Senior Member
An op-amp and a few resistors could be used to convert an analog signal to a digital signal. Since the thermistors that I've used in various designs have had almost linear output voltages based on temperature, the op-amp's comparator circuit could be biased with a resistor(s) to instantly swing the op-amp's output to a digital 'active' level. Total cost for components would probably be less than $2.00 at a Radio-Shack type store.
Senior Member
Mike, these standardised thermistors are non-linear, but in a good way. Their resistance increases dramatically from 550 ohm to 1330 ohm when exposed to temperature within 5 degrees C of the thermistor's nominal "trip" temperature. (These numbers are standardised by the EuroNorms).
I entirely agree with you that a circuit which detects this jump in resistance should be very cheap, but I am trying to protect a $700 spindle so I bought the cheapest brand-name thermistor relay (already packaged & calibrated) I could find, and the Siemens 3RN cost me about $100 out here.
The way that SB connects these thermistors to VFD's worries me because I don't know if the VFD's multi-function digital inputs will see 550 ohm as "closed" and 1330 ohm as "open". I am guessing that the VFD sees both those values as "closed" and therefore a hot spindle won't trip the VFD when it should.
Why have motor/spindle manufacturers moved to thermistors instead of thermoswitches? Probably because:
- they are much smaller (typically 1/8" as opposed to 3/8") and cause less of an intrusion into the tight windings inside the motor.
- they are extremely reliable and don't deteriorate with time or repeated switchings (no moving parts)
- and it seems that they are cheaper.
Senior Member
Done a little more homework. Seems that standard thermistor relays trip at between 3100 and 3600 ohm and reset at about 1600 ohm. This allows the temp to go 15 degrees over nominal on a single thermistor. If more thermistors are connected in series, the trip temp comes down.
Senior Member
Gerald, the non-linear output of the thermistors that you've described is a big plus. Because a digital circuit is basically a binary analog circuit, with LOW being 0 to 0.7 volts and HIGH being 2.2 to 5 volts (when using 5V as VCC), a non-linear analog circuit with high-gain in the critical part of the temperature range could be constructed as a voltage-divider circuit, and effectively become a digital circuit. (Back in my days designing process control circuits for photo labs, I often used the circuit that I'm describing to control the heater temperature in the film/paper dryer cabinets where variations of 10-degrees F. were not critical. However, in the chemical temperature control circuits, where the temperature had to be held withing 1/4-degree F., I used commercial units like the Siemens unit that you listed.) Spending $100 to protect a spindle seems like a wise thing to do. (Since installing my 3hp Colombo spindle, I've decided that most of the 'optional' equipment that 'might' be necessary for proper operation of the spindle should NOT be optional because - in my case - it was required to reduce EFI to acceptable limits in a residential area. In my part of the world, summertime temperatures are usually moderate enough that I don't have to worry about things getting too hot except for a few extra hot days. Down where you're located, I can see how temperature would be a major consideration.)
Senior Member
Mike, does your Colombo have a switch or a thermistor? I have already stalled my spindle and tripped the thermistor. It doesn't really matter how hot the day is, but rather how careless one is to answer the phone without stopping the machine.....
Cape Town does not get as hot as towns further north, near the Kalahari desert. (We even have a town with the onomatopoeic name Hotazel). But the ocean around here is cold and that keeps our town's temps moderate.
Senior Member
Gerald, as far as I know, the Colombo has neither. I'll have to pull the cover off the VFD and see how many wires go to the spindle. When I installed it, over a year ago, I simply checked that all of the screws were tight without even counting the conductors. The only connections that I'm sure of are the three legs of the 3-phase power, a ground wire and two conductors to the external fan. When I get back to the shop I'll have to take a look inside the VFD.
As far as shop temperature goes, I get really nervous when it get above 105 F. inside the shop. Even though 100 is a hot day here, my lack of air flow through the shop can let the heat build up quickly. Since I don't like running the spindle any hotter than 120 F, a fifteen-deqree margin is pretty slim. On those hot days I tend to take shallow cuts at more moderate speeds than on cooler days.
Senior Member
has anyone used ekstrom carlson spindles. i am looking for some feedbackhttp://www.ekstromcarlson.com/
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