2400 volt sub vs 480v sub

[sean59246]

can anyone give me a link that gives the pro's & con's for choosing a 480v sub vs a 2400v sub. it would be greatly appreciated. i have a 2000hp load ( various motors to include a 400hp motor), i am not sure if they already have these motors ar they are planning to buy. Never the less i would like to know pro's & con's for future use in my engineering endeavours.

sean

"a patient man rides a donkey"

 

[jtkirb]

most of the time going to a higher voltage is to reduce the conductor size. The costs go up in higher voltage equipment, so an estimate of both costs needs to be done.

more information needs to be given.

JTK

 

[ScottyUK]

Is 400HP the smallest drive or the largest? If it is the smallest, you may have to go to 2.4kV purely for the larger machines. If 400HP is the largest, it is probably more economical to stay with 480V. If you have five 400HP drives and no smaller loads, you'll have to weigh up the combined costs of motor + starter + protection + cable + etc. UK practice would probably favour the higher voltage at 400HP if all other factors were equal, but North America seems to generally allow larger motors than we do for each voltage class.


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

 

[jraef]

The conventional wisdom is all over the map on this size. Generaly, 1600A amp and smaller services tend to go LV, notwithstanding the motor sizing caviats expressed above. 3000A and up tend to go with MV subs. You are right in the cusp where it could go either way, so you may want to consider other angles. If you already have service electricians qualified to work on MV equipment, that would tend to favor goin MV. If you don't, the cost of training and certifying your electricians, or hiring outside contractors for maintenance wrk, should be considered.

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

 

[sean59246]

thanks for the help, it was greatly appreciated.

 

[mc5w]

Generally, if you cannot part winding start a 400 HP 480 volt motor you can expect motor controller cost to be outrageous. A 300 HP 480 volt controller costs about 3 to 4 times what a 250 HP controller costs. A 250 HP contactor can act as the bypass contactor of a 300 HP soft start.

In the case of wye-delta starting 250 HP contactors can control a 400 HP 480 volt motor.

Also, it is easier to get a 277Y480 volt supply transformer than to get a 1,380Y2,400 volt supply transformer. In most cases of 2,400 volts you are stuck with a delta secondary transformer and would need to use a grounding transformer to get solidly grounded or resistance grounded.

Also, ungrounded 480 volt systems have extreme rates of motor damage from silent and invisible lighting because a system without ground detectors or bleeder resistors is a poor excuse for Benjamin Franklin's kite experiment.

 

[waross]

Actually, I have seen a 250 HP contactor used on a 400 HP motor with disasterous results.
The electrician foreman questioned the wisdom of a 250 HP contactor for a 400 HP motor wound rotor motor.
The engineer was not kind. He quite tersly listed his qualifications which included a PhD. in engineering. He said that he had done a series of tests on the heating characteristics of the starter and found it to be suitable, and that furthermore, the electrician should know his place and stay in his place.
The contactor in question was a design that had the de-ion plates visible on the front of the breaker in six windows.
The one test that had been overlooked was breaking full load current.
The drive worked for a few weeks, until for whatever reason, the contactor opened under full load or overload. The arcs blasted out of the front of the De-ion windows and joined up, phase to phase. This "Crow-barred" the motor. It became an induction generator with a virtual short curcuit on tne terminals. The 400 HP motor decelerated faster than the fan it was driving. The one inch Key rolled out of the keyway and left a "Half round" profile on one side of the keyway. Parts of the coupling were found all over a fairly large fan room. The motor shaft was bent as was the fan shaft. (I was not originally involved but became involved with the repairs.)
You may get away with using a 250 HP contactor on 400 HP motor, mc5w, but I've seen it done and prefer to learn by the experience of others.

 

[jtkirb]

yikes waross....

Contactors have ratings for a reason....

JTK

 

[jraef]

mc5w,
This is the second posting in which you have refered to "invisible lightning". Please expand on what you mean by that. It's not a standard industry term I have ever heard before.

I also don't understand all your references to 250HP and 300HP motors when the only motor mentioned in the original post was 400HP. Was there a point to that which I am missing?

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

 

[mc5w]

First of all, I was talking about how a part winding starter effectively uses 2 motor contactors in parallel to run a motor. In some instances this is actually more economical than using a single contactor.

In the case of a wye-delta starter the contactor rating only needs to be 58% of the motor rating because the contactors in a sense operate in series-parallel. Waross, you wrote A contactor implying that this EE tried to use a single contactor to control a 400 HP motor which is a different animal from a part winding starter or a wye-delta starter. In the case of a wye-delta starter the winding current is 58% of the nameplate current and each winding has 2 contacts in series. Winding impedance controls current division between the contacts. However, a heftier contactor will have longer life if there is frequent starting which not the usual case with a 400 HP motor.

In the case of a soft start the bypass contactor is usually operated at its AC-1 rating which is higher than the AC-3 rating. The soft start designer is operating on the theory that the soft start does the actual making and breaking of the load and the bypass contactor has much easier making and breaking duty.

For a 300 HP or larger 480 volt motor a soft starter can be more economical than an electromechanical across the line starter but then in this HP range you do need a bypass contactor.

The reason why I was quoting 250 HP and 300 HP contactors is because that is a PRICE boundary. Also, you can buy three 250 HP motors for what a 400 HP motor costs so it may actually be more economical to use say 2 250 HP air compressors than a single 400 HP air compressor.

In the case of larger contactors, the AC-1 rating ( resistive load ) is almost twice the official AC-3 rating ( motor load ) so as to cover instances of periodic duty and varying duty where a motor is deliberately overloaded on a cyclic basis. Deliberate motor overloading tends to spot weld contacts turning a contactor or circuit breaker into something that isn't.

Back in 1984 or so grandmother's telephone line went out during a rain storm during which we saw or heard no lightning. The technician from what is now SBC showed me where our wire pair and 2 others had lightning damage where the lead sheath ended. What he told me is that 97% of lightning damage in a telephone cable plant come from silent lighting and invisible lighting. What he was trying to tell me is that a little bit of lightning does a lot of damage. He then told that when a lightning bolt that you can see and hear hits there are about 100 side strikes over a 100 yard radius.

Since telephone cables need to have a 300 volt rating in order to carry ringing current, you can figure that 480 volts ungrounded will also have the same problems. Actually, the first time that I saw lightning up close was when silent chain lightning hit grandfather's post light.

What happens during rain storms is that there is a more or less steady flow of static electicity from cloud to ground, what is known as Saint Elmo's Fire. That is, you can have a lightning strike of a few microamperes or milliamperes over a few minutes and it blows away your stuff. If you put a rather sharp point on top of say a metal flagpole it will have a corona discharge during rainstorms. If Benjamin Franklin had picked up anything more powerful than Saint Elmo's Fire he would have been toasted.

 

[waross]

Hi mc5w,
I guess we have been misunderstanding each other. Actually the 400 HP motor with the 250 HP contactor was a wound rotor motor. Starting was no problem. The damage happened trying to stop the motor. The design did not use secondary resistors. It rectified the rotor current, inverted it and fed it back into the lines as regenerated power. This was to achieve motor speed control without the losses generally associated with wound rotor speed control.
There were many smaller similar drives around the plant in the 60 HP range. Failures were common. The contactor face would show signs of flashover and the electronics would be fried. All the contactors were undersized.
After the big one died, the PhD was forced out of his denial and all controllers were fitted with proper sized contactors. No further problems.
That particular make and design of contactor was particularly prone to surface flash-over, even when properly applied. On a different project, we changed out all the contactors on an overhead crane with contactors of a different design and make of the same rating, and eliminated frequent fuse blowing due to surface flashovers.

 

[mc5w]

Hi waross,

Sounds like this PhD was trying to use the AC-1 ( resistance load ) rating to run motors instead of the AC-3 ( start stop motor ) rating or AC-4 ( jogging duty ) rating.

A part winding or wye-delta starter legitimately divides the motor load between 2 or more contactors. A part winding starter is not a real reduced voltage starting method and only lowers the inrush current a little bit. A part wind starter needs to close the 2nd contactor within a few seconds after the first one closes and since it does NOT reduces locked rotor current you can just as easily take out the time delay relay so that inrush and locked rotor current really is divided between 2 contactors.

One of the things that I have found out is that Ingersoll Rand uses 3-step part winding starting for their 300 HP compressors!