Star/delta or resistance starting ?

[Warpspeed]

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.

 

[rbulsara]

Just my opinion:

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.

 

[cbarn24050]

Hi, a dol starter would probably do ok. If you use star/delta do not let the motor stay in star for longer than the minimum you can get away with.

 

[Warpspeed]

Thank you for the info. Starting current is not really the problem, it will start o/k DOL, but the huge torque is hell on the drive belts. I have yet to try running it in star, and am well aware that the increased slip at full load might cook the rotor if it remains in star for very long. I will give it a try though.

 

[jbartos]

Suggestion: The pro's of the star-delta starter are:
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.

 

[cbarn24050]

Hi, if you are looking to reduce the torque on your drive system then you would be better using an electronic soft starter.

 

[EdDanzer]

Dodge makes a Flexidyne drive for electric motors to start heavy loads without reduced voltage starting.

http://www.dodge-pt.com/products/pt_components/flexidyne/flexidyne.html

 

[GGOSS]

Hello Warpspeed,

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

 

[Warpspeed]

Thank again for helping me chaps. The basic problem is inertia, but because it is a centrifugal blower the actual drive horsepower is the cube of RPM. This means it should be able to accelerate easily up to fairly high speed before the load overcomes available motor torque.

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.

 

[rbulsara]

Just checking......

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.

 

[Warpspeed]

Yes, completely throttling the blower at startup is essential, but it does not help with inertia though. I cannot direct couple the blower because it requires 5,700 shaft RPM to develop sufficient pressure (2.0 psi).

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.

 

[jbartos]

Suggestion: Please, notice that 5700 shaft RPM can be achieved by a squirrel-cage induction motor of the inverter duty type.

 

[Warpspeed]

Any type of variable frequency drive, or dc drive is going to cost a fortune, in fact about twenty times the cost of the secondhand three phase 2850 RPM induction motor that I now have.

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.

 

[jbartos]

Suggestion to the previous posting: Visit

http://www.saftronics2s.co.uk/softstarts/manuals/pdf/productguide.pdf

for: Paragraph 3.2

http://www.engineeringtalk.com/news/reo/reo101.html

etc. for more info

 

[Warpspeed]

Thank you jbartos, a very interesting read indeed.