, 36kA,50kA . it also depends on the short-circuit protection device. for MCCB SC protection, the rating of contactor is higher, For MPCB, the rating of contactor is lower and for Switch fuse unit, the rating is still lower.

The reason for the same is due to the let through energy being released from the arc during short circuit breaking, to the contactor. for overloads, OLR will cut out the supply to the coil of contactor. in siemens make MPCB we have a MPCB with relay function where MPCB will trip for short circuit only and will trip the contactor for overloads. because the contactor is designed for higher mechanical operational life compared to MPCB.

Thanks and regards

Register to Reply
Guru
India - Member - New Member

Join Date: Sep 2009
Location: Bangalore, India
Posts: 2710
Good Answers: 157
#19
In reply to #18

Re: how to select contactor for star delta starter??

12/24/2009 3:25 AM

Just amplifying a bit ....

Electrical interlocking is universal. Mechanical interlocking is optional. However, as a designer of LV switchgear, i was never very happy about the reliability of electrical interlocking, especially in large contactors where the travel of the tiny aux contact is much smaller than that of the main contacts.(i already siad this in an earlier comment, sorry fr the repetition). The aux contacts do not represent the state of the main contacts in such cases...consequently, the electrical interlock is not as safe as the mechanical one. For safety's sake, the small extra cost of the mechanical interlock is really value for money.

__________________
Nothing worthwhile can ever be taught, it can only be learnt.
Register to Reply
Anonymous Poster
#20
In reply to #5

Re: how to select contactor for star delta starter??

07/16/2010 12:49 AM

It is mandatory to pull first Star contactor and then Mains contactor?

My star contactor is having lower rating than mains contactor and i m pulling first mains contactor and then star contactor. Does it make any problem, i have taken around 12000 on/off cycles of star -delta on 20hP motor. My starter still works OK.

Register to Reply
Guru
India - Member - New Member

Join Date: Sep 2009
Location: Bangalore, India
Posts: 2710
Good Answers: 157
#22
In reply to #20

Re: how to select contactor for star delta starter??

07/16/2010 1:01 AM

Strictly speaking, it is better to switch on Star first and then Main. (Remember that whichever one makes last makes the starting current). Reason being, (as already stated by others) that Star being one size smaller, it lasts longer if the switching on of the start current is done by the bigger contactor, the Main contactor. However, since contact erosion on Make operation is not much, it is OK to switch on Main and then Star also if absolutely essential. However, there should be no big problem in rearranging the control circuit such that Star switches on first... please see whether this can be done.

Remember to use both mechanical and electrical interlocking between Star and Delta contactors though.

__________________
Nothing worthwhile can ever be taught, it can only be learnt.
Register to Reply
Participant

Join Date: Jul 2010
Location: Pune, India
Posts: 1
#21
In reply to #5

Re: how to select contactor for star delta starter??

07/16/2010 12:57 AM

It is mandatory to pull first Star contactor and then Mains contactor?

My star contactor is having lower rating than mains contactor and i m pulling first mains contactor and then star contactor. Does it make any problem, i have taken around 12000 on/off cycles of star -delta on 20hP motor. My starter still works OK.

Register to Reply
Guru
India - Member - New Member

Join Date: Sep 2009
Location: Bangalore, India
Posts: 2710
Good Answers: 157
#23
In reply to #21

Re: how to select contactor for star delta starter??

07/16/2010 3:36 AM

Here what i posted in another thread...you can study that one too ... gives the schematic of the control circuit. Please note that the Main contactor is switched ON by the aux contact of the Star contactor, so it is delayed by about 20ms...it takes the brunt of switching ON the starting current of the motor.

_________________________________________________________________________________________________

The K1T is a timer, shown as a separate product for this general purpose schematic. On smaller contactors, it is mounted on the Star contactor KM1 itself, so no separate coil is there for it.

i give below a few images which should explain the operating sequnce..

The pneumatic timer is an add-on accessory which is mounted on the star contactor. In such a case, the time-delay NC deenergises the star contactor and its NO energises the Delta contactor KM2. Mechanical and electrical interlocks are a must.

However, i have found it necessary to insert a "transition delay" when changing from Star to delta, to prevent any ionised gases in the star contactor causing a short-circuit. Such special timers are available in both electronic and pneumatic versions.

Hope this is somewhat clear? If not, please feel free to ask.

Cheers.

__________________
Nothing worthwhile can ever be taught, it can only be learnt.
Register to Reply
Guru
United States - Member - New Member Engineering Fields - Power Engineering - New Member

Join Date: Sep 2006
Location: California, USA, where the Godless live next door to God.
Posts: 3832
Good Answers: 568
#8

Re: how to select contactor for star delta starter??

12/21/2009 1:01 PM

I shudder to think that there are people still using electrical interlocks only. I always recommend, and use myself, both types of interlocking. I was assuming the electrical interlocking and transition timer were a given, shame on me.

The reason why mechanical interlocks are even made is because electrical interlocks alone are not considered fail safe. A welded electrical aux. contact can allow the closing of both contactors, which is a bolted short and could destroy not only the contactors, but the conductors and motor as well. That little piece of plastic interlock is extremely cheap insurance.

__________________
** All I every really wanted to be, was... A LUMBERJACK!.**
Register to Reply
Active Contributor

Join Date: Mar 2009
Posts: 16
Good Answers: 1
#9

Re: how to select contactor for star delta starter??

12/22/2009 7:15 AM

Thanks alot to everyone for these valuable inputs and suggestions. I would like to further ask is that to what range of Induction motor mechanical interlocking is preferable and electrical interlocking is preferable...?

Register to Reply
Guru
India - Member - New Member

Join Date: Sep 2009
Location: Bangalore, India
Posts: 2710
Good Answers: 157
#10
In reply to #9

Re: how to select contactor for star delta starter??

12/22/2009 7:26 AM

i personally believe that both are mandatory, irrespective of the motor size. In the event of star and delta contactors being ON at the same time, there will be a dead-short-circuit of the incoming supply, irrespective of the motor size. So, in the interest of safety, please use mechanical interlocks in addition to electrical interlocks.

Even the transition time delay is highly recommended. The star contactor may have dropped off, but the air in the arcing zone may still be somewhat ionised. If the delta contactor closes at this time, the arc will restrike in the star contactor resulting in a short circuit. Though, this is more likely with higher current ratings, say 50 hp and above.

__________________
Nothing worthwhile can ever be taught, it can only be learnt.
Register to Reply Score 1 for Good Answer
Anonymous Poster
#13
In reply to #10

Re: how to select contactor for star delta starter??

12/23/2009 12:55 AM

Thanks Mr. Sridhar.. just wanted to know.. what is the transition time.. is 1 sec ok or less...

Register to Reply
Guru
India - Member - New Member

Join Date: Sep 2009
Location: Bangalore, India
Posts: 2710
Good Answers: 157
#15
In reply to #13

Re: how to select contactor for star delta starter??

12/23/2009 1:03 AM

1 sec seems too high for LV. i did use 1 sec in a 3.3kV starter, because the ionisation in the large arc chute was dangerous even after 300 ms. That was needed, so the customer agreed for a 1 sec delay, and thankfully, the motor behaved ok.

However, the downside is that, depending on the load, the motor slows down. If it slows down too much in the transition time, when it is switched ON in delta, there will be a large current surge again...so the very purpose of star/delta will be lost. The transition time is really a trade-off, if you see what i mean.

For <50hp, i daresay 30 ms is enough, and above that 100 ms. Hope this helps.

__________________
Nothing worthwhile can ever be taught, it can only be learnt.
Register to Reply Score 1 for Good Answer
Active Contributor

Join Date: Sep 2011
Posts: 11
#24

Re: how to select contactor for star delta starter??

09/16/2011 5:31 PM

the main contactor must be 100% of the load which means that if i have a 90 kw motor the main contactor should be a 90kw contactor because at the start when the star contactor is activated the whole current of the star connection as mentioned in the name plate of the motor will flow through the contacts of the main contactor, when the delta contactor is activated the current will be divided between the main contactor and the delta contactor which makes it unnecessary to use a 100% for a delta contactor , in this case the ratio would be about 75% which makes about 67 kw(you might not find 67kw contactor but you should use one that includes this figure) ,as for the star contactor a 35% would do just fine which makes about 32 kW (again you should use a contactor that includes this figure), also you should take in consideration the current division between the main contactor and the delta contactor when choosing the over load protection, by dividing the current as mentioned in the motors name plate in case of the delta operating you can choose the right rang for the best protection.

Register to Reply
Guru
India - Member - New Member

Join Date: Sep 2009
Location: Bangalore, India
Posts: 2710
Good Answers: 157
#25
In reply to #24

Re: how to select contactor for star delta starter??

09/16/2011 8:12 PM

A small point...

A 100kw motor using a ΥΔ starter, delivers only 33kw in star ! It will go to full power (100kw) only after changeover to delta....

__________________
Nothing worthwhile can ever be taught, it can only be learnt.
Register to Reply
Active Contributor

Join Date: Sep 2011
Posts: 11
#26
In reply to #25

Re: how to select contactor for star delta starter??

09/30/2011 2:28 AM

I have made the star/delta connection and maintained many electrical panels of that connection several times, since I'm an electrician , so i assure you this is the right way to do it . you should keep in mined the fact that the 100kw will be divided between two contactors the main and star and after 6.5 seconds the star contactor will be turned off to switch to delta contactor and then the 100kw will also be divided but with different ratios between the main and delta contactors this time.

Register to Reply
Guru
India - Member - New Member

Join Date: Sep 2009
Location: Bangalore, India
Posts: 2710
Good Answers: 157
#27
In reply to #26

Re: how to select contactor for star delta starter??

09/30/2011 3:59 AM

While i do not question your credentials as an electrician, i must mention that i am a designer of contactors and starters, and i also assure you that :

1. In star connection at start, the motor draws and delivers 1/3 of its full rated power.

2. When the starter switches over to Delta, the motor draws full power, but since the contactors and the overload relay are usually wired within the Delta, you need to use contcators and relay which are only rated 1/√3 ≈58% of the full rated power of the motor.

__________________
Nothing worthwhile can ever be taught, it can only be learnt.
Register to Reply
Active Contributor

Join Date: Sep 2011
Posts: 11
#28
In reply to #27

Re: how to select contactor for star delta starter??

09/30/2011 1:40 PM

I'm happy to learn from you and share my humble experience with you , as i learned from you the star connection draws and delivers 1/3 of its rated power which explains to me why should the star contactor be 35% of the rated power which is about the 1/3 , but in case of the delta contactor i can say that the 75% is exaggerated but in terms of durability it would be welcomed since the rated power of the delta contactor is 58%, bearing in mind that the star contactor will be turned off after 6.5 seconds to turn the delta contactor on .

Register to Reply
Register to Reply 29 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Copy to Clipboard

Users who posted comments:

Anonymous Poster (4); fmalura (1); JRaef (2); kvsridhar (10); Mehul38 (1); nezihozfirat (2); Patrick Whowha (1); ranjithkrishna (1); shanbhag (4); suliaman (3)

Previous in Forum: dear friends   Next in Forum: What is Fire Rate kwh/m from Busway?

Advertisement