For those with experience in AC motors, what is the reason of this noise - Page 1
Forum section: electronics / repair

author: highcap
author_rank: Contributor
timestamp: September 26, 2024, 08:45:07 am
content: Bought this old bench grinder and after connecting it to power (single phase), this is what happens when connected (video attached).The motor is lacking a start capacitor so it needs a push. It makes that noise when connected, which otherwise disappears at power disconnection.Other than that, it has a few issues like slightly worn bearings (which I cleaned of old grease and re-greased) and a slightly bent shaft in the grinding stone area (probably from a drop on the stone from an angle).Electrically, there seems to be two identical windings with 3 wires each, coming from the left and right side of the winding, as seen from the top of the power switch box.One winding was not connected. The one which is connected to the power switch, measures like this:- Brown to Black 1nd = 28 Ohms- Brown to Black 2nd = 28 Ohms- Black 1nd to Black 2nd = openI have only connected Brown with either one of the Black wires.Video where the noise can be heard:https://youtube.com/shorts/7nsSNtZ2pHwAnd pictures attached.Thank you for any inputs!

author: CaptDon
author_rank: Super Contributor
timestamp: September 26, 2024, 01:19:46 pm
content: Was that thing dropped on a hard surface? It looks like the armature has a big scuff mark from rubbing on the stator. That would sure make some noise!! I have had some damaged motors and some with badly worn bearings/bushings that allowed the stator to scuff due to magnetic pull, but when power was removed the motor quietly coasted down.

author: highcap
author_rank: Contributor
timestamp: September 26, 2024, 01:27:21 pm
content: So this suggests the bearings are off the charts by enough to make the rotor being pullet onto the stator when plugged to power?The noise in the video clearly suggests some mechanical friction during powering up.I don't know it's history, but most than likely it was dropped, due to the (slightly) eccentric movement of the grinding stone.

author: BrokenYugo
author_rank: Super Contributor
timestamp: September 26, 2024, 01:39:44 pm
content: I'd guess between the 220Δ mark and three contact switch this is a three phase motor.

author: factory
author_rank: Super Contributor
timestamp: September 26, 2024, 06:16:08 pm
content: Quote from: BrokenYugo on September 26, 2024, 01:39:44 pmI think your right, also there are three sine waves shown on the spec. plate, I wonder where the cloth yellow wire goes after the switch? The other ends of the windings (cloth wires) appear connected together.Not sure I would trust a grinding wheel that might have been dropped either.David

author: CaptDon
author_rank: Super Contributor
timestamp: September 26, 2024, 11:58:20 pm
content: My wife is a German citizen. I showed her the picture. 'Stromart' means electricity or in this case type of electricity and you are correct in assuming three phase. It would appear to run at syncronis speed of 3000 RPM or 50Hz times 60 seconds. I find that strange since it looks like a squirrel cage armature I would expect a bit of slip under load. Being three phase it won't need a capacitor to determine rotation direction. Direction will be determined by the order in which the phases are connected to the three phase mains A,B,C, verses A,C,B. With four wires it would probably be wound in a 'Y' style with the center lead brought out. If it is being run in single phase you will ruin it if it isn't already wrecked. The phases would be 120 degrees apart so capacitor shifting one lead isn't correct. It really wants real three phase.

author: gupa
author_rank: Newbie
timestamp: September 27, 2024, 12:02:36 am
content: How many ohms will be measured on rag wires of the same color? Where is the capacitor hidden? Is star or delta connected? Calculate the capacitor according to the formula and probably buy a new one.It's probably connected to the star now to keep the revs low so the grinding doesn't kill a person.For a short time to sharpen one or two knives or a scythe or sickle can be wielded in this way. But the motor starts to heat up quickly due to bad induction.Edit: If a delta is connected, the next wire is to change the direction of rotation of the shaft by switching the capacitor to a different terminal. So maybe you just need to measure more coils on the new wires and supply only the missing capacitor with the direction switch.Edit: Is that a 1-0-2 switch in the picture? (three positions)

author: Kjelt
author_rank: Super Contributor
timestamp: September 27, 2024, 01:34:29 am
content: The plate on the motor states 220V 50Hz delta connection so single phase which normally needs a starter capacitor for the phase shift to the third connection.But do verify that the motor is still in delta configuration.

author: Xena E
author_rank: Frequent Contributor
timestamp: September 27, 2024, 06:59:26 am
content: No! As per Captain Dons post Its definitely a motor that was manufactured as 3phase.Six wires would be three windings, Delta connected 220V, (star connected it would be ≈ 380V.) That is normal.The switch *looks* like a cam type, if there are three contacts it proves the three phaseIf it were a capacitor start/run single phase, it would have a capacitor, if it were a split phase start motor the two windings would be different resistances and the motor would have a centrifugal switch.It's just been messed about with, and is now scrap.

author: highcap
author_rank: Contributor
timestamp: September 27, 2024, 08:21:12 am
content: Thank you for all your replies! Unfortunately I don't have 3 phase power available here, just single phase 220v.I took new pictures and measurements, I will try and answer all the above questions, here it goes:There is no start/run capacitors so those might have been externally, if any.There are two sets of 3 wires each coming from the windings (let's call those wire sets).Colors are Black, Yellow and White (white cloth turned to brownish partially).One set of 3-wires winding was connected together, so all 3 wires making contact.The power switch is a simple design which closes/opens its 3 contacts all at once.The other set of wires which were connected to the power switch are measuring roughly 25 Ohms all in between.If I were to unstuck the other 3-wire set, no measurement will happen when measuring wires in between within each set.When measuring wires between the two sets, all reads roughly 12 Ohms, color to color only so they're consistent. (e.g. Yellow from one set measures 12 Ohms with Yellow from the other set).My first test was the way it was setup and how it presumably ran before I got it: one set of wires was stuck together, and the other set had readings of 25-28 Ohms so I applied 220v onto it.Video of winding measurements:https://youtu.be/tq2VeAgLLqsVideo of contact function:https://youtube.com/shorts/8i0wIpBRGIcI attached a picture for those who can't open the videos.Thank you all for chiming in, I'm excited about your answers!

author: Ian.M
author_rank: Super Contributor
timestamp: September 27, 2024, 09:18:55 am
content: The electrics are pretty much irrelevant at this point. Its rotor assembly has a bent shaft, and if the bend is between the bearings, its scrap. If the bend is in the output shaft, it *may* be able to be straightened enough to get the runout down to something usable. However as you suspect its been dropped, the grinding wheel is compromised*, you know its got bad bearings and it doesn't appear to have a guard and rest. It is probably not economic to replace the wheel and bearings and make a guard and rest for it.Before OSHA, HSE and other national workplace safety bodies mandated proper guards for fixed grinders, that pretty much enclose the wheel, their wheels catastrophically failing at speed (aka: 'grenading') caused many grinder operator deaths and serious injuries. .* If a grinding wheel has had a hard knock, it may have an imperceptible crack in it which could result in its catastrophic failure when spun up or during use. Any wheel that has been dropped or chipped should not be used and should be destroyed by breaking in half and disposed of.

author: Xena E
author_rank: Frequent Contributor
timestamp: September 27, 2024, 10:22:33 am
content: Quote from: highcap on September 27, 2024, 08:21:12 amAs you have described this motor, with 12 ohms between each end of the three different coloured wires, it confirms it as a three phase machine as was previously said. The connection of the three wires together is star/wye.Sometimes one bodge used to get these three phase motors to run on single phase was to connect the mains across two windings and place a capacitor between live and the other phase, this would cause a phase shift in the third winding, enough to start the motor off load, but it would only give poor torque and would overheat the windings if loaded.If you want to restore this, and use it, I would recommend finding a VSD that outputs three phases from a single phase input.They are available to run small motors like this.I concur with Ians comments on the subject of safety...RegardsX

author: gupa
author_rank: Newbie
timestamp: September 27, 2024, 03:41:29 pm
content: Hello, I would also measure the ohms against the motor frame if the winding is not punctured. But it looks good. Now practice. wires from one hole that go are number 1 plus color cables from hole no.2 are cables no.2 plus color.Why am I writing this? In the triangle D, No. 1 of one color is connected to No. 2 of the other color. It is important to orientation the coils! No.1 and three colors are connected to the star and on the other side there are connectors with No. 2 ends coils.According to the flag, I took the liberty of looking for a reference to the theory in your language around thishttps://ifrunze.blogspot.com/2013/08/pornirea-motoarelor-trifazate-la.htmlEdit: also measure the switch if it has no connection to the frame. I would spray some oil on the mechanical parts.He put the balls of the bearings in vaseline and lubricated them with lubricants during operation.Edit:That electric motor is very old, so I would reach for it. Touch classes are higher today. It is better to control everything with a voltage tester with light (neon tester) before touching the surface of a running electric motor.Edit:The frequency converter switches very high, these motors can handle it, but the test neon indicator is active on the frame. It is necessary to additionally ground and be instructed about the possible penetration from the coils to the frame.

author: Jeroen3
author_rank: Super Contributor
timestamp: September 27, 2024, 05:11:32 pm
content: Quote from: Xena E on September 27, 2024, 10:22:33 amThis motor is too old to be run with a VFD without sine filter, the dv/dt of the switching will destroy the winding insulation. And buying a sine filter makes no economic sense for this thing.If you want to run this motor you can convert it to single phase with a capacitor. It will be derated.Remember, 220VΔ means 220VL-Lbetween the phases, this old. In a lot of places it's replaced with 400VL-Lbetween phases and 230VL-Nto neutral.Maybe 220VΔ is still common in some other regions in europe, I know Amsterdam and Belgium still have some.

author: Xena E
author_rank: Frequent Contributor
timestamp: September 27, 2024, 05:36:57 pm
content: Quote from: Jeroen3 on September 27, 2024, 05:11:32 pmThat is a very good point, thank you for picking that up, I don't know the date of this machine, but it may be that it is so very old, (pre 1950?) That it may have shellac insulation which could be destroyed.The 220 volt rating is Delta, this motor has all winding ends available and if connected star could be run at ≈ 380 volts supply: this is often an arrangement used with small motors even more modern ones. Larger motors are often wound so that in Delta they are for 380/415 volt supplies.RegardsX