MV Motors, star connected stator why?

[Flashover]

I am currently reviewing a spec for Boiler feed motors (5 MWatt). the spec calls for the stator to be connected in STAR, is there a reason for requesting star instead of delta connected stator windings?
also the choice between fluid coupling (voith) and variable frequency drive? what would your preferance be and why? i am looking for info and variable frequency drives, websites etc
any help appreciated
young electrical engineer
quitwhenurahead

 

[electricpete]

For medium voltage (>600v) motors, Star (wye) connection seems standard, whereas low voltage motors sometimes are connected either way.

I am not sure of the reason. Maybe it has someting to do with 3 harmonic currents.

I think an electronic variable speed has many more capabilities than a fluid coupling. The primary purpose of the fluid coupling is to reduce load on motor during starting... but you can't use it to vary steady state load.

There is also a new choice... magnetic couplings which actually offer variable speed operation. Discussed on the board at reliability-magazine.com. I think the site is magndrive.com

 

[jwerthman]

It generally doesn't matter to the user whether the motor windings are connected star or delta. From the terminal connections, the motor looks the same. However, for medium voltage motors above about 1500-2000 kW, is is quite common to include differential protection around the motor to detect stator faults. This is usually done with three flux balance current transformers, with the line and neutral end of each stator winding phase passed through a CT. While this can be done on a delta winding, it is much easier with a star winding.

I respectfully disagree with Electricpete on the fluid coupling. These are quite commonly used for continuous variable speed control of pumps and fans. Some of the newer designs rival the efficiencies of the VFD systems. One advantage is that if you need to operate at rated speed, the fluid coupling can be locked up with zero slip and virtually no losses. Most VFD's have quite good efficiency at full speed/load, but the zero slip fluid drive will beat it.

 

[electricpete]

good comments jwerthman. I am not that familiar with use of fluid couplings.... didn't know they could be used for steady state speed control.

 

[peebee]

Just a guess -- would star windings mean less voltage stress on the winding insulation?

 

[busbar]

The differential-protection aspect is a good point. It seems like wye-versus-delta windings in medium- (and low-) voltage motors has a lot to do with the human scale. For wye-connected stator coils, the coil voltage is lower and current higher than the delta equivalent. This requires a larger conductor for wye, and is likely easier for asemblers to handle and possibly more difficult to damage. In low voltage, the same idea can be applied, so it seems that smaller horsepowers in the US are typically wye-wound, for the enameled copper is easier to work with--less fragile. In larger delta-wound motors, the conductor material is smaller, so comparitively not too fat to handle.

Probably boils down ot pure ecomomnics, refined over decades of motor construction. Also, conductors in wye-wound stators are larger and less likely to experience damaging movement during magnetic stresses of starting, giving them potentailly longer life. {The J&P Transformer book mentions this...assumed concept extended to motors.}

 

[Flashover]

thx exeryone, i take it that the star stator winding is more so for convience of construction?

 

[busbar]

flash -- That may sum it up.

 

[jbartos]

Suggestion: Visit

http://www.aemdessau.de/katalog/asyn/english/a15.htm

(indicating convenience of star winding in range from 1000V to 6600V)
Essentially, the medium voltage often means smaller current. This is where the star connection is more economical. Voltage stresses around the neutral connections are smaller. Delta connections would have bigger stresses at delta connections.
Delta winding is more economical for bigger currents and lower voltges.
This winding aspect is more eleaborated on and visible at medium and high voltage transformers. Many high voltage transformers connected within transmission lines have star-star connections.