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| Transformer - strange current waveform and "overheating" ?? |
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| max_torque:
I installed an COTS 3kVA 240VAC single phase isolation transformer on a project: http://www.solahevidutysales.com/pdf/transformers/AutomationXfrmrs.pdf I wired the Taps to suit (240VAC primary, 240VAC secondary, ie 1:1 setup) The unit powered up, but sometimes trips the (12A rated) circuit breaker on it's primary feed when you first turn it on. Without any secondary load, the unit gets extremely hot after around 10 min of operation (i'd guess an external case temperature of around 80 degC). I measured the primary voltage and current, shown below: Yellow - Channel A = primary current via current probe at 100mV/amp Blue - Channel B = primary voltage it seems the current is extremely non-sinusoidal with 20A "Pulses" lasting 5ms lagging the voltage waveform, almost like something is saturating and the current is suddenly ramping up. The unit does provide a 240VAC output voltage, as measured on a multimeter - sorry, didn't get time to scope the secondary yet. So i guess questions to any transformer experts: 1) how hot should this sort of transformer run under no load? - rating is "80 degC rise at peak load 2) It's a 60Hz rated unit, we are at 50Hz, so i guess i'd expect a higher current ripple, but that should still be more sinusoidal surely? 3) Measuring the primary and secondary taps with an LCR meter shows inductances as expected (around 70mH Prim and Sec when wired 1:1). If taps were miss-labelled and hence mis-wired, i'd expect to see odd inductances across the taps? 4) Could a shorted turn cause this sort of behavoir? |
| The Soulman:
Most likely it is saturating because it's running on 50Hz instead of the intended 60Hz. Transformers these days are designed and build to such a precision that they function to spec but not beyond that, just to save few pennies on resources. |
| richard.cs:
Agreed, it looks like it's saturating. You don't say which part number you have but if it has an option to wire the taps for a higher voltage you could try that. A transformer designed for 240 V 60 Hz is only usable at 5/6 the voltage, or 200 V on 50 Hz unless it was over-specced in the first place. 80 C case temperature suggests a much higher winding temperature, if you leave it on long enough it will probably overheat and fail. You should probably just replace it with a transformer designed for 50 Hz use, it's not the right component for you. |
| max_torque:
I had considered saturation from 50hz operation, but it just doesn't feel quite right to me for several reasons: 1) The transformer is sold in the UK by a major distributor - (RS components) surely they would have noticed if everyone returned the units they bought? 2) There is no "min frequency" spec for the unit in any documentation for it. Surely, if it saturated at 50Hz, it would tell you that? 3) Looking at the wave form, a minor reduction in the time for each cycle (5/6ths) won't be enough to prevent the current rising suddenly, because the rise time only takes 1 ms? I can certainly try connecting to a high voltage tap on the primary and seeing what happens! |
| max_torque:
Unit bought from here: https://uk.rs-online.com/web/p/wall-mount-transformers/8908801 no mention of not being suitable for 50Hz. |
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