3 phase motor wiring

[csalp]

New to posting on this forum so hopefully without getting an earfull...

fuzzytrifle - top right of the plate identifies a 3 phase 50 htz motor, links shown connected in star.

If your replacement motor shows the same, connect it with the links and wires exactly as the old and it should be good to go. It may go the wrong direction, if it does swap 2 of the wires on the motor. If the plate on the replacement motor shows 230V 50 htz do not use it, you have the wrong motor.

Oh, by the way,
star - high current, low speed,
delta - low current, high speed.

Motors that use star/delta starting are run in star initially to aid starting.

 

[bernardgreen]

Assuming the motor is not rotating there is no generator effect so the full supply voltage is effective in creating current into the windings.

In star the voltage is across two windings in series so the current is lower.

In delta the voltage is across one winding so the current is higher.

When the motor is rotating the generator effect creates an opposing EMF ( voltage ) which reduces the effective voltage so the current is lower than when the motor is not rotating.

 

[Spark123]

Oh, by the way,
star - high current, high torque, low speed,
delta - low current, high speed.

Won't the speed be governed by the supply frequency, the slip being proportional to the load applied?
delta is high current, high torque.
star is low current, low torque (approx 1/3 of the delta current/torque values.)

 

[bernardgreen]

Yes, the slip does depend on the load and the power of the motor.

 

[matt1e]

In star the voltage is across two windings in series

Not strictly true Bernard but I catch yer drift

Matt

 

[echoes]

I found it interesting to see how many did not know what star and delta wiring was all about.

Indeed - particularly since I'm all but certain that we learnt about such things in the name of A-Level Physics many many moons ago!

Kind Regards, John

Maybe I'm too young, but I didn't learn about 3ph and star/delta until degree level. Physics at A-level didn't go very much beyond ohm's law.

 

[ricicle]

New to posting on this forum so hopefully without getting an earfull...


Oh, by the way,
star - high current, high torque, low speed,
delta - low current, high speed.

Oops - back to school for you !

Star connection = lower current and torque. The speed is only lower because the rotor is trying to catch up with the rotating field in the stator.
On some machinery with large loads the motor will still not be up to speed before the change from star to delta. Once at speed the rotor will nearly be at the same speed as the stator (it can never be at the same same speed otherwise the induction motor would not work)
This means the slip between the rotor and stator is at its smallest so the magnetic fields in the stator are not cutting across the windings of the rotor as fast so less current is induced in them. Hence less stator current.
When the motor is first started the magnetic fields in the stator are cutting through the rotor windings fastest (maximum slip) so the currents are big in the stator. That is why the voltage is reduced so that the currents are reduced.

 

[JohnW2]

Indeed - particularly since I'm all but certain that we learnt about such things in the name of A-Level Physics many many moons ago!

Maybe I'm too young, but I didn't learn about 3ph and star/delta until degree level. Physics at A-level didn't go very much beyond ohm's law.

I don't know how young you are, but I'm talking about the 60s "when exams were exams" Ohm's and Kirchoff's Laws etc. were very much O-Level. Quite involved aspects of AC circuits, including complex algebra treatments thereof, and certainly including 3-phase concepts, were definitely part of A-Level Physics back then.

Kind Regards, John

 

[matt1e]

I agree John
I saw an example of the 11 plus exam not that long ago (I missed out by a year)
I would love to see some of our newly A graded students have a go of it now, never mind 11 year olds

Matt

 

[RF Lighting]

I was at college 15 years ago as part of my apprenticeship and I don't think the even mentioned Kirchhoff's law. I still have all my notes from back then! I'll have a look tomorrow. IIRC we did some TP theory but very little mention of star delta starters an motor theory. I did four years at college. I dread to think how little they get taught these days. The last apprentice I worked with qualified after two and a half years

 

[JohnW2]

I was at college 15 years ago as part of my apprenticeship and I don't think the even mentioned Kirchhoff's law. I still have all my notes from back then! I'll have a look tomorrow. IIRC we did some TP theory but very little mention of star delta starters an motor theory. I did four years at college. I dread to think how little they get taught these days. The last apprentice I worked with qualified after two and a half years

I'm very conscious of how easy it is to just sound like an "ageing bore" but this does sadden me - and, in some contexts, also worries or even frightens me. Perhaps the greatest sadness derives from the fact that I am sure that the youngsters of today are at least as capable (and probably potentially as keen) as were my generation of acquiring all the education, understanding and knowledge that we did - so it is very much 'the system' that has let them down, definitely not "their fault".

Kind Regards, John

 

[ricicle]

In my experience nowadays most companies have an army of semi-skilled guys with watered down qualifications who have one or two 'engineers' back at base who are only a phone call away.
I think the onset of the mobile phone made this possible and it also allows companies to pay lower wages to the guys in the field thus saving them money. These companies still charge permium rates though.
To see if I am right next time you have a boiler breakdown or similar see if the guy can fix it without phoning someone !

 

[bernardgreen]

I'm very conscious of how easy it is to just sound like an "ageing bore" but this does sadden me - and, in some contexts, also worries or even frightens me.

I have the same sadness and worries. Especially where Hi_Tech is taught and it's use encouraged but without the essential basic knowledge needed to select the appropriate technology.
My school included 3 phase motors in third year physics linked to the use of them in the engineering workshops.

 

[csalp]

Wow, Wikipedia is a wonderful thing... Shall we move away from motor theory gentlemen and go to the dark practical side of things?

Let’s talk about running current because that’s what you are really interested in. Assuming the motor can achieve design speed under the required load, would anybody care to answer this for me... you have two identical motors under identical loads, one motor wired in star and the other in delta, which motor would have the lowest running current? Come on, you know the answer....

Now let’s talk about motor speed, under load conditions, a star motor MAY achieve design speed, a delta wired motor WILL achieve design speed. So if the motor has any sort of load on it, the star motor in not reaching design speed can be said to be slower than the same motor wired in delta.

Hence...

Star – high current, low speed
Delta – low current, high speed.

You may have noticed that I have removed the reference to torque. To find any information on torque availability against voltage will be virtually impossible and I can’t think of a decent analogy to argue my case, so I have removed any reference to torque.

Just to keep the cat amongst the pidgeons, I pose you a question; In a star/delta starter, where do you put the overload?

 

[ban-all-sheds]

I don't know how young you are, but I'm talking about the 60s "when exams were exams" Ohm's and Kirchoff's Laws etc. were very much O-Level. Quite involved aspects of AC circuits, including complex algebra treatments thereof, and certainly including 3-phase concepts, were definitely part of A-Level Physics back then.

I did my Physics A-level between 1969 & 1971, and I don't recall doing that.

I'm very conscious of how easy it is to just sound like an "ageing bore"

but this does sadden me - and, in some contexts, also worries or even frightens me. Perhaps the greatest sadness derives from the fact that I am sure that the youngsters of today are at least as capable (and probably potentially as keen) as were my generation of acquiring all the education, understanding and knowledge that we did - so it is very much 'the system' that has let them down, definitely not "their fault".

Because we are not allowed to have a system which recognises different levels of ability. Losing and failure cannot happen. Any suggestion that some people can go further than others is elitism. Everybody must be told that they can do anything and everything. All you need to do to get to university is to want to go. Can't have A-levels because they are too hard for some. Can't tell people that they should acquire a foundation of knowledge of a subject because that is egocentric denigration.