Need suggestions ideas for universal motor current sense circuit block - Page 1
Forum section: electronics / projects

author: jakeisprobably
author_rank: Regular Contributor
timestamp: March 22, 2021, 03:04:33 am
content: I came across this video from Mr. Johnnny today:I copied his circuit and attached it. I'm working on a similar circuit that uses a current shunt to regulate the motor speed instead of the tacho signal used in this circuit. Does anyone have a simple discrete current feedback block that would work well as a replacement for the tacho feedback here? For my current project I'm stuck waiting on extra U2010B's to ship from China. US stock seems to be nonexistent. I would probably attempt to etch something simple like Mr. Johnnny's circuit if I had a compatible circuit to work with. I'm like 7 levels down a rabbit hole for this project already, and too deep to take a really deep dive and have any chance of completing my string of projects. I'm curious if someone has a block they use for this in their bag of tricks or ideas to try. The only way I know how to do this is with an op amp, but I don't know a good way to use a jellybean like a LM358 with mains AC in a circuit like this, and ordering parts defeats the point.-Jake

author: moffy
author_rank: Super Contributor
timestamp: March 22, 2021, 08:32:06 am
content: Current in a motor is related to force applied or load, not speed. If you are driving a fan then load/current is roughly proportional to the square of speed.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 22, 2021, 08:43:21 am
content: Old-school triac control works and is still used, but in my opinion, if you go through the struggle of designing your own, I recommend something that performs way better and isn't any more complex to design.Rectify the AC, filter with a cap for smooth DC.Because an universal motor works on DC (even a tad better than on AC), build a DC motor controller: this is a half-bridge where one of the switches can be a diode, another will be a MOSFET.Measure current (typically from the low leg of the bridge, GND side).Use high-frequency (say, 5-10kHz) PWM. Regulate current using fast (aka pulse-by-pulse) current limit.Low-ESL DC link capacitance and tight bridge layout is of some importance.Advantages:* Little torque ripple* No sharp 60Hz harmonics buzz* Torque can be accurately controlled (speed control loop comes on the top, i.e., controls the torque setpoint)* Somewhat better efficiency than from triac-controlled AC.* Current never exceeds the limit, not even during start-up or stall; maximized efficiency ramp-up and operation to varying loads.* Everything you learn can be applied directly to "classic" permanent magnet brushed DC motors, and works also as a fundamental background for BLDC motor control.

author: Terry Bites
author_rank: Super Contributor
timestamp: May 10, 2021, 06:35:23 pm
content: how about:AMC1200seehttps://www.industrial-electronics.com/emct_2e_4b.htmlTriacs Arrrgh!

author: Benta
author_rank: Super Contributor
timestamp: May 10, 2021, 07:01:19 pm
content: @jakeisprobably:I think you need to read up on "Universal Motors 101".There isno wayto control universal motor speed by sensing the current. The tacho generator (or an optical speed sensor) is there for a reason.An unloaded ideal universal motor will theoretically spin up to self-destruction, no matter if you sense the current or not.The old (obsolete) Motorola TDA1085C should give you an idea on how it's done correctly:https://www.alldatasheet.com/datasheet-pdf/pdf/5758/MOTOROLA/TDA1085C.html