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Forum Index : Electronics : Diamond rotary glass grinder died
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| lizby Guru  Joined: 17/05/2016 Location: United StatesPosts: 3299 |
My wife does stained glass work. The bench rotary diamond grinder which she uses to clean up edges died. I've taken it apart. There are only 4 components: a fuse, a lighted switch, a KBJ608G bridge rectifier, and the motor. I've tested that the fuse is good and the switch works. The KBJ608G bridge rectifier can be had from Mouser for $1.79. I suppose the likely problem is the motor, but is there something I can do to test the rectifier? (The only oscilloscope I have is a PCB which plugs into a usb port on my laptop, and I'm not willing to try that with 120V 60Hz at the far end.) The only label the motor has on it is Type 4428/152. A cursory search on the internet didn't find anything about it. There are three wires going into the motor--GND from the mains connection (also connected to the motor's mounting plate), and the 2 wires from the legs of the bridge rectifier. I know next to nothing about motors--what type of motor is this which runs on rectified but unfiltered AC? PicoMite, Armmite F4, SensorKits, MMBasic Hardware, Games, etc. on fruitoftheshed |
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CaptainBoing Guru Joined: 07/09/2016 Location: United KingdomPosts: 2137 |
you can test the individual diodes of the bridge: test with a multimeter on diode test from both ~ tag to the + then swap the leads over and test the other way Repeat for the - tag remembering the polarity will be opposite. You should see about 0.6V on a forward test and o/c the other way |
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| Godoh Guru Joined: 26/09/2020 Location: AustraliaPosts: 498 |
If you use a multimeter you can test that there is DC going to the motor from the rectifier. Test first that there is AC getting to the rectifier. Then test the DC side. If there is voltage going to the motor then it is a motor fault. If it is an old grinder the motor is probably a brushed universal motor. In which case the brushes my have worn out. To check that take the back end plate off the motor and have a look, if there are brushes there see if they are worn down too much and are not making contact with the commutator. Pete |
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| lizby Guru Joined: 17/05/2016 Location: United StatesPosts: 3299 |
Thanks. Will test rectifier diodes tomorrow. There is definitely AC getting to the rectifier. With no capacitors in sight, will I actually be getting DC to the motor, or just repeated half sine waves? (Or is that just effectively very ragged DC?) Maybe my voltmeter will do its own averaging and see it as DC. Another task for tomorrow. The rectifier pins have wires soldered directly to them, with shrink wrap, so not easy to get to. There's no PCB. The switch has a plastic-covered spade receiver plugged directly into the spade on the switch, and the other end of the connector is soldered to a pin on the rectifier with shrink wrap on it. Good thought about brushes, but I'm not sure I can get to them. To remove the motor, I'd have to remove the diamond bit from the shaft, and it's fused in place. It is worn and my wife wanted to replace it, so I worked on it last year without being able to get it off. I'll have another try with WD40 and will see if I can get some kind of a terminal puller type of leverage. PicoMite, Armmite F4, SensorKits, MMBasic Hardware, Games, etc. on fruitoftheshed |
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| Godoh Guru Joined: 26/09/2020 Location: AustraliaPosts: 498 |
Most DC motors that have brushes have the commutator and brushes on the non drive end. So you may be able to get to them from the bottom of the motor, that is if the machine has a base that can be removed. If there are no smoothing caps then yes the DC will just be the sinewave shape with then negative half cycles turned into positive half cycles. The meter should still read it fine on DC. If your supply is 110 volts AC then you should read close to 100 volts DC. Just use a sharp knife and make a hole in the shrink wrap big enough for the meter probes to touch the terminals on the rectifier Pete |
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| davros Newbie  Joined: 24/02/2025 Location: CanadaPosts: 1 |
Pretty sure the one my daughter has the bits replace easily, fitting over a 1/4 inch shaft? |
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