Practical limits of AC universal motor speed control? - Page 1
Forum section: electronics / projects

author: e100
author_rank: Frequent Contributor
timestamp: December 02, 2022, 05:46:21 am
content: I have a 30 year old sewing machine that has a 240vac 90w brushed motor that I presume is of the "universal type". Speed control at the low end is imprecise.I'm guessing that it basically has the same design as a modern Dremel tool that also suffers from poor speed control at the low end. There is no 'super slow' mode, instead there is 'off', 'moderately fast' and various degrees of faster than that.If I were to add a rotational speed sensor to create a closed loop system would it be possible to improve the speed control at the low end, or am I fighting a losing battle due to the fundamental properties of universal motors and the limitations of triac and resistor control? Would using a variac make a difference?

author: Benta
author_rank: Super Contributor
timestamp: December 02, 2022, 10:54:59 am
content: Closed loop control would certainly improve the situation.An industry standard IC for this was for many years the TDA1085C, unfortunately obsolete.Check the datasheet, this might give some inspiration.

author: Circlotron
author_rank: Super Contributor
timestamp: December 02, 2022, 11:38:17 am
content: Sewing machine speed control at very slow speeds with a universal motor is absolutely awful. Press the pedal more and more to get started and finally gets going and suddenly runs way too fast. A very useful project would be to replace the existing motor with a stepper motor and suitable controller. No speed sensor needed. That would make all the difference in the world. You could inch along very steadily when necessary. Starting gradually would be a breeze.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 02, 2022, 11:44:51 am
content: Quote from: e100 on December 02, 2022, 05:46:21 amOf course, and significantly so.QuoteYou likely get a big improvement even with crappy triac controls, but if you want to do even better, you would rectify (and smooth with capacitors) the supply, because as the name suggests, universal motors work with DC. This removes the 100/120Hz torque ripple component. You can then buy or design an actual DC motor controller, which consists of a pulse width modulator running at say >10kHz, high bandwidth current sense for instantaneous motor current, and well tuned very quick feedback loop (hysteretic or PI loop) so that motor torque is always well constrained and under control. Then the speed controller will just control the current setpoint, and the loops work in cascade.Getting the speed control loop right for very slow speeds may be challenging, though. Having enough resolution in the encoder is a must, because if you get a pulse (position update) only every 100 milliseconds, it's hard to not make the loop oscillate. Combination of P, I, D and feedforward terms help.

author: mikerj
author_rank: Super Contributor
timestamp: December 02, 2022, 12:02:03 pm
content: The problem is the change in friction with the machine stopped and running. The speed of a universal motor depends on current and torque (load), so when you start the machine you ramp up the current until it starts turning, but as soon as it starts turning that drops the load on the motor so it quickly speeds up. Closed loop control is the only way to significantly improve this with a universal motor. Industrial sewing machines use AC servo motors these days which offer very precise speed control. You can buy import motors and controllers quite inexpensively to upgrade older industrial machines, but if you have a commercial machine with a built in motor that may not be possible.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 02, 2022, 02:01:20 pm
content: Quote from: Circlotron on December 02, 2022, 11:38:17 amBrushed motor with closed loop PWM is more than enough for hobby use.

author: David Hess
author_rank: Super Contributor
timestamp: December 02, 2022, 10:54:23 pm
content: On my old sewing machine the bobbin winder pulls out to engage a gear reduction in the motor for low speed high torque operation.What I think would work is to disconnect the field winding and drive it separately. They make controllers for field controlled DC motors.

author: Benta
author_rank: Super Contributor
timestamp: December 03, 2022, 12:28:29 am
content: Quote from: David Hess on December 02, 2022, 10:54:23 pmInteresting new approach. How would you do that on a universal motor? Field and armature are normally in series,

author: mag_therm
author_rank: Frequent Contributor
timestamp: December 03, 2022, 01:39:24 am
content: Quote from: Benta on December 03, 2022, 12:28:29 amI made a toolpost drill spindle in 2016 like that.Here is a dir with photos, circuit, and graphshttps://app.box.com/s/bqos9olnhxqp00zpskhr57fkl4ygupvg

author: james_s
author_rank: Super Contributor
timestamp: December 03, 2022, 01:46:52 am
content: The older ones at least just used a variable resistor to control the motor speed. I bet you would get a significant improvement just switching to a PWM control even without closed loop.

author: David Hess
author_rank: Super Contributor
timestamp: December 03, 2022, 04:10:58 am
content: Quote from: Benta on December 03, 2022, 12:28:29 amYou have to take the motor apart and separate them where they are connected, adding two wires. If the motor has 4 wires then they are externally connected.Quote from: james_s on December 03, 2022, 01:46:52 amMine is like that, however I thought those were field controlled. A rheostat would dissipate an incredible amount of power to control a series wound motor.

author: james_s
author_rank: Super Contributor
timestamp: December 03, 2022, 08:46:18 am
content: Quote from: David Hess on December 03, 2022, 04:10:58 amThe motors don't draw very much power in the first place and they're typically low duty cycle.