Help wanted, DC motor speed controller issue

I have an issue on a motor speed controller that I've been unable to solve (I seem to have created or uncovered a different issue!?)

This lathe has never run and the guy who gave me it had been tinkering with stuff, he perhaps knew even less than I do as I found one of the motor wires connected to field terminal by mistake, I'm not sure if this could have damaged something.

Any, there was a bank of 3 diodes and 2 thyristors, all but one thyristor seemed to function as it should so I went ahead an replace it with a like for like unit.

The machine was constantly outputting 110v DC seemingly regardless of pot position (but it did pop the 3a or 5a plug fuse when turned up despite not changing the voltage) - now after changing the tyhristor, it just trips the mains out.

I checked the motor brushes and communtator and after cleaning it all seems absolutely fine, it'll run from a little 9v radio battery smooth as silk, and generates quite a bit of power, when run from a 12v supply, more smooth running, more speed, more power, it looks good.

I'm afraid my knowledge on this repair comes from a google search and a bench test based on my limited knowledge of what is to me, a complex piece of electronics.

Can anyone recommend or offer any expert advice or diagnosis/repair services please, I believe these boards are known universally as KBIC or something. Mine is a BC2000-tb and there is a comprehensive manual online detailing many many different things, wiring configurations and all sorts, it seems like quite a useful board...... if it worked.

The controller should have two outputs to the motor. The one that goes to the field winding will be a fixed voltage. The one that connects to the brushes will be controlled by the speed potentiometer. Which connection did you get a constant reading of 110 volts ? I have not been able to find the schematic online for your speed controller But I do have a schematic of the KBIC 240D controller. If yours is a similar design it would explain why you get a reading of 110 volts rather than about 240. The 240D uses half wave rectification for the field supply so you will be reading the average voltage of half wave rectified 240 volts. I suggest that you measure the voltage to the armature and see if that is present and varies with the speed potentiometer setting. It would be helpful if you could provide a link to the information on your controller and any information on the motor.

Les.

If you just want it to work, rather than enjoy the fun of fixing it then a couple here at unspecified clearance price:

http://www.toolco.co.uk/categories/clearance-electrical-items

(Not a recommedation but popped up when I searched to see what it was you were struggling with)

How many wires go to the motor? You mention a field terminal there, but also later on that the motor runs connected to a 9V supply. If the motor has a wound field there should be 2 wires to the field and 2 for the commutator, though it's conceivable that they common one end of each winding to get only 3 wires. If it is wound field it probably wouldn't be keen on running from only 9V.

Hi John,
I asked for information on the motor in case it is not a shunt wound motor. It is possible that it is a series wound motor that someone has attempted to use as a shunt connected motor.

Les.

Hi,

The motor just has two wires, the documentation I found online for the BC2000-TA explained to me what the F+ and F- terminals were for, and that is how I knew whoever connected the motor had done so by mistake.

When I measured 110v I had the board wired correctly (to the best of my knowledge and by the book) - I assumed this was because one of the thyristors was stuck closed circuit and passing half the rectified AC sine straight though (again, I'm not knowledgeble on this stuff, but at the time I think I recall seeing that one thyristor was connected to one of the diodes, and the other thyristors was connected to the other diode, so I assume they combined the two halves of the AC in varying amounts, and the 3rd diode I guess did something else, but, I don't know, I'm not really clued up on complex electronics like this, I can wire up basic machines and follow diagrams, that's about it!)

This link is also referencing a KBIC, and later refers to two models, a BC2000 and a BC2200, and everything in it, seems to match almost exactly to the board in hand.

I have taken things like the rotary switch and NVR etc out of circuit although I have tested them all and they do work as they should so I'm focused on the board. There was also some sort of filter box or something, I recall trying with and without it, it made no difference before or after changing the S6025L thyristor

A very general comment: Depending on the design, power switching circuits can fail catastrophically if one element fails, and a fault can easily kill many other components around the failed one, faster than any fuse can operate.

At work many moons ago, I took over a colleague's repair of a switching power supply in a television monitor. I had to replace about four beefy output transistors that had failed. My colleague walked in and said "I just changed all those". "Yes, says I, and yours all blew again", (because the original fault had still been present.)

Hopefully not the case here, but just to warn you.

You have not answered the question of which connections measured 110 volts (Field connections or armature connections.)
You start by saying "This link" but you do not give any details for the link.
Two of the diodes and the two SCRs form a phase controlled bridge rectifier. This output will be to the armature and the average voltage output is controlled by how much delay there is between the zero crossing point of the AC waveform ant the point at which the SCRs are triggered.
The 3rd diode provides a half wave rectified of a constant voltage for the motor field winding.
As the motor only has two wires coming out of it you will need to dismantle it to bring out the field and armature connections separately.

Les.

The Chinese lookalike boards similar to the KBIC control boards are essentially copies using thru hole components, although some later derivatives use smd devices too.

The mains AC is used to drive a full wave bridge rectifier comprising 2 S*025L thyristors and similarly rated diodes.A third diode is connected directly across the output. The F+ and F- supply is generated using two low current diodes( typically around 1 amp).

The full wave bridge uses phase angle control with a zero crossing detector to generate the speed control.

A startup ramp is generated using a resistor and capacitor network that is adjustable to give a soft start, current overload sensing and motor load compensation is also featured on these boards.

The Seig speed controllers are similar but principally operate the same way.( are some subtle differences in how the 12v rail is provided).

Repairing these boards is not for the faint hearted.
Be also aware that the scr/diode devices that are heatsinked on the KB boards have isolated tabs, the Best controllers I’ve encountered are isolated using mica washer sets, the devices not having isolated tabs…Using the wrong type of device can result in failure of neighbouring parts.

Fake parts abound, beware where you buy replacements from

Edited By John Rudd on 27/01/2022 13:09:11

Sorry Les,

110v was measured at the armature connections.

I don't think there is a field, its a brushed permenant magnet motor, all in good condition and running smoothly to my untrained eye.

The manaul is here Link I didn't do the link correctly last time.

John,

I wrapped my head around the concept of the full wave bridge using SCR's to control the output, but I'm afraid the purpose of the diode across the output is beyond my understanding, when I think of a diode I think of a check valve (converting in to my more familiar mechanical/pneumatic engineering knowledge). Also these have 3 pins, I don't know if each one is a pair of diodes sharing a common annode/cathode or if one pin floats - I should know because I tested them and believed them to function, I have perhaps have misunderstood them but without reviewing the data sheet and what/how I tested them I can't say either way.

As for mica washers..... the board and wiring had been fettled and given that the armature was accidentally connected to the field terminal adjacent I would guess finer details like mica washers slipped the attention of whoever dissasembled it in the first place. That said, when I tested it, it was on a non conductive bench, out of the machine, the heat sink was attached, however where as the SCR's and Diodes connected dry, directly to the annodised aluminium, I slipped a 0.3mm thermal pad under them believing it would be useful?

I feel like I shouldn't be touching this at all but as it didn't work, I felt I had nothing to lose and when I identified a clearly faulty SCR I thought I'd cracked it! I hope I haven't done more harm that good!

I realised after re reading you first post that I assumed it was a shunt wound motor with you mentioning field connections. It is useful to know it is definitely a permanent magnet motor. What is the information on the plate on the motor. (Voltage, current wattage , horsepower.)
I had assumed that the motor did not run at all when connected to the controller. Is this assumption correct ?
Can you post the circuit of exactly you have wired it up. (This is with it wired outside the machine to simplify things.
You are correct in connecting a permanent magnet motor to the A+ and A- terminals.
What is the part numbers of the original diodes and thyristors and the one you fitted as a replacement ? I ask this as some devices have insulated tabs and some have the tab connected to one of the terminals. Depending on the type just an insulating pad between the device and the heatsink may not be enough to insulate it from the heatsink. The fixing screw could connect the tab on the device to the heatsink. A photograph showing the diodes and thyristors in YOUR unit would help to clarify this potential problem.
Can you measure the voltage between the potentiometer connections P1 and P3. Then measure the voltage between P1 and P2, first with the potentiometer fully anticlockwise then about half way and then fully clockwise.

Les.

Les, this information only partially covers your request, more information to come tonight.

No information on the motor plate, perhaps its once had a sticker on it, but I doubt it!

The lathe itself has a bit of info, listing 240v, 50hz, 3.5A and 50-2500rpm (via 2 belt ratios).

The speed controller board lists:

BC2000-TB
DC Motor Controller

Input 115/230VAC, 50/60hz, 1PH, 9A
Armature 0-90/0-180VDC, 6A
Field 100/200VDC, 1A

Best Control Inc. Made in Taiwan

The Original Diodes (still in place) read D8020L 9E5HB
The SCR's were S6025L 9A3LV, and I replaced that with an S6025L OE126 - and after reading your comments and looking into it, I believe the L equals relates to an insulated tab.

As for wiring, could I refer you to page 13 on the manual linked above, with the exception that my P1 and P3 terminals are reversed vs this image (if I am to trust the printing on the circuit board). And also my machine is configured with a single fuse in the incoming live, and another in the motor A+ side, neither of which have been problamatic so far (both F5A if I recall correctly but don't hold me to that). My 3pin wall plug had a 3A fuse which is perhaps a little low if the lathes 3.5A rating plate is to be trused but I thought it would be ok for testing without load. I did try a 5A, it blew that too.

I will recap the missing points tonight, retest the SCR's and Diodes and also retest to ensure I'm providing accurate information, because I don't understand fully what is going on it would to easy for me to get some information wrong as it's been a little while since I tested it.

Russ,

The following notes might aid your fault finding...

The component designations are for a KB controller, they may be different to that of the Best item...

(A) Check resistance across L1 and L2. Reading should show infinite or open circuit. Be sure to reverse leads across L1 and L2. Reading should still be infinite. Measurement values shown below are approximate only .
If "0" ohms is observed, check field diodes D4 and D7.
With (+) lead on L2 and (-) lead on L1,
If .66 ohms is observed, replace diode D7
If .56 ohms is observed, replace diode D2.
With (+) lead on L1 and (-) lead on L2,
If .66 ohms is observed, replace diode D4
If .56 ohms is observed, replace power diode D1.
(B) Check resistance across F+ and F-. With (+) lead on F+ and (-) lead on F-, reading should be infinite. With reversed leads, reading should be 1.2 ohms. If reading is less, check diodes D4 and D7 (see above).
(C) Check resistance across A+ and L2. Reading should be infinite. If reading is less than infinite, check SCR2.
(D) Check resistance across A+ and L1. Reading should be infinite. If reading is less than infinite, check SCR1.

Les, John, I'm afraid time, tiredness and the other half watching TV has got the better of me tonight so this will have to wait, I'll get grumbled out if I trip the RCD which can happen from time to time when I'm messing with stuff I shouldn't be although I don't recall this ever cause it to happen!

John, this testing seems interesting, I presume D4, D7 and D2 are my 3 diodes, I'll have a look and see if they're referenced similarly. Do you have any diagrams or could point me to an image online/describe which is which.

As the SCRs and diodes have the L suffix they do not need to be insulated from the heatsink.. So that is one of the possible problems eliminated.

Don't worry about P1 and P3 being reversed. That would just swap over the low and high speed ends of the potentiometer. I would expect a reading of about 10 volts between these two points. The voltage on P2 With respect to P1 or P3 should change as the potentiometer is rotated,

Les.