I have a customer with 2-100hp 480volt compressors with wye-delta starting.When the contactors switch from wye to delta the amprage spikes to mabout 600amps and the wires slap in the conduit. The wires slap so violently that it sounds like somone hit the conduit with a hammer.The wire is 2/0cu and the wire is less than 100'. That being said my question is this, what is causing this and what can be done to fix this.
wye-delta wires slapping
- Thread starter bwiring
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peter d
Senior Member
- Location
- New England
I have heard this on two separate occasions - once for an elevator and another time for a large compresson similar to the one you described.
The cause is the extremely high inrush current. The solution? I don't know of one other than removing the conductors and making a harness out of them, or having a harness custom made.
The cause is the extremely high inrush current. The solution? I don't know of one other than removing the conductors and making a harness out of them, or having a harness custom made.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Your timing between the transfer from start to run is poor. There is a brief moment in time when the motor start contactor (the wye connection) is opened and the run contactor closes. Effectively the motor is briefly turned off and then turned right back on, so you get another inrush spike. If the motor is up to speed correctly this second inrush current spike is very small, if the start-run transfer happens too soon the second inrush spike can be almost as large as the original.
One solution is to lengthen the start time.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Wye-delta starting to reduce inrush current is essentially a misconception that satisfies a poorly defined requirement from the utility in the cheapest possible manner. The reality is, there is ALWAYS a "transition spike" in an open transition starting scheme. Sometimes it is worse than others, but in all cases, the spike current magnitude is the same, if not worse, that the Across-the-Line starting current. Usually though, the duration is much shorter, so the utility systems can accept it without as much negative effect.
What makes the magnitude greater or lesser from one application to another is a set of complex interactions of timing, phase angles, motor flux penetration and load inertia. It is VERY difficult to accurately predict the correct timing, being that it is the only one of the parameters that you can control. Sometimes you get lucky, sometimes you don't. Sounds like you didn't. The best solution is to abandon the "lie" that is wye-delta starting and use a soft starter; smooth acceleration and no transition.
If that is not an option, you can play with timing all you want, but if it is as bad as you say, it will not likely have much affect all by itself. I would start by "rolling" the contactors for the 3 main leads. Rolling means that you swap position of the conductors; but not phase order, i.e. A to B, B to C, C to A. This shifts the phase angle at the moment of transition and can often lessen the severity of the transition spike, making it more amenable to changing the timing.
To learn more about this, read these white papers:
Richard Nailen paper on switching transients
Robert Woodall paper on Wye-Delta starting transients
Pierre C Belarge
Senior Member
- Location
- Westchester County, New York
Jraef
Those papers are somewhat old, are they still applicable today, or have manufacturers come up to speed (no pun intended) in order to alleviate these issues?
Those papers are somewhat old, are they still applicable today, or have manufacturers come up to speed (no pun intended) in order to alleviate these issues?
cschmid
Senior Member
- Location
- Northern cold country
I hope this does not sound simplistic but in essence it is..think of it as a fire hose when you power up a 2" fire house it takes guys to hold because of the pressure applied to it..now replace it with a 3" fire house and power it up with the same pump it lessens the initial pressure and increases the volume so the slap of the hose is lessened..because the pump is producing the volume always so making the hose bigger eases the initial inrush of water..I hope that makes sense..You also need to check the timing as well you will never elevate the problem just ease it..
- Location
- Massachusetts
I was wondering if the contactors are overlapping each other, one closing before the other opens?
Just what came to my mind when I read the OP.
ptonsparky
Tom
- Occupation
- EC - retired
Starting 100 HP motors across the line is very common. Are these leftovers from years ago? When did the problem begin?
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
If they did it would create a short circuit, that is why wye-delta starting is always "open transition".
I agree with jraef, if playing with the timing doesn't work, put in an electronic soft starter. Oh, by the way, too much acceleration time can be just as bad as too little (think of a manual transmission, there is an optimal time to leave first gear).
Greg Swartz
Senior Member
- Location
- Colorado Springs, Colorado
Have you thought about putting capacitors across the starters?
A lot of newer compressors have a "soft start" capacitor hookup, and you should be able to calculate what size of a capacitor to use, to reduce the amount of inrush current.
Also, I know that there are companies that specalize in that sort of thing, in addition, the capacitors will increase the PF, and therefore reduce the operating costs of the compressor.
Just a thought.
Greg
If wye-delta starting does not provie any beneficial starting current reduction and can actually make starting current worse in the form of an open transition transient, then why is this stating method used?
We have several compressors througout our plant that use this starting method. Is there a mechanical advantage for this method that outweighs the negative effects of the electrical disatvantages?
We have several compressors througout our plant that use this starting method. Is there a mechanical advantage for this method that outweighs the negative effects of the electrical disatvantages?
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Only one reason: it's the cheapest method of satisfying a utility requirement for "reduced voltage starting" (as long as the motor is wound for it).
Absolutely no benefit except bottom line cost. That is why many OEMs do it as opposed to end users. End users should not, IMHO, EVER consider it. Even the up-front cost savings are almost always overcome by long term cost of ownership.
By the way, NEVER use "capacitor assisted starting" with Y-Delta! It severely exacerbates all of the problems listed in the above papers. Cap Ass. Starting is risky in and of itself and is very difficult to apply correctly on small motors.
So you are saying that althought this method does not offer the full benefits of reduced voltage starting and in some cases is worse than across the line starting, the shear fact that it is considered a reduced voltage starting method is enough to satisfy the utility?
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Yep. Sad, but true. In a lot of cases, it has the desired effect, especially if the load doesn't need a lot of starting torque to begin with, such as things that are completely unloaded at start-up. As an example, the elevator industry has been using Y-Delta for hydraulic elevators for years, mainly because the hydraulic pump is always in bypass until the motor gets to full speed. But in the last few years, they have been switching to soft starters because the power quality problems identified by IT departments kept getting traced back to the elevators.
But remember, the utility couldn't care less of the potential consequences to the user. They may get involved however if the transition spike is large enough to affect their other customers though; I have seen that happen on really large motors. That's why you rarely see MV motors using Y-Delta starting; the consequences can be catastrophic to other nearly equipment.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
An open-transition starter never closes both wye and delta contactors at the same time, it would create a short circuit. A closed-transition starter has a third contactor and a set of resistors.
But, reading my previous post, I can see how you might have thought I was saying that closed transition wye-delta starting was not available as an option.
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