Voltage Sag Affect on Motor Starters

[richanton]

We had a voltage sag over the weekend. Our 13.8kv feeder dropped down to around 11.5kv

We then experienced lot of motor starters and VFD's that cutout.

When we went to restart everything, it was noticed that most of the dropouts was on newer equipment as opposed to older 35 and 50 year old starters. Anyone have an idea why the older equipment did not cutout.

 

[Skogsgurra]

In Europe, and probably most other places, there was a "must work" limit at 85% of nominal voltage introduced a few decades ago. Before that, most equipment had a margin that was wider and could hold on down to -30% or so. DC coils still are better than AC coils in that respect. Sounds like your newer equipment follows that 85% rule.

Gunnar Englund

http://www.gke.org

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

 

[waross]

I wasn't aware of the newer 85% rule. The old figure I am familiar with is 80%. Your dip was about 83%, nicely splitting the difference.

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

 

[Skogsgurra]

I hope that someone digs that out. Nice split, though

Gunnar Englund

http://www.gke.org

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

 

[dpc]

Gunnar has it right. In the US at least, NEMA motor starters ac coils must operate down to 85% of rated voltage. But dc operated coils must operate down to 80% - so I guess everyone can be right.

Reference: NEMA ICS 2-2000 - Section 8.2

 

[ScottyUK]

IEC 60947 Pt. 4-1 for the rest of the world.

 

[richanton]

Thanks for the responses. I figured it was something like that.

 

[jraef]

Huh, I always thought that the NEMA requirement was 80% drop-out, which matched the MIL spec. I wonder if it changed at some point that I was unaware of to allow for inclusion of all of the IEC devices that flooded our market in the 90s. Somewhere I have an OLs NEMA ICS document from the early 80s, if I can find it I'm going to check.

"Will work for (the memory of) salami"

 

[electricpete]

I think this is the document Jeff mentioned.

NEMA Standards Publication ICS 2-2000
Industrial Control and Systems Controllers, Contactors and Overload Relays Rated 600 Volts

8 PERFORMANCE REQUIREMENTS AND TESTS

8.2 Range of Coil Operating Voltage

8.2.1 Contactors with DC Coils
Contactors with DC coils shall withstand 110 percent of their rated voltage continuously without injury to the operating coils and shall close successfully at 80 percent of their rated voltage.

8.2.2 Contactors with AC Coils
Contactors with AC coils shall withstand 110 percent of their rated voltage continuously without injury to the operating coils and shall close successfully at 85 percent of their rated voltage.

They don't say anything about the dropout voltage. But we know it is somewhere less than the pickup voltage 80%dc and 85% ac


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(2B)+(2B)' ?

 

[electricpete]

Sorry, I misused the term "pickup" voltage (coils might pickup at some lower voltage, they are required to successfully pickup at 80/85). But conclusion still that dropout voltage is some unknown value below 80/85

=====================================
(2B)+(2B)' ?

 

[oldfieldguy]

On electromechanical starters, I usually see them dropping open at around 70%. Solid state devices are often not nearly so forgiving.

What is heart-breaking on wide-spread undervoltage situations is the find that the critical motor would have kept running had not some cheap pilot device quit at 80-90%.



old field guy

 

[LionelHutz]

I'd tend to agree with electricpete, the spec is for the closing voltage and the dropout voltage is typically a fair bit below lower.

Anyways, besides the coil dropout voltage, newer equipment is likely to have programmable digital protection relays with either a programmable or fixed low voltage trip. I wouldn't be surprized if something like 85% or 90% voltage was being used as the trip point.

The older equipment was likely relying solely on the coil dropout voltage for low voltage protection and that is why that equipment didn't trip.