for star delta stater

What voltage(s)?

If the contactor strips are still closed, how does the motor stop, unless there is some extreme overload such as mechanical binding? And, if that is the case, why doesn't a fuse group blow or a circuit breaker trip?

Details, details.

DVader seems to have zapped this guy twice. About a month ago, he posed a similar question, in both cases involving a "stater" rather than a starter among other confusions.

Yeah, and the delta contactor should have been the one with the highest amp rating.

you should use an interlock between star and delta contactors,

a mechanical interlock is more than necessary, what is happenning, you close the delta contactor BEFORE openning the star contactor, causing a schort circuit for a fraction of second, enough to melt and weld the contacts together...

please reconsider your control circuit

Dear Mr Chari,

Please go through this thread : http://cr4.globalspec.com/thread/55459

Next, go to LNTEBG website and download the maintenance instruction manual for starters. They, like all sensible people, use a star contactor one size lower than main and delta contactors. If your starter has a star contactor of higher rating, we are all very perplexed, so please explain why you did this? Obviously L&T will never make a starter like this, you must have got it made elsewhere.

i will make it easier for you to find L&T docs on the web. Please use this link : L&T

Good luck.

JRaef,

Best argument against the "star-delta mindset" I have seen yet.

I have seen requests for star-delta for machines as small as 2.5 KW.

For 400v/415v select a combined thermal overload relay range 12-18 Amps and set it at 15 Amps, this will save you from melting problem.

As for the current rating of the magnetic contactors ambiguity exists on various accounts in your description, also please note that for any industrial application the one rating given under AC3 duty as a minimum would be applicable (Do not take curent rating in AC1 duty catagory).

Check control wiring and rectify if defective also please check the time setting of the timer.

A small doubt here. As per one motor chart, a 20HP motor takes 34A FLA at 415 V. Assuming that the thermal relay is within the delta loop (as is usually the case), it sees 20A normally. Should the OP not use a higher rated relay ?

Welding of star and main contacts, as the OP reports, indicates something more than locked rotor condition. More like a terminal short, aggravated by improper short-circuit protection.

kvsridhar: 34 Amps ! You are making some assessment error.

I am having a slide rule in my hand which is from Fuji Electric Japan, it declares motor FLA 26.5 amps at 415 v, O/L heater range recomended is to be 12-18 A and setting current 15 A, However recommended size of MCCB is 60A and wire size is 5.5 mm sq.

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Knowledge sharing is a WIN-WIN situation

It is from Schneider Electric's Contactor catalogue. An extract of that table is here...

Of course, this mentions 380-415V.

Moreover, if you take pf and efficiency = 0.8, you should get ~ 33 A @ 415 V. At 26.5 A, even kVA ~ 19 ?

An overload can not melt and weld the contactor strips,

only a short circuit can do this.

i experienced a similar case, where i was called to resolve similar case on a 240hp motor, the problem was in the control system, the delta contactor was closing before star contactor open, this cause a short circuit for a very short time but enough to make the strips melt together sometime.

I added a mechanical interlock to prevent delta contactor from closing befor star open and re arragement of the control timing

Please calculate yourself by using the formulas;

HP x 746 = watts . (20 x 746 = 14920 watts)

Watts = sqrt3 x V x I x cos phi

By actual calculation;

Current at unity power factor is 20.7562 amps

Current at ,8 pf is 25.9453 amps

____________________________

Knowledge sharing is a WIN-WIN situation

....and at 0.8 efficiency of the motor, should be ≈ 33A ?

load at 25.94 amps or 34 amps will not make the contactor of 32A to melt every 3 days,

a 32A AC3 contactor is well adequate for a 20HP star delta starter, the cause of melting is not an overload,

Still one question I hope the OP will answer, how many start per hour is performed? if it is too high, it could be the reason

Of course. i happen to have tested this very contactor for AC4 life, it did reasonably well with its tips wearing out in about 17000 ops. So, no problem there. i do believe this guy has a serious shortcircuit problem with inappropriate SCPD. Only he can tell.

Unfortunately 1. This is one of those OPs who seems to disappear after the OP.

Unfortunately 2. I'm not so sure he/she can tell, anyway.

Vader, i am sure

Something is wrong with your slide rule. For one thing, if the FLA is 26.5, the OL heater range can't possibly be 12-18 A.

Do we even know yet the voltage of the OP's motor? The US National Electrical Code FLA for a 20-hp 460/3/60 motor is 27A; for 380-415 volts it would have to be higher. So the slide rule's FLA doesn't look right, either.

I am inclined to agree that the starter's switching sequence is wrong; that the delta contactor is cutting in before the star contactor drops out. That's not the only possible problem. What if the mystery voltage is 220, for instance?

Tornado: Remember it is a star-delta that means you have to divide FLA with 1.7321 (square root of 3) to size the heater.

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Knowledge sharing is a WIN-WIN situation.

Oops on me. I need to review which part of the circuit the overloads would be wired into.

Thanks! That was quick.

I'm with JRaef on scrapping the YΔ starter, anyway. For starters (pun intended), it has given the OP nothing but trouble.

i am all for it too. However, solid state starters are still relatively more expensive here in India. There are also bad examples of under-protected electronic components failing due to transient surges, which makes users wary of using them in remote areas. Plus unbelievable voltage swings at from day to night in rural areas. So, regrettably, ths YΔ will stay at least for a while more here.

In our company we had assembled a star delta starter for a 20 hp motor.....the rating of star, delta n main contactor was 40A. the overload relay rating was 23-32A.....the upstream protection for the contactors were 50A 36Ka Mccb in series wit Elcb........supply was 415v 3ph 4 wire...this panel was delivered n implemented successfully....the client was so happy tht he started giving all his future electrical projects to our company........

so check your starter wit the above mentioned details........may be something wil click yo brain

1.U WILL CHECK CONNECTIONS ARE TIGHT R NOT

2.10SQMM/6SQMM CABLE USING FOR 20HP MOTOR

3.RELAY RANGE 22 TO 32A.

4. CHECKING MOTOR &STATER TERMINAL CONNECTIONS PROPERLLY CONNECT R NOT