Does anyone use NTC resistors to limit inrush current on single phase AC motors as a soft start?.

[OperaHouse]

The question is commercially. My only experience is with power supplies. I got into a discussion about using these with air conditioners to limit startup current where inverters were unable to take the startup surge. Some have tried it. I thought they might help a little in much the way a long extension cord would. I prefer to do things electronically. Not interested in a technical discussion. They asked if I had ever tried it. I've never seen them used with motors, but I don't get involved much with motors. Have you seen these used in commercial products?

 

[Heaviside1925]

OP,
This post is now going to be a worm in my brain. I don't think I've dreamed it, but I seem to recall these being used in a similar application involving small HVAC compressor and inverters, possible on a PV supplied DC system....? I think they were performing like duty as to what you are describing. Maybe it will come back to me, but it will just be stuck in my brain for now.

 

[jraef]

I’ve never seen it done. I would think that would come with some undesirable baggage. For one thing, you would need to add a bypass contact, otherwise the resistor is just going to burn out in short order by being in the circuit all the time. So now you have another part to fail. You already have a number of moving parts in single phase motors (that would need soft starting), so you are exponentially decreasing the MTBF.


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

 

[Marke]

I have seen it done on very small machines, but then you could argue that it is not necessary as the current is always so low.

Mark Empson
Advanced Motor Control Ltd

 

[FacEngrPE]

The whole purpose of a soft start is to limit inrush, and starting current. Inverter drives control the motor power feed in a manner that provides better management of power flow into the motor than a soft start. I see no reason to add a resistive element to either.

 

[waross]

Inverter drive limit control the current by controlling the slip frequency/rotor frequency.
Within reasonable limits an inverter may produce full torque at rated current at any speed.

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

 

[OperaHouse]

The thermistor used is a 5 ohm 20A and they put two or three in series. They confuse limiting inrush with limiting current. They have to use two or three in series because the transition is so fast. At half current the resistance drops to 0.08 ohms. Motor won't do much with 15 ohms in series but all they can see is that initial current is limited. I'd think more in parallel would give a slower warm up. What they use costs $11 each and still should have a relay to short it out. You can get a cheap soft start on AliExpress for $35. Just curious if any of these were used commercially. Refrigerators have a similar thermal device for the start winding which is dirt cheap. Seems like something could be made with some more thermal mass to better match a motor.

 

[edison123]

NTC resistors OEM links (pdf alert)

https://www.eaton.com/content/dam/e...eaton-ntc-thermistors-app-note-elx1247-en.pdf

https://cdn.soselectronic.com/productdata/71/15/e533b543/s-237-5-b57237-s509-m.pdf

Muthu

http://www.edison.co.in