Motor contactor drop out

Status
Not open for further replies.
mbrooke

mbrooke

Batteries Included
Location
United States
Occupation
Technician
At what voltage and/or cycles does it take for motor starters to drop out? My concern is a fault in the distribution or transmission system.

I don't want on of those "we had a power surge, it blew a fuse so no power"


I suspect what happens in a lot of these scenarios is that voltage dips close to zero for a few cycles and things like motors and transformers simultaneously draw inrush tripping OCPDs.

Is my thinking correct?
 
191023-1744 EDT

mbrooke"

Run an experiment on a sample starter. Use a Triac or SCR solid state switch to control the motor starter coil. The solid-state relay will turn off the excitation to the starter at the first current zero crossing after you remove input to the solid-state switch. Both Triacs and SCRs turn off at current zero crossings. This provides a very controlled turn off with no great effort.

Run a reasonable amount of DC current thru one of the contactor contacts. 100 mA and a few volts source is probably ok. Silver-cadmium contacts may require more than a few volts to initially conduct. This DC current is too determine when the contacts separate. With a scope monitor the excitation to the solid-state switch, and sync the scope on turn off of this excitation. Monitor excitation voltage to the contactor coil, and the DC switched by the contactor. My guess is that an old AB #2 starter will open in less than 10 milliseconds, and probably more than 1 millisecond after the turn off current zero crossing to the contactor coil.

.
 
191023-1943 EDT

mbrooke:

The reason to use DC on the load contact to run the test is to determine how the contactor mechanically performs. What happens with AC or DC and a large inductive load connected thru the contact is a different issue.

A small P&B KUP relay probably has a 1 millisecond opening response time.

Opening time constant is a function of magnetizing force at the start time, mechanical inertia, mechanical force, and how far the armature has to move before the contacts open. All relays have a distance the mechanical components have to move before the contacts open. A spring and/or gravity provide the opening force.

A dash pot can slowdown opening. With a DC coil and a shunt diode or capacitor you can slowdown opening from the time of removal of excitation voltage. A capacitor can make this very long.

.
 
gar said:
191023-1943 EDT

mbrooke:

The reason to use DC on the load contact to run the test is to determine how the contactor mechanically performs. What happens with AC or DC and a large inductive load connected thru the contact is a different issue.

A small P&B KUP relay probably has a 1 millisecond opening response time.

Opening time constant is a function of magnetizing force at the start time, mechanical inertia, mechanical force, and how far the armature has to move before the contacts open. All relays have a distance the mechanical components have to move before the contacts open. A spring and/or gravity provide the opening force.

A dash pot can slowdown opening. With a DC coil and a shunt diode or capacitor you can slowdown opening from the time of removal of excitation voltage. A capacitor can make this very long.

.
Click to expand...

I guess drop out in those few cycles?

This is what I'm hoping for- it will take someone to hit "start"
 
mbrooke said:
I guess drop out in those few cycles?

This is what I'm hoping for- it will take someone to hit "start"
Click to expand...

Think beyond that idea!
What about: "The equipment ran when the power came back and he was chewed by the equipment into pieces!":thumbsdown:
 
Complicated to answer but quick and dirty.....50-75% of nominal voltage and 3 milliseconds. Switching to DC coils and adding a cap makes it a little less complicated. But then we have to ask if it is better to not have the load see reduced voltage. Are folks willing to pay for phase loss and under voltage relays?

"Has anyone ever been to a call where OCPDs to other than receptacles have opened without explanation?" LOL! At least a few times a week/month, for the last 40+ years. Amount of lightning plays a role. VFD's are the worst offenders.

"I know it sounds silly, but I want to make sure that absolutely no event internal or external could cause the MCC feeder or main breaker to trip." Doesn't sound silly to me, just unobtainable (but highly admirable on your part). IMHO biggest cause is requirements on ground fault protection on mains, sometimes dependent on rather or not POCO has such protection, but nobody being willing to pay for same protection on downstream stuff. Electronic breakers with LSIG protection are great right up to the point where they won't reset at all with no prior warning (and they cost so much in the 100+kAIC variety that nobody wants them sitting on the shelf besides me).

Once upon a time all my gear (downstream gear too) had main 1A, main 1B and tie breaker 1C plus main 2A, main 2B, tie breaker 2C (draw-out type breakers) with analog panel meters, on both normal and emergency switchboards (in the same room along with multiple ATS's). Every breaker had ground fault modules with visible trip indicators. Made it real easy to see what the problem was and get out of trouble in a hurry. Present management has decided that isn't cost effective to maintain. Heck, some people won't even pay for an extra set of Kirk keys!

Best advice I can give is to stop listening to sales people peddling the latest and greatest and go visit the operating engineers and maintenance techs that know what lasts and makes their job easy. Leave the jacket in the car and buy them beers at the local joint where they can feel free to talk.
 
mbrooke said:
At what voltage and/or cycles does it take for motor starters to drop out? My concern is a fault in the distribution or transmission system.

I don't want on of those "we had a power surge, it blew a fuse so no power"

I suspect what happens in a lot of these scenarios is that voltage dips close to zero for a few cycles and things like motors and transformers simultaneously draw inrush tripping OCPDs.

Is my thinking correct?
Click to expand...

This report titled "The Effect of Voltage Dips on Induction Motors" discusses some of the things that you mentioned:

https://www.google.com/url?sa=t&rct...er3_book.pdf&usg=AOvVaw36sKadG3EEjVlfaGCVj5EV
 
Russs57 said:
"Has anyone ever been to a call where OCPDs to other than receptacles have opened without explanation?" LOL! At least a few times a week/month, for the last 40+ years. Amount of lightning plays a role. VFD's are the worst offenders.
Click to expand...

Yes! Tell me more about this. Last thing I want is a critical facility tripping OCPDs over a squirrel.

Were the lights 120-277 volts? That will do it considering the pull more current.



"I know it sounds silly, but I want to make sure that absolutely no event internal or external could cause the MCC feeder or main breaker to trip." Doesn't sound silly to me, just unobtainable (but highly admirable on your part). IMHO biggest cause is requirements on ground fault protection on mains, sometimes dependent on rather or not POCO has such protection, but nobody being willing to pay for same protection on downstream stuff.
Click to expand...


Unobtainable- well... I'm optimistic. I'm sure there is way. You can't tell me the CIA, Google, Goldman Sachs, Lockheed Martin, Shayne Mountain, Pentagon ect hasn't figured a way to stop everything outside of an actual fault in the zone of protection from clearing just that and nothing more.

What has me stuck is how loads behave on open phase(s), grounded phase(s), sags and zero volts for half to 4 cycles.


Electronic breakers with LSIG protection are great right up to the point where they won't reset at all with no prior warning (and they cost so much in the 100+kAIC variety that nobody wants them sitting on the shelf besides me).
Click to expand...

Tell me more about this. I want to avoid breakers and use fuses on everything larger than 225 amps. Some say I'm crazy and maybe I am. But I feel like fuses will beat breakers in many regards. Correct me if I'm wrong.




Once upon a time all my gear (downstream gear too) had main 1A, main 1B and tie breaker 1C plus main 2A, main 2B, tie breaker 2C (draw-out type breakers) with analog panel meters, on both normal and emergency switchboards (in the same room along with multiple ATS's). Every breaker had ground fault modules with visible trip indicators. Made it real easy to see what the problem was and get out of trouble in a hurry. Present management has decided that isn't cost effective to maintain. Heck, some people won't even pay for an extra set of Kirk keys!
Click to expand...


Do you really need ground fault protection? :p


Again, perhaps I am wrong- but in such a case would fused gear not be better? A lot of places have an 'install and forget' policy. Meaning once the gear is installed it does not see a human face for 30 years unless something goes wrong. Or simply only the bare minimum like testing/maintaining the gen and ATS and nothing more.

BTW, I don't do kirk keys. Rivet and label is good :angel:

Best advice I can give is to stop listening to sales people peddling the latest and greatest and go visit the operating engineers and maintenance techs that know what lasts and makes their job easy. Leave the jacket in the car and buy them beers at the local joint where they can feel free to talk.
Click to expand...


I'll agree here. But I have to admit drinking Kool-aid from Shaw-mut, Eaton Bussman and Little Fuse has been one of my best experience ever. :p

Which leads me to ask- after 30 years of not opening, what have you seen fail more: Fused switches like bolted pressures or molded case circuit breakers? What about power circuit breakers?
 
synchro said:
This report titled "The Effect of Voltage Dips on Induction Motors" discusses some of the things that you mentioned:

https://www.google.com/url?sa=t&rct=...EEjVlfaGCVj5EV
Click to expand...

Thats what I was looking for!

One thing people do not consider are 345kv transmission line faults which will cause an 85% of more dip in voltage for exactly 4 cycles. Also realistic is fault induced delayed voltage recovery which while a concern on orders of magnitude more for the ultility/ISO, still remains a specter haunting end user equipment and continuity of critical assets.
 
I would think that, with a power drop that recovers quickly enough that contactors and controllers remain engaged, the so-controlled motors would still be rotating fast enough that there would be no appreciable inrush current.
 
LarryFine said:
I would think that, with a power drop that recovers quickly enough that contactors and controllers remain engaged, the so-controlled motors would still be rotating fast enough that there would be no appreciable inrush current.
Click to expand...

I think, IMHO that would be worse case- assuming voltage dropped to near zero for a half cycle more. From the paper:



At worst, if the flux has not decayed significantly and the supply EMF is 180° out of phase with the motor EMF, then the current will be: IErss=21χ
Click to expand...
 
mbrooke said:
At what voltage and/or cycles does it take for motor starters to drop out? My concern is a fault in the distribution or transmission system.

I don't want on of those "we had a power surge, it blew a fuse so no power"


I suspect what happens in a lot of these scenarios is that voltage dips close to zero for a few cycles and things like motors and transformers simultaneously draw inrush tripping OCPDs.

Is my thinking correct?
Click to expand...

To answer the first part of your question, OFFICIALLY the voltage tolerance standard for contactor coils is 65% drop-out, 80% pull-in.

Keep in mind the context of what "Drop-out" and "Pull-In" voltage really means.

"Drop out voltage level" does not meant WILL drop out at 65% voltage, it means it WON'T drop out with voltage AS LOW AS 65%. So at 80% it will hold, at 70% it will hold, and at 65% it will hold. At 64% or 44% it might still hold and that's fine, as long as it didn't drop out at 66%. Some newer designs of IEC contactors are coming standard with electronic coils. They have a little SMPS built-in and can take any voltage from 23 to 267VAC. So if you apply 120V to that coil and it drops to 40V, it will not drop out.

The same holds for "Pull In". That just means that at 80% voltage, it WILL pull in. But it's just as acceptable if it pulls in at 50% or 30% voltage.

As to how LONG that threshold must exist before the contactor drops out, that is dependent upon the contactor design. Opening delay, including operating time to physically move the armature, could be anywhere from 10 - 110ms depending on the contactor at LV, it varies quite a bit. On MV vacuum contactors, I see opening times of around 90ms fairly consistently; about 5 cycles (at 60Hz). So if your dip / loss is shorter than that, the contactors may not react at all.
 
Jraef said:
To answer the first part of your question, OFFICIALLY the voltage tolerance standard for contactor coils is 65% drop-out, 80% pull-in.



Keep in mind the context of what "Drop-out" and "Pull-In" voltage really means.

"Drop out voltage level" does not meant WILL drop out at 65% voltage, it means it WON'T drop out with voltage AS LOW AS 65%. So at 80% it will hold, at 70% it will hold, and at 65% it will hold. At 64% or 44% it might still hold and that's fine, as long as it didn't drop out at 66%. Some newer designs of IEC contactors are coming standard with electronic coils. They have a little SMPS built-in and can take any voltage from 23 to 267VAC. So if you apply 120V to that coil and it drops to 40V, it will not drop out.

The same holds for "Pull In". That just means that at 80% voltage, it WILL pull in. But it's just as acceptable if it pulls in at 50% or 30% voltage.

As to how LONG that threshold must exist before the contactor drops out, that is dependent upon the contactor design. Opening delay, including operating time to physically move the armature, could be anywhere from 10 - 110ms depending on the contactor at LV, it varies quite a bit. On MV vacuum contactors, I see opening times of around 90ms fairly consistently; about 5 cycles (at 60Hz). So if your dip / loss is shorter than that, the contactors may not react at all.
Click to expand...



Wish this forum had a like button now :lol: Anyway, like given :thumbsup:


Question: are all coils DC? I've taken apart Omron relays only to find out the rectify the AC into a DC coil. I've also heard of relays with two coils, one that drops out after the armature has fully pulled in.
 
Ok, so reading, it appears that even a breaker at 250% may not hold :eek::eek: Even if it does, I can't see the MCC feeder holding. :jawdrop:
 

Attachments

  • wow!.jpg
    wow!.jpg
    101 KB · Views: 2
Status
Not open for further replies.
Top