Star Delta conductors

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chris kennedy

chris kennedy

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Miami Fla.
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Retired electrician
In a star start delta run application is there any advantage or disadvantage to having all 6 conductors in the same raceway?
Thanks
 
No advantage or disadvantage I can think of, other than conduit fill. S-D starting is usually going to be a larger motor, so 6 CCCs for a 300HP motor in one conduit is going to mean a big conduit.
 
Just don't forget that you have six current carrying conductors and an ampacity adjustment of 0.8 is required.
 
I have never worked with star delta, but don't they by the very nature never have more than 3 current carrying conductors at one time?
 
The delta connection is made at the motor starter so all 6 conductors make up to different terminals in the motor J-box, therefore all 6 carry current.
 
Yes they do so I agree 6 CCC's. They can each be sized to 72% of the FLC.
 
It may be obvious, but with two conduits you'd want the conductors going to the line (i.e., power) in one conduit (e.g., to motor terminals 1, 2, 3), and those to the terminals that are switched (4, 5, 6) in the other conduit. That's so that each group of 3 conductors will have their magnetic fields cancel out.
If you just had to distribute the six conductors between three conduits, you'd put a pair of conductors going to each winding in a separate conduit. In other words, pairs 5-2, 4-1, 6-3 each in its own conduit.
 
Just curious but in the last 20 years the only new Y start Delta run motors were on fire pumps ( was told this might help selecting a smaller Size emergency generator ). The last new building did have a soft start for the 250 HP Fire Pump motor. ( building had two 2,000 KW diesel generators ). With the price and so much possible energy savings would only go with a VFD. That new bui!ding had VFD'S on every motor 5 HP & larger and 60 HP & over had 18 pulse drives except for two 1,759 HP 4,160 volt chillers that had the normal ( 6 pulse ) VFD'S.
 
VFDs win hands down on centrifugal pumps and fans where flow control is part of the application or speed control improves a task. But if there is no value in varying the speed, they actually waste energy and cost too much. I personally prefer soft starters in that case vs Y-D for a number of reasons. Yet sone people stick to the old ways because they are either cheap (Y-D is still the least expensive form of Reduced Voltage starting, if only by a few percent), or they are technophobes. For sone reason the large refrigeration compressor industry is hanging on to Y-D in many areas, as are sone elevator mfrs.

Fire pump controllers need separate and expensive UL and FM listings, which are very difficult to achieve for any solid state controller like a VFD or Soft Starter. With a relatively short product design lifespan on electronics, the low sales volume on fire pump controllers makes it difficult to make the investment in testing, only to have to possibly start all over in 5 or 10 years when the components are obsolete. So you will still see Y-D there for sone time to come.
 
Jraef said:
VFDs win hands down on centrifugal pumps and fans where flow control is part of the application or speed control improves a task. But if there is no value in varying the speed, they actually waste energy and cost too much. I personally prefer soft starters in that case vs Y-D for a number of reasons. Yet sone people stick to the old ways because they are either cheap (Y-D is still the least expensive form of Reduced Voltage starting, if only by a few percent), or they are technophobes. For sone reason the large refrigeration compressor industry is hanging on to Y-D in many areas, as are sone elevator mfrs.

Fire pump controllers need separate and expensive UL and FM listings, which are very difficult to achieve for any solid state controller like a VFD or Soft Starter. With a relatively short product design lifespan on electronics, the low sales volume on fire pump controllers makes it difficult to make the investment in testing, only to have to possibly start all over in 5 or 10 years when the components are obsolete. So you will still see Y-D there for sone time to come.
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One thing that I never liked about the Y start Delta run several times I have seen where in older installs the timer failed and the motor burnt out because it stayed on the Y connection. Worst case a fellow sparky replaced a critical motor and only checked direction then walked away. A few minutes later they restarted the production line and replacement motor burnt up due to only running on the Y connection. We were taught to Megger every motor after replacing along with checking ampere on every phase at least twice after start up. Getting back to fire pumps we had the control panel with built in ATS transfer switch replaced on a 100 HP fire pump. They used the cheapest Joslyn Clark garbage contactors. On our 3 month fire pump PM'S we had to remove covers on the contactors. Cheap Joslyn Clark covers could not be removed to check contacts. We always did voltage drop measurements on contacts while motor was running along with IR scans.
 
don_resqcapt19 said:
Just don't forget that you have six current carrying conductors and an ampacity adjustment of 0.8 is required.
Click to expand...
Just for discussion purposes, since Wye-Delta starting requires, as the name implies, a switching of the motor windings from one connection (wye) to another (delta). Switching the motor from wye to delta by the open transition method allows it to be completely disconnected from the power source for a brief time period before being reconnected delta.

So technically there are 6 CCC in the conduit and invokes Table 310.15(C)(1) Adjustment Factors for More Than Three Current-Carrying Conductors, the 6 conductors are never carrying current at the same time, only 3 at a time.

Maybe a work around, not requiring the table ?

In reality, I've not used a Y-D starter in many years, since VFD's are relatively cheap, but it was something that crossed my mind.
 
ron said:
So technically there are 6 CCC in the conduit and invokes Table 310.15(C)(1) Adjustment Factors for More Than Three Current-Carrying Conductors, the 6 conductors are never carrying current at the same time, only 3 at a time.
Click to expand...
Correct me if I'm wrong but I don't believe that this is true. When on Delta run all six conductors are carrying current at 58% of the motor rating. That's why those conductors are sized at 72% (58*125%=72%) of the FLC.
 
…the 6 conductors are never carrying current at the same time, only 3 at a time.
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No, that’s not how Y-Delta works. You are maybe thinking of 2 speed 2 winding motors, different beasts.

Current flows through all 6 conductors, but in an “open transition”, there is no current on 3 of them for that transition time, usually less than a second as it switches from the Wye connection pattern to the Delta pattern.
 
ron said:
Just for discussion purposes, since Wye-Delta starting requires, as the name implies, a switching of the motor windings from one connection (wye) to another (delta). Switching the motor from wye to delta by the open transition method allows it to be completely disconnected from the power source for a brief time period before being reconnected delta.

So technically there are 6 CCC in the conduit and invokes Table 310.15(C)(1) Adjustment Factors for More Than Three Current-Carrying Conductors, the 6 conductors are never carrying current at the same time, only 3 at a time.

Maybe a work around, not requiring the table ?

In reality, I've not used a Y-D starter in many years, since VFD's are relatively cheap, but it was something that crossed my mind.
Click to expand...
Every one I have worked on had all six conductors carrying current in both the wye and delta connections. In the wye one set of conductors is connected to the 3 lines and the other set is connected together by the shorting contactor to create the wye. When you switch to delta the 3 conductors that were shorted in wye are connected to the 3 lines.
 
don_resqcapt19 said:
Every one I have worked on had all six conductors carrying current in both the wye and delta connections. In the wye one set of conductors is connected to the 3 lines and the other set is connected together by the shorting contactor to create the wye. When you switch to delta the 3 conductors that were shorted in wye are connected to the 3 lines.
Click to expand...

I agree. The 6 conductors going to the motor consist of 3 pairs, with each pair going to its own separate winding that is isolated from the other two windings. Therefore the amount of current through the two conductors of each pair is always identical at all times, no matter what position the wye-delta switch is in (and even during an open transition interval).
 
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