star delta modified to Direct on line 1

[emadmoad]

I alwasys have wondered, what would happen if I modified the star delta circuit of a 75 KW motor to a direct on line circuit ?
Is there any risk on the motor winding??

 

[aolalde]

The mechanical stress of the windings is proportional to the square of the line currents. That means the motor elements will suffer elevated stress but generally motors are constructed to withstand full voltage start.

I do not expect a sudden failure but a reduced life expectancy, specially when the motor starts frequently.

 

[Marke]

Hello aolalde

Star dleta starters usually cause the motor to suffer a sever current and torque transient at changeover and I have seen this cause much damage to the motor and to the driven load. Is DOL going to shorten the life relative to open transient switching with a star delta starter??

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[Marke]

Hello emadmoad
If you change from a star delta starter to a DOL starter, the motor will draw Locked rotor current to almsot full speed and will develop a much higher start torque.
This will result in a shorter starting time and may put additional strain on the mechanical system although from my experience, most damage occurs at the transition from star to delta due to the "auto reclose" effect.
The severe current and torque transients at the changeover will be eliminated and so the peak current and peak torque will be less than with the star delta starter.

The questions to ask are why was the star delta starter fitted in the first place? Why do you wish to make the change? If it was fitted to reduce the start current, or to reduce the start torque, then you would be better off to fit a soft starter to the application.

What is the application? let us know what the machine is and perhaps we can comment further.

Best regards,


Mark Empson

http://www.lmphotonics.com

 

[aolalde]

Hi Marke.

"Star dleta starters usually cause the motor to suffer a sever current and torque transient at changeover and I have seen this cause much damage to the motor and to the driven load".

Not if a closed transition starter is implemented.

 

[rbulsara]

with all due respect..to aoladle, my experience is also affirms marke's view. Motors are designed to be started DOL and I do not think it is detrimental to the motor, especially compared to star-delta starter.

A closed transition star-delta may be a different matter, but by that time I would go with a soft starter.

 

[RRaghunath]

Whether Star-Delta or DOL is not an issue decided based on the motor spec or the driven equipment specs. These are designed stand DOL, any way, without any problem.

Star-Delta starting is always a dictated by the power source (how strong is it in the area) and the unacceptable (to the subject motor as well as other equipment connected to the same power source)voltage drop that DOL starting could cause due to the large inrush current associated.

In case the motor is drawing power from the public distribution network in the area, the connection agreement with the ditributor may also be stipulating the type of start to be used (such as Star-Delta) depending on the motor kW rating.

raghunath_n00@rediffmail.com

 

[emadmoad]

Thank you all for ur replies
my supply is from a site generator,
the motor which is 75 kw moves a concrete (ready mix) mixer ,this is my load .
concerning the star delta why it is fitted in the first place -it is manufactured by siemens .
when the short circuit happened and the contactors were burned , I modified the star delta to work DOL

 

[mc5w]

However, in the case of 100 Horsepower and larger air compressors the wye-delta starter saves having to install heftier gears in the rotary screw pump. If you try to start across-the-line you will actually destroy the gears. You will either shear off the teeth or the starting torque will squeeze out oil from between the teeth allowing metal to metal contact what is known as lubrication failure. The oil circulation also depends on the pressure difference between the inlet and outlet - the compressor pump also serves and an oil pump.

 

[aolalde]

Thank you mc5w, People must understand that each device has specific requirements and limitations. We can not recommend a golden rule like " DOL starting is better than Y-D"

 

[mc5w]

In the case of a rotary screw air compressor a wye delta starter is cheaper than heftier gears in the air pump. Most large rotary screw refrigeration compressors are also wye-delta start for that reason.

Actually, when a rotary screw ammmonia compressor starts you have to throttle it if the liquid receiver in between the evapoarators and the compressor has had any chance to warm up. If you do not the liquid ammonia in the receiver will boil over which slugs the compressor. Ammonia refrigeration systems recirculate liquid from the receiver back to the evaporators. This is necessary both because of multiple evaporators and so that an expansion valve that is opening too much will not disable the system.

 

[ScottyUK]

mc5w,

The load in question is a concrete mixer - why all the talk about compressors? Not exactly similar loads!



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[jraef]

Good point from ScottyUK. We should all endeavor to keep on track. I know I am guilty of rambling off on tangents at times myself! I'm taking heed...

 

[itsmoked]

Lets not forget that lots is learned from other's experience's.

I'm not here just to answer specific questions to those who don't know something.

You pay people for THAT abuse.

Frankly people's experiences are more interesting than answering a lot of the same questions. Without the experiences this would be a very dry forum.

I don't mind if these things go off track... In fact more is often taught when they do. A noticable part of the time the OP never comes back, or never provides any further guidance/required info so why not take the opportunity to meet our fellow regulars?

 

[aolalde]

I agree with itsmoked.We should take advantage of this forum to exchange experiences whenever an oportunity arises.

 

[ScottyUK]

Hey guys,

I'm not sugggesting for a moment that mc5w's contributions aren't informative and interesting - some of them are really useful - but they sometimes aren't related to the thread. I'm not trying to stop the occasional wandering of the thread, but equally it should stay vaguely on target too. Feel free to disagree if you think I'm wrong - but can we take the discussion out of this thread? I'm trying to practice what I preach!



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[itsmoked]

Have not aolalde, Marke, rbulsara, and rraghunath already provided solid, clear answers to the original post and then some? Isn't it time to allow this thread to shift?

As for taking this discussion out of the thread: Where would this conversation, with these participants, occur otherwise?

ScottyUK; I do agree fully with you about off topic responses in the first few responses. This certainly doesn't help! It can even stall the thread.

The expansion valve stuff was... umm Oblique..

 

[noel0589]

I still wonder about Y-D Open transition transient torque vs. DOL starting torque. What's worse? 1-2 cycles of auto reclose current / torque? or 5-6 cycles of starting current / torque. (5-6 cycles for motors I deal with)
I didn't originally design the elevator systems I deal with using submersible motors/pumps, but I think they designed with the intent to minimize starting torque. The motors run in Y for 0.3seconds to allow full speed rpm under no load... then under delta load is applied. But it looks like using wye delta open really defeats the purpose due to the auto reclose effect. I've seen some job sites where the entire power unit would shake a lot when delta would make. any thoughts? I know solid state soft starter is better, but is DOL starting the middle ground here? The motor can reach full speed rpm loaded in .100 seconds.
Thanks all!

 

[itsmoked]

noel0589;
Well you get the Y opening this leaves the motor un-synchronized with the line.. Then you reconnect it to the line D.. This can occur at some horrible phase relationships like 180 degrees.. This can cause a large(luckily short) fault current that shakes the works.

Some here call Y-D a lousy solution especially for preventing power system transients.

About the only point I can see for the hassle is if a load must be brought up slowly to prevent something like lubrication failure mentioned recently.

 

[Marke]

noel0589
See faq237-613. Much damage to equipment occurs due to the reclose transient. - much more than due to DOL torque.

Best regards,

Mark Empson

http://www.lmphotonics.com