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DC injection braking and a burned out transformer

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-20190425transformer.jpg

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0 Chris Pearson over 4 years ago

I have stripped down the lathe, replaced bearings and seals, etc., and turned my attention back to the electrics this weekend.

I have replaced the transformer and the DIB works, but it is terribly fierce and brings the spindle to a dead stop almost instantaneously. (This brief clip shows a similar lathe stopping quickly - just before the end.)

The new transformer is rather bigger than the old one, so I may have over-egged the pudding, but would the lower resistance in the secondary (0.49 Ω as opposed to 1.16 Ω) make much difference? I selected a 230/110 V transformer and it is giving an output of 116 V today.

Then I wondered whether the wiring diagram might be wrong - was the output voltage in error? So I unwound the old transformer, which was a messy and tedious job with all the scorched paper and so on. The secondary had 123 turns of 21 SWG. The outer part of the primary had 29 + 55 + 268 turns of 28 SWG plus a further 352 turns of 25 SWG. It was this innermost part with the thicker wire that was connected. With 240 V input, that gives 84 V output, which is not consistent with the wiring diagram's figure of 130 V, but not grossly different from the new set up.

Looking online, I have found a few references which suggest that the DC current should be 2 - 2.5 x the full load current. The motor's plate gives 6.4/3.7 A at 240/415 V respectively. 6.4 x 2.5 = 16 A and in fact I measured the max current as 17 A over about 2 seconds when the brake is applied, so again, not grossly excessive.

Now what should I do? I think that the brake needs to be tamed somewhat. I don't want to go to the expense of buying another transformer if I can help it, but I was wondering whether to put something like a 4 Ω resistor in the braking circuit.

Sensible advice gratefully received.
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0 Chris Pearson over 4 years ago

I have stripped down the lathe, replaced bearings and seals, etc., and turned my attention back to the electrics this weekend.

I have replaced the transformer and the DIB works, but it is terribly fierce and brings the spindle to a dead stop almost instantaneously. (This brief clip shows a similar lathe stopping quickly - just before the end.)

The new transformer is rather bigger than the old one, so I may have over-egged the pudding, but would the lower resistance in the secondary (0.49 Ω as opposed to 1.16 Ω) make much difference? I selected a 230/110 V transformer and it is giving an output of 116 V today.

Then I wondered whether the wiring diagram might be wrong - was the output voltage in error? So I unwound the old transformer, which was a messy and tedious job with all the scorched paper and so on. The secondary had 123 turns of 21 SWG. The outer part of the primary had 29 + 55 + 268 turns of 28 SWG plus a further 352 turns of 25 SWG. It was this innermost part with the thicker wire that was connected. With 240 V input, that gives 84 V output, which is not consistent with the wiring diagram's figure of 130 V, but not grossly different from the new set up.

Looking online, I have found a few references which suggest that the DC current should be 2 - 2.5 x the full load current. The motor's plate gives 6.4/3.7 A at 240/415 V respectively. 6.4 x 2.5 = 16 A and in fact I measured the max current as 17 A over about 2 seconds when the brake is applied, so again, not grossly excessive.

Now what should I do? I think that the brake needs to be tamed somewhat. I don't want to go to the expense of buying another transformer if I can help it, but I was wondering whether to put something like a 4 Ω resistor in the braking circuit.

Sensible advice gratefully received.
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DC injection braking and a burned out transformer

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