Motor Thermal Protection

Are all 2 wire motor thermal protectors PTC's? I understand there is also a simple bi metal temp switch called a Klixon also? We have a few motors with RTD's also.

I am connecting 2 5HP Baldor motors and 755 drives and it appers the 755 only has a PTC input?

How can I tell if it's PTC or Klixon? What if it is Klixon how would it connect to a 755?

Can I tell with a meter if it's a Klixon or PTC for sure? Don't see anything that says on the nameplate or data sheet.

A Klixon is a simple switch it looks the same as these tactile switches, but the plate is of bimetall, if it gets too hot, it will open.
yes it should be possible just to connect a klixon
a Pt100 is not possible on a PTC input.
yes a klixon is below 50 ohms, while a PTC is above 1000 ohms.

Based on the info above they are 1.8 Ohm and appear to be bimetallic. In the 755 manual I only see PTC inputs. Would the bimetallic still go on the PTC terminals and work? if not where would they wire on a 755?

Tim Ganz said:

Based on the info above they are 1.8 Ohm and appear to be bimetallic. In the 755 manual I only see PTC inputs. Would the bimetallic still go on the PTC terminals and work? if not where would they wire on a 755?

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Yes. Think of it this way; with a PTC thermistor, the resistance increases as the temperature increases. At some point the resistance gets to the point where it exceeds the threshold that you were looking for in the PTC sense circuit of the drive, and the drive detects that as a fault.

With a Kilxon (bi-metal snap switch), the resistance is low as long as the switch is closed, and when it snaps open, it is infinite. Infinite is greater than whatever that threshold was no matter what, so the condition is true and the fault is initiated.

Sometimes people try to use PTC thermistors as analog input devices, and it can be done if you want to go through the highly complex math, but it is complex because the resistance change is extremely non-linear and the "curve" is very close to vertical, meaning the difference between hot and cold resistance is not great. So they are better used as "knee point" detection devies, because they will pass through a threshold very quickly.
This is a typical PTC thermistor curve.

The threshold temperature used for selecting the PTC thermistor would be the Tp point on that graph, so you can see that the resistance changes very rapidly before and after that point, making analog measurement difficult at best. So it works more like the Klixon device anyway, just no moving parts.

jraef said:

Yes. Think of it this way; with a PTC thermistor, the resistance increases as the temperature increases. At some point the resistance gets to the point where it exceeds the threshold that you were looking for in the PTC sense circuit of the drive, and the drive detects that as a fault.

With a Kilxon (bi-metal snap switch), the resistance is low as long as the switch is closed, and when it snaps open, it is infinite. Infinite is greater than whatever that threshold was no matter what, so the condition is true and the fault is initiated.

Sometimes people try to use PTC thermistors as analog input devices, and it can be done if you want to go through the highly complex math, but it is complex because the resistance change is extremely non-linear and the "curve" is very close to vertical, meaning the difference between hot and cold resistance is not great. So they are better used as "knee point" detection devies, because they will pass through a threshold very quickly.
This is a typical PTC thermistor curve.

The threshold temperature used for selecting the PTC thermistor would be the Tp point on that graph, so you can see that the resistance changes very rapidly before and after that point, making analog measurement difficult at best. So it works more like the Klixon device anyway, just no moving parts.

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That makes perfect sense JRaef thanks. One more question in the Pram list I see 205, 251 for config action and status of the PTC but no where to set the temp that it would trip at (if I were using a PTC) is this a hard value? Does not seem to be listed anywhere if so or is it calculated somehow from motor data such as HP / RPM/etc? Just wanted to know for future reference.

Looking at AB Technote 600886 in the PTC status parameter there is Ok,Short and over temp. Would the normal operation of the motor running cool and the Klixon closed be detected as a short? Would that fault the drive?

Tim Ganz said:

That makes perfect sense JRaef thanks. One more question in the Pram list I see 205, 251 for config action and status of the PTC but no where to set the temp that it would trip at (if I were using a PTC) is this a hard value? Does not seem to be listed anywhere if so or is it calculated somehow from motor data such as HP / RPM/etc? Just wanted to know for future reference.

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If using a PTC thermistor, the temperature value that raises the resistance is based on the design of the thermistor. So the circuit in the detector is just looking for that resistance value. It is not adjustable as far as I know. The nominal value it looks for is 1.8k ohm, trip is at 3.1k ohm, reset is at 2.2k ohm.

It is listed in the installation manual, but not ion the programming manual if that's where you looked. 750-IN001H-EN-P

Tim Ganz said:

Looking at AB Technote 600886 in the PTC status parameter there is Ok,Short and over temp. Would the normal operation of the motor running cool and the Klixon closed be detected as a short? Would that fault the drive?

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That only applies if you have installed the "ATEX" (IEC explosion proof motor) option card. If a motor has an ATEX listing, it will never have a Kilxon, it will only have a PTC, and the PTC detector in the drive needs to be able to detect a failure of the circuit or component in it. Different PTC programming parameters, which are masked if you don't have that option card installed.

If you don't use that option card, then the PTC programming is at 40 and 41, which do not include the extra option to detect a short. You can read the resistance value it is seeing on 42.

yes tim it will show short, however you can set this as normal operation .
another way is to put a resistor in series with the klixon, use a resistor to get rid of the short warning. (about 1000 ohm.)

shooter said:

yes tim it will show short, however you can set this as normal operation .
another way is to put a resistor in series with the klixon, use a resistor to get rid of the short warning. (about 1000 ohm.)

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Yes, however, no need for it unless that ATEX option board is installed. The basic PTC circuit that comes with the drive does not detect a short in the circuit.

jraef said:

Yes, however, no need for it unless that ATEX option board is installed. The basic PTC circuit that comes with the drive does not detect a short in the circuit.

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It's the standard card not the ATEX. I did not catch that the short detection is ATEX only. Thanks jraef.