3 Phase Electric Motor Wiring?

[ryanm]

Please can someone help and explain wiring a 3 phase motor, as I can't really find anything in laymans terms.

For example, a motor like this:

http://www.electricneutron.com/wp-content/uploads/2011/04/nameplate-voltage.jpg

If running through a contactor, I would wire it in Star

If running through a single phase input/three phase output inverter, I would wire it in Delta

If running through a three phase input/three phase output inverter, I would wire it in Star

The above has always worked. My theory is that Star is for 400v, and Delta is for 230v (230v L, N, input on the inverter)

Is it that simple?

Under what other circumstance, would you wire that 3 phase motor in Delta?

Thanks

 

[ProDave]

Depends on what the motor is wound for.

Have you heard of a star / delta starter?

 

[ryanm]

No, never seen one.

From that link I posted, it shows a name plate of a typical motor I see. Is that not saying it can be wired in Star or Delta? (I mean the way you configure the links across the 6 terminals)

Thanks

Thanks Canoeboy for the clarification

I wouldn't know how to use a capactor and run it off a single phase input

 

[ryanm]

Thanks for the diagram.

So with my example, you would wire the motor in Delta, with 230v Live and Neutral, and put a capacitor over one of the windings.

 

[Technician]

Be careful about wiring motors in star when the label states delta only.

Damage could ensue.

A transition from star to delta, as Canoe's diagram indicates, means that

the phase current in the motor is increased.  This means an increase

in Torque because Torque is proportional to the Phase Current squared.

 

[Technician]

Just to clarify;  Canoe's diagram refers to the Steinmetz connection.

Torque/speed characteristics for Star and Delta wound machines

LOOK similar to Canoe's diagram.

I once had to write a commentary/explanation on the manner in which

copper and iron losses were distributed in both connections for a

large motor and the (possible) consequences of leaving a large motor

in Star for too long.

 

[ryanm]

Brilliant, thanks for the replies again

A few more questions I thought of though, if hopefully someone could help:

 - Star Delta starters: Are these used on larger motors, and you would used a star delta starter to start the motor (due to torque?), then once it is running, it would switch over? How does this physically work, do you have two contactors in the control panel on a timer or something to swap them over? I've never seen one before, but read about them

- Inverter protection: If you used a single phase input, 3 phase output inverter, and ran a 0.75kW motor from it for example, the output on the inveter would be approx 2A at 50Hz..

How do you calculate the input current for the invter? I don't know what the formula is, but surely it isn't as easy as 3 x 2A?

Thanks a lot

 

[ryanm]

Thanks again Canoeboy.

Regard the Star Delta starting: Why do you have three? Thats confused me slightly, as if the motor started in Delta, then after XX seconds switched over to Star, what is the third contactor needed for?

The inverter question was just a thought I had to be honest, so no manual. I didn't know if you were running a certain motor on the output, you should spec the protection to the motor and the inverter, rather than the inverter iteself.

Thanks a lot

 

[Sidewinder]

Star delta is star first then delta.

The main contactor feeds one end of the set of windings, the other being connected to the star & delta contactors, which switch the configuration from star to delta after the time duration set on the timer, this is motor & load dependent.

With an inverter the upstream circuit is not protecting the motor, you would design the supply circuit to meet the input requirements of the inverter.

After the inverter, you would meet inverter manufacturers requirements.

Forget BS7671 after the inverter, you can't apply it.

Also, you are generally moving away from circuits covered by BS7671 here anyway.

 

[apprentice87]

HI Ryanm,

Just to try to rephrase what has been said in more "laymans" terms, [i am a layman so would know!!]

First, the three contactors...

If you look in the terminal box of a motor, you will find 6 terminals [usually] Imagine that you connect the three incoming phases to the one row of three terminals, say the top row. If you connect the little brass links across to the other three, you have just wired the motor in "delta" and the motor will need 400V.

If instead you connected the three incoming phases to the one row of three terminals, but then just linked the "other" three to each other, [instead of to the "first" three] you have connected the motor in "star" What you have done, is you have connected the "mains" to one end of all three windings, and by linking the "other" row of terminals together, you have formed the "star point" if you follow me.

Best way to see it is; The first contactor, the "line" contactor, powers the whole lot up, and as Sidewinder has said, it connects the incoming power to one end of all three windings, one phase to EACH of the separate windings.

The "Star" contactor closes at the same time, and connects the "other" three ends of the windings together [just like you could with the "links" in the terminal box] to form the "star point", so the motor starts in star. Then after a time preset with a timer, the "star" contactor opens, and the "Delta" one closes and quickly reconnects the motor in "Delta" instead, [again, just like you could with the links in the terminal box] so that the motor keeps on running.

Idea is, in delta the motor runs on 400v yes?? In star though, it would need more like 690V.... So why would you do that???? Well, by wiring the motor so it needs 690V [but only giving it 400v] you have greatly reduced the power of the thing, this means that it will start a lot more gently and avoid a sudden, violent start that might damage the machine it is connected to, and also, just as importantly, it vastly reduces the starting current needed by the motor. In practice, all motors above about 7.5hp are started "star/delta" to avoid the high current demands that would other wise be needed if you just switched them on, "DOL" [direct on line]

Once the motor is running, the timer then swaps the two contactors, opening one [disconnecting the "star" wiring] and closing the other [connecting the delta wiring], so now the motor runs in "delta" on its "proper" voltage, and develops full power..

Here is a link to a diagram of how the thing would be wired so you can visualise it all.

http://i1178.photobucket.com/albums/x373/resistance87/stardelta.jpg

john...

Look carefully at the diagram... Can you see that the "star" and "delta" contactors are just doing exactly what you could do yourself by changing the links in the terminal box??? They just do it more quickly than you could !!!!!

john...

 

[Technician]

Just an addition to posts by Sidewinder & John (Apprentice 87)

I have read a good deal about this and other starting methods.

The Star-Delta method is perfectly serviceable but power is

dicsconnected during the change-over from Star to Delta. The

rotor is magnetised during this time, caused by current circulating

in the rotor bars.

Upon re-connection the rotor field may oppose the new stator field

and the stator field has to reverse the field polarity in the rotor.

Some think this imposes a disadvantage on this method and cite

the transformer method as superior (Korndorffer) for larger motors

because for the entire start sequence, power is never disconnected

from the supply.

Source;  John Say; Performance and design of alternating current

machines.  (old book)

 

[apprentice87]

Hi all,

The first thing i did after i left school, after a stint in tech, was working in a small machine shop. We had two lathes, I had the biggest one and it had star delta starting. Love to know what make the lathe was, but i cannot remember.. Anyway, the starter was nothing like the "modern" ones.  You had a lever that operated in a quadrant, say about a third of a full circle and, so far as i can remember, two buttons. From what i can remember you had to swing the quadrant lever to the position marked "Start" and press a button, As soon as the motor was up to speed. maybe 2 seconds perhaps, you had to quickly swing the lever to the position marked "run" let go of the button and off you went!! To stop it you simply pressed the other button.

They had a large Pearns Richards horizontal borer i used to use too. Cannot remember how you started this, so i assume it had one of them "new fangled" "Automatic" starters. After all, there was a brass plate attached to the thing stating that it had been reconditioned in 1962!!!

I remember year later driving past a local machine tool dealers, and there was my lovely borer cut in half in a large skip. I know it was "my" one, as, apart from it looked the same, [from 50 yards] it was sky blue. How many sky blue borers you ever seen?? I was so sad......

They was better days them....

john....

 

[Technician]

I remember those starters John;  I made complex sketches of

them with all the drawings of how the interlocks worked.  As time

went by I worked on the "Korndorffer" ones and I was at pains

to point out to anyone who would listen that because of the current

flowing in the transformer during the start sequence, the number

of start attempts was limited to 2 per hour.

Funny things happened when this advice was ignored.........It was

there on a lovely big label on the core.

 

[NozSpark]

I remember using a starter similar to that on a 200V 400Hz rotary invertor (?) many years ago, I bet its still there and in use today

 

[Sidewinder]

Technician,

That depends on whether you use an open or closed transition Y/D starter!...

 

[Sidewinder]

Leave him out of this will you!

He has definitely not moved with the times, however, he does have a lot of experience on the old kit!

He has a mate who only ever now works abroad who won't move with the times too, they both insist that MV is still in the International Standards as a voltage band, when it's gone according to every current Std. I've seen!

 

[Technician]

Technician,

That depends on whether you use an open or closed transition Y/D starter!...

I will look into this Sidewinder;  thanks for the information and after

that I will try to get out more...... ut the kettle on