Triac-based SSR to protect low voltage starting circuit of generator - Page 1
Forum section: electronics / projects

author: brammekeuh987
author_rank: Newbie
timestamp: March 03, 2021, 09:37:51 am
content: HiI'm currently making a starting circuit for the generator of a water turbine. In some circumstances the turbine needs a bit of speed to start generating electricity. It's the electric equivalent of a starting motor of a combustion engine. The 3 phase starting chip (starting motor) is the BD63242EFV (https://www.rohm.com/products/-/product/bd63242efv). These pins are only tolerant to lower voltages (20V), so when the generator starts to produce more than 20V it'll burn the circuitry. So whenever the start circuit stops we need to disconnect the starting chip from the generator (as with a starting motor).I already implemented this 'disconnect' with a regular relay, but I would like to test the same with an SSR.Here below a suggestion for the circuitry, I tried to simulate this in spice (not this actual circuitry, but the full implemented one), but it keeps giving convergence errors whenever the "startCircuit_relais" comes on, my guess is that these convergence errors are due to simulation numerics, and not because of the circuitry. Therefor my question: Would below circuitry work?Explanation of symbols:Phase1, 2, 3 are to the generator, U, V, W are to the starting chip;Situation 1 is when the generator is driven by starting chip, Situation 2 is when the generator is producing;Only phase 1 is made completely, the others are copies of phase 1;MOC3023SM is an optocoupler for triacs;L0103MTRP is a triac;C30 and R10 are snubbers;R14: the starting chip sends a positive pwm-based voltage to the different phases (shifted 120°). The return path must be through the other phases and not through the ground. Otherwise the in-built current restrictor is bypassed and the chip burns. It took me 7 chips to discover this (this is not mentioned in the manual). And this is an issue. The generator is mounted (and therefor electrically connected) to the turbine frame which is placed in the water. The star connection of the generator is connected to the housing (I dismantled the generator but couldn't find the star point, so I couldn't disconnect it) and is always grounded to the earth. Therefor I cannot ground the controller PCB directly, and a (eg) 1K resistor is needed to not let the ground voltage of the controller pcb float. So the R14 resistor is not in between the mosfet and the "zero" of the 5v, but between the "zero" of the 5V and the actual earth ground;R12: as because of the return current which needs to be through the other phases we need to place R12, to make sure the gate gets triggered whenever a positive voltage is given on the chip side of the triac.Cheers!

author: David Hess
author_rank: Super Contributor
timestamp: March 04, 2021, 10:28:00 pm
content: I think the limit for low voltage operation comes from the MOC3023SM and the forward voltage drop of the TRIAC gate so it should work at 24 volts and down to 5 volts.

author: brammekeuh987
author_rank: Newbie
timestamp: March 05, 2021, 01:35:48 pm
content: Hi DavidThat would be perfect since I supply the driver with 12V.I guess the 5V is because of two times passing a triac. I can't find the forward voltage in the data sheet (https://www.littelfuse.com/~/media/electronics/datasheets/switching_thyristors/littelfuse_thyristor_l01_datasheet.pdf.pdf), but since a SRC has two junctions in its path ( look here:https://electronics.stackexchange.com/questions/15044/triac-heatsinking) a typical value of 2 * 0.7V per Triac, so a total of voltage drop of 2.8V would be achieved. Is this the correct train of thought?I don't understand the limitation for 24V, since the datasheet (http://www.farnell.com/datasheets/2281290.pdf) shows a "Off-state terminal output voltage" of 400V.But anyway, this circuit should work, which is nice. I'll let you know when it's built!UPDATE:I tried simulation some more and it doesn't look like this is going to work since i cannot control sufficiëntly the current through the gate of the triac. Since it needs to be controlled from both sides (passively on the starting chip side and actively (optocoupler) on the generator side. In the end it would be a lot more expensive in comparison with an ordinary relay, I guess.

author: Medved
author_rank: Contributor
timestamp: March 09, 2021, 06:43:35 pm
content: I would stick here with a classic relay. The SCR's have the bad habit of spuriously turning ON from time to time, mainly when there is some noise on the generated line (someone connected/disconnected some load,...), whatever noise suppression you use there. And the first time this happens, bye-bye the starter inverter...If you really don't like relays that much, better use MOSFETS (an antiseries pair for each phase), with some 10..100nF between Gate and Source these at least won't activate. But still beware of their capacitance passing the high dV/dt spikes back to the inverter.