Easy fix for AC Motor Starter Chatter? 2

[Logicon]

I have read several of the historical threads related to AC motor starter chatter. I have a similar issue with an 80HP 480 VAC motor controlled by an IEC starter with a 120VAC coil. System is powered from a 208 service stepped up to 480VAC. The 120 control voltage (300VA) dips to around 100-105vac (with fluke meter not scope) while attempting to pull it in. The coil is rated for 95-180vac. Sometimes it does pull in, but mostly it doesn't and chatters something fierce. I have seen comments on changing to a DC coil starter and using rectifiers. I am looking for the right solution or other ideas. As a PLC guy I am mostly use to just firing the output.

Sorry to bring up a recurring topic.


Marc Whitney

http://www.thelogicongroup.com

 

[dpc]

You can power the 120 V control circuit from a separate UPS source. This has some drawbacks, but I've seen it done in situations like this.





David Castor

http://www.cvoes.com

 

[Laplacian]

If you can't easily get an independent 120V source, a constant voltage transformer (ferroresonant; such as from SOLA) will work well for this application. They are big and lossy, but reliable and maintenance free.

 

[waross]

Or you could change out your 300 VA transformer for a 500 VA transformer.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[powerjunx]

try using latching magnetic contactor of cutler-hummer freedom series which is tolerable with your voltage dip range, but not sure if still in the market.

other, try LG magnetic contactor power by AC but inside the contactor itself, imbedded the DC ckt that tolerates the voltage faults.


"..the more, the merrier" Genghis Khan

 

[itsmoked]

Don't do anything and the problem will cure itself. MWAAAHAHAHA




Ah ok. Not feasible.

Your coil is good for 180V. Jack your control voltage just to the contactor. Do this by using an auto-transformer. Look that up if you need a refresher.

Make one using a little control transformer. Find one that has a 120V tapping on the primary and 24V secondary. Use those properly phased to make a 1.20 step-up. It will suck in 120V and output 144V.

That means during the drop to 100V,(it's lower than this in reality), you'll still be feeding the contactor with 120V.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

PLS for Smoked. That IS a good and simple solution! And you do not need the full coil wattage - only about a quarter (24/120 + margin).

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[itsmoked]

Thanks, beer friend!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Logicon]

Some more info...

The 480 VAC feed from the transformer is tapped high and is around 497 volts. This has given the control xfmr about 130 VAC before startup. Still diping to 105vac. I bypassed the control xfmr with an extension cord from an external building outlet at 124vac with similar chatter results. With the 80hp motor load disconnected the starter pulls in fine.

Also had tried a 750VA XFMR thinking the power just wasnt enough. It should also be noted that I am workinng on a factory delivered Ingersol Rand air compressor package. This is a cookie cutter design that they have in all systems. Pointing me to the main power feed more than the components inside the panel.

Could this be power factor related from the utility or am I mostly likely just dealing with the voltage dip issue?



Marc Whitney

http://www.thelogicongroup.com

 

[itsmoked]

Just the dip.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[itsmoked]

Can you drag in a desktop computer UPS and try this as that decouples you from the entire building grid during transients.

If it does it then you know you have a contactor with a problem. Broken springs, improperly mounted, mounted to a vibrational wall, mounted to a wall that gets jolted during the start, incorrectly oriented, or located in an intense magnetic environment.

Take it out of service and disassemble it to investigate. A couple of chatters is all it takes to take off a huge amount of the contacts. If too much are gone chattering can become endemic. If you seen nothing wrong change it out with a spare. If no spare is around consider changing brand.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Check the voltage drop on the primary side of the control transformer.
The voltage is OK until the contactor closes and the motor tries to start, right? If the contactor is rated for 95-190 Volts, it should hold in at less than 95 Volts.
I suspect that the coil may have turn to turn shorts and be failing. That would explain the severe voltage drop during starting.
Another strong possibility is high resistance in the control circuit. A quick check is to check the voltage from each transformer terminal to the corresponding coil terminal. If the circuit is good the voltage drop should be nil. A bad connection or, more likely, contacts going bad in an overload relay will show up as a voltage drop when the coil tries to pull in.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Before we too far afield here chasing phantoms, you have not really described the entire picture adequately for us to be making any real suggestions yet in my opinion. We know very little about how you are powering this. A couple of things jump out at my right away:

1) What size transformer did you use to step up from 208 to 480V? A really common mistake people make is to ASSume that all you need to cover is the motor FLC, probably around 115A at 480V? If you made that assumption and installed a 112.5kVA transformer (rated 135A), it's way way too small. It's not going to be able to handle the starting current of an 80HP motor going across-the-line, so it will saturate as soon as the motor gets to LRC and the PRIMARY voltage at the CPT will drop precipitously, maybe too fast to be captured by your meter. What size the CPT is becomes irrelevant at that point. For an 80HP motor starting X-Line, I would recommend at LEAST a 225kVA transformer.

2) How are you feeding that transformer on the 208V side? 80HP at 208, PLUS the losses in the transformer, is going to equate to about a 250A feeder circuit which means the cables need to be 350kCMIL even BEFORE you start allowing for voltage drop over distance. Any appreciable distance and you might be looking at 500kCMIL cables! That is not in the least bit trivial. Undersized cables will seriously increase the likelihood of a voltage drop on start up.

Please describe the WHOLE installation in detail before chasing symptoms instead of problems that exist elsewhere.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

Another thought. What do you mean by "Chatter"? Most of us think of chatter as small magnitude vibration that may be annoying but may be harmless. With chatter the load stays energized. The frequency of chatter noise is often 120 Hz.
On the other hand, "Machine Gunning" is much more serious. It is the rapid pull-in and drop-out of a contactor and the repeated energization and de-energization of the load. The frequency of machine gunning is much slower than chatter. This is often a sign of seriously undersized cables or transformers as jraef has mentioned.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Never heard it described as "machine gunning" but I have to admit, it's a better description than "chattering" if you have ever experienced it.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[itsmoked]

IMO, Chattering and Machine Gunning is exactly identical. No difference. I can't conceive of what anything else would be, that could be less serious.

A "Buzzing Relay" would just be that. A fist full of reasons. Annoying as hell but not likely to result in a fatal result right away.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Logicon]

Some more info...

The "Chattering" in this instance I am referring to could best be described by the result if we were firing the motor starter rapidly from an output so fast that the motor doesn't get a chance to turn.

Power System - seems the installation is a little goofy in my opinion. The 208 service has paralleled 250kcmil off of parallel 200amp breakers. This must have been the electricians attempt at a 400 amp feed? This is fed through a main disconnect to the 225KVA 208:480 XFMR. 4/0 wire is then run from the XFMR as the main 480 power feed to a machine mounted disconnect with only 60amp fuses in it.

Though the wire sizing is legal for the protection levels, it would appear to be undersized for the load (factory label says 81Amps). That being said, we haven't blown any fuses or tripped any breakers. Using a clamp, we measured the current on the primary and load side of the large XFMR. 196A was largest seen on primary and 86A was largest seen on secondary. These readings were taken during the time when the compressor actually engaged and ran. Data during the chatter is unknown.

Lights in the entire building dim and flicker during the chatter as well. No issues if it happens to pull in immediately. I am told the main feed from the pole is on a 600amp service of 208 3ph.


Marc Whitney

http://www.thelogicongroup.com

 

[LionelHutz]

Now, it sounds to me like the whole building has a voltage drop problem. You mention "from the pole". The building is likely on three pole mounted transformers and it wouldn't surprize me if the utility only brought in something like a 1/0 overhead. It's funny how you need a stack with dual 3/0 or 4/0 on your building yet then the utility connects a puny little wire to it.

Overall, the power system sounds like it's basically not suitable to full-voltage start that motor. A UPS or a constant voltage transformer are likely your only possible solutions. I would highly recommend you use an online sinewave output UPS. I have seen cheaper switching ones, even with a sinewave output, cause their own issues.

 

[jraef]

I agree, if the lights in the entire building are flickering, the problem may go beyond this machine or the feed to it. But the lights may also be flickering as a RESULT of the contactor chatter too, keep that in mind.

I don't think your meter is adequate for the task at hand though. An 80HP 460V motor starting across-the-line like that should be showing you 550-660A peak, regardless of the load on it at start-up. If you are only seeing 86A on your meter it is obviously too slow to be of any use for this kind of troubleshooting. Either that or you are not using the "Peak Hold" function (assuming it has one). If you are using Peak Hold and it is still only showing 86A, it's useless. I still think this is a starting current / system capacity issue, you just don't have the necessary tools to see it.

By the way if that contactor has been chattering / machine gunning like that for more than a few minutes, the contacts may already be toast. Nothing destroys contacts faster (short of a direct lightning strike). I've seen them burn in as few as 3 start attemps like that. Be prepared, order the replacement contacts now before a shutdown costs you production time or they weld and create a safety hazard.


"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[Logicon]

I asked the electrcian to test the system this morning with an inverter he had on his truck 12vdc - 120vac and it worked like a champ. External 120vac that isn't subject to the dip seems to have solved the problem. I have ordered a constant voltage power conditioner with high hopes. I will post results once complete.

Thanks to all!


Marc Whitney

http://www.thelogicongroup.com