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Star-Delta Transition Timing

03/03/2020 10:24 PM

I have been working on a 75 kw air compressor that uses a star-delta starter. The unit is apx 6 years old and i'm told that pretty much from the start the unit has periodically tripped the 200 amp thermal magnetic trip breaker. Within the last month or so this unit has started tripping the breaker almost every time it starts. The peak/inrush current draw at star start is 336 amps. The starter timing is set at star for 10sec, and star-delta transition time was set for 150ms. After some research I decided to adjust the Star-Delta transition time to 75ms, my thinking was that the cause of the tripping is being caused by the voltage potential deference between the applied voltage from the delta starter versus the induced voltage in the motor. my question is how does this transition timing effect the motor/starter/breaker? just playing around with it i decided to record some readings while forcing the unit to trip. I set the transition timing to 150ms and when the delta energized there was no indication of high amp load yet the breaker would trip. And with it set to 75ms the motor transitioned through to delta with out tripping the breaker and the amp load at delta was 140 amps.

as you can see from the above graph when the trip took place there was no increase in amp load to the circuit.

This graph is of a normal successful start.

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#1

Re: Star-Delta transition timing

03/04/2020 6:53 AM

I don't have a clue but welcome to CR4.

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#2

Re: Star-Delta Transition Timing

03/04/2020 12:44 PM

I would think it has something to do with the length of wire run and the harmonics generated and it's relationship to the time...

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#3

Re: Star-Delta Transition Timing

03/04/2020 3:00 PM

There is a 30 degree phase shift that takes place when transitioning from Star to Delta windings inside of the motor. So if the transition time is too short, the magnetic fields of the windings from being in Star are not yet collapsed when the starter reconnects in Delta, so it is like connecting two generators out of phase; there is a potentially huge spike of voltage and/or current that takes place inside of the motor. That voltage spike can then go into the line and back into other equipment, or the current spike creates a massive torque spike that can (I've seen it) shear the motor shaft.

If the transition time is too long, then you run the risk of the motor slowing down too much and end up defeating the purpose of having a reduced voltage starter by making the motor have to re-establish the magnetic fields again and pull locked rotor current anyway.

Bottom line, you DON'T want to mess around with the transition timing. The time TO transition should be based upon the Star connection getting the load to at least 80% speed. Most people don't take the time and effort to find out what that is, so they guess, and it goes badly.

I have given up on using Star-Delta for that reason years ago, I only use Solid State Soft Starters.

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#4

Re: Star-Delta Transition Timing

03/05/2020 8:04 AM

Were the transition time = 1.6 + (20*N) milliseconds, where N is an integer, then there is the least chance of the motor tripping on overcurrent, as the timing ensures that the phase shift needed takes place with the minimum of disturbance to the system.

However, real-world contactors cannot perform to that sort of accuracy.

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#5
In reply to #4

Re: Star-Delta Transition Timing

03/05/2020 8:49 AM

...for a 50Hz installation.

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#6
In reply to #5

Re: Star-Delta Transition Timing

03/05/2020 9:11 AM

60hz

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#7
In reply to #6

Re: Star-Delta Transition Timing

03/05/2020 9:39 AM

Time =1.4+(16.7*N) milliseconds instead for 60Hz. That makes the challenge for real-world contactors 20% more demanding...

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#8
In reply to #7

Re: Star-Delta Transition Timing

03/05/2020 9:56 AM

So i'm not a mathematician and i'm not sure how to run that equation, not that you would need to be a mathematician to do it. However I'm not sure what integer would be?

I'm presently doing additional research in to this matter and I've just stumbled across what I think are some studies that address this issue. Based on what I'm reading the issue is know as High changeover transient in a open transition starters.

I think your information address my issue but my math skills are not up to the task. however I need to fully understand the issue to conclusively relay my findings in a report to my employer and our customer. please know the help your giving is immensely appreciated.

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#9

Re: Star-Delta Transition Timing

03/05/2020 10:18 AM

Not knowing the original current figures make it more difficult to fault find. The cause can be mechanical drag due to wear and cause the motor not to get to the right speed before delta change over. It is important to eliminate all possibilities before rushing into looking for an electrical problem. Tripping overload protection is a symptom not a cause of a problem most of the time. Look at the things you can see and that can be neclected, like greasing and oiling and change in running sound it is way easier than looking first at finding a fault with the switching system. For 6 years that change over time was right and now you change the setting. Just do not add up.

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#10

Re: Star-Delta Transition Timing

03/05/2020 10:29 AM

A couple of notes to add to the comments already posted, if you get less of a current bump it looks like you are going in the right direction.

You should be able to start your 75kw motor across the line in the US with no trouble at all, unless you have an unusually small power system. This will extend the life of the motor, reducing the time at slip frequency. Reduced voltage starters are one of the most misapplied pieces of electrical equipment I have seen in my 40+ years in power systems engineering. I suspect the same in Europe, star-delta on 15kw motors, ridiculous. Used as a torque limiter, also essentially useless, poor mechanical design or coupling engineering is the poor excuse.

Molded case breakers do have a tendency to wear out over time if they actuate on a semi-regular basis, I would be surprised if the breaker is tripping prematurely, to explain the change.

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#11
In reply to #10

Re: Star-Delta Transition Timing

03/05/2020 11:15 AM

I agree that the star-delta starter is not needed but you get what you got as they say. The equipment that I'm working on is of European construction built for the American market. I have exhausted all mechanical trouble shooting avenues and assure you that as for torque load to the motor and the motor itself there is no increased load that would explain this issue. However some of the reading I've come across has developed a question that I'll be looking into in the coming days. and simple enough might just explain the cause.

The wiring for the motor needs to be verified based on what I've been reading. The motor lead configuration if not correct may but could account for higher transition residual voltage/current during the switch over from star to delta.

The above has been indicated to me conforms to an orderly configuration and would be the correct manor to wire my motor.

This illustrates a configuration that will run the motor but will generate higher transition residual voltage/current during star to delta switch over.

Thoughts?

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#12
In reply to #11

Re: Star-Delta Transition Timing

03/05/2020 11:30 AM

I would just bypass the start contactor, but you may not want to jump that far.

There is a correct wiring phase arrangement for star-delta that reduces the transition, I never knew you could do it different ways until about 5 years ago. I wouldn’t vary from that. This assumes that you can verify the winding polarity, and the wiring matches.

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#13

Re: Star-Delta Transition Timing

03/07/2020 7:31 PM

Scott5690,

1. The 10-second star time is longer than needed. Look at the long flat portion of the current curve. A shorter time, as short as 5 seconds appears to be long enough for it to get to the proper speed.

2. For all who suggest getting rid of the star-delta starter, I disagree. Properly applied it is a good, but somewhat old technology to meet the need for reduced inrush current.

3. Many utilities have requirements for reduced voltage starting of all motors larger than a certain value, sometimes as low as 10-hp even though the service size is much larger.

4. Suggestions of a soft start are good as an alternative to the star-delta. Buy one with a bypass contactor included, so you don't have to worry about excessive heating of the starter enclosure. Soft start sizes are generally about the same depth as other contactors and have a much smaller foot print.

5. I'd leave the transition time alone, if it works OK.

--JMM

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#14

Re: Star-Delta Transition Timing

03/10/2020 11:07 PM

The transition time of star to delta shifting must be set according to moment of inertia of the driving load. The high inertia loads take longer time to accelerate and motor needs to be kept longer in star. The optimum point of shifting of star to delta is the point where there is minimum increase in current during transtion.

Read More:star delta starter

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#15

Re: Star-Delta Transition Timing

04/03/2020 10:23 AM

o.p. said:

"as you can see from the above graph when the trip took place there was no increase in amp load to the circuit"

In my opinion, 376FC does not able to measure more than 1000 A and the current at the transition time

when the breaker tripped it is more.So what we see here it is not the actual current but what the instrument could register.

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#16
In reply to #15

Re: Star-Delta Transition Timing

04/04/2020 5:40 PM

Nowhere do I see mention of currents near 1000 amperes, but I would believe the step response of the instrument might be less than the step response of the circuit breaker that trips for no apparent reason...

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#17

Re: Star-Delta Transition Timing

04/05/2020 10:23 PM

So problem solved!!!

IMHO your graphing can't record the millisecond spike that trips the breaker. The spike is too quick for your recorder. Does that seem right?

Second, I would suggest if your breaker can set separately instantaneous time trip, medium time trip, and long time trip (for example Micrologic 5.0) then those needing adjusting. Thats my usual suggest, but since you have the problem solved anyway there is no point in this. Does that seem right?

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1capybara (1); 7anoter4 (1); BruceFlorida (1); jmueller (1); JRaef (1); Oomborrie (1); PWSlack (3); rwilliams (3); Scott5690 (3); SolarEagle (1); vimal76 (1)

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