Missing link - Star/Delta 1

[kensea]

Hello all
First l'd like to thank you all, having been an avid viewer of this site for 1 and a half years l have gained much valuable knowledge. As an electrician, but budding engineer l hope l'm not intruding on your forum. My question pretains to reversing motor direction when using star/delta starting. I have always been told to reverse two motor winding lead connections on both sides of the motor terminal box, every schematic l have ever seen illustrates this also. Recently l have tried to demonstrate this on paper to see the difference between this and just reversing two of the incoming phases to the starter (as you would with a DOL delta connected motor). I can see that with star/delta method that as well as a rotational phase shift at the motor windings, there is reconnection of winding configuration to the two windings that have swapped phases in respect to the one that remains unchanged. However, when only swapping two of the incoming phases, the winding config. remains unchanged and there is a phase sequence rearrangement, but l cannot link these two together . Can someone please point me in the right direction as to why Star/delta motor reversing must be done this way and how it differs from DOL delta motor reversing. l've tried researching this but now turn to you all.

Thanks Kensea.

 

[aolalde]

I hope this explanation is understandable.
For three-phase single voltage motors with 3 terminal leads (T1,T2&T3), switching any couple of the power supply lines (L1,L2 &L3) will reverse the Voltage Phase Sequence imposed to the motor windings.
When a motor winding is designed for Wye-Start, Delta-Run it has six terminal leads (T1 to T6). The matching array to keep the same Voltage Phase sequence in the winding is L1-T1, L2-T2, L3-T3 (4, 5&6 shorted neutral) for Wye and L1-T1&T6, L2-T2&T4, L3-T3&t5 for Delta. If this condition is satisfied the motor runs in the same direction for both connections.
Now to reverse properly the Voltage Phase Sequence applied to the winding, two pairs of motor terminals need to be interposed. For instance it could be T1&T6 with T2&T4.

 

[Marke]

Hello kensea

If you reverse the phases applied to the whole starter, the motor will operate in reverse.

Reversing the phase sequence to the whole starter will cause the transition from star to delta to alter from a trailing phase connection to a leading phase connection.
When you change from star to delta, there is a period of time when the motor is open circuited and acting as a generator. there is some argument that copnnecting onto the trailing phase will reduce the switching transient, but in practice that result is questionable.
If you are concerned about the transition to the following phase, then you must wasp two motor windings. If you are not, (most installations that I have come across do not take any notice of this) then reversing the applied phase sequence is OK.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[kensea]

Thank you to both aolalde and Marke for your responses, that really had me stumped.
Marke
Thanks for the insight to the intrigate workings at play, that's what l love about the electrical industry and what is the driving force behind becoming an engineer, to get away from the "she'll be right" attitude.
aolalde
Re did the drawings with your suggestions and got correct voltage phase sequence in both wye and delta when interposing T1&T6 with T2&T4. However l'm still stuck on one point. I was working backwards from a respected engineering firms schematic on motor direction changes when using a wye start/delta run starter and get the following connections L1-T2&T4, L2-T1&T6, L3-T3&T5 (voltage phase sequence in the same diection for both wye/delta connections) and L1-T1&T5, L2-T2&T6, L3-T3&T4 (voltage phase sequence opposite in wye to above, but the same in delta to above). Have l mis-interrupted or will the motor direction follow the initial magnetic rotation direction set up by the wye connection? In this example the T1&T4 and T2&T5 phase connections have been interposed.
Thanks Kensea.

 

[aolalde]

Kensea.

That is interesting; looking the proposed connection it will not change the rotation in delta since you are making a double swap of phase sequence. Connecting T1&T5, T2&T6, and T3&T4 has reverse phase rotation as compared to T1&T6, T2&T4, and T3&T5. But you changed the lines L1 and L2 too. That double change will keep the same rotation in delta. However when the wye connection is made the rotation will change since T1 and T2 are reversed to L1 & L 2.
Does it make sense?

 

[kensea]

Thanks again aolalde
Yes, makes sense and was my take on the situation. I have drawn, drawn and redrawn the circuit and found my point of confusion. I was swapping the phase connection wires to the motor terminals (L1 to T2, L2 to T1 & L3 to T5, L1 to T4) and not interposing the motor lead connections (T1&T6 with T2&T4), leaving the phase connection wires where they were. But this is the strange thing, this is how the vender supplied schematics show it. This is their recommendations, L1-V1U2, L2-U1W2, L3-W1V2, which l agree will work. For reversal, L1-U1V2, L2-V1W2, L3W1U2, which will result in reversal in wye to the above, however, cause a voltage phase sequence change (and motor reversal)when switching from wye to delta. Am l mistaken? Unless l can prove this, with someone's higher technical knowledge, three motors are going to be wired as per vendor recommendations. These motors are direct coupled to water pumps, if the motor direction required, is the one where it starts one way in wye initially, then changes direction in delta, it brings about a number of concerns.
1/ Due to the di/dt of "throwing" the SCIM motor rotating field in reverse, will this bypass the thermal O/L and trip the C/B on overcurrent?
2/ Cause undue stress on the motor windings/insulation and reduce motor life expectancy?
3/ Spin the centrifical pump's impellor's off the shaft, causing mechanical damage and LRC. To cut a long story short, if this happens there are no spares, it will take approx. 2 days to fly in parts and hundred's of thousands of $$$ in downtime of a 3.3Mtpa process plant.
Thanks Kensea

P.S. I voted both you and Marke a star, but doesn't seem to have worked.

 

[aolalde]

I have no idea why they change totally the connections just to reverse the rotation.
However when I refer them to the winding each phase groups polarity works OK.
Apparently the connections proposed; L1-V1U2, L2-U1W2, L3-W1V2 and to reverse rotation L1-U1V2, L2-V1W2, L3-W1U2 work properly.
I will try to make a physical test as soon as I have a Y-D motor on hand.

 

[ScottI2R]

Kensea,
After confering with myself and my wife (does motor stuff), we have come up to the the following conclusions RE: your LAST post.
Question #1)Assuming the OL's are last in line to the motor, YES.(Unless all involved are rated to handle the current draw)
Question #2)Yes, due to the excess current draw during decel and accel during the direction reversal.(Generally, we stop ours before reversal, although these were NOT pump apps.)
Question #3)Wife says possible, but is this under load or not at the time? And just how fast do you plan to reverse how much motor?

Sorry, I hate to try to answer a "?" with a "?", but I do not know what you are working with.
Let me know what you come up with.

Scott


In a hundred years, it isn't going to matter anyway.

 

[kensea]

Thanks to all who replied
aolalde
I appreiciate your time, cause of that double transposing of phase rotation you explained, to me it seems the motor would reverse in the wye/delta changeover, but, this vendor must sell lots of these units and l have researched similar schematics from other suppliers, l was attempting to understand the finer workings at play and have enjoyed the exercise, have learnt some interesting facts and will keep at it till resolved. I look forward to your field findings and will post results of what happens when our field tests are done (nobody else seems concerned here and are prepared to lay blame on the vendor if something untoward occurs after the fact). I know these are simple wye/delta starters (not my choice) and a single solid state soft starter with some bypass contactors and control gear would have been better suited and equally cost efficient, but it has been intriging none the less.
ScottI2R
2/ The motor reversal when changing from wye start to delta run is not a required or suitable situation to occur, but one from my limited knowledge, can see occurring(although l think i'm missing something), and l was trying to determine the effects on the system if this was to occur.
3/ Yes, the motor(SCIM 22kw @ 415v @ 50Hz @ 1470RPM) will be under load at the time, pumping water with a slight SG above 1 from a suction of -3m to -4m to a discharge head of approx. 8m-9m, don't know the pump specs sorry, but it is a direct coupled centrifical pump to the motor so must have an Ns curve near 1470RPM and usually these pumps have open vane impellors installed, and if the transition time b/t wye/delta is set correctly l would assume it to be running at approx. 85% of full speed if/when the change occured.
Thanks again
Kensea