Saturday, August 4, 2018
General transformer problems
Temperature is too high
- Short circuit in the winding, at primary or secondary. Once shorted, the current will be high and cause rise in temperature. If this continues, not only the insulation layers will be burnt, and the adjacent winding will be burnt due to high temperature, and the vicious circles continues, until the whole transformer is burnt. This high temperature happens whether it is loaded or unloaded. The only fix is to remove the bobbin and remove and replace the burnt winding(s), or all altogether.
- High temperature may happen at small transformers and goes un-noticed, where there are a lot of turns in a single winding. If the adjacent magnet wire got short-circuited, since the voltage is low, wires are long and thin, the temperature may not reach the exterior, but it will not work for long.
Note: When there are shorts in the transformer, the no-load current will be higher than normal.
- Magnetic flux density (B) set too high - especially when using unknown transformer core/lamination. If B is set too high, causing the turns to be lesser than ideal. With reduced primary turns, the no load current will be too high, causing increase of transformer temperature. Even if secondary are unloaded, the transformer temperature will rise high. If you reduce the primary voltage up to a certain point, the temperature will be at normal temp, for such kind of transformers.
- Overloaded transformer - self explanatory
- Magnet wire is too thin - high DCR causing more power dissipated at the wires.
Electrocute feeling when touching the core
- If one touches the core and feels a slight tingle or eletrocute feeling, this is due to parasitic capacitance between the winding and core. You can measure a certain Vac value on the core with respect to ground. This may be resolved by adding grounding to the core or mount it at a grounded plate.
Magnetic flux leakage is high - causing noise
-When powered on, the field generated by the winding should follow the path formed by the core, and should have low magnetic path resistance, to enable a high efficiency transformer. Due to various reasons, if the path is not smooth, leakage form where magnetic flux to goes through the air instead (magnetic flux leakage). That could happen to uneven surface between the cores (gap), or cores not tighten enough with the E/I lamination, or the air-gap laminate is too thick, and various other reasons.
- If the transformer works alone, the impact is not much and may not be noticeable. If it is mounted in jam-packed equipment with lots of sensitive electronics, the magnet flux leakage will be detrimental, causing AC hum to the adjacent components (therefore, giving EI transformer a bad name).
- To check for magnetic flux leakage, one can wind a 200-300 turns of wire on an iron nail, connecting it to a high impedance sensitive headphone, and move this to a powered transformer with magnetic flux leakage, and see if there are audible hum generated in the headphone.
- To avoid this, the lamination must be even so that the interface is as close as possible. This is even more important when using refurbished cores. It has to be clean enough and polished to ensure the closest interface. OTOH, transformer designer should not design it close to the maximum magnetic flux density. Also, the copper band will come in handy to prevent such leakage from affecting the environment. Distance may help too since the density drops as it moves further away from the transformer.
Noise when powered
- Ringing / hum when powered - this could be due to loose lamination. Lamination could get loose in many ways - abuse, drops, not tightened enough, and etc. Low grade core with un-eveness in the cores (not flat enough).
- Short circuit in the transformer, where there is high current flowing., causing the lamination to rattle, and heate up.
- Misaligned lamination - you can see the un-eveness alignment of the lamination when the whole transformer is constructed. Such zig-zag lamination will cause noise. This should be easy to see externally.
So, you have been empowered now with the knowledge to know the general transformer problems. Choose wisely.
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