Motor Theory

[DM7.ENG]

Hi

I previously made a similar post howeverjust want to ensure ive grasped this,


Found an explanation of star delta on awebsite although don’t think it explains the concepts all that well.
As far as I know the relationships for3phase star are IL=Iphase, Vphase = Vline/√3
And for Delta VL=Vphase, Iphase = Iline/√3(For balanced loads) and apply KCL otherwise.
The concept stated the voltage across a star winding is 230V if Vline = 400 but doesn't that only have relevance whendescribing the voltage/current characteristics of a system requiring a neutraleg. Between the secondary windings of a transformer connected in star & aTP&N DB.

If we connect a 3 phase motor In starlike above there Is a potential difference of 415V between winding U andwinding W therefore, as there is no neutral connection and the 230V is with respectto ground/Neutral then surely that concept changes for motors?

Additionally Would I be right in sayingif we had a name plate stating (400VDelta) (690V star) and we connect it in star then what we are saying Is…..
As the voltage propagates down L1 at thattime domain in the cycle, It see’s the resistance of all 3 windings incomparison to the single winding that would be between any 2 phases in delta and for this reason the current Drawn is 1/3of the current that would be drawn in delta.
Thus the windings may be rated for 660Vstar but will see 400V across them and hence they are under excited?
I apologies if I am not making myselfclear, I understand the concepts but there is creases in my understanding thatI want to iron out.

Thanks

 

[Marvo]

If we connect a 3 phase motor In starlike above there Is a potential difference of 415V between winding U andwinding W therefore, as there is no neutral connection and the 230V is with respectto ground/Neutral then surely that concept changes for motors?

I'm a bit confused by your question, I'm wondering if you're confusing transformer connection arrangements with motor connection arrangements when, in many ways, they're different.

If a 3-phase motor is connected in star with no neutral at the star point, if the winding resistances are equal and if the supply phase voltages are equal ie the load is perfectly balanced then the star point will stay a zero potential wrt ground. If you connect a neutral to this star point no current should flow in the neutral unless something happens that makes the load unbalanced ie a winding fails.

 

[DM7.ENG]

To summarize I came across a drawing online of a star connected motorwith VL = 400V & VPhase = 230V. The problem I see with that is that wouldbe fine for like you said a transformer as it has a star point neutralconnection as the 230V is a measurement between phase and neutral but as thisis a motor there is no neutral and hence does the windings not see the full400V across them? i.e. if we measure the voltage between L1 and L3 we will get400V so is this not the voltage that exists across windings U and W?

I'm a bit confused by your question, I'm wondering if you're confusing transformer connection arrangements with motor connection arrangements when, in many ways, they're different.

If a 3-phase motor is connected in star with no neutral at the star point, if the winding resistances are equal and if the supply phase voltages are equal ie the load is perfectly balanced then the star point will stay a zero potential wrt ground. If you connect a neutral to this star point no current should flow in the neutral unless something happens that makes the load unbalanced ie a winding fails.

 

[Darkwood]

In both diagrams put 400v between L1 and L2 ..

In Star the voltage goes through 2 windings but in delta it only goes through 1 ....so if the star is rated to 680v and you input 400v you get a limiting effect on the inrush because you are undervoltaging the motor then when it changes to delta it can run at full power as the voltage suits the winding arrangement.

 

[Marvo]

...... as thisis a motor there is no neutral and hence does the windings not see the full400V across them? i.e. if we measure the voltage between L1 and L3 we will get400V so is this not the voltage that exists across windings U and W?

The windings in star connection are effectively in series between the phases so the voltage between any two phases at a given point is always divided between more than on winding. The phase voltage is 380v(voltage between phases) but the line voltage across a winding will always be less than the phase voltage in star connection. The issue is that the three phases are constantly at 120 degrees out of phase but any resulting current flow will always be through more than one of the windings.

 

[DM7.ENG]

Thanks Guys thats pretty much clarified everything I was unsure of.
So the steady state current will pretty much be determined by VL/ Z where Z is the impedance of the windings in that series loop in

The windings in star connection are effectively in series between the phases so the voltage between any two phases at a given point is always divided between more than on winding. The phase voltage is 380v(voltage between phases) but the line voltage across a winding will always be less than the phase voltage in star connection. The issue is that the three phases are constantly at 120 degrees out of phase but any resulting current flow will always be through more than one of the windings.

 

[Darkwood]

Thanks Guys thats pretty much clarified everything I was unsure of.
So the steady state current will pretty much be determined by VL/ Z where Z is the impedance of the windings in that series loop in

You need to remember that the resistance of the windings may have little bearing on the actual current flow as it can be often found to be a very low resistance of a few Ohms because it relies on the build up of the magnetic field to counter the current flow. If you have a standard motor to practice on you will probably find the resistance of the windings are next to nothing.

 

[shanky887614]

You need to remember that the resistance of the windings may have little bearing on the actual current flow as it can be often found to be a very low resistance of a few Ohms because it relies on the build up of the magnetic field to counter the current flow. If you have a standard motor to practice on you will probably find the resistance of the windings are next to nothing.

how does the magnetic field limit the current flow?

 

[telectrix]

by creating an reactance/impedance

 

[Darkwood]

how does the magnetic field limit the current flow?

A bit loose in my explanation, I meant the generation of the magnetic field and its interation with the rotor to create a turning force limits the current flow, remove the rotor and you have just got 3 windings with very little resistance and then ohms law will take over - very high current which will either burn out the winding and/or take out the fusing.

I was keeping it simple so as not to let it get confusing, motor theory can get very complex very quickly so was just expressing the resistance of the windings may not have a calculating factor on the current drawn by the motor.