Solid-state TRIAC-based inrush current limiting for inductive load - Page 1
Forum section: electronics / projects

author: arcfour
author_rank: Newbie
timestamp: January 26, 2021, 04:45:15 am
content: Recently my father mentioned he had a power tool with a large motor in it that drew enough current to sometimes trip sensitive 15A breakers in some houses (it draws more like 8-12A once powered up).Everyone knows about relay-based soft start designs, but I recently got some powerful TRIACs and was thinking about using one of them for this application. The basic idea is that the device's inrush current is limited through a high-power resistor, then after 300-500 ms the TRIAC shorts out said resistor, connecting the device directly to the mains. The delay is achieved by charging up a large electrolytic capacitor to the breakover voltage of the DIAC. I'm using a capacitor instead of a resistor to drop voltage/limit current into the control circuit to reduce heat losses incurred using a simple resistor.My prototype of this works pretty well. The only concerns I have are heat dissipation in the TRIAC (but I've got heatsinks) and seeing if I can reduce the size of the X2 capacitor as 2.2uF is pretty big and possibly overkill. The datasheet for the TRIAC lists a wide range of gate trigger currents depending on temperature and which quadrant it's in, so I oversized it to ensure I can supply enough current to turn it on and keep it on. Since I don't technically need to switch 25A I can probably use a different TRIAC with a lower gate trigger current but I wanted to overspec this to ensure it lasted a while.Here's the schematic:What do you guys think? Is this a stupid idea? Any suggestions for how I could improve it? I was hoping to keep the design as simple as possible...I've seen designs using an SCR or some BJTs instead of a DIAC but this seemed to work well enough for my purposes that I didn't bother testing them. This is basically a dimmer without the potentiometer and with DC-based control...can't get much simpler than that.

author: WattsThat
author_rank: Frequent Contributor
timestamp: January 26, 2021, 06:23:54 am
content: Not a power guy so I cannot comment on the actual concept but... couldn’t you reduce the size of X2 and C2 with some series resistance in front of C2? It just feels like a lot more gate current is available than is needed with X2 being what 1200 ohms at 60Hz? Too late here for clear thought...

author: arcfour
author_rank: Newbie
timestamp: January 26, 2021, 07:28:20 am
content: Currently it draws about 40mA gate current. That's high but not outside the range of values given in the datasheet, and I want to make sure it stays triggered. Sure I could use a smaller cap or add a series resistor, but then I run the risk of it not staying on. I'd have to test to see what the lowest gate current would be for reliable operation.

author: eblc1388
author_rank: Frequent Contributor
timestamp: January 26, 2021, 12:37:10 pm
content: Quote from: arcfour on January 26, 2021, 04:45:15 amBut you'll need a sizable heatsink. The typical forward voltage drop across the TRIAC at full load is somewhere between 1.5V ~ 2V. At 12A load current, the TRIAC is dissipating 24W for no good reasons at all.Alternatively, if you use an AC timer to bypass the resistor, the timer only takes a few Watts or less. That's an easy choice to make. Just wire the timer coil in parallel with the load and its delay close contacts across the series resistor.

author: jonpaul
author_rank: Super Contributor
timestamp: January 26, 2021, 12:55:50 pm
content: Hello: IF the breaker trips on the motor, the circuit is not adequately rated.Either the breaker or the wiring is inadequate.PLEASE CONTACT A LISCENCED ELECTRICIAN IN YOUR AREA.Never use a triac or SCR on inductive loads: Asymmetry in device or trigger phase causes a DC bias and saturates the motor or transformer core.The saturated core will cause breakers to trip, and may damage the motor.Inrush currents can reach 100s A.Kind Regards,Jon

author: arcfour
author_rank: Newbie
timestamp: January 26, 2021, 03:45:35 pm
content: Quote from: jonpaul on January 26, 2021, 12:55:50 pmI have measured current draw with a shunt and my oscilliscope. It's just inrush current that trips the breaker; current is well within the capabilities of a 120V/15A circuit and the one he usually runs it from has no-to-minimal other loads on it. A 20A breaker handles the inrush fine. I don't know what the trip curves for his breakers look like but I'm guessing they are fairly sensitive. The fact that it sometimes works fine on a 15A circuit but sometimes doesn't further confirms this is an issue of inrush current to me.I have never heard about that particular issue with TRIACs switching inductive loads—in fact I was able to find many examples of them being used to control very large inductive loads like motors. Can you please explain a bit more?

author: arcfour
author_rank: Newbie
timestamp: January 26, 2021, 03:53:48 pm
content: Quote from: eblc1388 on January 26, 2021, 12:37:10 pmFair, I had considered that. If I actually give this to him I'll probably use a relay instead, but I was interested in seeing if this was a reasonably-designed alternative, ignoring heat dissipation issues with the TRIAC.I do have an optocoupler with ZCD. I wonder if I were to use that + a capacitor to phase shift it 90 degrees, if switching the load on at the peak of the A/C sine would be a viable alternative...

author: eblc1388
author_rank: Frequent Contributor
timestamp: January 27, 2021, 05:57:51 am
content: Quote from: arcfour on January 26, 2021, 03:53:48 pmThere is another aspect that will affect your decision.There is probably a power switch on the power tool itself. Whatever circuit you have finally decided to use would means that the circuit needs to be installed*after*that power switch for it to function. For TRIAC circuit this might be more troublesome on a portable tool because of exposed connections and heat dissipation requirement.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 30, 2021, 08:48:14 pm
content: The inrush current should not turn off the protection if this current does not approach the short-circuit current.The thermal protection in the power supply circuit is not triggered instantly when the rated current is reached.TRIC has a voltage drop and power is dissipated on it. Why don't you use a relay if you only want to short out the limiting resistor?

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 30, 2021, 08:51:16 pm
content: Quote from: arcfour on January 26, 2021, 03:53:48 pmYou can turn on TRIAC whenever you want. On the phase shift, dimmers are built. But turning on non-zero generates transients characteristic of TRIAC and creates strong interference.