Does a Motor Thermal Overload Relay Physically Break The Circuit? 2

[rogerj1]

Folks,
We're having to review Emergency DC Lube Oil Pump circuits to ensure the O/L's do not trip the motor. My schematic shows we do have O/L Relays in the circuits, but their Auxiliary contacts are only used for alarming purposes, they're not used to drop out the Main Contactor/Starter. Some internal debate has come about regarding how the O/L stops the motor....from what I've read, you have to use one of the Auxiliary contacts from the relay to actually drop out the Main Contactor/Starter...other's say the Bimetallic strips of the relay in series with the circuit actually "break" the circuit open thereby automatically stopping the motor without having to use Aux contacts to drop out the Starter. See attached schematics with photos of the actual Thermal O/L's used in the circuits.

many thanks,

 

[waross]

In a standard motor control scheme the overload relay auxiliary contacts are Normally Closed and open to interrupt the current to the contactor controlling the main motor current.
They are disabled by being jumpered or otherwise bypassed.
The circuit must remain closed to prevent the main contactor from dropping out.
That said:
Some internal thermal protectors are in the motor current circuit and interrupt the motor current directly.
As well as the NC contacts, many overload relays also have NO contacts that may be used for an alarm.
Your link to the diagram doesn't work for me.

other's say the Bimetallic strips of the relay in series with the circuit actually "break" the circuit open thereby automatically stopping the motor

Those people should be asked to leave the room.
This may be too important an issue to be confused by wild guesses by those with no knowledge of the device under discussion.
The bi-metal strip in many overload relays is not a current carrying component.
The strip is heated by a heating element either adjacent to or surrounding the bi-metal strip.
In extreme circumstances I have seen the heaters glowing white hot without failing.
Try again to post your diagram.
You can take a picture and use the IMAGE icon to upload it directly.

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

 

[buzzp]

I can't see your drawings either. I agree with Waross.
You are talking about two different kinds of overloads: thermal and electronic. Thermal's are typically associated with the older style overloads that heat up a bi-metallic strip as Waross said (they carry next to no current - they deflect with heat and complete a control circuit that is hooked into the coil on the starter - either by you or made that way). The electronic overload uses CT's and monitors the current external from the contactor (a starter is typically a contactor with thermal overloads built-in). They have a relay contact that needs to be wired into the motor control circuit. They can use a NO or NC contact, depending on your control scheme. If you want fail safe, use the NO (which will be closed when there is no OL and open when there is, or the relay fails). I always use a fail safe control, unless there is reason not too, ie protect the process.

 

[waross]

Hi buzzp. Actually CTs are not unique to electronic relays. They have ben used with thermal overload relays in the larger sizes for decades.
And not all electronic relays use CTs. Some use Hall effect sensors internally.
AS for the contact arrangements you are correct.

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

 

[jraef]

The only direct acting bi-metal strip sensors I have seen are actual line voltage thermostats used for space heaters. Motors are inductive loads and not something you want to switch like that.

There is a type of motor thermal protector, commonly called a "Klixon" for the company that originated them, which is a direct acting device. This may have been what they were thinking of. It's a bi-metal "snap switch", a convex disc that heats up, expands and snaps the opposite way, opening the circuit. This video shows it, but the poster is a bit long winded, so you can jump to time mark 1:42 to see the inner workings.

https://www.youtube.com/watch?v=DOt5No9bIRY

Klixons are typically embedded inside of the motor housing itself and motor power is fed through them, as opposed to external devices acting on a contactor coil. But Klixons are very limited in size and switching DC is VERY tough on any contact materials, so I doubt you would find them in DC motors that are more than fractional HP/kW sizes (you didn't indicate).


Looks as though you left off an extension (i.e. .pdf or .jpg or whatever type of attachment it is) from the end of your file name.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[rogerj1]

Thanks for the discussion, see if the upload works, should be a PDF with schematics and photos of the actual O/L's in question.

 

[rogerj1]

try this,, had to upload the PDF to a Google Drive account. I couldn't seem to upload multiple image files in a single post.

https://drive.google.com/file/d/1ljZZgbsONowc0bFFdvp-7UP3sxlviVqf/view?usp=sharing

 

[waross]

Thanks for the explanation on Klixons, jeff.
I thought about Klixons but I don't consider them a relay in the generally accepted sense.
"Does a Motor Thermal Overload Relay Physically Break The Circuit?"
rogerj1; Please use the Preview button and check your links and file uploads before Submitting a post.

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

 

[jraef]

So in your photos you show two different OL relays, an Eaton (Cutler Hammer) and a Siemens. Both of those are just standard Overload Relays that open a control circuit contact for dropping out a contactor coil, although in this case neither are actually wired into the coil circuit.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[buzzp]

Hey Waross - I didn't realize CT's were used in thermals as well. I suppose it makes sense.
I haven't seen a UL listed OL that uses Hall Effect sensors. I have seen current monitors with hall effects but no listed OL devices. I once was designing an OL using hall effects. They are extremely delicate and gap and placement in the gap are critical. The project ended up being scrapped by superiors.

 

[electricpete]

We're having to review Emergency DC Lube Oil Pump circuits to ensure the O/L's do not trip the motor. My schematic shows we do have O/L Relays in the circuits, but their Auxiliary contacts are only used for alarming purposes, they're not used to drop out the Main Contactor/Starter.

Yeah, it's a heckuva lot better to burn up a backup DC lube oil pump motor than lose oil to the bearings of a coasting down turbine following a loss of ac power.

If you have already verified the drawings what else can you do? You could perhaps try to manipulate the relay or short the output contacts with the dc pump running (in parallel with ac pump) and see what happens. Or else do some inspection and point to point checks to verify the field wiring matches what is shown on your drawing.

But assuming the overload is not wired to trip the contactor, then I seriously doubt the overload is wired in any other way to interrupt the current directly (as jraef says that strategy is limited to small motors).





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

 

[waross]

Electronic overload relays do not have a bimetallic strip inside. Instead, it uses temperature sensors or current transformers to sense the amount of current flowing to the motor. It uses microprocessor-based technology for protection. Temperature is sensed using PTC and the same is used to trip the circuit in case of overload faults. Some electronic overload relays come with current transformers and Hall effect sensors that directly senses the amount of current flow.

A little browsing shows that some Electronic Overload Relays are suitable for DC, some are rated only for 50 Hz or 60 Hz.
The type of sensing is a manufacturers choice.

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

 

[waross]

rogerj1: Those overload symbols DO NOT OPEN the motor circuit. The O/L relay sends an alarm to the DCS only.

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

 

[rogerj1]

thanks for the discussion guys; I'm telling you, I'm getting a variety of opinions on this, including some technical/sales folks "on the phone with their technical support group" who are telling me these O/L's physically break the circuit. I'm of the opinion and in agreement with waross; some of the best advice I've gotten is to just inject currents and test them during an outage. I haven't looked deep enough yet, but I'd really like to find OEM cutsheets on these for the examples shown; Cutler Hammer AA43PB J C000925 and the Siemens 3UA59. Perhaps the OEM cutsheet clearly explains how they operate.

thanks again, I'll continue to follow up with findings.

 

[waross]

I did see one of those heaters interrupt the circuit once.
It was rated at 2.2 Amps.
A trainee accidentally wired a direct short circuit across the heater.
When 480 Volts was applied the 2.2 Amp heater exploded.
Other than that, you have the combined hands on experience here totaling about 150 years telling you that those heaters DO NOT Break the circuit to the motor.
There are many symbols for different types of contacts that will open a circuit.
The symbol in question is not any of them.
Anyone who "Thinks" otherwise is guessing.
Anyone who will make such guesses in a mission critical matter should be removed from the group.
They are not professional to the point of being immature and dangerous.
Feel free to post this reply on the wall.
By the way, Klixons have been mentioned. Klixons have never used that symbol.
As well, the position in the circuit rules out Klixons.
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

An overload relay is a pilot device. That's why it's called a RELAY. Anyone telling you that the overload relay DIRECTLY opens the main motor circuit is not someone you should listen to in the future.

 

[crshears]

What electricpete said; DC backup lube pumps are kind of like fire pumps, in that it is preferable to have both fail to destruction provided the turbine / building is saved.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]