Converting RTD to PTC in VFD driven motor

Which did you order when you purchased the machine? The normal provisions by the manufacturer are going to either be a thermostat (Normally closed, or normally open) which acts like an on/off switch at a set temperature. Or it will have been what you found: a resistance temperature detector (RTD) - either a button or stick type, depending on the winding.

Check your drive documentation: there is often a way to connect things properly. Might require an additional summing relay somewhere along the line.

Converting energy to motion for more than half a century

" So we have pulled 1Pair cable to the motor."
"But in the motor we have found PT100 RTD. So There are 3 pairs at the terminal box. Now we have a problem. How can I over come this problem?"

Pull out the single pair cable and pull either 3- or 4-wire cable in its place.

You can use the two wires you have to power one of these:


How many sensors do you have? It sounds like you have two. Can these sensors be replaced or would that require breaking down the motor?

You can get a (relatively) inexpensive RTD monitor relay, then wire the trip contact of the relay to the PTC input of the drive. I use these when I need that capability.



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

With only two wires available, it would have to be loop powered.

The obvious best solution is pull the right wires but we've all had this one dropped in our laps so I'm curious to see what workarounds people use. My personal vendetta ks against process switches. We use them for alarm points. I don't like switches because you can't see if they're working. Being able to replace 2 wire switches with 2 wire sensors can save a lot on labor while making upgrades.

Dear Mr. Histor (Electrical)(OP)11 Jan 23 10:41
"... 55kW motor with VFD drive. In the VFD drive we had considered PTC Thermistor protection provision. So we have pulled 1Pair cable to the motor....But in the motor we have found PT100 RTD. So There are 3 pairs at the terminal box. How can I over come this problem?"
I suggest the following for your consideration.
1. At the motor terminal box, connect three PT 100 in series. Now having only two terminals instead of six. Attention: three PT 100 connected in series = 3 x 100 = 300 ohm, at 0 ^o C. Assuming all three windings are heated up with the same temperature-rise, the new resistance value will be 3 x of each PT 100. In case only one winding is faulty, the total new resistance = R1+R1+R3 ohm. Protection is still in place.
2. Connect the motor terminal box (2 terminals only) with one pair (2 wires) twisted and screened wire to an external RTD relay. See learned advice by by Mr. jraef You can get a (relatively) inexpensive RTD monitor relay, then wire the trip contact of the relay to the PTC input of the drive.. Attention: Take note that the new setup is a PT 300 !. If the RTD monitor relay can only accept/calibrated for PT 100, you may be able to scale it down by voltage divider method.
Che Kuan Yau (Singapore)

Q for drive gurus

A PTC is just an ON/OFF switch for a single preset temperature. Unlike RTD's where the drive can actually measure the winding temps and thus control it for alarm/trip, how does connecting a PTC to a drive help in anyway? Can't the PTC just use a shunt trip mechanism to protect the winding?

Muthu

@ Mr. edison123 (Electrical)13 Jan 23 00:27
"#1......A PTC is just an ON/OFF switch for a single preset temperature....#2. Unlike RTD's where the drive can actually measure the winding temps and thus control it for alarm/trip, ....#3. how does connecting a PTC to a drive help in anyway? Can't the PTC just use a shunt trip mechanism to protect the winding? "
I see your point. I wish to submit the following for your consideration:
1. Yes, you are correct.
2. Yes, you are correct that a RTD can actually measure the winding temperature...... But (a) take note that the VFD in question has provision for PTC input only. (b) BTW , most "standard" VFD on the market do have external on/off command provision as standard. But, NOT having provision for RTD input.
3. The RTD changes the resistance value with temperature change. It does NOT have any on/off switching function. Therefore, by itself, it does NOT operate a shunt/under-voltage release. RTDs are connected to a RTD relay which monitors the resistance and effect a NC/NO contact switching , when the resistance exceeds the set value. Where the resistance is converted to temperature.
4. My proposal is to connect the RTD to a RTD relay. The relay NC/NO voltage-free contact is connected to the FVD PTC input. Therefore, the winding temperature is indicated in the RTD relay, NOT on the VFD.
Che Kuan Yau (Singapore)

The PTC is just a resistor with a resistance value that changes rapidly when it reaches a specific temperature, so it needs to be paired with something that is monitoring the voltage through the circuit to know when that resistance changes. A lot of VFDs (but not all) have a dedicated PTC input set up to do that or can have one of their analog inputs configured for PTC. I tend to use the PTC input option for external OL protective devices because that usually is tied to a display readout that indicates an over temperature condition.


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

@ Mr. jraef (Electrical)13 Jan 23 23:35
"....The PTC is just a resistor with a resistance value that changes rapidly when it reaches a specific temperature, so it needs ...... #2. A lot of VFDs (but not all) have a dedicated PTC input set up to do that or can have one of their analog inputs configured for PTC...... "
Thank you for your learned advice. I have the question:
1. Mr. Histor states that (a) the VFD has provision for PTC input, which implies that there is No RTD input provision. (b) the motor is installed with six pieces of RTD of PT 100, and (c) only (two conductors) had been installed between the [VFD and the motor].
2. Based on this (a). (b) and (c) pre-conditions, please advise how to resolve the issue.
Che Kuan Yau (Singapore)

You may use a resistor with the same ohmic value as one of the wires in place, connected across the drive terminals in place of the missing wire.
Be aware: The third wire is used to provide compensation for the resistance of the field wiring.
If you have extreme temperature variations, the change in resistance of the field wiring will be uncompensated and may cause measurement errors.
#16 AWG copper has a resistance of 1 Ohm per 250 feet.
If you use a solid jumper, the error will be about 2% on a 100 Ohm RTD
If you use a 1 Ohm jumper, any errors due to ambient temperature change will be quite small.
Double check my calculations.
It looks as if a 1 Ohm difference in resistance will cause about 2.5 degrees C error in indicated temperature.
You can probably live with that.
I would.
I may do a work sheet, looking up SI values and converting to Imperial, and after a calculation convert back again. Feet to meters, Celsius to Fahrenheit. If you have a page filled with calculations showing the expected results, no one will take the time to check your figures.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!