What Is a Transformer? A device designed to transfer energy from one electrical circuit to another.
One of three basic uses for this: To increase the input voltage (“Stepping Up”) Used by power plants in order to send power large distances efficiently To decrease the input voltage (“Stepping Down”) Steps down power to 120v for your home To transfer electrical energy without a physical link (a “Buffer”) Used for sensitive electronics like computers, audio equipment, to prevent interference from other devices/electrical spikes.
How Does It Work? Four Main Principles Magnetic Flux A quantity of magnetism – the strength and extent of a magnetic field. Change in magnetic flux is the result of a magnetic field intensifying or weakening. For example, moving a magnetic closer or further from an object. Back EMF By-product of magnetic induction. When a magnetic field is created, a current is induced in the object being affected by the field. This current then creates its own magnetic field, which opposes the original inducing magnetic field. Faraday’s and Len’s Laws Any change in the magnetic environment of a coil of wire will cause a voltage (emf) to be "induced" in the coil. Emf = -Number of turns * ( Magnetic flux) / ( Time) Power and Conservation of Energy Power (w) = Voltage (v) * Current (I) If Power remains constant (conservation of energy), then Voltage and Current are inversely related.
If Np (number of primary turns) = Ns (number of secondary turns), then no change in voltage or current. If Np > Ns, voltage is decreased and current is increased. If Np < Ns, voltage is increased and current is decreased. If Np = Ns, then energy is simply transferred and no changes take place. Remember that: Vs/Vp = Ip/Is = Ns/Np However, it’s not so simple…
Efficiency – Special Considerations AC is the most common – remember, an emf is only produced when there is a magnetic flux. AC changes direction 60 times a second. DC can be used with a rapid switch. No change in direction means less induction, can be used in un-synched power grids, not prone to cascading failure, can transmit at higher voltages. Alternating Current vs. Direct Current Counter EMF means current induced within transmitting bar, caused by dipoles being rapidly reversed by AC. This leads to a lower rate of transmission, heat loss, vibration. Typically several thin slices of metal are lamented in order to reduce eddy currents. Eddy currents: Current flowing through windings creates resistive heat, which means loss of energy in transmission. Winding Resistance Magnetic flux in the core causes the core to rapidly expand and contract slightly, which results in frictional heat loss. Magnetostriction
Not 100% of the flux is transmitted to the secondary. This “leakage flux” can induce eddy currents in nearby structures (such as support structures), which can be converted into heat. Stray Losses Due to rapidly changing magnetic field from primary and secondary, vibrations can be generated which convert into heat. Mechanical Loss Hysteresis properties are properties of matter that to do not immediately react to a force, or do not immediately return to a previous state once experiencing a force. An example is silly putty, which will assume a new shape if pushed in. Affects the efficiency of flux, slower rate of change in dipoles increases eddy currents. Hysteresis Properties Different kinds of cores: Steel, Solid (iron), Air Different cores are useful for different functions, such as different frequencies, transmission efficiency, consistency, resistance to heating, etc. Core Material Some cores can become permanently magnetized, and will inhibit flux until overcome when used. Permeability Efficiency Continued
Real World Examples – Power Plants Power plants step up voltage to tens of thousands of volts for efficient transfer (high voltage means low current, which means lower resistance). Voltage is stepped down using a series of sub-stations, by the time it reaches your house it is at 120v. Power meters often use an isolation transformer that is 1:1, in order to protect the house from interference from the main line, and visa versa. Power generators are often three-phased; they have windings places 120 degrees apart. A three-phased transformer consisting of three coils is required to match this. Advantage is up to 173% more power, because power is available between any two of the three phases
Real World Examples – Your Home Chargers/ Power Supplies Most electronics run on less than 120v, like your cell phone charger or laptop. In other areas, like Europe or Asia, a transformer is required because standard household voltage is not 120v (vary between 100v and 240v). Motors Some appliances that use electric motors, such as your hair dryer, washing machine or fridge, require high currents and low voltage to operate Halogen Light Bulbs Operate on only 12v as opposed to florescent bulbs, which can run on 120v. Power Tools Many power tools operate at an industry standard 220v