AC/DC, Magnetism & Electrical Machines

DC flows in one direction. A battery-powered circuit is familiar: a battery maintains polarity, and conventional current flows from its positive terminal toward its negative terminal through an external circuit. Batteries, solar cells, and DC power supplies are common DC sources. DC voltage or current need not be perfectly constant; it simply does not periodically reverse direction.

AC periodically changes direction. In a common AC system, voltage and current vary with time, flowing one way during part of the cycle and reversing during another. Household outlets supply AC. In the United States, ordinary household power is approximately 60 Hz, meaning 60 cycles per second.

DC is useful for batteries, many electronic circuits, computers, portable electronics, and solar panels. AC is useful for power distribution, household electricity, many motors, and conventional transformers. Equipment often converts outlet AC to DC internally.

One hertz is one cycle per second, so 60 Hz is 60 complete cycles per second. Typical U.S. household supply is 60 Hz; some countries use 50 Hz. The common AC power waveform is a sine wave: it rises to a positive peak, returns to zero, falls to a negative peak, and returns to zero to complete one cycle.

Frequency is how quickly cycles repeat. Amplitude is the maximum size of a changing quantity. Higher frequency means more cycles per second; greater voltage amplitude means a higher peak voltage. Household AC voltage is commonly specified as RMS (root mean square) voltage, its effective heating comparison with DC across a resistor. No RMS calculation is needed here.

Magnets have north and south poles. Like poles repel; opposite poles attract. Outside a conventional bar magnet, field lines point from north toward south, and closer lines indicate a stronger field.

Iron, nickel, and cobalt respond strongly to magnetic fields; copper is not ferromagnetic. A current-carrying wire creates a magnetic field around it. Reversing the current reverses that field's direction.

A solenoid is a wire coil that produces a magnetic field. A suitable iron core can strengthen it. Increasing current, increasing coil turns, or using a ferromagnetic core can make a conventional electromagnet stronger when other conditions are similar.

Electromagnets are used in relays, motors, lifting equipment, solenoid actuators, and electrical machines. A permanent magnet needs no continuous current. An electromagnet's field can usually be switched, adjusted, or interrupted by controlling its current.

If a magnet moves toward or away from a coil, magnetic flux through the coil changes and can induce voltage. If the coil is a complete path, that voltage can produce current. Magnetic flux is the magnetic field passing through an area. Changing magnetic flux produces induced voltage.

Induction can occur when a magnet moves relative to a coil, a coil moves relative to a field, or the field through a stationary coil changes. A stationary magnet beside a stationary coil does not continuously induce voltage if flux is unchanged. Faster flux change, more coil turns, or greater field change can increase induced voltage. Lenz's law states an induced current's magnetic effect opposes the change that produced it.

A transformer has a primary winding, secondary winding, and magnetic core. AC in the primary creates changing flux through the core and induces voltage in the secondary. A step-up transformer has more secondary turns and increases voltage. A step-down transformer has fewer secondary turns and decreases voltage. Transformers do not create energy.

For an ideal transformer, V_secondaryV_primary = N_secondaryN_primary. The voltage ratio approximately equals the turns ratio. A conventional transformer requires changing flux, so it uses AC or another time-varying input; constant DC produces no continuous secondary induction after the initial transient. Higher voltage can lower current for a given power transfer, reducing transmission-line losses.

A motor converts electrical energy into mechanical energy. Current-carrying conductors in suitable magnetic fields experience forces, producing motion, usually rotation. A simplified motor has a magnetic field, current-carrying coils, and a rotor. Some DC motors use brushes and a commutator to reverse current at appropriate points.

A generator converts mechanical energy into electrical energy using electromagnetic induction. Motion from wind, water, steam, or a hand crank produces changing flux through windings and induces voltage. An AC generator produces alternating voltage; a DC generator uses a suitable commutator or conversion method for a unidirectional output. Some machines can operate as either under suitable conditions.

Resistance opposes current; voltage and current in an ideal resistor rise and fall together in AC. Inductors oppose changes in current, and inductive reactance increases with frequency. Capacitors oppose changes in voltage, and capacitive reactance decreases with frequency. An ideal inductor becomes a conducting path after steady DC settles; an ideal capacitor blocks steady DC after charging. Impedance is total opposition to AC, including resistance, inductance, and capacitance, and is measured in ohms (Ω).

Electronic devices often need DC from an AC outlet. A rectifier converts AC to unidirectional current, filters can smooth it, and a regulator may maintain DC voltage. An inverter converts DC to AC, such as battery electricity to AC for compatible equipment.