Jerking motor in the drill press - Page 1
Forum section: electronics / projects

author: diy_debik
author_rank: Newbie
timestamp: October 19, 2023, 05:14:32 pm
content: I've got a drill press with a semi-alive control board and decided to make my own board, with PID speed control and stuff. The current schematic is as follows:The SCR is controlled by the Arduino which receives pulses from the zero-cross detector and sends the opening pulse to the SCR after a certain delay. The code is tested with a scope and signal generator, and the control board was tested with resistive load and everything works fine in this regard. However, when I connect a shunt DC motor from the drill press and reduce the power below a certain point, the motor starts jerking, as if the SCR is not closing after the cycle ends and stays open for a few cycles. The only out of the ordinary thing that I've noticed is the shape of the output voltage in the rectifier that powers the motor. During normal operation, it has a plateau like shown on picture below (also why is it on the left side? at low power settings the SCR is opened closer to the right portion of the wave...). However, when the shape becomes a clean rectified sine, the motor makes a jerking motion.I've tried a lot of things to no avail, like writing a frequency locked loop to eliminate the effect of possible glitches in the zero-cross detector, replaced the SCR, powered the field winding from a separate rectifier, tried different ZCD circuits, all to no avail. The only two things that made a difference is skipping half of the ZCD pulses, which made the jerking less pronounced, and this also allowed the motor to work smoothly at very low speeds (like 20 RPM out of maximum 3200). Adding a diode in parallel to the motor made jerking shorter and more frequent, however, this makes reversing impossible.The first revision of the board used a BJT and obviously had a bit different circuit to drive its base. That didn't work, transistors just kept dying, they didn't overheat, didn't release any smoke, just all their pins short circuited between each other. In the schematic shown above I've also tried to install an IRF740 FET instead of an SCR and got the same story, it just quietly turns into a slightly warm jumper wire after a few seconds of operation.Any ideas how to make the circuit work? The only idea I currently have is to try to build an SCR-controlled rectifier instead of using a low-side switch but I'm just randomly trying everything at this point...

author: Benta
author_rank: Super Contributor
timestamp: October 19, 2023, 08:28:36 pm
content: As the type of motor seems to be a secret: impossible to comment.

author: moffy
author_rank: Super Contributor
timestamp: October 19, 2023, 09:46:11 pm
content: The inductance of the motor will resist current and for low phase angles the SCR might not latch or turn on reliably. The way to help the SCR is to send it multiple gate pulses close together not just a single pulse, a burst, that way if it doesn't latch on the first gate pulse the second one will be there etc.

author: diy_debik
author_rank: Newbie
timestamp: October 20, 2023, 09:44:06 am
content: Quote110V 2.9A brushed shunt-excited DC motorQuoteThe SCR turns on just fine, it doesn't turn off (apparently)

author: Psi
author_rank: Super Contributor
timestamp: October 20, 2023, 10:09:23 am
content: maybe try pull the SCR a bit negative to turn it off, instead of assuming it will turn off at zero crossing.

author: moffy
author_rank: Super Contributor
timestamp: October 20, 2023, 11:06:18 am
content: Quote from: diy_debik on October 20, 2023, 09:44:06 amToo much motor inductance, the current is continuing conduction into the next cycle, not passing through zero so the SCR stays latched. That's a problem with using rectified DC into an SCR you don't have a negative cycle to force turn off. A different type of switch like a MOSFET or IGBT might be better but that would require a redesign.

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: October 20, 2023, 11:38:52 am
content: I think I would use a FET or IGBT at a higher frequency (1-20KHz) PWM. I don’t see that zero cross buys you anything here.

author: Ian.M
author_rank: Super Contributor
timestamp: October 20, 2023, 11:39:54 am
content: What was the original board? I've got a sneaking suspicion that it was probably a KBIC clone*, which if adequately rated and properly set up for the application have pretty good low speed performance and are fairly robust, far more so than most MOSFET motor controllers of comparable specs. at a comparable price point. If so, its probably worth fixing it and if an Arduino 'in the loop' is still desirable, generating a control voltage for its speed input from an optoisolated PWM.* see:http://andysmachines.weebly.com/variable-speed-controls.htmlNote the use of two SCRs in a controlled full bridge rectifier, specifically to give enough dead time for commutation.

author: diy_debik
author_rank: Newbie
timestamp: October 20, 2023, 12:45:36 pm
content: QuoteI haven't really looked at it when I was building my controller... the whole drill press (and its control board) were made in low quantities for the factories that needed them, so it's pretty obscure. There's a weird mix of parts that were made from 60s to 90s, there's 3 custom transformers with a huge mess of wires, I really don't want to waste time trying to repair this...One thing I can tell for sure is that original board has a controlled mid-point rectifier, like this:Are there any advantages in this method over low-side switching?QuoteI did, initially. Tried IRF740 and MJE13007, both die with no smoke, with all their pins short circuited between each other. The protective diodes and RC snubber were present and did not help.

author: Zero999
author_rank: Super Contributor
timestamp: October 20, 2023, 01:01:19 pm
content: It's due to the inductance of the motor keeping the SCR on.How about using a TRIAC before the bridge rectifier?

author: Ian.M
author_rank: Super Contributor
timestamp: October 20, 2023, 01:32:51 pm
content: ... or adopt the dual SCR mid-point converter topology, which like the controlled full bridge I mentioned earlier, has the major advantage of allowing far more time for each SCR to commute, and reverse biassing it after commutation so it cant retrigger. The only issue is the gate drive needs to be relative to their cathodes, i.e. the motor positive, which is easiest to achieve with an isolated DC-DC converter off your logic Vin. Both gates can be driven with the same waveform - before commutation, it doesn't matter and after commutation a reverse biassed SCR cant trigger - so only the forward biassed SCR will trigger. Simply duplicate the circuit from your gate driver emitter to the SCR(s).

author: diy_debik
author_rank: Newbie
timestamp: October 20, 2023, 04:48:13 pm
content: QuoteLike this? The ground of 15V rail should be connected to the rectifier output?QuoteI've looked more closely at the original board and it has diodes in series with SCRs, as if the idea was to reduce reverse biasing, any ideas why?

author: Ian.M
author_rank: Super Contributor
timestamp: October 20, 2023, 04:59:13 pm
content: Yes, you are on the right track with the 15V supply and gate drive. That setup should work with your existing firmwareThe diodes were probably to allow cheaper SCRs with a lower peak inverse voltage rating to be used as the disadvantage of the midpoint converter is the CT transformer doubles the voltage the SCRs must withstand vs a controlled full bridge. The SCRs will still have *some* reverse bias due to diode leakage, reverse recovery time and junction capacitance

author: diy_debik
author_rank: Newbie
timestamp: October 22, 2023, 05:20:52 pm
content: The controlled rectifier is certainly a major improvement, thanks for the advice! The motor became a lot more stable at low speeds, however, it's now a bit unstable at high RPMs, as if there's very brief losses of power going on. Right now I'm blaming a very sloppy setup, the Arduino is hanging on 10 cm wires, picking all sorts of interference. I'll make a proper PCB, look more closely inside the motor, and post an update.Also when I connected the diodes in series with SCRs, the motor became even more unstable, it wasn't jerking but more slowly gaining and losing speed. Very weird...