DC motor driven with an AC square wave signal (±V)? - Page 1
Forum section: electronics / general

author: AWa.
author_rank: Newbie
timestamp: January 17, 2025, 04:24:21 pm
content: Hi,I saw a DC motor driver that uses as output 20 kHz AC square wave signal to drive the motor.At 50% (average voltage equal 0) the motor doesn't turn and current was limited by winding inductance at approximately 15% of nomina current.If proportion between positive and negative signal changes the motor start turn in some direction.What is the point of such control and what benefits can it bring?

author: IanB
author_rank: Super Contributor
timestamp: January 17, 2025, 04:47:02 pm
content: It would be a crude way of providing variable speed control with reverse (like pulse width modulation, but bi-directional).The idea is that the inductance of the motor windings would filter out most of the AC component and just pass the residual DC component that makes the motor move.But it's not very efficient, and will probably make the motor run hotter than with a normal DC supply.

author: thm_w
author_rank: Super Contributor
timestamp: January 17, 2025, 11:54:53 pm
content: Does it provide some kind of holding current? More than shorting the windings.Thats all I can think of, otherwise better to use PWM.Do you have a photo, part number or anything.

author: langwadt
author_rank: Super Contributor
timestamp: January 18, 2025, 12:15:41 am
content: Quote from: thm_w on January 17, 2025, 11:54:53 pmit provides fast vs. slow decay of the motor current, so if affects the motor response

author: tom66
author_rank: Super Contributor
timestamp: January 18, 2025, 12:57:06 am
content: This is actually how many cheap class D amplifiers work, using the loudspeaker as the filter inductor. It's generally inefficient for any significant power level, and has high EMI risk, but it can be reasonable enough for a low power application.https://www.ti.com/lit/an/sloa145/sloa145.pdf?ts=1737085943096

author: langwadt
author_rank: Super Contributor
timestamp: January 18, 2025, 01:43:46 am
content: Quote from: tom66 on January 18, 2025, 12:57:06 amand how just about every stepper, AC, DC, or brushless, motor is driven

author: SteveThackery
author_rank: Super Contributor
timestamp: January 18, 2025, 10:47:43 am
content: Quote from: langwadt on January 18, 2025, 01:43:46 amEh? I'm confused. The OP is describing aDCmotor being fed withAC. They weren't talking about stepper, AC, or brushless motors.Obviously DC motors are frequently fed with pulsating DC, but I've never heard of feeding a DC motor (presumably permanent magnet, brushed) with AC.

author: Benta
author_rank: Super Contributor
timestamp: January 18, 2025, 11:28:27 am
content: Sounds more like a servo drive to me.

author: langwadt
author_rank: Super Contributor
timestamp: January 18, 2025, 01:14:20 pm
content: Quote from: SteveThackery on January 18, 2025, 10:47:43 ampulsating DC from both ends of the motor, the voltage across the motor is AC

author: tom66
author_rank: Super Contributor
timestamp: January 18, 2025, 02:55:21 pm
content: Quote from: langwadt on January 18, 2025, 01:43:46 amTrue, though stepper and brushless motors usually are only driven when they need to move. The drive signals will be open circuit (or shorted if braking) otherwise.

author: langwadt
author_rank: Super Contributor
timestamp: January 18, 2025, 03:01:32 pm
content: Quote from: tom66 on January 18, 2025, 02:55:21 pmsteppers are driven all the time whether they are moving or holding, same for brushless in a servo

author: tom66
author_rank: Super Contributor
timestamp: January 18, 2025, 06:12:38 pm
content: Quote from: langwadt on January 18, 2025, 03:01:32 pmI disagree. You don't have to drive a stepper all the time, but you will be able to freely move it if the drive signals are removed. Whether that is an issue for the application depends on that application - it's not desirable for a 3D printer bed, for instance. But once the print has finished, most firmware will turn off the steppers to allow the print bed to be manually moved.

author: langwadt
author_rank: Super Contributor
timestamp: January 18, 2025, 08:33:59 pm
content: Quote from: tom66 on January 18, 2025, 06:12:38 pmsure just like you don't have to power a light bulb all the time, you only need only need power it when it has to do what it is meant to doyes, you can move a belt drive machine with the steppers off, just keep in mind it pushes energy into the supply

author: AWa.
author_rank: Newbie
timestamp: January 19, 2025, 08:12:03 pm
content: Quote from: tom66 on January 18, 2025, 12:57:06 amHi,Not exactly, in my system Out+ is always the negation of Out-

author: AWa.
author_rank: Newbie
timestamp: January 19, 2025, 08:14:14 pm
content: Quote from: SteveThackery on January 18, 2025, 10:47:43 amYes, this is a permanent magnet brushed motor and it works with this control.I just don't understand why they control it this way.

author: IanB
author_rank: Super Contributor
timestamp: January 19, 2025, 08:37:46 pm
content: Quote from: AWa. on January 19, 2025, 08:14:14 pmThere are various answers in this thread. Do you have a better understanding after reading those answers, than before? If not, what questions remain?

author: AWa.
author_rank: Newbie
timestamp: January 21, 2025, 08:43:25 am
content: Quote from: IanB on January 19, 2025, 08:37:46 pmI have not found any rational justification for this type of control.

author: SteveThackery
author_rank: Super Contributor
timestamp: January 21, 2025, 01:34:43 pm
content: Quote from: AWa. on January 21, 2025, 08:43:25 amI agree: the question has not been answered. It seems a crazy way to drive a DC motor.

author: IanB
author_rank: Super Contributor
timestamp: January 21, 2025, 01:50:24 pm
content: Quote from: AWa. on January 21, 2025, 08:43:25 amSo then, provide better information, rather than asking everyone to guess. Where did you see this driver? In what context? What was its purpose? What was the intended application? You can't just "see" something without context, like it just fell out of the sky and landed at your feet.I gave you one possibility above, that it would allow the motor to have variable speed and directional control, perhaps with the ability to have some holding torque, which suggests possibly a servo application. But since you don't say where you saw this thing, it can only be speculation.

author: AWa.
author_rank: Newbie
timestamp: January 21, 2025, 03:01:39 pm
content: Quote from: IanB on January 21, 2025, 01:50:24 pmI can't give any more precise details.Standard DC motor control, speed and direction, blockage detection.I think we can close this thread.Thanks to everyone for trying to figure this out.

author: jmelson
author_rank: Super Contributor
timestamp: January 22, 2025, 05:59:27 pm
content: This is called synchronous antiphase. The motor, due to inertia, averages out the DC component of the square waves sent to it. If the positive part of the wave is greater than the negative part, the motor develops torque in one direction. The big plus of this scheme is that there is no dead spot near the servo null position, as you might get with some sort of sign/magnitude drive scheme. However, it does cause a lot of heating in the motor windings and armature iron, as well as the power transistors. Many commercial servo drives needed to add a series inductor in one of the motor wires, to combat this. Also, lots of potential for EMI.Jon