Fuse keeps blowing - Page 1
Forum section: electronics / beginners

author: Leuven
author_rank: Regular Contributor
timestamp: June 02, 2013, 07:48:12 pm
content: I have a small variable speed lathe which keeps blowing fuses - but only when I crank the pot up. It has a DC controller board inside, which takes 240AC and puts out 240V DC at the motor. I've isolated the board from everything except for the pot, so at least I have a good indication there is a short on the board itself.I have replaced the 2 mosfets marked M (first time it blew it took one of the mosfets out and so the variable speed). I then replaced the original slow-blow fuse with a standard one and now at least it leaves the mosfets alone when it blows.I also can't see anything wrong with the the other 3 diodes attached the the black heat sinks (bridge rectifier?), can't see any shorted coils or caps except for cap C19 where I found continuity between the 2 legs. I thought the cap is shorted so I took it out, but the continuity comes from the board. Cap is good. Does that mean I'm getting close?Mind you, it works fine until I crank the pot about 30%. Any pointers?Thanks!

author: SeanB
author_rank: Super Contributor
timestamp: June 03, 2013, 04:04:29 pm
content: First things are is the motor itself good? Does it still have good brushes, good commutator and such? Are the field windings down to earth or one of the rotor windings? If it is a 4 wire motor ( separate field windings) then check the resistance of the field windings, and the resistance from the commutator to the case with an insulation tester. Turn the motor over a turn or two during this time to check all the windings on the rotor.Take a 100W mains ( I assume 220V local mains) incandescent lamp ( or close to it, 60-150W will do, not CFL or LED, incandescent or halogen) and connect in series with the motor winding leads. This will show if there is a shorting winding in the motor as you turn up the wick. No load on the motor the light should be quite dim from low to full speed, if it lights up bright at some point then you probably have a dying rotor and need to get on fleaBay to get a new OHL motor for the little lathe.Post photos of the motor and the baby lathe please, there are a lot of mechanically minded on here as well.

author: G7PSK
author_rank: Super Contributor
timestamp: June 03, 2013, 04:10:58 pm
content: Check the brush gear as well particularly the commutator, clean out between each section as you can get carbon build up causing shorts and arcing over the brushes. Also check the bearings as tight bearings will overload the motor.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: June 03, 2013, 05:07:17 pm
content: Pretty sure that's a classic 7x8/10/12/14 240v minilathe controller board.The minilathe Yahoo group should give you a pretty quick answer I would think. Its been years since I've been on it but it was very active. You need the 7x12minilathe group. One of the others is kinda useless.EDIT: One thought bubbled up from my memory; you can test the controller independently from the motor by attaching an incandescent light bulb to the output rather than the motor. I would think 100 or 150W should load it enough so you can be reasonably sure where the problem is.

author: Paul Price
author_rank: Super Contributor
timestamp: June 03, 2013, 07:14:58 pm
content: If the fuse blows, it must mean too much current, so where is the problem, controller or motor?Get a flashlight and make a visual inspection of the brush contacts and the contacts rotating under them. You look for pitted, irregular, very blackened, missing, splashes of solder, or eroded commutating contacts. You can turn the motor by hand. You can hear and feel any irregularities that suggest a damaged motor armature.It is very possible that a capacitor snubberacross the motor drive output connection or across the voltage power switch FETs is open and causing the failure. A scope can show excessive spikes that could blow a power FET. In series with a snubber capacitor will usually be a small wattage resistor that is not uncommon to have opened circuited.Testing the output with a resistive load like light bulbs will not stress the controller with inductive kicks and surges. Testing it with another motor a similar size and type would be a better test, if you are lucky enough to find one.You could also have a motor with just one wire shorted to the next, just two wires touching in a winding that is short circuiting the output or else ashorted commutator contacts(solder or accumulated carbon and/or copper dust) can short adjacent contacts on the rotating armature under the brushes) causing current surges in excess of normal at low speed settings and quickly blowing the fuse at highest speed settings.Unless you canscope the current into the motor with a current probe, you might find this problem is difficult to troubleshoot, and finding a motor problem does provide a method to fix it, it might be easier to swap another motor.Use any scope to probe into this problem:Make a simple current transformer by winding just a few turns of wire upon a ferrite or metal core and next to it or over it, a secondary of several turns of wire. The secondary of this makeshift transformer must be, for safety reasons, made of insulated wire(but you only need a few turns) and so now you have an output isolated from the AC mains and this is your current sniffer winding fed to a scope.Then you connect the primary of this transformer in series with one of the two power leads going to your motor. Crank up the speed to the motor slowly and you can see how the motor behaves in commutation of power. The waveform may be full or spikes, but should be consistent. A large difference in current will show up as a big spike during every armature rotation cycle and tell you where the problem is. If it is the motor, the trouble will be be clearly visible on the scope as a surge.

author: Leuven
author_rank: Regular Contributor
timestamp: June 04, 2013, 03:56:21 am
content: Quote from: SeanB on June 03, 2013, 04:04:29 pmQuote from: G7PSK on June 03, 2013, 04:10:58 pmQuote from: Paul Moir on June 03, 2013, 05:07:17 pmIt is indeed a 7x14 chinese Sieg. I will post picture with the whole setup later on. I think I have an account on the Yahoo group but I figured this is strictly a controller problem, and a random one at that. Otherwise, I know this lathe inside out. I must have spent more time repairing and upgrading it than actual work done.Quote from: Paul Price on June 03, 2013, 07:14:58 pmThe problem is,the motor is disconnected, along with everything else except for the pot and the chuck safety switch. The fuse blows at zero load basically, it's the DC voltage increase that causes a short not the load. If I power the board with the pot down to min and keep it about there (that means about 20V DC), the fuse doesn't blow.I will post pictures with the board so we know what we're looking at. I have measures most of the diodes/coils/caps and found 1 cap and 1 diode shorted out - but due to the circuitry not the components themselves. I took them off the board and they're on spec. Measuring between the 2 points on the board (where these components reside) shows continuity in both cases.I'll have to order some more mosfets in the meantime. I did some live testing and the fuse blew again, this time taking both mosfets out.Again, I will post clear picts so please bear with me.

author: 4to20Milliamps
author_rank: Regular Contributor
timestamp: June 04, 2013, 04:43:37 am
content: It looks like this is the same control board but it's hard to tell:http://wiki.bootstrap.je/display/Mill/Motor+Controller+Adjustmenthttp://wiki.bootstrap.je/download/attachments/1736718/XMT%202335%20-%20siegc3.pdf?version=1&modificationDate=1296945595000&api=v2http://littlemachineshop.com/products/product_view.php?ProductID=2525

author: Paul Moir
author_rank: Frequent Contributor
timestamp: June 04, 2013, 07:41:25 am
content: QuoteI know what you mean. I have a non-Seig "7x8" which is a 7x10. Lapped gibs, fine feed, DIY quick change toolpost, ball bearings on the cross & compound, saddle lock, and all dialed in. It's sweet to use but I use my 10x20 99% of the time now.Sorry I had missed that you were running the controller without a load. Clearly you have a controller problem. But there used to be a guy on there who did a bunch of controller repairs (I think he was retired and specialized in them for fun & a little cash) and seemed to know just about every failure mode. Was very open with DIYers who could sling a multimeter.

author: Leuven
author_rank: Regular Contributor
timestamp: June 09, 2013, 01:47:08 am
content: Quote from: Paul Moir on June 04, 2013, 07:41:25 amYep, lapped gibs and bed, tapered spindle bearings, ball bearings on all axis - even in the handles, carriage lock, carriage stop (took useless factory one and reworked it), carriage gibs properly shimmed + self-locking aerotight nuts. In addition to that, I bypassed the gear system and went for direct motor-to-spindle timing belt transmission (4:1 ratio, 1500rpm max) which improved the smoothness and cut stability no end.A very useful tool indeed. I've been working at the car today replacing some of the rear axle bushes and I could have done with turning some sleeves and washers to help out. It would have turned a 4 hour job with loads of cussing and swearing into a surgical 30 min job.

author: Leuven
author_rank: Regular Contributor
timestamp: June 09, 2013, 02:00:55 am
content: Anyway, the 2 mosfets have been replaced and here's the board. To recap, I only have connected the 3 leg pot and a safety switch (senses if the chuck safety guard is lowered and releases power).@ 4to20Milliamps That is the exact board, thanks a lot for that link!The fault manifests as follows: I plug the board into the mains and is all good until I start cranking the pot and at about 30% travel the fuse blows.For my part, the only thing I see is continuity on the board at cap C19 (cap is good when taken of circuit) but wouldn't know where to go from there. Other that that, I'm armed with a meter and hopefully some of you might have an idea of what's going on.

author: Leuven
author_rank: Regular Contributor
timestamp: June 09, 2013, 02:37:37 am
content: Another thing, there was a slightly blackened area (top corner, R1 and R2 resistors) and after some scraping and looking at the schematics now + measuring continuity, it looks like pin 1 and 2 from connector J5 are bypassing the 2 resistors R1 and R2 completely - as per image. For what it's worth, R1 and R2 both measure 5Kohm in circuit.