Not bad at SCR circuits, Ken- the thyristors used in the AC drive units on these locomotives and subways use SCRs in the crowbar and precharge circuits... the older drives used a variation called a Gate Turn On thyristor (GTO), which has been put to obsolecense by the economization of Insulated Gate Bipolar Transistor (IGBT) technology.
SCRs don't switch 'off' until the pass-through current is cut-off, and depending on the SCR, there's a bit of time-delay before they unlatch... Are you using SCRs to establish the 'RUN' vs. "STOP" situation?
Terry I don't reverse polarity ... I'm purely STRAIGHT ... <font size="-2">(oh dat wuz a gud'un)</font>
Dave ferget it ... I changed to another design and I think it'll fly. I spent a sleepless night on the first 2 ideas and they just wouldn't work out. Man I like drawing schematics on da 'puter in PSP !
Hey if you've got a cross ref on SCR's and TRIAC's and IGBT's see what these numbers show "SPC 8531". My old ECG doesn't show it nor does a 2 year old version of GTE cross ref on the 'puter. I use to use Part Miner when it was FREE !!
I've got 7 old RCA parts that don't show up even in an RCA book but I tested them to be SCR's but I don't know the AMP rating but they should be pretty good as they're stud mount.
Here's what I ended up with and am about to put the iron to here in a bit.
No "iron" needed when you use a lot of jumpers with clips and already have half of it made up , just have to rob it off another motor
Anyone find the fault in that drawing ?? I did after a couple of failed trys. Now it's time for the real circuit on a board. It's nice having more parts in stock than Radio Shack ... what a freakin joke !
Ken- Terry's alluding to one of the SCR's attitudes... they'll trigger not only from a high breakover voltage (across A-C), but also from a fluctuation which causes the A-C relationship to swing enough to cause a differential voltage from GATE.
In the case of your DC motor controls, the DC motor can generate a substantial voltage and current from dynamic braking action, and from transients.
Consider the DC motor as being an ignition coil... the shunt and field windings around iron core, with a substantial field built up in motoring. Now, open the current path... what happens to the magnetic field? it collapses... right across it's own windings... WHAM! The voltage HAS to go somewhere, and it occurs EXTREMELY fast. The SCR's taking an awesome beating, and if it's surviving, it's quite a tough soldier.
What you SHOULD do, since you're not using speed-control, is just use a pair of contactors... one for braking, the other for running. If you really must use SCRs, at least put a low-value high-wattage resistor across the motor leads (always in-circuit) so that when you cut power to the motor, it'll drop SOME of it's field through the resistor (snubbing!) to keep from giving the SCRs a big Owie. A good WAG for resistance... take the resistance of the motor, and multiply that by 8... and use that resistance. Don't forget to apply Ohm's law to calculate current through the resistor, and from there, calculate wattage. Be very conservative, as the resistor WILL get warm.
If you want some contactors, I've still got a big pile of surplus in my shed, probably have some 400w shunt-resistors, too...
okay I see it'll do no good asking questions here because nobody is considering how the circuit works. I know that the dc motor will cause a reverse voltage, that's how it breaks itself against the breaking resistor ... BUT it can't trip the gate UNTIL I hit the stop switch ... as I said it is sometimes tripping at start up, if it was tripping at Stop then the circuit would be working. At stop is when the motor will feed back the self generated voltage.
I do have a relay schematic drawn but I want something faster than that.
I am using a variable speed control before the bridge.
A diode across the motor would only keep it from feeding from terminal to terminal which wouldn't work if using a F/R direction PLUS it has to feed back into itself to brake.
I tried a diode and I tried a resistor between the Anode and the motor POS to limit or stop any feed back into the Anode but it still turns on at start up. The Cath is limited by the braking resistor also.
I guess the next circuit will need an optocupler in the circuit.
I think my 5 ohm 250 watt (not rated on drawing) braking resistor will handle the heat.
If I put a resistor across the motor terminals then there would be no need for the braking circuit , and that is what the braking circuit is doing when it's turned on. There is a resistor across the filter cap to burn off the voltage when the motor is turned off so , altho it has to run back through the circuit control SCR it is connected to the motor terminals.
The thing that turns on SCRs by themselves is usually the rate of change of the voltage (dv/dt). The inductive kick has a high dv/dt and must be smoothed. A series inductor will slow the dv/dt down, back-to-back zeners directly on the motor will suppress the amplitude. They make some snubber device, the name of which escapes me right now (lost those brain cells a long time ago), that basically acts like back-to-back zeners. I think it is used in TVs, UPS's, etc. Also make sure your wiring is up to snuff - don't run the power currents next to the gate wiring, use twisted pairs.......
I think, since you are not using the scrs for speed control, I would do as Dave suggests and just use a contactor to engage the brake.
BTW, I just finished laying down 600 sq ft of wood floor and will have time now to wire up my 3 phase converter and let my knees recover. The logic is based on Dave's but I am using the original motor starter contactors on the mill to handle the heavy chores. So my extra relays just do logic stuff. I hope to test in a few days.
Notice the reverse characteristics are the same as discussed previously for the rectifier or diode, having a break over voltage with its attending avalanche current; and a leakage current for voltages less than the break over voltage. However, in the forward direction with open gate, the SCR remains essentially in an off condition (notice though that there is a small forward leakage) up until the forward break over voltage is reached. At that point the curve snaps back to a typical forward rectifier characteristic. The application of a small forward gate voltage switches the SCR onto its standard diode forward characteristic for voltages less than the forward break over voltage.
Obviously, the SCR can also be switched by exceeding the forward break over voltage, however this is usually considered a design limitation and switching is normally controlled with a gate voltage. One serious limitation of the SCR is the rate of rise of voltage with respect to time, dV/dt. A large rate of rise of circuit voltage can trigger an SCR into conduction. This is a circuit design concern. Most SCR applications are in power switching, phase control, chopper, and inverter circuits.
The CEMF of the motor could cause the SCR's to close unpredictably. I think you will need a voltage clamp. Like the double Zener diode thingy Jim talked about.
The double-zener clamp is advertised under MOVistor, as well as other names. There's other variations... I suspect the motor's kick will be enough to whip the MOV after a few dozen cycles, though. Ken- running an optoisolator won't solve the problem, but if you're not willing to go to contactors, put a 50-ohm 20w wirewound or a similar lightbulb across the motor leads, with a MOV across the wirewound- perhaps that'll soften the blow enough. Another thing to try, is to stick an NE-2 neon tube across 'em... that'll help 'clamp' voltage at 80v, perhaps it'll take enough of the 'lick' off to keep from getting the Dv/Dt gate trigger...