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Mini lathe

Speed board/ motor

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Arckivio Funiciello24/03/2019 21:14:13
9 forum posts

Hello everyone. I joined this forum tonight to start this topic about my mini lathe. Not so much because I want help, as I'm pretty sure it's being skipped & I'll put any accessories I bought on Ebay, but because I am just so curious as to what it's done wrong, what I've done wrong,or whatever. Started with my motor brushes flashing, not sparking but really flashing & tripping breakers. I tested the motor on a bench power supply up to 50 volts & worked fine.

Debating whether or not to buy a whole box, (147 quid) or a new speed control board, (88 quid). I tested DC output from the board and it was showing 115 volts with the potentiometer switched fully off, no motor & if I turned the pot on it tripped my breaker so I ordered a new board. All the while Am***al doing an impeccable impersonation of a deaf chocolate teapot! I tested new board and seemed OK, 0 to 180v on pot with no load. I connected the motor & I'm pretty sure it worked but when I assembled everything I stalled the motor a couple of times but only for a second or 2. Motor stopped working. I tested impedance on motor & measured 11 ohms. I eventually got it working but connected to the board it draws 130ish volts with pot at lowest setting with no torque at all & brushes not sparking but smelly. Turned right down, the motor stops, but any voltage going to motor & it starts going potty & just keeps upping the voltage all by itself. Tried the board using another 550w motor in a powerhead & it runs it fine, keeping the correct speed with loads of torque. If anyone can put me out of my misery I'd really appreciate it.

Neil Wyatt24/03/2019 21:57:27
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Moderator
15814 forum posts
672 photos
73 articles

Hi Arckivio

If you are lucky, the motor has just worn down the brushes - this causes dramatic sparking, uneven running and can even damage control boards (I've had it destroy a washing machine control board...)

If you are unlucky, the motor has shorted or burnt out one or more armature windings.

Don't use the motor until you have cleaned the commutator of any carbon dust and fitted new brushes. This is very easy to do and can miraculously rejuvenate a poorly running motor.

There's a good chance your original control board may be OK too.

Neil

Arckivio Funiciello24/03/2019 22:07:19
9 forum posts

This is why I'm so confused. Brushes are probably 3/4" long as this thing has seen less than 10 hours use in total. I also cleaned the commutator as my first step in troubleshooting. It still seemed to suck whatever voltage it fancied from the control board but the other motor worked. The previous board was showing 115 volts when it should have been zero with no motor connected. If I bin this without finding out what happened, it'l haunt me forever! Thanks for the reply.

Martin W25/03/2019 00:53:17
784 forum posts
29 photos

Hi

If the speed control board drives a similar motor with no problems then the problem is the motor. I take it you are using a Digital Multimeter to measure the volts across the motor and this can give erroneous readings when measuring switched waveforms, even the so called true RMS meters as they are limited to a range of form factors (waveform shapes). If the motor is not running smoothly then as Neil suggested it could be the brushes or the motor connections to the brushes, shorted turns on the armature, carbon build up around the brushes causing tracking to earth. As I said above If the electronics drive another similar motor without any problems then the fault is with the motor.

HTH

MartinW

XD 35125/03/2019 03:41:01
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1228 forum posts
83 photos

Have you tried the old controller in reverse to see if there is a difference ? If as you say the controller works fine with another motor then it must be ok , the transistors inside it will either go dead short and the motor will run flat out or they go open circuit ( usuall case ) and the motor won’t run at all . The motor will be possibly be powered by a H bridge confguration unless it uses a switch to reverse the motor . You need to test the resistance of each winding on the armature not just across the motor terminals as one winding could be shorted or gone open circuit . If you have been using the lathe to do a lot of low speed work the fan on the motor of most mini lathes is not very good so the motor can overheat pretty quickly and this fries the insulation on the armature windings , it also tends to cook the commutator . You also need to check that the commutator is machined true as any runout will cause the brushes to bounce causing arcing and reducing the life of the commutator and brushes . What do the brushes look like ? Is the end that contacts the commutator pitted or burned ? Some photos may help us to help you . If all else fails - VFD & 3 ph motor conversion works well .

Arckivio Funiciello25/03/2019 06:24:05
9 forum posts

Morning. I was working on something at a pretty slow speed when the motor initially started flashing so the insulation melting would make sense. It does have a switch to reverse the motor & the original speed controller did trip in both directions with this motor. I will definitely give measuring across the armature so I can finally sleep peacefully. Thanks

SillyOldDuffer25/03/2019 09:17:04
4098 forum posts
830 photos

First off, bad luck - I hope this horrible experience doesn't put you off lathes completely!

You have a thoroughly odd set of symptoms, my vote would be for a faulty motor except you've had it running off a 50V supply.

If it's not the motor and it's not the control board, then you need to look carefully at the wiring harness and how the motor and board are mounted. Check for likely shorts or cross-connections. Things like damaged insulation, exposed wires and cables nipped during re-assembly. (Trust nothing - factory mistakes aren't unknown.)

Unlikely the problem is down to the motor mounting, the harness is much more suspect. But particularly check the way the control board is mounted - the tracks underneath and to the sides should be well clear of the lathe's metalwork. The peculiar behaviour you're seeing may be due to an intermittent contact between the board's internal circuitry and the lathe. Not heard of it on a lathe, but other equipments have been reported with board bottoms shorting as a result of using stand-off pillars that are too short, missing, or made of steel rather than plastic. Also, contact due to excessively long bolts, or because an insulating plastic sheet is missing, or because swarf has got inside.

I hope you fix it - I had loads of fun with my mini-lathe.

Dave

Arckivio Funiciello25/03/2019 17:28:25
9 forum posts

It hasn't put me off lathes as I need one for some of the kinetic arts projects I do even though I'm a woodturner really & have my beloved union graduate for that. I just don't really want to throw more money at this little thing & get a bigger one. I will be measuring resistance on the commutator hopefully one evening this week. Will take a while as there's a lot of divisions.

AdrianR25/03/2019 17:41:32
131 forum posts
3 photos

Another thing to look out for are brushes sticking and not pressing against the commutator properly.

Arckivio Funiciello25/03/2019 17:47:25
9 forum posts

Just found a really good video about fully testing an armature. I'll be doing all the tests

Grizzly bear25/03/2019 19:30:12
195 forum posts
6 photos

S-O-D wrote; " You have a thoroughly odd set of symptoms, my vote would be for a faulty motor except you've had it running off a 50V supply".

If its shorted, it could probably cope with low voltage. Just a thought!

Arckivio Funiciello25/03/2019 20:04:50
9 forum posts

Yes I had it running at 50 volts but maybe stalling it a couple of times because i messed up while re installing it,, may have been the final nail in it's coffin. I can't wait to test it & see if it's shorted somewhere. If there's nothing wrong with it, I wont know what to think. The original board probably damaged it when it made the brushes fire like a giant Tesla coil!

Phil Whitley25/03/2019 20:23:53
813 forum posts
105 photos

Sounds like you may have a duff diode or rectifier, you are getting an AC leak to a DC motor? I am no electronics wizz, but many on here are. I have just rebuilt a 200amp 3phase mig, and was advised to watch for arcing at the DC wire feed motor caused by faulty rectification!

John Rudd25/03/2019 20:54:34
1364 forum posts
58 photos

Arckivio,

I sent you a private message, check the flashing envelope at the top of the page.

Arckivio Funiciello26/03/2019 22:29:40
9 forum posts

Test results are in! There is no continuity between commutator bars & the motor housing. Impedance between each bar & the one next to it is between 1.4 to 1.6 ohms, apart from 2 where it is 6.3 ohms. Not sure if this means there is a short circuit or if it's open circuit or if it's the cause of the issue with this motor. Hopefully someone will know....Thanks for the help so far

Emgee26/03/2019 23:27:53
1084 forum posts
199 photos

Arckivio, did you check the commutator bars directly opposite each other for continuity ?

You should get the same resistance reading on all pairs.

Emgee

Arckivio Funiciello26/03/2019 23:34:25
9 forum posts

No, only those next to each other. It's actually quite difficult to check those opposite each other as there's so many. I'll try that tomorrow night & I'll start earlier when I'm not so sleepy! I'm sure it will be the 2 dodgy ones I found tonight that wont measure correctly with those opposite.

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