A very handy circuit for diagnosing brushed motors such as in power tools - Page 1
Forum section: electronics / projects

author: DDT
author_rank: Contributor
timestamp: May 08, 2022, 08:00:30 pm
content: My old faithful 1500W AEG core drill (DB2E 1500) stopped working a while ago. This was from the era when AEG used to make high quality tools in Germany now it’s owned by TTI. I decided to give it a go with repair as the unit was very expensive (500£) and the replacement parts are still available.I started with the usual suspects, check the plug, cord and the switch, all seemed fine. Next, I checked the carbon brushes, gears and also the speed control unit. They were also in a good condition. At this point the only remaining suspects were the armature and stator. First, I visually inspected the stator and no burn marks or charred wire on either stator or armature. The Ohm reading of both of the windings on the stator were also consistent.The usual test for an armature is to successively read the resistance of the adjacent commutator bars. Any huge variation is indicative of either a shorted or open coil. Unfortunately, this is not a good test as you are working with low ohm range and instrument or user error can mess up the result. Additionally, if you have only one or two shorted wires within each coil (very common), like in my case, you can’t tell with a multi meter.In the past they used a device called Growler to diagnosis armatures but they are rare now. I continued my search for a tool that I could actually build and would be able to reliably detect fault and preferably in the form of an electronic device. I came across this gentleman on YouTube who had such a device and was kind enough to show the circuit diagram. The issue was that the video was in Russian and also, they wound their own custom inductors using magnet wire.After several trial and error, I came up with a working device using a modified circuit that works with jellybean parts. It detects shorted coil within armature and stator and it is very sensitive and reliable.Please read the notes in the diagram and make sure you also follow the steps in the instruction note. The device seems to work better with the radial type (shown in the circuit diagram) inductors. The optimal size is 8x12mm or 9x12mm. Not very critical, but seems to have better sensitivity. If you have smaller inductors (6x8mm), you might need to lower the distance between inductors from 27mm to 21mm. Further testing needed.Theory of operationI am not an engineer, just a hobbyist who like to make functional and useful circuits. This is how I think the device works so my apology for any error or mistake.The left side of circuit, with L2 highlighted as transmitter in the diagram, is a sine wave generator. The oscillator generates a signal of 10-100KHz, depending on the values used in this part. I get 54KHz as shown on my oscilloscope. Using larger values of C5, C6 gives lower frequency, lower amplitude but cleaner sine wave. I didn’t dive very deep to determine the optimum values, what is shown worked for me.The receiver section with L1 receives the signal and this is adjusted using a multi-turn pot, after which, the signal is rectified through D1. The presence of shorted coil between L1 and L2 affects how the L1 receives the signal depending on the mode of operation explained in the instruction notes. When the signal reception is disrupted to L1, Q3 and Q2 receive no base current so they are in off state, and the green LED is on. With the signal of sufficient strength, Q2 turns on. In this condition, the red LED shines and the base of Q1 is grounded so the green LED is off. See the instruction file for more photos and how to operate the device.I tested my suspected armature. It showed shorts in four locations which means a pair of coils shorted. I noticed the iron body of armature was shiny on front side. I suspect the back bearing went bad and the body of armature was rubbing against the inside of the stator. The resulting friction caused overheating and few winding wires were shorted as the result. I tested a few good armatures and the device works like a charm. No false positive at all. You can test the device by artificially shorting two commutator bars and see how your device functions. Or place a piece of closed loop wire on the stator winding.Thank you for your time reading this and I hope you make this device and have a success like I did. It's rare to see a DIY circuit works this well. You never know when you might need such a device but since universal motors are so ubiquitous in power tools, blenders and many household items, it would be very handy to have such an item.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 09, 2022, 02:34:47 am
content: Quote from: DDT on May 08, 2022, 08:00:30 pmHave you tried an inductance measurement?

author: DDT
author_rank: Contributor
timestamp: May 09, 2022, 01:11:56 pm
content: I haven't, but I'd imagine it wouldn't work for the same reason as you can't rely on inductance test for shorted transformers and inductors. That's why they use the ring tester for testing the ferrite core transformer, like the one Bob Parker made ages ago and sold now as "blue2" ring tester. The ring tester also doesn't work on iron core transformers, and by extension on armatures and stators due to the low Q factor. You are looking at only one or two shorted turn within a several hundred turns. The variation between readings of ohm or inductance would be just too small to rely upon, not to mention you have to read many adjacent commutator bars, which can involve many factors; time, user error, instrument accuracy, cleanness of comm bars, presence of carbon dust between comm bars etc.Since the shorted coil forms a loop, it would be better to use a method that ignore the above factors and directly look for the said loop. That's how a Growler functions. There are also some commercial units like the Bosch 1609244c02 that work similarly to the circuit that I've posted.

author: strawberry
author_rank: Super Contributor
timestamp: May 09, 2022, 01:24:57 pm
content: looks like metal detectorcan test motor in series with resistor, when high loses(shorted turn) motor wont run even with small help

author: DDT
author_rank: Contributor
timestamp: May 09, 2022, 01:49:42 pm
content: Not exactly a metal detector. The device ignores the iron body and healthy copper windings, only looks for a shorted loop. You can test the motor by the method you mentioned, but you need to be able to tell where the fault is coming from. The stator or the armature. You can get confirmation of short in few seconds with this tester with the exact location. If it's in one winding of the stator, for example, you might be able to even rewind that part yourself.

author: PaulAm
author_rank: Frequent Contributor
timestamp: May 10, 2022, 02:00:49 pm
content: Well that is really cute, I like it.I've had a number of small motor repairs, but it has been pretty obvious what went wrong (open winding, burned commutator segments, etc), but I can see the utility of this when faults are not so obvious. You can always replace both the stator and armature, but that can get needlessly costly.

author: DDT
author_rank: Contributor
timestamp: May 10, 2022, 05:32:29 pm
content: You're absolutely right. I found the replacement armature for 25£ on ebay which came with bearings already installed on it. If I needed a stator, however, it was only available on a spare parts website, with the asking price of 70£, and in case of not knowing which part was bad, I'd had to order the stator as well. The tester saved me money. To be honest if it was a cheap drill, I'd have threw it away. But this is a high quality expensive drill and now it's working again.

author: wizard69
author_rank: Super Contributor
timestamp: May 11, 2022, 01:00:02 am
content: This is pretty cool. Build yourself a fixture, with centers to rotate the armature on, then you have a nice little test bench.

author: Kerlin
author_rank: Regular Contributor
timestamp: May 11, 2022, 09:13:39 am
content: Check what professional motor winders use, its called a growler.There is a clear reason they use it.Untill you have used it you can't understand how good it is.Easy to make.

author: DDT
author_rank: Contributor
timestamp: May 11, 2022, 01:36:06 pm
content: Quote from: Kerlin on May 11, 2022, 09:13:39 amActually, I can understand how good a Growler is. I used one years ago. I briefly mentioned it in my first post. It's basically a large transformer without a secondary winding. The shorted coil of armature acts as a secondary winding when it's placed in the magnetic field of the Growler, and this induces a current (and magnetic field) in the shorted coil. When a steel object, usually a hacksaw blade, placed parallel on top of the armature, it starts to vibrate due to said magnetic field. This is indicative of a bad armature.A Growler can be made by a microwave oven. This involves removing of the secondary winding, and cutting a precise 45 degree V groove on top corner of the iron core. You also need to regulate the current that goes into the primary. What I posted is a better alternative for those who don't have access to a commercially made Growler and don't want to mess about with the main voltage and modifying oven transformers. Just a few jellybean parts and you have an electronic device that achieves the same results.

author: DDT
author_rank: Contributor
timestamp: May 11, 2022, 03:29:09 pm
content: In case anyone is interested in the improvised growler that I mentioned, this is a crude diagram of the device that works very well but personally, I wouldn't bother.

author: Shock
author_rank: Super Contributor
timestamp: May 12, 2022, 07:12:31 am
content: Aka the name of every ex wife.

author: DDT
author_rank: Contributor
timestamp: May 12, 2022, 04:17:07 pm
content: Quote from: Shock on May 12, 2022, 07:12:31 amHaha, I see what you did there. It's indeed very aptly named. I remember my uncle had one in his workshop, branded Snap-On. It had an angry disposition, specially when encountered a shorted turn.