What kind of motor is this? Meant for AC, but can be driven with DC? - Page 1
Forum section: electronics / beginners

author: help_me_pick_username
author_rank: Contributor
timestamp: December 23, 2019, 03:12:48 am
content: I took apart an old vacuum cleaner today. I found a tiny PCB with a triac and a DIP8 IC (couldn't find datasheet) that I'm guessing is some sort of MCU for turning off the motor if it's been running for too long? (Does that seem like a plausible explanation?).Anyway, I found a motor. It's pretty big and heavy, I've attached some pictures.I measured the resistance across contacts 1 and 4 (these were the two contacts that were connected to the PCB), and got 1.2Ω. I decided to power it with my DC bench power supply and see what would happen. To my surprise, at about 6.5v, the motor started spinning, drawing about 1.2A. I kept raising the voltage and the current remained constant at about 1.2A, and the motor kept spinning faster. The highest I went was 32v, which was the highest my PSU went, and it was still drawing 1.2A, and the high pitched whine was annoying, so I didn't run it for very long.I don't know how the two contacts of either side are connected (continuity tester said each pair was connected, with a few ohms of resistance, but this could just be the coils, so I don't know), but the motor only worked when powered through contacts 1 and 4. I tried to power through 1 and 2, but it didn't run. Another thing to note is that the motor only runs in one direction. If I flip the DC voltage on 1 and 4, it still spins the same way.So I have a few questions. What type of motor is this? How is it running on DC when it is meant for a much higher AC voltage? There are two brushes and a commutator, I thought that was a brushed DC motor thing?I was wondering if the two coils on either side of the motor (the ones connected to contacts 2 and 3) were acting as permanent magnets, thus creating the same design as a brushed DC motor...? Is that even possible?Since this is a pretty large motor, will I have to worry about damaging my power supply while driving it? Inrush current shouldn't be an issue since my PSU has current limiting, but what about when turning off my power supply? Will I have to worry about the negative voltage spike damaging my PSU? I watched this video by ElectroBOOM:https://www.youtube.com/watch?v=yO9xIVv8ryc&vl=en.In it, he solves the negative voltage spike by placing a diode across the contacts. Should I do this when powering this motor with my PSU?If I have made some incorrect assumptions in this post, please feel free to correct me, I do not have much experience when it comes to AC or DC motors.

author: Nusa
author_rank: Super Contributor
timestamp: December 23, 2019, 03:19:11 am
content: https://en.wikipedia.org/wiki/Universal_motor

author: james_s
author_rank: Super Contributor
timestamp: December 23, 2019, 06:48:20 am
content: Universal motors are ubiquitous in domestic vacuum cleaners, food mixers, blenders and various other things that need a powerful, compact, cheap motor that does not require a particularly high duty cycle.They will typically run waaaay below rated voltage. Not as powerful as on the full rated voltage of course but most will spin on 5% of rated voltage.

author: help_me_pick_username
author_rank: Contributor
timestamp: December 23, 2019, 08:10:41 am
content: Since the motor is rated for 240v AC, does that mean I could (theoretically) run it at 240v DC?

author: Zero999
author_rank: Super Contributor
timestamp: December 23, 2019, 08:22:44 am
content: Quote from: help_me_pick_username on December 23, 2019, 08:10:41 amYes it should be fine off 240VDC. I've tested one before running from a bridge rectifier connected to the mains, with no smoothing capacitor. A smoothing capacitor would increase the average voltage towards the mains peak voltage of 340V, which would cause the motor to run faster and hotter, than what it was designed for.Often a TRIAC is used to switch the motor, so the on/off switch can be smaller. Did you get the part number for the 8 pin DIP? Perhaps someone here can find the data sheet. I could also be an opto-isolator.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 23, 2019, 08:23:28 am
content: Series-wound brushed DC motor. Yes, you can run it at around 240V DC, but a slightly less will probably do the same work with a bit better efficiency, because it will run a bit better on DC.A bit of background:An electric motor works by providing magnetic fields that pull each other together, or push each other apart. A motor has two parts, rotor and stator; both provide magnetic fields that repel of attract each other. By changing one of the fields with the correct timing, the rotation can go on and on.A permanent magnet DC motor uses magnets to generate a constant field, usually the magnets are on the stator. In the rotor, windings generate the changing field; the brush assembly is responsible for providing the correct timings, energizing the windings in sync with the actual position. Now, if you reverse the terminal connections, the direction and the sequence of the changing rotor field swaps, and the motor runs the opposite way.A series-wound brushed DC motor, on the other hand, does not use permanent magnets; both parts of the motor use windings. Furthermore, the windings in stator and rotor are electrically connected in series, so that the same current flows through both. Now, if you reverse the applied polarity, both magnetic fields swap their polarities, and the net result is the same as before. Such motors are widely used in older-generation electric vehicles (up to hundreds of kW), but also in practically all power tools, vacuum cleaners, etc.Of course, when the direction of rotation is the same, swapping the polarity at 50 or 60 Hz just means high current ripple, causing torque ripple. These machines run smoother and better on DC, but they are designed to be "good enough" with direct AC input.Why use brushed motors? The answer is simple; single-phase mains-driven 50 or 60Hz AC induction motors totally and utterly suck! Their power / weight ratio is disastrously bad, and the torque / weight ratio is even worse, torque vs. rpm characteristic means that they only provide torque near the rated full rpm, so can't be used in power tools where the mechanical load may cause the tool to run at reduced rpm; they "stall" and stop working.If you need any torque out of an electric motor, you want to have at least two phases, i.e., dimensions of the magnetic field vector. An actual single phase machine is unable to start if it happens to be in the unfortunate position. In a 50/60Hz single phase ACIM, which actually internally is a two-phase machine, the second phase is generated using a capacitor, and it will never be at optimum 90 degree angle, and will be weak.A vacuum cleaner with a 1 kW AC induction motor wouldn't be very portable, neither in size nor weight. A drill weighing 5kg is impossible to use.A brushed motor isinternallyan AC motor, in the end; the windings are switched on and off to provide alternating fields. The designer can use any arbitrary number of "phases". The brushes are an old-school variable frequency drive, which apply correctly timed currents in the windings. This means, a brushed motor can use higher rotating magnetic field frequency, and are not limited to 50 or 60 Hz, so they are able to get beyond the 3000 rpm / 3600 rpm top limit.Using higher rotating field frequencies means delivering the same power with lower magnetic field density, meaning less iron material needed for the same output power. It's the same reason why high-frequency SMPS supplies are used today instead of 50/60Hz transformers.Of course, using carbon brushes is a very old-school way to do this, but it works, so whatever.Expect to see a gradual change to electrically commutated permanent magnet AC motors, also called "BLDC", even in the cheapest things. At some point, the required MOSFETs, position sensors and driver ICs are cheaper than the carbon brush and copper commutator assembly.

author: james_s
author_rank: Super Contributor
timestamp: December 23, 2019, 07:17:13 pm
content: Yes they will run on DC, in fact they will run slightly more efficiently. I had a food mixer that touted that it contained a "Integrated Circuit"! Turns out it had a bridge rectifier, I guess that's technically an IC.I equate universal motors to 2-stroke gasoline engines. They're cheap, compact, light weight and very powerful but they're also noisy and rather crude things.

author: help_me_pick_username
author_rank: Contributor
timestamp: December 26, 2019, 09:36:36 am
content: Quote from: Zero999 on December 23, 2019, 08:22:44 amI have attached a picture of the control PCB. The text on the IC reads 501B-8P, 173KNHZ0A . e3.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: December 26, 2019, 09:41:42 am
content: Fun fact. Rock band AC/DC was so named because of a vacuum cleaner, apparently, due to it's anti Tesla/Edison preference.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 26, 2019, 10:00:05 am
content: When running power tools or vacuum cleaners off a DC supply, do remember that any mechanical switches must be bypassed because the contacts weld stuck on DC. Any electronic speed controllers must be bypassed, as well.

author: james_s
author_rank: Super Contributor
timestamp: December 26, 2019, 04:52:30 pm
content: Quote from: Ed.Kloonk on December 26, 2019, 09:41:42 amI thought it was a radio?The classic "All American Five" radio design with 5 tubes having their filaments wired in series and no power transformer would run just fine off AC or DC and was typically labeled as such.