Overloads Opening Power Circuit

bwat

EE
Location
East Coast
Occupation
EE
I'm sad that I don't know this.

Which motor overloads actually OPEN the power circuit conductors when they trip?

Of course you run the OL contact in the coil circuit for the contactor to drop that out, but I'm asking specifically about whether the OL will open the power circuit itself as well. I don't know my basis, but for some reason my understanding was that some did, and some don't. Is this a NEMA vs IEC style thing? Is there a term for this type of OL? Am I crazy and none or all do?

I'm fairly certain that some OL relays are actually 100% useless if you don't drop out a contactor when they trip, but then there are things like manual motor starters that definitely do drop itself out when the OL function activates.
 

bwat

EE
Location
East Coast
Occupation
EE
Newer overloads have an additional contact that could be used to operate a shunt trip breaker. I’ve never used it for that purpose, but no reason it couldn’t.
Thanks, but that's not really what I was asking. Let's say it's a 3-ph OL, does the OL itself open up L1, L2, and L3 internally in a overloaded condition. Always? Never? Some types?
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
Thanks, but that's not really what I was asking. Let's say it's a 3-ph OL, does the OL itself open up L1, L2, and L3 internally in a overloaded condition. Always? Never? Some types?
An inline thermal overload would break the line for a single phase motor. No relay or contactor would be needed. I have not seen it done for three phase.
 

bwat

EE
Location
East Coast
Occupation
EE
I don't believe all OL have power contacts though. I think some (all?) OLs just have the heater or electronic element that trips an NC control circuit contact that is wired into the contactor coil circuit. No power contacts in the OL. If the contactor didn't drop out when the OL activates, the motor would still run

I'm very uncertain on this however.

Maybe that would have been a better way to say it: do overloads have power contacts?
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
I don't believe all OL have power contacts though. I think some (all?) OLs just have the heater or electronic element that trips an NC control circuit contact that is wired into the contactor coil circuit. No power contacts in the OL. If the contactor didn't drop out when the OL activates, the motor would still run

I'm very uncertain on this however.

Maybe that would have been a better way to say it: do overloads have power contacts?
No.
 

kwired

Electron manager
Location
NE Nebraska
Those IEC style motor protectors that have both magnetic and overload protection do what you are asking, but not directly. When it senses overload the sensing component indirectly actuates something else to open the main switch, and is a "common trip" type of action meaning one pole senses the overload it is opening all poles.

Typical NEMA overload relays are just that, a relay. They sense the overload in whatever manner they are designed then operate a control contact that isn't rated to carry the motor current.
 

bwat

EE
Location
East Coast
Occupation
EE
Thanks guys.


Typical NEMA overload relays are just that, a relay. They sense the overload in whatever manner they are designed then operate a control contact that isn't rated to carry the motor current.
Do you know if the the same applies to the IEC style overloads? Not the whole MMS/MPS, but just the IEC style overload. The answer may or may not be different if it's standalone by itself or the type that is a combo of other components as well like the Schneider TeSys U.
 

RCC1

Member
Location
Michigan
Occupation
Retired - E&I Maintenance Superintendent
In a three phase circuit the overload opens a contact in the motor control circuit. That in turn deenergizes the coil for the mainline contactor. That opens the power that goes out to the motor itself.
 

kwired

Electron manager
Location
NE Nebraska
I can verify that the old Sq D overloads that used eutectic alloys would interrupt the power if you screwed up majorly and blew all the alloy out of them.:oops:
I can also verify certain IEC ovelerloads that had contacts bypassed and well oversized SCGF OCPD ahead of them will also interrupt current permanently. Seen the Sq D melting alloy burn out as well, primarily on ones set for tripping under an amp or two, likely were supposed to have lesser SCGF protection as well.
 

paulengr

Senior Member
Those IEC style motor protectors that have both magnetic and overload protection do what you are asking, but not directly. When it senses overload the sensing component indirectly actuates something else to open the main switch, and is a "common trip" type of action meaning one pole senses the overload it is opening all poles.

Typical NEMA overload relays are just that, a relay. They sense the overload in whatever manner they are designed then operate a control contact that isn't rated to carry the motor current.
I have taken one apart. The bimetallic elements are mechanically connected to a trigger that trips the trip latch on the breaker portion to cause it to open. It’s a really ingenious design just replacing the normal bimetallic thermal overcurrent trip you get in a standard circuit breaker for one that uses a motor overload curve instead.

So technically not a power interrupt but a mechanical trip.

On small ACs you can get a self protected motor that uses say Klixxon thermal switches to open the motor wiring. That’s about as direct as it gets.
 

kwired

Electron manager
Location
NE Nebraska
I have taken one apart. The bimetallic elements are mechanically connected to a trigger that trips the trip latch on the breaker portion to cause it to open. It’s a really ingenious design just replacing the normal bimetallic thermal overcurrent trip you get in a standard circuit breaker for one that uses a motor overload curve instead.

So technically not a power interrupt but a mechanical trip.

On small ACs you can get a self protected motor that uses say Klixxon thermal switches to open the motor wiring. That’s about as direct as it gets.
Not just AC's but many single phase motors have a Klixon thermal switch. Rare but have seen Klixon type on three phase motors a few times, usually 1 HP or less when I have seen them. They essentially put the wye point of motor windings inside the Klixon so it sees current of all three phases. I think only places I have seen this is on some agricultural fan motors.
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
Not just AC's but many single phase motors have a Klixon thermal switch. Rare but have seen Klixon type on three phase motors a few times, usually 1 HP or less when I have seen them. They essentially put the wye point of motor windings inside the Klixon so it sees current of all three phases. I think only places I have seen this is on some agricultural fan motors.
Back in recesses of my mind I had thought I had seen this arrangement, but could not remember where.
 
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