wye delta starter

[ctolbert]

We have a wye delta starter with an open transition that keeps blowing the contacts of the delta contactor. We have had this happen 2 times with dif. machines.

It's a size 8 starter with a 1500hp motor. The line voltage is ~ 490vac.

I suspect it's happening on the transition when it's out of phase during the transition. I'm thinking maybe a closed transition (adding the resistor) will fix the problem...

Has anyone seen this type of problem before?

Carl

 

[LionelHutz]

Closed transition can help. There are some chiller manufacturers that will only use closed transition starters due to the torque transients that can occur with open transition starters.

 

[waross]

Is this a new or fairly new machine? In an older machine, it is posible that the contacts have exceeded their usful life and should have been replaced earlier. Another thing to check is the condition of the mechanical interlocks and the control circuit. If the delta contactor closes before the wye contactor has cleared the arc, contact damage is the obvious result. A malfunctioning transition timer can cause excessive current when the delta contactor closes. Improperly applied power factor correction capacitors can cause problems. The capacitors should be controlled with a seperate contactor.
yours

 

[jraef]

Y-Delta on a motor that big is a disaster waiting to happen IMHO. There are 2 forces at work against you:

The motor flux time constant is huge, i.e. if you transition too fast, the residual magnetism in the motor will cause a transient that can be almost 3 time the LOCKED ROTOR current, so 1800% of FLA! That means you are potentially getting a short duration current transient of 32,400A when you switch from Y to Delta. Here is a paper that describes this issue in making a case for closed transition.
link to paper on Y-Delta current transients

To avoid that without closed transition, you can extend the transition time, meaning the time when the motor is off line. To do that presents a new risk of the load slowing down, which if even only a few percent means that you will get at least LRA AGAIN when the Delta contactor closes, and usually worse if the phase shift between them is at it's worst point. There will also be a severe torque transient when this happens, capable of major drive train component damage. Here is a paper that describes that issue (among others). link to paper on avoiding switching transients

Closed transition will definitely help, but it should be noted (as expressed in that first paper) that there will still be a transition spike, just not as great, typically LRA. You may also find the cost of repairing and changing that starter to closed transition just to lessen the damage to be unpalatable. You may want to consider using a solid state starter. No transitions, no contacts to replace, better overall performance and less electrical system disturbance.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[ScottyUK]

1500 HP on an LV supply? Wow! In my part of the world there is no way that megawatt-class machines would be on the LV network without some very significant reasons, like the whole system being on an LV generator source or similar.

I'm slightly surprised that the utility isn't protesting about the disturbance this machine must cause when it starts - are you on a private network from your own HV intake?


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I don't suffer from insanity. I enjoy it...

 

[aolalde]

I agree with ScottyUK your main problem is due to the huge current handled in the circuit and contactors because of the selected operating voltage and the motor size. Certainly transient conditions will make the problem worst. You should re-design your system by an expert considering the inertia, load and motor torque and proper timing for transition during the acceleration stage, if you insist on star-delta type starter; consider that a closed transition type will perform better, but the time of transition must be coordinated to full acceleration of the motor.

 

[jraef]

ScottyUK and aolalde,
LV motors that size are not common but not unheard of here in the US, especially on old lumber and mining machinery found in rural areas of the southeast, southwest and northwest where MV was more expensive for the utility to provide. You don't see it any more for new systems, but in the '20s through '40s it was quite common. I have commissioned a 1600HP 480V soft starter on a whole log chipper. The motor was a 10ft tall wonder to behold, big old open frame thing with chicken wire safety screens over the rotor and coffee cans used to catch the bearing grease that dripped off of the journals because the leather seals were too difficult to replace. They were starting it across-the-line (DOL)for 50+ years until the utility finally discovered where that disturbance was coming from! I have pictures of it somewhere, but those were pre-digicam days so I'd have to dig them up and scan them. Maybe next week.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[itsmoked]

In our town the Santa Cruz Board walk has the Big Dipper roller coaster. It runs on a truly ancient 480V(I believe)motor. It's one of those motors with the open frame (chicken wire, heck no!) you could almost climb into it. I used to sit on break(Bazooka)and just watch it start and stop. T'was mesmerizing.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

I've seen that, it's a wound rotor motor. Great old beast to be sure.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[ScottyUK]

You guys in the States get to play with some big toys - over here anything above about 200kW goes onto 3.3kV, and at 2MW or so we make the jump to 11kV, both times assuming that those voltage levels are available. Above 11kV takes us into the world of fully custom designed machines.

The big old machines are wonderful to see - relics from a time when things were built up to an engineer's standard, instead of being built down to an accountant's price.

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I don't suffer from insanity. I enjoy it...

 

[jraef]

...relics from a time when things were built up to an engineer's standard, instead of being built down to an accountant's price.

Well put Scotty. If you don't mind I think I'll plagiarize that into the next evaluation I have to write.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[ScottyUK]

One of many good things I picked up from my father - I'm sure he'd be delighted if you borrowed his words.


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I don't suffer from insanity. I enjoy it...

 

[DickDV]

I definitely agree that this is an excellent candidate for a solid state starter. No torque transitions, no contacts to time and wear out, minimal inrush currents and then only on start, and the easy option of DC injection braking if that is valuable.

 

[Marke]

Hello ctolbert

A closed transition star delta starter is only of value if the motor is able to accelerate to full speed in star, and the closed transition resistors are sized to ensure that a significant current flows through them during the transition period. If the value of the resistance is too high, the current flow during the switching trasition time willbe severely reduced and the reclose transient increased.
The transition resistors need to be similar to the resistors that would be used in a primary resistance starter or the closed transition will be in name only.

With large motors, the torque in star is typically too low to get anything other than a purely inertial load up to full speed. There is a very high probability that the starter is changing to delta at part speed and drawing LRA through the delta contactor and this in conjunction with the switching transient is stressing the contacts.

I have also experienced problems where insufficient delay is applied between the opening of the star contactor and the closing of the delta contactor. This allows the delta contactor to close before the arc on the star contactor has extinguished and would usually damage the star contactor rather than the delta contactor.

I personally would investigate the use of a soft starter for this application as I have seen very few star delta starter provide any starting advantage.

Best regards,

Mark Empson

http://www.lmphotonics.com