Can a thermistor be used to build a soft start?

If a high amp thermistor was inline with a motor could it be use for a soft start?

Kevin Beitz said:

If a high amp thermistor was inline with a motor could it be use for a soft start?

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Probably. The old series string filament tube TV sets used a similar device to limit the current surge through cold filaments. They are called Globar resistors.

Bill

If sized right, a "negative temperature coefficient" part could work. That is an initial higher resistance, and drops with temperature.

They work best when bypassed with a relay after the surge is over, because they take a while to cool and go up in resistance. So if turned on after a short off time, they are not effective.

For three phase, there might be an imbalance if they were not matched.

Theory is one thing, practice is another. You will find that "high amp" NTC thermistors also have a low amount of initial resistance as well. I looked at one that I wanted to try for this purpose once and ordered a 40A rated NTC resistor. Great... but turned out it was only 1ohm of initial resistance at 25deg.C... basically useless.

Jraef said:

Theory is one thing, practice is another. You will find that "high amp" NTC thermistors also have a low amount of initial resistance as well. I looked at one that I wanted to try for this purpose once and ordered a 40A rated NTC resistor. Great... but turned out it was only 1ohm of initial resistance at 25deg.C... basically useless.

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That is true, although one does not generally need a lot of resistance. Many "soft start" schemes only reduce the voltage to perhaps 60%, so one needs only a few ohms to be as good as they are.

The bigger problem is the restart issue, and also finding thermistors which will take the inrush current. Most of the NTC thermistors ae made to fix capacitive inrush, so they tend to be rated by the amount of bus capacitance, which amounts to a "joule" (total energy) rating. Figuring the equivalent out for the motor, given the start up time to reach idle speed, and so forth, is a nuisance because there may not even be just one answer.

The ratings are also rather accurate... I sized one for a unit once and then the bus capacitor was upsized maybe 50%. The NTC was not changed, but should have been. We found in testing that the difference was enough to kill the NTC after a reasonably low number of starts. The NTC had been sized closely to the original bus capacitance, to cut the inrush, and the larger capacitor drew a longer pulse of current that overheated the NTC.

Motors are somewhat variable in the start current, depending on various voltage and load issues, so unless you know closely what will be the current pulse, you may not get what you want.

Oh, yeah.... they are unstable if parallelled, since they have a negative tempco, so that's not a way to adjust current capability.