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View Full Version : wye delta vs full load current connection







engrp
08-01-2012, 01:13 AM
Hello again, fellow members. is it the norm to experience a 15 to 17 % reduction in running current when a motor is connected in wye delta , as against full load connection ?

install monkey
08-01-2012, 10:46 AM
https://rapidshare.com/files/2367871387/Star_Delta_Starting_and_Dual_Voltage_Motors_Explained.pdf;
may help

nike123
08-01-2012, 10:47 AM
I am not aware of any arrangement of windings in 3 phase motor that are connected in why-delta.
You could only chose one type (star or delta) of connection for starting and other for running, or for using different phase voltage.

Star delta start
(http://www.patchn.com/index.php?option=com_content&view=article&id=42:star-delta-start-up-principles&catid=18:motors-and-drives&Itemid=116)
Therefore , I don't see relevance of your question.
Rephrase your question.

chemi-cool
08-01-2012, 03:56 PM
Refrigeration compressors i general are ot wired for star\delta.
Its a common mistake of electricians and tech with low knowledge of refrigeration electrics.

Most refrigeration compressors have a Y YY start. It can be 50% - 50% or any other % arrangement.
My favorite start is YY with soft starter.

Refer to name plate on any compressor how to wire the start.

goshen
08-01-2012, 08:42 PM
Hi engrp
as chemi wrote ,star delta is being phased out ,soft sarting is the future!
good luck

chilliwilly
08-01-2012, 10:43 PM
Yes but you would expect to see up to 2/3 reduction of delta (mesh) running current when in wye/star. Like the other posts say, soft starting has been around for a few years now. But so have invertors and they will be the future as the speed control for the motor can be so low it can be stalling without causing an overload or torqueing out.

It is still more economical to actually start large motors of 300KW upwards with star delta/wye mesh or with a variable auto transformer. Two speed motors are still in use today to reduce the in rush of starting current when connected in star delta/wye mesh.

engrp
08-01-2012, 11:59 PM
thanks to all the posters, and sorry if my terminology is in error. anyway, before , two of our recip compressors were using soft starters (danfoss brand). after a few years of use, both units needed repairs, but the cost is almost 80 % of the bnew unit. i did some research on soft starters, and from my understanding, you will have savings during startup of the system. but in our case, once we fire up the compressor, it operates 24/7, unless there is a power interruption or a scheduled maintenance. so instead of buying or having the soft starters repaired, i had it replaced with wye delta(or star delta, or again , correct my terminology, the box contains 2 big magnetic contactors and 1 smaller magnetic contactors + other smaller parts). unless there is savings during at 100 % capacity of the compressor, please correct my interpretation of soft starters. i also read about the frequency inverters or variable frequency drives (i think they call this vfd's). of course, the sales engineers would say that this is applicable , even to recips (ammonia compressors we use, 125 hp above). but when i asked from the compressor manufacturers, they say that vfd's are only applicable for screw comps. if any of you guys are using vfd's on recips, i would like to hear from you. again thanks to all

chilliwilly
09-01-2012, 12:23 AM
You can use vfd/vvf invertors on any type of drive, except some of the older motors 20+ years or older don't like the low voltage and low frequencies they tend to hum and vibrate more due to their type of construction. Mits use invertors on their recip pots on all their mid to top of the range air con systems.

Magoo
09-01-2012, 12:28 AM
I totally agree with Goshin, these days soft starters are seriously cheaper than a star delta system , auto trani what ever old tech systems. Apart from the actual component costs and installation cost, you dont get huge inrush and transistion spikes/ currents. With the HEM motors these days those spikes can destroy a motor prematurly. Particually with vector shifts on star delta [ wye delta an american terminology ] unless you want to spend more money on a closed trasition change over. With the soft starter use a parallel line contactor as well.

chilliwilly
09-01-2012, 02:34 AM
Up to 300 kw soft starters are economical for installation costs, but anything larger tried and tested contactors are more economical in the long run. In a rubber compounder factory where I used to look after the maintenance, there wasn't such a thing as a soft starter that would start a 800 horse motor. Eurotherm, Dowding and Mills, and Berl advised us to stick with contactors and auto transformers, as the distribution was set up for such load demands. It had 2x 11kv subs for the building, so no penalties for exeeding kva demand.

And connecting several SCR type soft starters banks in parralell with the main contactor just wasn't economical, and eventually one of the soft starter banks would fail and wouldn't allow the motor to start. Even the Ellison starters and wound rotor motors were more economical to maintain rather than upgrade to SCR technology. And the 3kv motor on one of the 500 litre Banbury mixers, was also more economical to stick with autotransformer starting.

The only motors with soft starting applications with variable speed control on them were the ones of up to around 100 kw, and the 750 horse tandem drive for the 300 litre Banbury was a DC drive, which is similiar to a soft start but DC drives are in a world of their own. The rest were low speed high torque hydraulic drives fed by feed and charge pumps running flat out constantly and started via star delta control.

engrp
09-01-2012, 08:45 AM
thanks again for your comments. i just noticed in chilliwilly's comments, that he was referring to recips for airconditioning systems. we are using ammonia based recip compressors , which are always on, unless for power outages or maintenance activities. unless i again misunderstood the concept of the soft starters, the savings would be gained if the system starts and stops repeatedly. i understand that the soft starter would greatly help at startups, but what if our startups are very seldom ? please enlighten me further. before asking in this forum, i also asked some of the ice plant operators here in our country, and all the plant operators i asked all use wye delta starters(sorry again, if i used wrong terminology). somewhere also i noticed a post saying that , unless our ammonia recip compressor is more than 20 years old, vfd is also applicable to us. our latest purchase was a mycom 8cyl wbh series compressor. before proceeding with the purchase, i asked the local mycom distributor if i can use vfd on the said compressor, and they said, it is not applicable (vfd's) for recip compressors. grasso and sabroe (local distributor), when i also consulted them about this matter also said the same. is there a conspiracy ? or are this local distributors of compressors only misinformed ? from the other posts , big motors were mentioned(300 kw above), but i did not notice application of soft starter or vfd's mentioned in refrigerarion plants (ammonia based). did i miss this item (use of vfd's and/or soft starters on ammonia based recip compressors) from your posts above ?

shooter
09-01-2012, 08:50 AM
use the relais all industries still using it a lot.
As screw compressors start unloaded, no problem for them.
A VFD is very good when starting under load, it will take the high inrush current, however they do need some cooling and that is about 5% of power. In a relais set this is way under 1%.
A screw should always be on max rpm. for economic reasons, however it is possible to lower the speed. Minimum is 25%.

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10-01-2012, 11:51 AM
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NH3LVR
11-01-2012, 01:21 AM
A screw should always be on max rpm. for economic reasons, however it is possible to lower the speed. Minimum is 25%.

I disagree with that. Speed control saves a lot of energy over using the slide valve. Our State Government is partially funding the VFD portion of a screw installation through tax credits. Screws are very inefficient at low loading percentage.

engrp
11-01-2012, 07:29 AM
thanks for the post, nh3lvr. what you are mentioning are screw comps. we use recips . i am inquiring about actual experience of other fellow members on use of vfd's on recips. i am not convinced with the cost savings , if there will be any, on soft starters, since we seldom shutdown the recips. if maybe, the compressor starts/stops a couple of times, then there may be some savings to be generated. sometimes the recips runs for almost 2 months without shutting down, so maybe soft starters are not that attractive to us.

Magoo
11-01-2012, 11:41 PM
Now you mention that you have recips, soft starters with a parallel line contactor is the best option rather than direct-on-line [dol] Set the ramp time to 5 seconds minimum , which will drop the inrush current peak loadings. That will save money on the power bill

D.D.KORANNE
12-01-2012, 08:14 AM
FOR RECIP COMPRESSOR ,YOU WILL HAVE TO ASK THE LOWEST RPM OF COMPRESSOR FROM THE MANUFACTURER .THIS WILL HELP YOU SET THE FREQUENCY AT START UP . OLDER COMPRESSORS HAD HAD LOWER SPEED LIMITATION , MOREOVER THE IMPORTANT POINT TO REMEMBER IS THE SUCTION PIPE SIZING FOR THE PERMITTED LOWEST SPEED BY THE COMPRESSOR MANUFACTURER . New versions of recip compressors permit lowest speed to 750 rpm .