Inside Delta Connection 2

[VicP]

I have heard references to using an "Inside Delta" connection when replacing a wye delta starter with a solid state soft start. Can anyone explain what this connection is or provide links to information on it.
Thanks

 

[Marke]

Inside delta connection, or six wire connection, is where the controlling elements of the soft starter are connected in series with the windings of a delta connected motor, rather than in series with the supply as is the standard connection.
By connecting the SCRs in series with the windings, the current controlled by the SCRs is reduced and therefore a smaller SCR can be used, or a larger motor can be used for a given soft starter size.
You must have a soft starter designed for this type of operation, many three wire units will be damaged if used inside delta and certainly will not give adequate control.
With inside delta connection, when the starter is turned off, there is still full voltage on one end of the motor windings, so a contactor must be used on the supply.
There is an app note at

http://www.aucom.co.nz/technotes/me00055.pdf

Best regards, Mark Empson

http://www.lmphotonics.com

 

[jbartos]

Suggestion: Visit

http://www.benshaw.com/products/lv/rsm.shtml

http://www.benshaw.com/literature/index.shtml

http://www.grossautomation.com/abb/pdf/AC1061.pdf

(ABB literature)
etc.

 

[infomaniac]

In regards to Marke's comments, very well explained. To add one additional comment, the current inside the delta is reduced by 58% in relation to the motors rated current. For example, if you have a 100A rated motor, the current inside the delta is 58A. This is why a smaller softstarter can be used when connected in series with the motor windings. I have written a 'white' paper that shows all the mathematical calculations behind the idea if you would like to see it, VicP.

 

[Marke]

Hello infomaniac. If you consider a steady state sinusoidal current flow, then the current inside the delta loop is as you say, root three of the line current (58%) however this does not apply when considering the ratings of soft starters applied to inside delta. (I know that a number of soft start manufacturers imply that it does). If you are correctly rating the starter for that application, you select the SCRs etc to limit the maximum junction temperature of the SCRs to their maximum operating temperature which is typically 125C. The maximum junction temperature occurs during a "hot" start, or a start from a previous run. The junction temperature is determined by the heatsink temperature, power dissipated and thermal resistance between the junction and the heatsink.
When you connect the SCRs inside the delta loop, the shape factor of the current waveform through the SCRs is considerably different from that when the SCRs are controlling line current. The conduction angle at half voltage in normal operation is in the order of 150 degrees, while the conduction angle at half voltage inside delta connection is about 90 degrees. For the same average current, the on time is much shorter so the instantaeous current is much higher leading to a higher power dissipation and therefore a higher junction temperature. It is safer to select the starter on the effective current being 2/3 rather than 1/rt3. This will ensure long term reliability similar to the normal connection.
Best regards, Mark Empson

http://www.lmphotonics.com

 

[jraef]

VicP,
Be very carefull about using "Inside the Delta" soft starters without the isolation contactor as Marke explained. You will see in some of the website referenced that several manufacturers do not mention this issue at all, probably because it adds cost and somewhat defeats the purpose of using this technique. If there is no contactor, and one SCR shorts in the soft starter, you WILL loose the motor! The only other alternative is to have shorted SCR detection and a shunt trip mechanism on your circuit breaker feeding the motor. Even with this, post signs warning electricians that the motor leads are hot even if the starter is "off".

Marke,
Wow, I had never considered that issue about SCR sizing inside the delta but you are absolutely correct. I learn something new every day.

Infomaniac:
Not to be nit-picky, but the current is not reduced BY 58%, but TO 58%, which would be reduced BY 42%. Most of us knew what you meant, and your example was correct. I just felt compelled to clarify that for the newbies. Subvert the dominant paradigm... Think first, then act!

 

[infomaniac]

Jraef, You are correct! The current is reduced to 58% not by 58%. Typo on my part.

In regards to the isolation contactor, the better solution is to use a 'fault' contactor that is installed on the three 'hot' motor leads. With this type of configuration, the fault contactor is designed to open anytime the softstarter fault relay changes state due to a fault. This will de-energize the fault contactor and open the three 'hot' leads. This is very common practice (espcially in the hydraulic elevator industry where many hundreds of thousands of softstarters are being used inside the delta) that works very well. We have a lot of experience with connecting softsarters inside the delta and have no problems at all with this configuration.

 

[tmahan]

A second solution to the issue of hot lead to the motor is the use of a fault contactor. This contactor connects three of the lead directly to the line and the circuit can be easily designed to result in the contactor opening once the deceleration of the motor is complete. This eliminates the issue of hot leads while maintaining power to the softstarter as well as the concern over shorted SCR's

 

[infomaniac]

Marke,

I agree with your phase angle change inside the delta completely. That is why our softstarters are programmed to adjust the phase angle in the algorithm when it is placed inside the delta windings (there is a dip switch on the softstarter to let it know that it is being connected inside the delta windings).

In addition, most softstarters are all rated 115%-125% continuously so even if the phase angle where not considered, the softstarter would not normally have problems.

The lack of problems is evident due to increased usage by large hydraulic elevator manufacturers such as Otis, Kone, and Thyssen-Krupp. These larger manufacturers have almost entirely switched their motor control schemes to softstarters for hydraulic elevators where the softstarters are used inside the delta of the motor windings. They have been using softstarters for over a decade in this configuration and consider them superior to normal Y-D starters. The installed base is over 50,000 pieces per year.

 

[jraef]

The "Fault" contactor is exactly what I meant when I referred to it as an isolation contactor, probably not the best choice of terms for me since isolation contactors have another meaning when discussing soft starters. My bad.

The important point that I was making was that several soft starter manufacturers push the inside-the-delta configuration without mentioning the fault contactor, which I think is risky at best. I wanted the uninitiated like VicP to be wary of those that hawk the cheapness of it by leaving out important components.

I agree that there is nothing wrong with the concept or success of this application from an engineering standpoint. I do, however have a problem with widespread use in an unfamiliar workplace environment, and by that I mean the general US industrial market. I believe that the success of this in the elevator industry, even in the US, is due chiefly to the fact that elevator mechanics and electricians are very familiar with Y-Delta motors and starters because that is what is commonly used in their industry. The same may also be true of the refrigeration industry and a few others, where OEMs have a specialized maintenance / service force. My caution comes from seeing many many "industrial" electricians who have been given a Y-Delta motor to replace, and did not know anything about them or how to connect them. One can argue that it should not be so, but I have seen this problem in the lumber, rock crushing, air compressor, pumping, injection molding, metal working and several other industries. As a rule, US Electricians are not experienced enough with Y-Delta because they don't run into it often enough. This may come as a shock to people in Europe and Asia where it is common, but trust me, outside of a few specialties we do not use Y-Delta very often in the US. So even if you do understand it, chances are the next guy will not, and damage to the motor, controller and machine may result. For my money, connect a full sized soft start to the motor in Delta only. It also opens up the ability to replace the motor in a hurry with a standard delt wound motor. Just my 2 cents.

Subvert the dominant paradigm... Think first, then act!

 

[infomaniac]

jraef, I agree with your concerns that a lot of people are unfamiliar or even understand what is really going on with the inside delta idea. I believe the people that try to do this on their own without help are asking for trouble. However, when our customers contact us with an application that has the possibility to use a softstarter inside the delta, we ask a lot of questions. If we feel they are capable and understand the concept after we explain it, we will push the softstarter solution. Otherwise, we just tell them to permanently wire the motor as a delta configuration and use a standard three lead softstarter connection method. In other words, we pretty much evaluate the customers knowledge and push the solution that makes sense.

 

[jbartos]

Suggestion: The softstarters have some electronic artificial intelligence built-in. Visit

http://www.nsoem.com/Benshaw/literature/manuals/890001-15-xx.pdf

for more info. Certainly, a catastrophic failure can happen in the softstarters; however, it can happen in other starting arrangements too.

 

[GGOSS]

Hello All,

I am a new member to this forum and am looking forward to communicating with you all in future. This post will be short as there is much to do and very little time to do it.

To Marke, keep up the good work. It's nice to know that there are still some people out there that understand power electronics.

To jraef and infomaniac, I find your discussions re the use of 'fault' or 'isolation' contactors very interesting. You will note with all soft starters there is a leakage current that flows even when the SCRs are in the 'off-state'. Therefore if we do as you propose and place a fault contactor in the three hot wires, every time we open this contactor we are in affect opening the output of the soft starter, causing its output voltage to become equal to the line voltage. This presents a potentially hazardous situation, hence the practice should not be adopted!!!

The best solution is to install a contactor (or motor operated circuit breaker) into the line circuit, ie not within the delta loop. Furthermore the contactor or circuit breaker should be de-energised whenever there is no need for the soft starter to control the motor. In other words the contactor should be de-energised on each stop, not just when there is a fault condition.

Inside delta or 6 wire connection is not new technology. It is also very easy to explain and apply if you know how. The 'trap' is that most 6 wire soft starters currently marketed will only operate correctly in one of the six possible connection formats. Unfortunately when these are misconnected all the good smoke tends to escape(they fail)and the user is left with an undesireable experience. I must say this is a product / technology weekness.

I have been directly involved with inside delta connected soft starters for 20 years now. To the best of my knowledge there is only one 6 wire soft starter product available on the world market that will operate in all 6 possible connections formats correctly and is automatically configurating.

I hope that you will find at least some of the above helpful to you.

Regards,
GGOSS

 

[infomaniac]

GGOSS, we have also been involved with wye-delta softstarters for many years and have used the fault contactor scheme forever. There has never been any magic smoke let out of the box with the thousands of units that are sold in this configuration. Again, I point out the fact that the largest elevator companies in the world have stnadardized on this design over a decade ago. We have personally been involved with some of the testing methods. These elevator companies (Otis, Kone, Schindler and Thyssen-Krupp) would not have continued to use this design scheme or place their reputation on a product deisgn of this nature if it was not acceptable and reliable. Food for thought....

 

[GGOSS]

Hello Infomaniac,

Please re-read my post.....this time carefully!

Like you, I am an advocate for 6 wire soft starter installations and have promoted this technology for 20 years. In fact as far as the Australian market is concerned, there is no-one who has done more to establish 6 wire connection as a credible alternative than I.

My comment regarding good smoke escaping relates to the many 6 wire soft starters presently on the market that will only work in one of the six possible connection formats, it has absolutely nothing to do with inside delta operation or the use of fault contactors.

On the subject of fault contactors, I would ask that you draw the electrical circuit to give yourself a full appreciation of why it is un-safe to place the fault contactor in line with the three hot wires only. You will see that this method does not provide off-state isloation. In addition, you might want to consider what would happen to the motor if the soft starter does not provide SCR failure protection and an SCR fails. In such cases the fault contactor will not open and the motor will be severly damaged.

I hope this clarifies my position.

Regards,
GGOSS

 

[jbartos]

Suggestion: The soft starters without any contactors on their input side and output side have a galvanic connection upstream and one downstream. It may be overlooked, and it may also be objectionable and dangerous. Therefore, to match the contactor quality wye-delta start and control, the soft starter should ideally have input and output contactor to have it galvanically isolated from the source and load/motor. Although, I see advantages of soft starters, I also see their significant drawbacks.

 

[Marke]

Install the contactor so that it is controlling the three phase input and there is no problem.
Done this for 20 odd years. Mark Empson

http://www.lmphotonics.com

 

[infomaniac]

GGOSS, I did read your email carefully and understand what you are trying to say but again, the method of simply adding a fault contactor on the hot leads has become an industry acceptance and works fine with very little problems. It is a cost/risk benefit and so far the industry is saying the risks are neglible as evidenced by the increased use of the softstarter/fault contactor design. There are several softstarter manufacturers including Siemens, ABB, Allen bradley and S&S that all offer this configuration as standard. All the Big Boys are not going to place their reputation on an unacceptable design. In regards to your statement that only one manufacturing can run in all six wiring configurations, there are more than one that I can document.

 

[GGOSS]

Risk to personal safety is not an option!

There are statutory requiremtns in certain parts of the world that demand an isolation contactor be used even for 3 wire soft starters.

Rather than continue this argument, could you refer me to a link/drawing to ensure that we are not all arguing for the same connection scheme, but have misunderstood each others posts.

Regards,
GGOSS

 

[infomaniac]

GGOSS, I agree personal safety is above all. But remember, almost everything built in this world has a cost/benefit/risk assesment (i.e., bridges and buildings built in an earthquake prone areas, planes, cars etc....).

In any event, we need not argue and I did not try to make it appear so. If the posts seem offensive, I assure you they are not so not hard feelings. I repsect eveyones opinions. After all, if we did not have varying opinions, then many of the engineering wonders of the world would never have existed.

In regards to a drawing, you can go to ABB's, Allen Bradleys, Siemens, or S&S web sites and search for softstarters. In the softstarter sections, just look for the models designated for hydraulic elevators and/or inside delta configurations. You will find plenty of documentation on the designs which are essentially all the same.