AC Bench Grinder Motor Getting Hot - Question About Why - SOLVED - Page 1
Forum section: electronics / projects

author: bostonman
author_rank: Super Contributor
timestamp: April 20, 2025, 12:32:03 pm
content: I have an 120V AC bench grinder, it's junk; doesn't have the shields or tool rest. It was being tossed at an old job so I grabbed it. My point is that I'm not desiring to fix this necessarily, but wanted some technical feedback for the symptoms it has.Normally I've used it for some simple quick grinding even though it doesn't have a tool rest. Recently I ran it for about ten-minutes and noticed it got extremely hot to touch, would labor to grind, and then take time to build speed. At one point after getting very hot it wouldn't rotate after turning on power.Yesterday I disassembled it to check if the bearings were possibly frozen causing the motor to struggle. The bearings spin fine and there isn't much to the motor, just the rotator (or is it an armature even though it doesn't have the windings) and the stator. Not sure if this is considered brushless since it has a stator and no brushes.I'm assuming the previous owner jammed the grinding wheel and the motor "burned".My question is: what causes the motor to get hot and/or lose power over time, would the wire in the stator change resistance or is it something more mechanical like the magnets on the armature wear out causing a lack of magnetic field?

author: WattsThat
author_rank: Frequent Contributor
timestamp: April 20, 2025, 12:39:03 pm
content: Since its 120v single phase and you said it had no brushes, it has to have at least a capacitor for starting and or running, depending upon design, size, etc.The capacitor(s) are likely bad or the centrifugal switch that disconnects the start capacitor once the motor is up to speed is not functioning correctly. The switch is typically on the non-drive end of the motor shaft.Look for a lump on the top/side of the motor, the capacitor(s) are inside that lump.

author: bostonman
author_rank: Super Contributor
timestamp: April 20, 2025, 12:59:16 pm
content: QuoteWow, I didn't think of the startup capacitor. My initial thought were bearings, then I assumed the wires in the stator were getting hot and thought: the only reason they'd get hot would be due to changing resistance, but didn't think that's possible.I found the parts list and the startup capacitor is in the base. I found an image of the capacitor online, but let me see if I can see the value/voltage on this one providing it's not burnt.If hypothetically it isn't the startup capacitor, what else would go bad in a motor to make it labor like that (I'm asking for learning purposes)? Could the coil actually change resistance and/or the magnets wear out over time?

author: wilfred
author_rank: Super Contributor
timestamp: April 20, 2025, 01:06:26 pm
content: Does the motor have an axial fan to pass cooling air? It could be running slower than normal because of the capacitor or bearings. Or it may be clogged up blocking the cooling airflow.

author: bostonman
author_rank: Super Contributor
timestamp: April 20, 2025, 01:10:57 pm
content: The fan is mounted on the armature, so it spins at the same rate.There isn't much to it after removing the covers. Just the bearings on each side and the armature slides out. Inside has the stator; and now I learned the capacitor is in the base.I also blew out the unit with air and very little dirt came out.

author: moffy
author_rank: Super Contributor
timestamp: April 20, 2025, 01:17:22 pm
content: Most likely it's the starter cap, not wanting to turn and getting hot, like a stalled motor if the cap and winding can't produce enough torque.

author: WattsThat
author_rank: Frequent Contributor
timestamp: April 20, 2025, 01:25:36 pm
content: If the caps(s) are okay, the obvious problem, since you said the guards and rests were gone, is an oversized abrasive wheel having been mounted that presents more inertia which requires more torque than is available from the motor.Checking amps with an empty shaft would be a good starting data point.

author: WattsThat
author_rank: Frequent Contributor
timestamp: April 20, 2025, 01:40:10 pm
content: A asynchronous motor has no permanent magnets, it is an electro-magnet created by the windings and the iron laminations of the stator - which induces current into the rotor which is also a stack of laminations - which has a very low impedance winding contained within. The rotor windings (as it were) are usually cast aluminum with the individual bars shorted together at both ends. This pattern of bars with the rings on the ends is where the nickname of “squirrel cage motor” originates.Those rotor bars and shorting rings can crack and go high resistance or worse, open circuit, due to age, physical shock and other factors that lead to issues like low torque or the inability to start.One analogy of an asynchronous motor is that of a transformer where the secondary is shorted with it rotating about the primary.

author: Gyro
author_rank: Super Contributor
timestamp: April 20, 2025, 01:53:09 pm
content: If it is lacking torque while running and "would labor to grind", you need to look at the run capacitor rather than the start capacitor (if present). This will be a film type, not a bipolar electrolytic. It's important not to confuse the two.EDIT: Yes, looking at your parts list, it is a film type run capacitor. I have no idea why it says 'obsolete, no replacement available' - you need to look for a polypropylene film cap of the same value and voltage rating, taking care to make sure it will physically fit, you do find different form factors.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: April 20, 2025, 02:23:29 pm
content: (Especially) the "cheap" bench grinders have a low ED, and are only meant to be turned on for low durations. An ED as low as 20% is (unfortunately) quite common for these things. This means you can use it for 5 minutes, and then have to let it cool down for 20 minutes. If you use it under "full" power for 10minutes, it's quite possible you've overheated and damaged the motor yourself.Also note that normal induction motors always have a "working axle" on one side, and a fan on the other side to keep the motor cool, but bench grinders usually have a wheel on each side, and no fan at all, and neither do they have the cooling ribs found on most motors.

author: bostonman
author_rank: Super Contributor
timestamp: April 20, 2025, 02:44:03 pm
content: I'll look today, but for $15 or so dollars for a new cap, the grinder isn't worth putting much into it because (as mentioned initially) it's missing the tool rest, shields (which probably breaks off anyway), and, until now, didn't think it had much power.What is "ED"?

author: langwadt
author_rank: Super Contributor
timestamp: April 20, 2025, 02:52:46 pm
content: Quote from: bostonman on April 20, 2025, 02:44:03 pmduty cycle, I believe ED comes from "Einschaltdauer" ~on time

author: bostonman
author_rank: Super Contributor
timestamp: April 20, 2025, 10:44:51 pm
content: I feel silly because I should have known the grinder has a capacitor.As everyone suspected, the capacitor is junk. The end looks like popcorn, it's cracked, and even let out some smoke when I ran it without the cover.Per the diagram on the parts document and visual inspection, it doesn't have a run capacitor or a centrifugal switchThe marking on it is: ADM TYPE, 10uF, +/- 10/5 (I assume +10% and -5%), 250V, temp range, frequency (50/60Hz), AID FLE [can't make out the rest because it has gunk over it], and a backwards R and backwards L (I assume the company logo).I didn't plan to fix this since it doesn't have a tool rest or shield, but throwing away stuff that can be fixed feels like a sin. Maybe it will be worth investing a few bucks for a replacement capacitor.As mentioned from someone, this is suppose to be a polypropylene film cap. I don't know much about "carefully" selecting capacitors besides buying typical axial electrolytics, so want to confirm since this is a high voltage capacitor.Can I buy any polypropylene film cap from DigiKey (obviously providing it meets the voltage, temp rating, and dimensions), and believe this is non-polarized, correct?

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 20, 2025, 11:30:44 pm
content: Just search for any 10uF motor run capacitor, perhaps go for a higher voltage rating so that it's operating under less stress.

author: bostonman
author_rank: Super Contributor
timestamp: April 21, 2025, 03:12:01 am
content: Looks like Home Depot sells 10uF 450V capacitors - oval shape.Well, maybe not "sell", but they can be ordered and shipped to the store for free. I might try one of those, they are only $10, so it saves ordering online and paying shipping.

author: bostonman
author_rank: Super Contributor
timestamp: May 01, 2025, 01:27:43 am
content: I ordered a capacitor from "Walmart" and it's still waiting to be shipped. The price was $11 I think with free shipping. Physically it's a bit larger, but should fit fine.Upon realizing AC motors have a startup capacitor (and run capacitor), I began thinking more about the 90 degree phase shift in motors.Unless I'm wrong, some motors don't have startup capacitors such as hand drills, hedge trimmers, etc... Working on this, earlier I decided to measure the phase shift on various power equipment (see attached files).First I established a baseline with a 100W light bulb to confirm voltage and current are in phase (although I didn't save the file until the end of my tests). Next I measured a hand drill at half speed and full speed. This didn't give the results I expected since it didn't have a 90-degree phase shift; but does imply it's going through a TRIAC.The drill press and band saw were as expected (although not a perfect sine wave - but this is probably the inductance of the motor). The shop vac and mitre saw didn't give me the expected results.If a single phase motor needs a 90-degree phase shift, how are certain motors starting without a capacitor such as the shop vac and probably the hand drill? Also, why is the mitre saw in phase (technically different by a few degrees, but not 90)?

author: Ian.M
author_rank: Super Contributor
timestamp: May 01, 2025, 01:56:31 am
content: That's because many tools intended for intermittent use don't use induction motors.https://en.wikipedia.org/wiki/Universal_motor- has brushes, easy to speed control.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: May 01, 2025, 02:39:43 am
content: Quote from: Ian.M on May 01, 2025, 01:56:31 amYou can even run aninductionuniversal motor on DC, so using an SCR as a controlled rectifier to vary motor speed will do it too.

author: Analog Kid
author_rank: Super Contributor
timestamp: May 01, 2025, 02:43:53 am
content: Quote from: timeandfrequency on May 01, 2025, 02:39:43 amDon't you mean a universal motor rather than an induction motor?They're called "universal" because they'll run on both AC and DC. They're the ones with commutators and brushes.Trying to run an induction motor on DC will only turn the windings into a heater.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: May 01, 2025, 03:30:36 am
content: Quote from: Analog Kid on May 01, 2025, 02:43:53 amYes of course, I corrected my post.Thanks.

author: bostonman
author_rank: Super Contributor
timestamp: May 02, 2025, 02:53:24 am
content: Think I need to learn much more about motors than initially thought and will read more.The most simple question regarding my waveform captures is due to never measuring current on a scope with a clamp on current meter; or really never needing to pay attention to such detail because I'd just assume the odd shaped sine wave was just due to inductance and AC noise.Referring to the mitre saw waveform, at the zero crossings points, the slope has a slow rise and then is more vertical; same with the falling side.The mitre saw doesn't have speed adjust (just on and off), so it probably doesn't have a TRIAC. If it were the inductance of the motor windings, I'd expect overshoots and undershoots. What exactly is going on with the slow ramps?As for the other waveforms, I guess maybe more reading is needed on my part but I've started off a bit confused from what I've already read. From what I can tell though, the grinder (as I already know) has a single capacitor (not sure if this is considered a startup capacitor or run capacitor). The bandsaw and drill press doesn't appear to have a capacitor (but maybe it's somewhere I haven't seen yet). Also, both the bandsaw and drill press appear to have similar 1725RPM motors and both are approx. 10A each (the drill press doesn't have a label but I've measured the RPMs and the current is close to the bandsaw).In all three cases, the motors (I believe) are brushless, and, if the bandsaw and drill press don't have capacitors (again, I could be wrong), then why does the grinder need one?

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: May 02, 2025, 02:20:55 pm
content: Hello bostonman,Without snapshots of your waveforms, brand and model number of your tools + pictures of the motor plates, providing relevant and fair answers is probably doomed to failure.Not to mention thattothe topic expanded from a grinder to a whole collection of toolsQuote from: bostonman on May 02, 2025, 02:53:24 amBecause it is likely not the same kind of motor.

author: bostonman
author_rank: Super Contributor
timestamp: May 02, 2025, 03:21:06 pm
content: QuoteI thought the power tools all used similar motors and/or configurations.QuoteI didn't mean to. It was more of a case where I learned /remembered/realized the motor has a capacitor and then expanding my curiosity to the other power tools.Growing up I always had DC motors as toys (although I had an Erektor set with an AC motor - which I still have). Sometimes I'd pull them apart and see the brushes and armature. This has resulted in thinking only one type of motor exists and/or visualizing all motors to have brushes.Now I'm trying to get out of that mental state so I can understand motors a bit better.

author: Analog Kid
author_rank: Super Contributor
timestamp: May 02, 2025, 05:09:02 pm
content: Quote from: bostonman on May 02, 2025, 03:21:06 pmNope.Generalizing here:High-speed and variable-speed tools (routers, electric drills (but not drill presses), sawzalls, circular saws (portable), Dremels, etc., use universal (AC/DC) motors with brushes, though newer ones are using brushless motors.Larger fixed-base tools--drill presses, table saws, jointers, planers, bandsaws, lathes--use (usually) fixed-speed induction motors, which may or may not have start & run capacitors. (AC-only.)

author: seanf
author_rank: Contributor
timestamp: May 02, 2025, 06:29:01 pm
content: Electrical Machines, Drives, and Power Systems by T. Wildi, has a bit on motors.

author: bostonman
author_rank: Super Contributor
timestamp: May 07, 2025, 02:58:39 am
content: I installed the new capacitor and the grinder seems to be working much better.I haven't had a chance to fully test it, but it starts faster and the motor doesn't change pitch.Thanks for the help!

author: WattsThat
author_rank: Frequent Contributor
timestamp: May 07, 2025, 11:30:25 am
content: A happy ending! Thank you for posting.

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: May 07, 2025, 12:00:15 pm
content: Hello bostonman,Great success !Yoy may now update the title of this post and add as prefix or suffix the word we want to see as much as possible :

author: bostonman
author_rank: Super Contributor
timestamp: May 07, 2025, 12:50:45 pm
content: Good call and I just did.

author: bostonman
author_rank: Super Contributor
timestamp: May 08, 2025, 12:32:47 am
content: BTW, I had one other question.Since this grinder has only one capacitor, is this considered a start capacitor, run capacitor, or another name?

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: May 08, 2025, 02:40:29 pm
content: Quote from: bostonman on May 08, 2025, 12:32:47 amGenerally speaking :A start capacitior is connected to the windings for a few seconds, only during startup.A run capacitor is connected as long as the motor is energized.

author: bostonman
author_rank: Super Contributor
timestamp: May 09, 2025, 12:47:53 am
content: Ah perfect.Now at least I can (hopefully) use the correct terminology going forward.