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Motor Terminal

10/14/2014 2:11 AM

difference between motor terminal arrangment when it direct online start and when it have star delta starting?

is there two different terminal box of star delta starting motor?

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#1

Re: motor terminal

10/14/2014 5:37 AM

Star:
.____. .
.____. |
.____. |

Delta:
.____._.
.____._.
.____._.

That's the best I can do with mere typography. The 6 dots to the right are the motor terminals.

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#2

Re: Motor Terminal

10/14/2014 10:59 AM

Most motors have 6 terminals. For DOL starting there are 3 supply leads, and the motor can be wired star or delta (using appropriate bridge connectors as Tornado says), to suit the supply voltage.

If it's star-delta start it must run in delta. Then there are 6 supply leads, each wired to 1 terminal, but it has to be done correctly.

If the motor has only 3 terminals it can only be started DOL.

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#3

Re: Motor Terminal

10/14/2014 11:33 PM

There is no relation between motor terminal box and type of starting(DOL or STAR-DELTA).Motor starting method is achieved in the starter cubicle and not in the Motor terminal Box.IN motor terminal box normally you can find the 6 terminals of the winding.You can connect it either for star or delta connection as per the requirement.

V.Natarajan

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#4

Re: Motor Terminal

10/15/2014 1:28 AM

Star Delta Motors

First you do not need TWO junction boxes at or for the motor only one is required, Second, as stated you will have 6 cable ends or 6 motor winding ends (see picture 1)

They could be marked A:AA. B:BB, C:CC or any other type of lettering to ind the winding ends.

In this first photo this is a single junction box for a motor with 6 connections denoting each of the motor windings, as you can see three on the top and three on the bottom bolted connection block within the motor junction box.

STAR connections with the connection strip between the TOP three motor winding connections

Same motor,same junction box.... remove the connection strip between the top connections (marked with a RED X) and replace the connection between the top connections and the bottom connections as shown with the RED lines

Now that is how YOU would change a STAR motor to DELTA. For STAR/DELTA starting, you would need to run ALL six cables (one of each end of the motor windings) out of the motor junction box to a starter, that would START the motor is STAR and after 2 seconds change over the DELTA, and basically the contactors in the starter would do the same as I have shown you in the two photos.

Basic diagram for S/D starter

So to explain, you would pres the start button and KM3 and KM! would close to start the motor in STAR.

After a given time, KM1 would open and at the same time KM2 would close placing the motor into DELTA

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#5

Re: Motor Terminal

10/15/2014 2:33 AM

Hi,

The principle of star-delta is quite simple. Each motor stator has 3 sets of windings. Imagine each winding having a "head" U1, and a "tail" U2; Therefore, similarly for the other 2 sets of windings, you'd have V1, V2 and W1, W2.

The idea of star connection is to expose each winding to phase-neutral voltage which is sq.rt 2 of the line voltage. To achieve this, use a contactor to join the "heads" OR "tails" together, ie U1-V1-W1, or U2-V2-W2.

For the delta connection, use another contactor to join the "head" of 1 set of coil to the "tail" of the next, ie. U2-V1, V2-W1, W2-U1. Use a timer to switch between star and delta contactor and you'll have a star-delta starter.

The reduced starting current comes from exposing the motor windings to phase voltage (ie.sq.rt 2 x line voltage) during the star phase.

Regards.

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#6

Re: Motor Terminal

10/15/2014 9:46 AM

It seems to me MG-1 standard three-phase induction motor is provided always with two windings per phase [that means 6 terminals].The IEC 60034-8 recommends how to mark a single winding induction motor terminals-that means could be 3 [or four] terminals of a three-phase induction motor provided only. In practice I saw many 3 [or four] terminals motors. If there are only 3 -for Y without accessing Y point terminal- it is no apparent difference between Delta and Wye connection.

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#7
In reply to #6

Re: Motor Terminal

10/15/2014 10:59 AM

You said.."standard three-phase induction motor is provided always with two windings per phase [that means 6 terminals].

That is WRONG!

A Standard three phase motor has ONE winding per phase with an end and a start of that winding, and ONE winding for each phase, so three, phase windings with two connections, that is the reason for the SIX connections. Please see my line diagram in my previous answer.

Many 3phase motor have the STAR or Y point connected internally so you will only see THREE connections, maybe FOUR if you count the earth connection.

You said.."In practice I saw many 3 [or four] terminals motors. If there are only 3 -for Y without accessing Y point terminal- it is no apparent difference between Delta and Wye connection."

Then you do not know what you are talking about, and I am not going to explain the difference between a STAR connected motor and DELTA.

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#8
In reply to #7

Re: Motor Terminal

10/15/2014 8:49 PM

The correct connection is one point but the voltages of the rating plate need to be carefully checked too.

For example a 400/690 V 50 Hz motor will be rated at 400 V in delta and 690 V in star. If started directly with a 3x 400 V 50 Hz grid it will be connected in delta and for 690 V 50 Hz in star (note that 3x 690 V 50 Hz feeding is rather used for larger motors, including also Variable Frequency Drives).

Star/Delta starting simply first applies 400 V grid voltage to the windings connected for 690 V (i.e. a SQRT(3), about 1.73 times, higher voltage) before switching the windings over to a 400 V configuration in delta.

Small motors can be rated 230/400 V and those cannot be started in star/delta with a 400 V grid as they need to be connected in star for 400 V but it's not an issue as small motors don't require star/delta starting. Before switching from start to delta a very short pause is required to avoid locking contactors (which are normally interlocked at least mechanically).

More generally, in many applications star/delta starting is replaced by softstarting if conditions allow it.

The examples are for European IEC voltages, I don't know the U.S. voltage well enough, sorry.

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#9
In reply to #8

Re: Motor Terminal

10/15/2014 11:06 PM

I would go back to school if I was you, you know nothing about what you speak of... "a short pause".... Rubbish!

A short pause will stop the motor!

As for the rest of your comments.... WAY off target!!

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#12
In reply to #9

Re: Motor Terminal

10/16/2014 1:59 AM

The very short pause I mentioned is required to allow the star contactor to open its contacts before those of the delta contactor close. If the star contactor is still closed when the contacts of the delta contactor close you can imagine what happens. :) Of course, as mentioned, usually those contactors are mechanically interlocked but if you power both contactors at the same time it can (but must not necessarily) prevent the still closed contactor from opening.

The "short pause" I mention is indeed handled by typical electronic star/delta timing relays.

I made the comment about the voltages of the motor rating plates because many motors have been damaged due to incorrect voltage application.

If I'm wrong please enlighten me.

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#10
In reply to #7

Re: Motor Terminal

10/16/2014 12:28 AM

Sorry brich actually, you did not referred to any standard. Please refer to MG1 ch.2.60 General Principles for Terminal Markings for Polyphase Induction Motors. 2.60.1.1. Terminal Markings Using "T" and to IEC 60034-8 ch. 6.8 Terminal marking figures ch. 6.8.1 Three-phase asynchronous machines.

And as I am not familiar with Welsh so forgive me I cannot directly answer to your angry dissertation.

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#11
In reply to #10

Re: Motor Terminal

10/16/2014 1:06 AM

My Friend.. I'm not angry, just frustrated that you quote standards as if they are the be all and end all.. That tells me you have NO practical experience, and what you know or have learnt so far is from a book.

Well thats fine, but regardless of the standards, the markings will FOLLOW the same principals, regardless of country or who wrote them.

you write with authority, but I scratch the surface and you have no substance to your statements as you have NO practical experience, so my friend your comments IMHO are dangerous and misguided.

In my first reply I showed a motor terminal box... no markings on the cables! So where are your standards now?

Futhermore you have gone WAY OFF the OP's original topic. Come back when you know what you are talking about!

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#13
In reply to #11

Re: Motor Terminal

10/16/2014 3:33 AM

I am sorry to disturb the other post writers and I am sorry to occupy myself with such a nonsense affirmations. But for the record only I have to precise some things.

How from my reference to the above standards you could conclude about my experience

in the electrical machine field?. First of all I am Master in Electrical Machine Design and Engineering and I have 15 years experience only in machine fabrication field and another 30 years experience in Power Station Design field.

I understand your theoretical knowledge is limited- as per your declaration- however I doubt about your declare experience. You said "They could be marked A:AA. B:BB, C:CC or any other type of lettering to ind the winding ends."

It is wrong! See ade_jkrs post to learn how to mark terminals according to IEC 60034-8.

And for US standard the terminals are marked "T1" to "T12"[See NEMA MG1 Standard].

Your photo are for 3 line supply DOL start in Delta or Wye [not Star/Delta] since you use

fixed connecting links.

What I am sure your manners are not Italian as the motor in your photo is.

By the way name "brich" seems to me Romanian [some thing like "razor"]. So I think you are not an electrician but a barber [could you be from Seville?]

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#14

Re: Motor Terminal

10/16/2014 12:48 PM

@brich

Please let me know where I was wrong in my replies above as only criticizing people doesn't help much.

The start/delta changeover delay I mentioned depends on the used time relay, Typical value is around 50 ms. There are star/delta relays with fixed time and others with settable delay (for example between 40 and 100 ms, the larger the contactors the longer the required delay).

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#15
In reply to #14

Re: Motor Terminal

10/16/2014 3:49 PM

Plimos.

You are not wrong.In fact you are perfectly right.There will a small pause(say a few milli seconds)during change over from Star to Delta.This is absolutely necessary as other wise there will be a 3 phase short and it will blow the fuse or trip the breaker.During change over Star contactor will open first and Delta contactor will close within the said time pause.Otherwise you will never be able to start the motor.

During the transition period the motor may decelerate slightly ,but as it has already attained some speed it will continue to run and accelerate to its full speed as the delta contactor will be closed with in the time pause

V.Natarajan

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