Practicals, transformers and too much heat...

[jonathanhill]

A show that I was working on last year transferred to town, and a decision was made to retro-fit the twenty nine low voltage practical circuits from cheap dimmable electronic transformers to toroidal transformers. This was mainly to eliminate the 30-way load box that had been living on fly floors on tour. Twenty two of the circuits were using 12V 20W lamps, the other seven were 12V 50W MR16 lamps. Appropriate toroidal transfomers were selected from CPC's catalogue, and six way transformer boxes, Soca in, Speakon out, were constructed (Speakon was chosen as it is a four-pole connector and seven of the practicals were double circuit) and then installed into the venue.

 

During the production week, a problem was noticed with the seven double circuit footlight practicals. A number of the toroidal transformers overheated, initially thought to be a dead short within the SELV side of the circuit. This was investigated and the 'faulty' units and the SELV feeds from the transformers were rewired or replaced. The problem persisted over three double circuit units and on both circuits of those units. No other practicals were affected.

 

Does anyone have an explanation for this occurence? Due to a lack of time on site and other jobs requiring attention I did not have time to fully investigate the cause and eventually the toroidal transformers for the double circuit units were all re-fitted with the dimmable transformers (and the load-box re-instated). This sorted the problem but I would dearly like to know what was causing the overheating.

[adam2]

How many lamps, of what wattage were on each transformer ?

Toriodal transformers are available in a wide range of wattages, with the smallest being only 50 watts.

 

Are you certain that all the "20 watt" lamps are in fact only 20 watts and have not been replaced with larger ones.

 

Are the transformers supplied from a dimmer, even if on full, a dimmer might distort the waveform suffiently to cause heating.

 

Are the transformers sufficiently ventilated, most such transformers are used in ceiling spaces with plenty of ventilation, if enclosed in a box they may need a cooling fan.

 

Are they in fact overheating, some types can get alarmingly hot and still be within spec.

[jonathanhill]

How many lamps, of what wattage were on each transformer ?

Twenty two 12V 20W lamps on twenty two 30VA transformers, seven 12V 50W lamps on seven 50VA transformers.

 

Toriodal transformers are available in a wide range of wattages, with the smallest being only 50 watts.

 

Are you certain that all the "20 watt" lamps are in fact only 20 watts and have not been replaced with larger ones.

Yes.

 

Are the transformers supplied from a dimmer, even if on full, a dimmer might distort the waveform suffiently to cause heating.

 

Yes, all supplied from house dimming, Permus as I recall.

Are the transformers sufficiently ventilated, most such transformers are used in ceiling spaces with plenty of ventilation, if enclosed in a box they may need a cooling fan.

Six transformers per box (3 x 30VA, 3 x 50VA) for three boxes, five transformers per box (5 x 30VA) for two further boxes.

Are they in fact overheating, some types can get alarmingly hot and still be within spec.

 

Oh yes, getting so hot that the plastic coating melts.

[Andrey]

Hi, Jonathan.

 

I think your problem is in using of toroidal transformers. It could not be controlled by Phase Control Dimming systems, because that style of dimmers when not in FULL has non harmonic form of voltage on its outputs. That’s why transformers are heating. Do you remember dimmer levels on channels where transformers were broken? It must be lower than on channels where transformers worked well (I think well worked circuits had levels close to FULL). To solving overheating of transformers try to use sinewave dimmers.

[jonathanhill]

Hi, Jonathan.

 

I think your problem is in using of toroidal transformers. It could not be controlled by Phase Control Dimming systems, because that style of dimmers when not in FULL has non harmonic form of voltage on its outputs. That's why transformers are heating. Do you remember dimmer levels on channels where transformers were broken? It must be lower than on channels where transformers worked well (I think well worked circuits had levels close to FULL). To solving overheating of transformers try to use sinewave dimmers.

 

All channels were either on or off, except for the 50W lamp in the centre footlight. Most birdie transformers are toroidal type, and only three units were affected.

 

The show has now closed.

[Andrey]

All channels were either on or off, except for the 50W lamp in the centre footlight. Most birdie transformers are toroidal type, and only three units were affected.

 

The show has now closed.

 

It is very strange. Only I can to suppose – it’s a fallible interturn insulation inside of transformers.

[jonathanhill]

One thing that I didn't have time to do was to change the dimmer hook-up for the faulty units to see if it was a dimmer issue.

[David Duffy]

Maybe not related but, something to remember with all toroidal transformers. Do not let the top of the mounting bolt make contact with the metal case. If it does, the bolt acts as a shorted turn and high currents will flow, causing damage. ie. The transformer must be mounted from one side only, not both.

[themadhippy]

You haven't created a short circuit with the fixing bolt through the middle of the transformer by any chance?

[jonathanhill]

Transformers were mounted as per instructions using th neoprene washers etc.

[boswell]

The only point that I can add is that you had 50w bulbs on 50va transformers ie 100% loading so the transformers would be running at the limit even on hard power. I would have used >50VA transformers, around 60-80VA to allow some spare capacity and cooler running

HTH

[jonathanhill]

The only point that I can add is that you had 50w bulbs on 50va transformers ie 100% loading so the transformers would be running at the limit even on hard power. I would have used >50VA transformers, around 60-80VA to allow some spare capacity and cooler running

HTH

 

Fair point, but as with most things, cost dictated. Interestingly, the overheating also occurred on the 30VA transformers that were running 20W lamps. Bags of headroom there.

[boswell]

According to your original post (as I understand it) only the 50VA transformers actually failed, so although the 30Va ones ran hot, they did not overheat to the point of failure.

[jonathanhill]

According to your original post (as I understand it) only the 50VA transformers actually failed, so although the 30Va ones ran hot, they did not overheat to the point of failure.

 

What may not have been clear is that the failures were on units which had two individual circuits, one 12V 20W lamp (30VA transformer) and one 12V 50W lamp (50VA transformer). Failures occurred with both circuits on three units (which were in different six-way boxes).

[KevinE]

The only point that I can add is that you had 50w bulbs on 50va transformers ie 100% loading so the transformers would be running at the limit even on hard power. I would have used >50VA transformers, around 60-80VA to allow some spare capacity and cooler running

HTH

 

Lighting transformers should be sized according to the lamps in use and not 'under run' for the sake of the transformer, because toroidals arent particularly well regulated and an under loaded one will lead to poor lamp life due to elevated sec voltages. This is why a blown bulb in a daisy chain of halogens should always be replaced as soon as possible to prevent the others being overrun, the exception being electronic transformers which of course have a degree of regulation within stated loads eg 15-50VA or 60-100VA.

 

Were they specifically lighting toroidals? If so, they usually have a one-shot thermal fuse in them as they're meant to be installed under floorboards or in ceiling voids and must not catch fire during an overload!

 

Is it possible the dimmer channel was faulty slightly? Ive had this before where a triac has been replaced for the wrong one or an RC network has failed so that the dimmer becomes 'fussy' about what sort of load it will drive without distorting the waveform so much (eg half-waving) that the load overheats. It can happen to conventional transformers as well.