Log in to the online community

Want to post a reply? You'll need to log in

DC injection braking and a burned out transformer

11 Replies

  • New Question

DC injection braking and a burned out transformer

Posted by Chris Pearson on Apr 25, 2019 10:56 pm

I have been fettling a lathe which was fitted with DC injection braking. I was a bit unsure about the condition of the components in an enclosure on the back, but having looked up a bit about motor control circuits and having learned about ladder diagrams, I have satisfied myself that I know what has happened.

At some stage, a transformer (240/130V) has burned out. That leaves two questions: (1) why? and (2) what is the spec of the old one?

The horizontal surfaces of the enclosure and its contents have been covered by a thin brown layer, which I assume is vapourised enamel from the transformer windings, but there is no evidence of any further damage.

Father taught me never to replace a fuse without finding the fault which blew it. Along the same lines, I want to be satisfied that none of the other components has caused a short circuit. However, it appears that it is the primary which has overheated. So my first question is what would happen if the secondary of a transformer is shorted. Would it be fried? Woud the primary be fried? Or would both be fried?

My second difficulty is determining the spec of the transformer. The circuit diagram has "N" and "240" on the primary side, and "Z" and "130" on the secondary side. I assume that this means that the secondary provides 130 V, which after rectification, is fed to the motor. The problem is that there are no markings on the transformer. The manufacturer of the unit ceased trading 10+ years ago, so no help there. The motor plate specifies 6.4/3.7 A. The transformer wires (both sides) are about 4.2 sqmm CSA so good for 3 - 5 A (?). Based on a weight of 4.0 kg and the transformer's dimensions, and comparing them with current models; I am guessing that a 320 VA transformer is required. My second question is whether my above reasoning is sound.

Before anybody else says it, yes, I think that I could get a modern module which will do the job, but isn't it more satisfying to fix rather than replace?

eabde54935c495ab8feb4fb009758b5e-huge-20
 

Re: DC injection braking and a burned out transformer

Posted by mapj1 on Apr 26, 2019 7:58 am

Ah I love the smell of hot varnish first thing in the morning...
Based on many teenage attempts to rewind transformers that looked a bit like that, I'd suggest this has most likely suffered from shorted turn syndrome - where the insulation has failed between adjacent layers of  winding. Once a shorted turn has formed, the primary current is very high even under no load, and total failure soon follows. 
If the failure is in the outer layers  it is more often due to mechanical damage, if internal failed, and more cooked inside than out, then usually slow cooking due to poor cooling/sustained overload.
The wattage you can estimate from the iron area of the central leg as this will give you a volts per turn, and then if you can estimate the wattage from no of turns and  wire size.  Equally, assuming the C to T ratio of the laminations is pretty much optimum, then a Tx of the same weight is a good measure.
The modern trend is to include a thermal trip in the winding bundle, which makes this sort of thing much less dangerous  than it once was. If you are short of space a toroid and a fan might allow more watts per cubic inch, which translates to an ice-cold solution, which in turn means reliability.

What is the 2ndry load - can you sketch out the rectifiers and so on - it may never have been man enough, or there may be a failed diode in the rectifier causing an overload.
IF you can get a rectifier part no. we can get a supporting figure for the design load. If  it has been off for a few years, be aware that electrolytic caps tend to fail with a bang if brought back on the full volts suddenly - I sometimes  run old electronics if the 1960s an earlier with a lamp in series to give a trickle charge for a few mins before first power on. Also if there is a fault, then the lamp comes on, which  is a safe failure mode.
regards Mike

Re: DC injection braking and a burned out transformer

Posted by Zoomup on Apr 26, 2019 8:07 am

Apart from what Mike has said, perhaps the transformer has a duty cycle of use of perhaps 10 minutes per hour or so. If overused it does not have time to cool down. I installed a new transformer for a  E.L.V. shower pump recently that had a duty cycle of 15 minutes per hour, and that is a new modern design.

Z.

Re: DC injection braking and a burned out transformer

Posted by mapj1 on Apr 26, 2019 8:48 am

There are too many wires in that photo vs your description - where do they all go ?



for double wound design, for a toroid 4 kg is about 400-500watts at 50Hz, for a conventional core like this, more like 250VA-300, so your estimates are about right, except, that would only be an amp at 230V or two (115V) in the windings, which does not match your observation about wire size.  Therefore  is it possible it could be an auto transformer rather than double wound ?  I know it has failed, but have you tried with a meter to see how do the windings meggar up - are primarily and secondary isolated? Or are these just short tails of multi-strand, that are sized for mechanical reasons, and the windings are actually in something much thinner.?

If it is for the motor, then perhaps someone has stalled the motor in the past.
regards Mike

Re: DC injection braking and a burned out transformer

Posted by kfh on Apr 26, 2019 9:00 am

Further to Zoomup's comment. I recently replaced a DC injection module in a saw and was surprised at the low  No. of stops per hour both the replacement and other modules I looked at were rated at, from memory about 5. These were all modern electronic modules crammed onto the smallest circuit board possible and i would have expected an old  transformer type to be more robust.

I agree with your sentiment about repairing rather than replacement. Keep us informed of your progress. 
Kevin

Re: DC injection braking and a burned out transformer

Posted by Chris Pearson on Apr 26, 2019 11:41 am

Gentlemen, thank you for your responses.

Oops! I slipped a decimal point there. The CSA of the connecting wires is 0.42 sqmm. I have pulled it apart some more. The CSA of the secondary is 0.49 sqmm.

The extra wires in the photo are redundant - presumably for different supply voltages.

The CSA of the central leg is 2574 sqmm. I haven't counted the number of turns yet.

Over-use in the past is a distinct possibility. It came with a capstan attachment, so it may have been used for making batches of small components, which would have required frequent stopping and starting. (The thing that I miss from my Myford Super 7 is the clutch which allows the motor to keep running. It also gives a better degree of control.)

The rectifier is a KBPC2502. It conducts in the correct directions.

A section of the circuit diagram is below.

Whilst it wouldn't be a disaster if the injection brake were left inoperative, it would be nice to fix it and I am confident that it can be done.

a2342dedc00c98ea0f292e7e5d5bee36-huge-20
B is the brake relay.

Re: DC injection braking and a burned out transformer

Posted by Sailbadthesinner on Apr 26, 2019 12:16 pm

I hope it was never wired as shown in the drawing.  The bridge is incorrect!  As shown it gives a short across the secondary for one half cycle.
It can't have been wired this way originally as it would fail immediately but if it was rewired to the drawing that would explain its demise.

Re: DC injection braking and a burned out transformer

Posted by mapj1 on Apr 26, 2019 1:32 pm

Well spotted- the AC inputs should each see one kathode and one anode, and the positive output should be 2 kathodes, and the negative output should be 2 anodes.

The  data sheet for the rectifier indicates it is a 25 amp part, rated at  a mere 200v for reverse breakdown.  A rectifier rating of 25A forward feels like overkill, given what you said about the wire sizes, though that rating assumes you have it on a 25 degree C cold plate. However the 200V reverse, (noting a 110V RMS is more like 145v pk) seems to leave very little margin for the sort of 'flash and pop' surges often seen when contacts close on motor shuddering to a halt.  (the data sheet notes that 140V RMS is the limit for the 200V part.) However, if it passes the 'am I a diode' tests with the meter, then you are OK to keep it in there. (there are plenty of similar parts made of varying ratings  examples of bridges from the CPC catalogue. )


The current rating of wire when wound into a transformer is a funny function of the wire packing density, and its ability to cool via conduction through the rest of the winding, and the fact that the skin depth is far more serious than normal, because the magnetic field vs current is higher than the free space case. However the key thing is that the BS7671 wire rating values are miles off.
Luckily others have done the work for us , and there is a good reference for wire size versus cu rent in winding for low power 50Hz mains transformers here,  though in summary about 3-4 A per mm2 seems about right and if your wire sizes are  CSA in  mm2, then suggests only an amp or 2 for the windings you have. (I did say the BS7671 values were miles out for transformers didn't I..) This is more like 250VA, so not fitting that well with the 4 kilo weight. It may never have been fully optimised, if it predated CAD, that is quite likely.
Indeed if you are very brave, his "how to roll your own" article from the same website is very informative too see here.  Nowadays I only would only ever countenance this if I had a need to replace some special part for for some very odd voltage, as life is too short, and there are plenty of transformers in the catalogues, but I remember winding replacement transformer of a couple of hundred watts  with about ten secondaries to restore an oscilloscope many years ago, and it taking forever (probably about 2 weeks of evenings) just to wind the thing. I have used a lathe on freewheel as a manual winding machine, but even so it quickly gets painful, and hand winding never seems to manage to get quite as many turns in as the original manufactured item.


 
regards Mike

Re: DC injection braking and a burned out transformer

Posted by Paul Skyrme on Apr 26, 2019 7:46 pm

Just remember that there is no requirement per se for a DC injection brake on a metal working machine.
In fact, I know of one example where by the clever persons fitted a brake and unscrewed the lathe chuck the first time they tried it!...
Also, note that if this is not a DIY project that there is legislation around this which may need to be complied with that you need to be careful of.
A DC injection brake is part of a safety function, maintenance is one thing, refurbishment is another, and requires careful consideration to ensure compliance.
Paul Skyrme MEng, BSc(Hons), IEng, MIET, MSOE, MBES, TechIOSH, CMSE®. #e5 Founder Member

Re: DC injection braking and a burned out transformer

Posted by Chris Pearson on Apr 26, 2019 11:10 pm

Paul Skyrme:
Just remember that there is no requirement per se for a DC injection brake on a metal working machine.
In fact, I know of one example where by the clever persons fitted a brake and unscrewed the lathe chuck the first time they tried it!...
Also, note that if this is not a DIY project that there is legislation around this which may need to be complied with that you need to be careful of.
A DC injection brake is part of a safety function, maintenance is one thing, refurbishment is another, and requires careful consideration to ensure compliance.

I take the point, but no risk of unscrewing the chuck. I have considered switching the supply to the brake. When in use, it would be energized, but before routine stopping it could be isolated so that the lathe could slow down gently. It would appear to be better for the drive and bearings, etc.

Of course the easiest approach would be to remove the whole thing, but having turned my attention to the purpose of the box on the back, I find myself driven to repair it. Isn't that what we do? 🙂

Re: DC injection braking and a burned out transformer

Posted by Tony S on Apr 27, 2019 1:22 pm

Paul Skyrme:
Just remember that there is no requirement per se for a DC injection brake on a metal working machine.
In fact, I know of one example where by the clever persons fitted a brake and unscrewed the lathe chuck the first time they tried it!...
Also, note that if this is not a DIY project that there is legislation around this which may need to be complied with that you need to be careful of.
A DC injection brake is part of a safety function, maintenance is one thing, refurbishment is another, and requires careful consideration to ensure compliance.

You beat me to it. I’ve seen a lathe chuck come off, two brick walls didn’t stop it. It went in a nice graceful arc across the workshop before vanishing though the first wall.
 

Re: DC injection braking and a burned out transformer

Posted by mapj1 on Apr 29, 2019 3:49 pm

of side interest , there are other ways of slowing an AC machine, including to put a capacitance and a load  accross it so it becomes an induction generator - this is most effective at higher speeds, as the generated voltage is speed dependant, and DC injection is better at stopping the shaft dead when it is already slowing.
various patents have expired .
 DC braking 1958  dc we understand
combining the bet of two methods DC braking which unlike normal DC injection,  still works in a powercut.
 
 1948 auto generator DC breaking 
In accordance with the present invention, direct current for dynamic braking of the rotor 11 following interruption of the line circuits at the contacts 14 is derived from the kinetic energy of the rotor itself thereby avoiding the use of external power for dynamic braking.  
I suspect with modern polymer film capacitors for generation and electrolytics for the DC smoothing capacitor this has become a lot more practical now than it would have been when first proposed - a set of  60uF paper capacitors for half horse power at 220v phase to phase would have added a significant volume and cost.
regards Mike

Share:

Log in

Want to post a reply? You'll need to log in