Working on an AC Motor Speed Controller, need some help understanding it - Page 1
Forum section: electronics / projects

author: bushidov
author_rank: Newbie
timestamp: October 04, 2022, 03:20:20 am
content: First time poster.I've attached a schematic that I drew up when tracing this motor control board to one of our old industrial pre-feeders. It gets AC 120V in (L1 and L2) with a varistor and then goes into what I am guessing is a full-wave (not full bridge) rectifier via D1 and D2. The P1 and P2, which also ties into the trimmer pots R3 and R4, goes to the off board potentiometer, which controls the speed of the pre-feeder motor. It appears that the motor is something like 100VDC (not AC). A- and A+ are the motor leads.I'm used to DC type circuits and have been avoiding AC circuits, but I am trying to figure this one out. I am sure I "drew" the schematic correctly, in so far as that I know that is how things traced out on the PCB. I am sure of the capacitor and resistor values, as I was able to read the values of the mylar caps and measure the resistors (all through-hole).I am going to take a stab and guess that I didn't draw the schematic "cleanly", and that's just because I am unfamiliar with SCR and SUS circuits.Can anyone help explain what this thing is doing, or how it works, or even a better way to organize the schematic?Lastly, does anyone know what I can use as an alternative to the SUS1 2N4990?Thank you all for your time with this.

author: bdunham7
author_rank: Super Contributor
timestamp: October 04, 2022, 03:51:21 am
content: In brief, D1, D2, D5 and D6 form a bridge rectifier (without a filter cap) with the negative side going to the motor and the positive side going to the triac control circuitry. Only the negative side is high-power, the positive side of the bridge that supplies power to the motor is formed by the two SCRs. The triac controls the SCR gates, providing phase control for that section of the bridge. I'm not sure at first glance exactly how the triac control part works, is there possibly and error in how it is drawn? Could you have the gate and cathode/anode mixed up?The 2N4990 is still available from Newark and probably elsewhere.

author: bushidov
author_rank: Newbie
timestamp: October 04, 2022, 03:09:41 pm
content: Thanks for the reply. Yeah, now I see the full bridge rectifier. It's odd they used two 1A diodes and two 3A diodes, but it works.I updated the schematic, so it looks a bit better. The "Triac" 2N4990, is specifically a Silicon Unilateral Switch (SUS), so I changed the schematic to reflect that. I am not sure the pinout on pins 1 and 3 (anode and cathode) as the datasheet didn't look clear on the orientation of the picture where the pins are. However, I drew out the PCB in EAGLE and the silkscreen on the PCB matches the real one that I traced, so if I reproduced the PCB, I know it will work, but my schematic "might be" backwards.I've never made a motor controller that uses SCRs and SUSs, so this is new territory for me. I am used to MOSFET drivers or H-Bridge. I'm also wondering where the smoothing capacitor is for smoothing out the full bridge rectifier.This controls a DC motor rated for 120V DC, but uses this board to convert the 120V AC to do so.

author: bdunham7
author_rank: Super Contributor
timestamp: October 04, 2022, 03:40:45 pm
content: Quote from: bushidov on October 04, 2022, 03:09:41 pmThere are actually two bridges and they share the 3A diodes for the negative half. The 'big' bridge positive side is the phase-controlled SCRs. There won't be a filter capacitor because the control circuit needs won't work with uninterrupted DC--it needs the full-wave rectified AC, lumps and all.

author: bushidov
author_rank: Newbie
timestamp: October 05, 2022, 02:04:40 pm
content: Excellent. That helps in the understanding.

author: gcshum
author_rank: Newbie
timestamp: March 30, 2023, 11:12:46 pm
content: Hi bushidov,Do you think you can send me the Eagle file?Thanks!