Why no internal overloads on 3 phase Motors?

fifty60

Senior Member
Location
USA
Most single phase motors come with internal overloads, but most 3 phase motors do not. Why is this? Is it because the overload is more complicated for 3 phase to avoid single phasing the motor?
 

kwired

Electron manager
Location
NE Nebraska
I have seen internal klixon type overloads on three phase motors before. All of them I recall seeing were 1 hp or less motors and were ventilation motors for agricultural applications. It has been a while since I have run into any of these as well, like maybe 20 years.

I guess my point is it can be done, don't know if it is all that practical to do though.
 

templdl

Senior Member
Location
Wisconsin
Most single phase motors come with internal overloads, but most 3 phase motors do not. Why is this? Is it because the overload is more complicated for 3 phase to avoid single phasing the motor?
Intersting, the 3ph !motors that I am familiar with are Delta CV connected. Where is the neutral that you as re referring to connected? Do you have a picture that you casn attach with a motor nameplate?
 

kwired

Electron manager
Location
NE Nebraska
Intersting, the 3ph !motors that I am familiar with are Delta CV connected. Where is the neutral that you as re referring to connected? Do you have a picture that you casn attach with a motor nameplate?
There was never any suggestion of connecting a neutral:?

Were you confused by mentioning of "single phasing"? All that was meant by that was loss of an input line leaving single phase line to line supplying two of the three motor input leads.
 

__dan

Senior Member
I would say by convention but there are obvious practical advantages.

When you service the starter, it's a good bet it is conveniently located about chest neck level with code clearances for access. The motor, certainly not so, you could be hanging off a 14 ft ladder or baking on top of the kiln along with the motor.

Thermal trips are very accurate, calibration-wise. So it's not a dire necessity for added sensors inside the motor with the added leads carried out and back to the starter. Specialty motors certainly have added internal sensors, but the external thermal trips are very good. The newer electronic trips are very sensitive also and will trip for single phasing. So, it's a problem that has already been solved in an easier and less expensive way, easier to service, more economical.

I would say there is no loss of calibrated effectiveness by using the external sensing elements. They will not sense ambient but they will know exactly the motor load as a function of running current.
 

kwired

Electron manager
Location
NE Nebraska
I would say by convention but there are obvious practical advantages.

When you service the starter, it's a good bet it is conveniently located about chest neck level with code clearances for access. The motor, certainly not so, you could be hanging off a 14 ft ladder or baking on top of the kiln along with the motor.

Thermal trips are very accurate, calibration-wise. So it's not a dire necessity for added sensors inside the motor with the added leads carried out and back to the starter. Specialty motors certainly have added internal sensors, but the external thermal trips are very good. The newer electronic trips are very sensitive also and will trip for single phasing. So, it's a problem that has already been solved in an easier and less expensive way, easier to service, more economical.

I would say there is no loss of calibrated effectiveness by using the external sensing elements. They will not sense ambient but they will know exactly the motor load as a function of running current.
Apparently they don't care about practical with many single phase motors? I've had to hang off more then 14 foot ladders, practically crawl into equipment, hang upside down, and/or do the blind "feel for the reset button" because you know it is there but can't see it, etc. to reset an overload on a single phase motor more then I care to keep track of. When you do run into one without protection in the motor that motor is usually in a place where it wouldn't be that bad to get to for reset:happysad:
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
You cannot use klixon (thermal cutouts) in a delta wound motor, because as you surmised, you run the risk of single phasing the motor. They do make klixon switches for 3 phase motors that are wired to the Wye point of a Wye wound motor. They are typically only offered on what are referred to as "integral HP" motors, meaning 1HP and under. After that, the physical size of the klixon device itself becomes prohibitive. Once you get above a few Amos of running current, the arc that will be drawn when opening a circuit needs a lot more air gap to quench it quickly. That's why you use a contactor. So if you have to use a contactor anyway, it's a lot more practical to put the overload sensing device on the contactor than it is to use a klixon wired out from the motor to an external contactor.

Data sheet from Klixon showing 3 phase version, notice that they only show a Wye wound motor. When you think that through, you can see why it only could work that way.
http://www.sensata.com/download/phen3.pdf

I also noticed when looking this up that the Kilxons say they go to 5HP, but I've never seen one used above 1HP.
 
Last edited:

topgone

Senior Member
You cannot use klixon (thermal cutouts) in a delta wound motor, because as you surmised, you run the risk of single phasing the motor. They do make klixon switches for 3 phase motors that are wired to the Wye point of a Wye wound motor. They are typically only offered on what are referred to as "integral HP" motors, meaning 1HP and under. After that, the physical size of the klixon device itself becomes prohibitive. Once you get above a few Amos of running current, the arc that will be drawn when opening a circuit needs a lot more air gap to quench it quickly. That's why you use a contactor. So if you have to use a contactor anyway, it's a lot more practical to put the overload sensing device on the contactor than it is to use a klixon wired out from the motor to an external contactor.

Data sheet from Klixon showing 3 phase version, notice that they only show a Wye wound motor. When you think that through, you can see why it only could work that way.
http://www.sensata.com/download/phen3.pdf

I also noticed when looking this up that the Kilxons say they go to 5HP, but I've never seen one used above 1HP.
As far as I know, temperature sensors/imbedded RTDs are installed on windings of big, 3-phase motors. Especially on expensive motors/units that people find hard to secure replacements, some specifiers require motors with built-in thermal monitors that can be interlocked with the motor starter.
 

kwired

Electron manager
Location
NE Nebraska
As far as I know, temperature sensors/imbedded RTDs are installed on windings of big, 3-phase motors. Especially on expensive motors/units that people find hard to secure replacements, some specifiers require motors with built-in thermal monitors that can be interlocked with the motor starter.
Such sensors however cannot shut down the motor alone and need to interface with a controller somehow.
 

topgone

Senior Member
Such sensors however cannot shut down the motor alone and need to interface with a controller somehow.
Correct. The monitor taking signals from the temperature sensors gets to be "interlocked" with the starter trip circuit.
 

templdl

Senior Member
Location
Wisconsin
There was never any suggestion of connecting a neutral:?

Were you confused by mentioning of "single phasing"? All that was meant by that was loss of an input line leaving single phase line to line supplying two of the three motor input leads.
You are correct, I have to go back to see were I got the idea were there was a neutral involved. It may have been another subject and zI just mistakenly replied to this subject. Yes I agree that my reply was irrelavent to this subject. I can both draw and read both 2 and 3 wire motor control ladder diagrams in my sleep.

One comment that I could add is that I have been included on a rare occasion a temperature sensor in a 3ph motor with a n/c contact that would be interfaced with the motor starter itself to disconnect the motor starter should a given motor temperature be reached. But as was previously mentioned all 3 phases we would be disconnected which prevents single phasing.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
You are correct, I have to go back to see were I got the idea were there was a neutral involved. It may have been another subject and zI just mistakenly replied to this subject. Yes I agree that my reply was irrelavent to this subject. I can both draw and read both 2 and 3 wire motor control ladder diagrams in my sleep.

One comment that I could add is that I have been included on a rare occasion a temperature sensor in a 3ph motor with a n/c contact that would be interfaced with the motor starter itself to disconnect the motor starter should a given motor temperature be reached. But as was previously mentioned all 3 phases we would be disconnected which prevents single phasing.
Right. You CAN put temperature switches inside of a delta wound motor and run them out to a contactor to turn it off if any of them open up. You just can't use the type that actually open up the winding circuit itself, like a Kilxon does. So again, if you need the contactor anyway, the external OL relay is easier to implement. Where you usually see the embedded temperature switches is when the motor is subjected to high ambient heat, ie the motor may be over heated without being over loaded.
 

templdl

Senior Member
Location
Wisconsin
Right. You CAN put temperature switches inside of a delta wound motor and run them out to a contactor to turn it off if any of them open up. You just can't use the type that actually open up the winding circuit itself, like a Kilxon does. So again, if you need the contactor anyway, the external OL relay is easier to implement. Where you usually see the embedded temperature switches is when the motor is subjected to high ambient heat, ie the motor may be over heated without being over loaded.
If I can recall with the larger MV machines RTDs are even used.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
If I can recall with the larger MV machines RTDs are even used.
RTDs are another level of protection. You need an RTD Scanner or in most cases, a Motor Protection Relay that monitors them. the MPRs often cost more than the motor starter itself, I just bought some that were $7K each, not including all the little doo-dads that have to go with them to make them work.

That's when you get down to the nitty gritty of looking at the ACTUAL temperature of the windings, not an estimate based on current or a sloppy little Kilxon. Big MV motors are usually critical path equipment for a facility where down time will cost a lot of money and they do not want to lose the motors, but neither do they want to take them off line unnecessarily. So they will employ little tricks like "voting" of the RTDs, meaning 2 out of 4, or 3 out of 6 RTDs have to "agree" on the temperature increase before taking action, which avoids a shutdown from a false reading. They will also know the EXACT thermal damage curve of the motor and run right up to that without going over, using the RTD accuracy to take full advantage of everything that motor can deliver without sacrificing itself.
 

templdl

Senior Member
Location
Wisconsin
RTDs are another level of protection. You need an RTD Scanner or in most cases, a Motor Protection Relay that monitors them. the MPRs often cost more than the motor starter itself, I just bought some that were $7K each, not including all the little doo-dads that have to go with them to make them work.

That's when you get down to the nitty gritty of looking at the ACTUAL temperature of the windings, not an estimate based on current or a sloppy little Kilxon. Big MV motors are usually critical path equipment for a facility where down time will cost a lot of money and they do not want to lose the motors, but neither do they want to take them off line unnecessarily. So they will employ little tricks like "voting" of the RTDs, meaning 2 out of 4, or 3 out of 6 RTDs have to "agree" on the temperature increase before taking action, which avoids a shutdown from a false reading. They will also know the EXACT thermal damage curve of the motor and run right up to that without going over, using the RTD accuracy to take full advantage of everything that motor can deliver without sacrificing itself.
To my point. Yes, it much more complex to do so but it provdes the opportunity to monitor motor winding temperatures individually more accurately providing the benifits as you so apply illustrated. These large machines are not mass prooduced like NEMA frame 480v3ph motors, are engineered for the applicaton and expensive.
 

iwire

Moderator
Staff member
Location
Massachusetts
We have drifted quite a bit, my take on the OPs question is that the OP was asking about smaller low voltage motors based on the fact they used single phase motors as an example.
 

templdl

Senior Member
Location
Wisconsin
We have drifted quite a bit, my take on the OPs question is that the OP was asking about smaller low voltage motors based on the fact they used single phase motors as an example.
Yes, I totally agree. I must have to admit that this is not all that unusual should the first inquiry be compared to the last post in a good number topics which appeared to be common place on the forum. The last post quite often is completely off topic going well into the weeds from the first after pages upon pages of posts as it happens so often on this forum. In the interest of consistancy Is the policy of this forum being tightened up now and being policed closer? With all due respect maybe the topic should be closed when you, the moderator, determines that the posts start to get off topic in an effort to not confuse the first poster.
 

kwired

Electron manager
Location
NE Nebraska
At least we were still talking about motors and methods of overload protection which was the general subject of the first post.
 
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