(Another) Mini Lathe Speed Controller Problem

[SillyOldDufferModerator @sillyoldduffer]

Posted by John Haine on 15/02/2022 09:37:10:

…

These sense resistors go in series with the motor and need to be matched to the motor power, which is why it's a plug-in component (though it probably gets hot too). I think these controllers keep the motor speed approximately constant by having in effect a negative output resistance which cancels out the motor's internal rotor resistance, so that the armature voltage is a good indication of speed. This resistor trims the output impedance by adding a small positive value to match the motor. The fact that it has blown up shows …

John answered the question while I was wondering what a 'Horsepower Resistor' was, for reference the remains are shown in Philip's photo below:

It's a 0.1Ω (may be sold as 100mΩ ) resistor, which are widely available in the UK from electronics suppliers such as RS. What I'm not clear of is the wattage.

Guestimating, assuming the motor is 250W (⅓HP) at 180V, maximum current (R) would be:

250/180= 1.4A and as W=I²R

1.4 x 1.4 x 0.1 = 0.2W dissipation.

Philip's resistor is labelled 120V and ⅙HP (125W), in which case:

125/120 = 1.25A and

1.25 x 1.25 x 0.1 = 0.16W dissipation.

On the face of it, a 0.1Ω 1W resistor would do the job (5x safety factor). But I'm wondering if the resistor is also used as a fuse, and the wattage should be lower because it's meant to run hot? I think the component might be dual-purposed as a fuse because it's lifted away from the board presumably to reduce damage, the resistor body looks like a flash catcher, and the pink spray of copper on the lid reminds me of a blown fuse.

Worth Philip trying a new 0.1Ω resistor but I fear the accident has damaged many components.

Dave

Edit: I hate smileys!

 

 

Edited By SillyOldDuffer on 15/02/2022 10:40:50

[Les Jones 1Participant @lesjones1]

I don't think just replacing the resistor is a good idea. From the way it has been destroyed I think the fault current was much more than a stalled motor would cause. I also think it is VERY likely that the diodes and SCRs that form the controlled bridge rectifier will have been damaged. It is also likely that the LM324 IC has been damaged. I suggest that you first test the diodes D13 and D14 and the SCRs. When first testing it with power applied I suggest connecting a 100 watt incandescent lamp in series with the mains input to limit the current. Also disconnect the motor from the control board and connect a small (Say 15 watt.) mains voltage bulb in it's place. Don't be tempted to fault find using an oscilloscope UNLESS YOU USE AN ISOLATING TRANSFORMER TO SUPPLY THE BOARD. This is because the common negative line will NOT BE AT GROUND POTENTIAL.

Les.

[Anonymous]

A resistor like that should be capable of dissipating several watts. They are designed to run hot, often a couple of hundred degrees, or more. I'm surprised it has failed in the way it has, with no heat discolouration. In my experience power components fail like that when short term grossly overloaded.

I would agree with Les regarding his diagnosis. I would also support his warning about debugging. Personally I'd use a high voltage differential probe with an oscilloscope rather than an isolation transformer, but that's my preference:

Good luck with trying to repair the board.

Andrew

[Ian PParticipant @ianp]

From the OP's own description of his electronic skills I agree with earlier replies in that repairing this controller is bets left to experts or just replaced.

Coincidentally I have just com across an article in 'Electronic Weekly' that has some information on these types of speed control (this one on an CMD10 (Seig X1).

Discussion on similar (or same) controller

Ian P

[Anonymous]

I have the professional kit needed, but personally wouldn't mess about trying to repair the board. I'd rather make parts than repair electronics. Don't like doing it in the day job, let alone at home.

Wow, Electronics Weekly has gone downhill? There used to be two weekly electronics papers for the professional; Electronics Weekly and Electronics Times (?). I skimmed them for interesting components, and count the number of pages of job ads, to get an idea of the state of the jobs market. No point in actually applying for one of the jobs, as most didn't exist. They were simply listed to sucker you into going onto the books of one or other of the recruitment agencies. Cynical? Me? Never!

Andrew

Edited By Andrew Johnston on 15/02/2022 19:11:09

[Philip Coupland 3Participant @philipcoupland3]

I’m very grateful for all the helpful contributions made – thank you gentlemen.

I made further investigations this evening and found a possible cause – a curly piece of swarf, which had got in the back of the motor and was making a contact between one of the brushes and the casing of the motor. Happily the motor seems to be still in working order and spins happily with a 12V DC supply. Once this electrical problem is resolved I’m going to ensure that the motor is properly separated from such nasties. Incidentally, I wonder if there is any benefit from adding a fuse in-line with the positive supply wire to the motor – would that prevent this happening again?

The plan at them moment will be to try out the replacement resistor when it arrives. If that doesn’t work I shall probably have to buy either a replacement board or I may buy a complete control box (Amadeal) as its not a huge amount more and will avoid any problems with wiring in a board that may have different connections to mine.

[old martParticipant @oldmart]

There is a website, mini-lathe.com, which might be helpful.

[Philip Coupland 3Participant @philipcoupland3]

The suspected culprit – next to the bearing

[John RuddParticipant @johnrudd16576]

Philip,

I sent you a private message.

IanP,

The description of the controller discussed in the article is based on a PWM based controller using mosfet power devices rather than the KB clone which is Phase angle control using a pair of scr’s in a full wave bridge.

[duncan webster 1Participant @duncanwebster1]

With all the posts we get about failed motors and controllers I'm glad to have good old simple reliable squirrel cage motors on my machines.

[roy entwistleParticipant @royentwistle24699]

Duncan

Roy

[Ian PParticipant @ianp]

Posted by duncan webster on 16/02/2022 00:01:12:

With all the posts we get about failed motors and controllers I'm glad to have good old simple reliable squirrel cage motors on my machines.

Nothing wrong with squirrel cage motors especially if they are three phase and driven by a VFD, reliable too!

Ian P

[CirclipParticipant @circlip]

Not much to go wrong with an eight speed 'Crash' gearbox, – – – providing you don't try to change speed with the machine running. I know it's heresy not to accept the power (or not) of electronics but hey ho Luddites rule.

Regards Ian.

[Anonymous]

Posted by Ian P on 16/02/2022 10:16:22:

….especially if they are three phase and….

…..run from a 3-phase supply.

The cause in this case might have been an external short caused by swarf, In which case the reliablilty of the electronics is not so relevant. The issue with power electronics is that it is easy to create a design that works, but more difficult to decide what fault conditions need to be protected against, and to modifiy the design accordingly.

Andrew

[Philip Coupland 3Participant @philipcoupland3]

An update.

The replacement KB Electronics horsepower resistor that I ordered yesterday from the helpful and efficient Axis Controls of Southport arrived this morning and I fitted it this evening.

I’d checked the motor for any more shorts and tested the board first just connected to a voltmeter. When powered up, turning the potentiometer caused the voltage to increase and then when I connected the motor (detached from the lathe) that also speeded up to a high rate in response to the control.

However, we are not quite there yet. When the motor was reinstalled and the drive belt fitted, I found that it would not speed up more than a little. It will certainly be encountering some more resistance than previously because of the new roller bearings fitted and perhaps the new metal high-low gears that I’ve replaced the plastic ones with. But the bearings haven’t been over loaded.

What I’m wondering is whether this is something that can be resolved by an adjustment on the circuit board or does it indicate that there are further electronic components that require replacement?

Once again, thanks to all who’ve contributed to this thread.

[Les Jones 1Participant @lesjones1]

I would NOT advise adjusting any of the potentiometers. Have you got the correct horsepower resistor to match the power rating of the motor on your lathe ? If the value of this resistor is too high it will limit the current (And hence the torque.) to too low a value. I am surprised the board works as well as it does after such a fault.

Les.

[Philip Coupland 3Participant @philipcoupland3]

Posted by Les Jones 1 on 16/02/2022 20:07:51:

I would NOT advise adjusting any of the potentiometers. Have you got the correct horsepower resistor to match the power rating of the motor on your lathe ? If the value of this resistor is too high it will limit the current (And hence the torque.) to too low a value. I am surprised the board works as well as it does after such a fault.

Les.

Thanks Les.

Yes, the resistor is identical to the one that was blown.

I was surprised too that everything seemed fine until I put load on the motor.

The bearings aren’t tight in the headstock – they turn easily but I found that when released the locknut in the spindle the speed certainly increased although still not within the normal range.

[Ketan SwaliParticipant @ketanswali79440]

Posted by Philip Coupland 3 on 16/02/2022 18:12:07:when I connected the motor (detached from the lathe) that also speeded up to a high rate in response to the control.

If you are prepared to take the risk to do following test: how easy or difficult it is to stop the motor by hand (wearing appropriate leather gaunlets) at slow to medium speed.?

If difficult to stop, then issue may be mechanical.

Have you checked connecting pulleys to see if any have worm by the keyway?

Ketan at ARC

[Anonymous]

Ketan beat me to it!

Debugging a system is a matter of making measurements, thinking logically and not jumping to unjustified assumptions. We could assume that the resistor is the correct value, although to be sure we could measure it. The next stage is to do as Ketan suggests and load the motor when it is running on it's own. Can be done by hand, or hold a couple of pieces of wood either side of the spindle. If the motor slows more than it should then either the motor and/or the board is faulty. If the motor doesn't slow then the problem is the lathe spindle.

If the motor/board has a problem I'd start by making measurements. However I assume the OP doesn't have appropriate test gear? i agree with Les, don't fiddle with settings on the board. That is simply attempting to paper over cracks, rather than addressing the root cause of the cracks.

Andrew

[oldveloParticipant @oldvelo]

Hi There is a correct way to tune the circuit to match the motor but not for the faint hearted or inexperienced.

It involves locking the motor rotor and setting the Amperage output to be LESS than the maximum of the motor rating by adjusting the correct Potentiometer.

I have the manual for adjusting a KB circuit on file. PM me for a copy if you want use it. I Strongly suggest the you have the setup performed by a qualified Electrician with the proper equipment to do the job safely.

One modification to a DC motor drive I keep on banging on about is to fit an Amp Meter to the motor leads to measure the motor amps under load.

Use this to set the power feed speed to be able to read that the power used is less than the motor output to prevent overloading.

A photo in my album will explain it better. The motor leads also have fuses on both motor leads

Edited By oldvelo on 17/02/2022 06:27:57

Edited By oldvelo on 17/02/2022 06:28:21

[Philip Coupland 3Participant @philipcoupland3]

Posted by oldvelo on 17/02/2022 06:11:21:

Hi There is a correct way to tune the circuit to match the motor but not for the faint hearted or inexperienced.

It involves locking the motor rotor and setting the Amperage output to be LESS than the maximum of the motor rating by adjusting the correct Potentiometer.

I have the manual for adjusting a KB circuit on file. PM me for a copy if you want use it. I Strongly suggest the you have the setup performed by a qualified Electrician with the proper equipment to do the job safely.

One modification to a DC motor drive I keep on banging on about is to fit an Amp Meter to the motor leads to measure the motor amps under load.

Use this to set the power feed speed to be able to read that the power used is less than the motor output to prevent overloading.

A photo in my album will explain it better. The motor leads also have fuses on both motor leads

Edited By oldvelo on 17/02/2022 06:27:57

Edited By oldvelo on 17/02/2022 06:28:21

Thank you again OldVelo.

I think the manual you mention is one that I've found on the manufacturers site (https://acim.nidec.com/drives/kbelectronics/-/media/kbelectronics/documents/dc-drives/manuals/kbic_manual.ashx?la=en). I expect that I'll have a go, after I've marked the starting positions on the 'trimpots'.

My comment about my electronics education having stalled at the batteries and bulbs stage is the literal truth – I remember struggling and failing to understand the working of resistors in the Ladybird Book of the Radio. However, despite not having a brain that likes maths, I can follow a recipe (although perhaps not if translated from Chinese).

I'll have a look at your photos. I have already fitted one fuse to the + power lead to the motor.

[John HaineParticipant @johnhaine32865]

Motor controllers don't necessarily react happily to the circuit to the motor being interrupted whilst running, your fuse could be a bad move…

[Philip Coupland 3Participant @philipcoupland3]

Posted by John Haine on 17/02/2022 09:37:23:

Motor controllers don't necessarily react happily to the circuit to the motor being interrupted whilst running, your fuse could be a bad move…

Hello John and thank you for your comment.

The manufacturers of the board say that: 'It is recommended that both AC line and armature fuses be installed'. I interpreted the latter as meaning a fuse in-line with the positive power wire to the motor – is that incorrect?

[John RuddParticipant @johnrudd16576]

Posted by John Haine on 17/02/2022 09:37:23:

Motor controllers don't necessarily react happily to the circuit to the motor being interrupted whilst running, your fuse could be a bad move…

KB electronics, the designer and manufacturer of these boards, recommend the fitting of fuses, line side and armature side, obviously of the correct rating for the motor being served by the controller.

[John HaineParticipant @johnhaine32865]

Good point, and I should have remembered that I did manage to blow the motor side fuse on mine!

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