Star/delta or resistance starting ?
Star/delta or resistance starting ?
(OP)
What are the pros and cons of star delta starting a three phase induction motor, as opposed to resistance starting? And what criteria should be used for selecting suitable resistance values? The application is a high pressure centrifugal air blower operating at 5,700 RPM powered by a 7.5 BHP motor, the problem is one of high inertia at startup.





RE: Star/delta or resistance starting ?
I never used a resistance starter. For 7.5 HP, star-delta should do just fine. Closed transition even better.
What is the upstream service or transformer size? Even why not consider DOL start for such a small motor? It is cheapest, provides maximum toruqe and comes to rated speed faster. All you need to do is priovide a suitable branch circuit protection , fuse or breaker.
If you must use a reduced voltage starter, now-a-days Solid State Reduced Voltage starters cost even less than a star-delta starter and provides much more flexibility to set the ramp and also no bumping the motor like when it happens when changing from star to delta. Closed transtion Y-delta starter will be much more expensive.
RE: Star/delta or resistance starting ?
RE: Star/delta or resistance starting ?
RE: Star/delta or resistance starting ?
1. Reduced motor starting current to 1/3 of the delta connection run
2. Can be made automatic or manual to switch from the motor star connection to the motor delta connection.
3. It may be more reliable than the electronic soft starter.
4. Etc.
The con's of the star-delta starter are:
1. Creates the relatively large current spike during the star to delta transition.
2. It may be more expensive than the soft starter.
3. The speed transition can be relatively abrupt.
4. The motor load must be such that the star winding start will actually start the motor.
5. Etc.
RE: Star/delta or resistance starting ?
RE: Star/delta or resistance starting ?
http://www.dodge-pt.com/products/pt_components/flexidyn...
RE: Star/delta or resistance starting ?
In order to answer your question correctly one would need to review the torque/current vs speed curves of the motor and the load torque vs speed curve for the fan. Without that information available and from personal experience I would suggest you stear clear of star/delta starting for this application. Whilst connected in star the motor will produce 1/3 torque. This would generally be in-sufficient to accelerate the blower beyond say 40 - 50% speed and therefore when you transition to delta you can expect the torque shock on the belts to be very similar to that currently being experienced with DOL.
Incidently you should also be aware that aside from the step up in current and torque to near DOL levels when you transition, there will also be a current and torque transient (if the star/delta starter is not closed transition). The magnitude of these transients is quite significant and very damaging ie the current transient can be as much as twice that of DOL starting, the torque transient can be as high a four times that of DOL starting.
Star/delta starters often cause more more mechanical damage that DOL. They should only be used on very light load application requiring no mnore than 30% torque to accelerate to speed. For applications requiring much more starting torque (as would be the case for your blower), beware!
Regards,
GGOSS
RE: Star/delta or resistance starting ?
I have yet to try starting in star, but will today. Cost is also a big factor, and a simple contactor and delay circuit is pretty simple. I suspect the torque/speed/current relationship between resistance starting and star/delta starting to be quite different.
The problem is that there may be over ten starts per hour, and I want to get reasonable belt life. I also plan to increase the number of drive belts from two to four.
RE: Star/delta or resistance starting ?
I am sure you are already doing it, but starting the blower with its damper 'closed' would reduce the starting torque requirement considerably. In fact it is the norm.
Also consider direct coupling the motor, rather than using belts.
RE: Star/delta or resistance starting ?
I did look at the possibility of using a three phase rectifier/SCR combination with a dc brush type motor, but cost and complexity ruled that out. It would have had excellent starting torque though, but I do not really need that.
With star/delta starting, and some beefier drive belts I think I will probably be o/k.
RE: Star/delta or resistance starting ?
RE: Star/delta or resistance starting ?
Anyhow, the motor starts up very well in star connection, with the blower reaching 85% pressure in about twenty seconds. That is about 92% of final rpm, so switching to delta at that point should not cause any major dramas. A contactor with changeover contacts and a time delay should do the trick as far as I can see.
RE: Star/delta or resistance starting ?
http://www.saftronics2s.co.uk/softstarts/manuals/pdf/pr...
for: Paragraph 3.2
http://www.engineeringtalk.com/news/reo/reo101.html
etc. for more info
RE: Star/delta or resistance starting ?