Can a transformer be used with dc? what happens is a transformer designed for 120v ac is connected to a 120 v dc line?
No, it can't. If it is connected to 120VDC, the output will give an initial pulse that decays to zero and stays there. The input will draw far more current than designed, overheat, and either open up or catch on fire. Here is how a transformer works with AC: The input winding generates a magnetic field (because it is an electromagnet). The field alternates because the applied current alternates. The alternating magnetic field couples into the output winding where it induces an electric voltage that alternates because the magnetic field is alternating. Now here's the kicker. The voltage induced in the output winding is proportional to the rate of change of the fluctuating magnetic field. If the field is static (ie, steady, as is the case if the input winding is powered by DC), then the rate of change is zero and the induced voltage is zero. That's why the transformer can't transform DC. Other interesting facts: The input winding converts electrical energy into energy in the magnetic field that is then converted back into electrical energy in the output winding. The input winding resists DC current based on the winding's resistance. If the input current is alternating, then it resists the current additionally based on the inductance of the winding, and the higher the frequency, the more it resists. A transformer is designed to work on a specific frequency (eg, 50Hz or 60Hz). Operating it at 0Hz (DC) will allow too much current to flow. The higher the applied frequency, the more power the transformer can transfer. Think of it as though the transformer passes a bucket of energy from input to output on each cycle. The more cycles per second, the more energy per second, aka the more power. At 0Hz (DC), it isn't passing any buckets of energy. Hope that helps.
