# A.C Generator ## Generator Effect - When a coil of wire rotates inside a magnetic field by an external force, an e.m.f. is generated in the wire which causes current to flow within the coil - The **generator effect** converts mechanical energy to electrical energy - This principle can be used to: - Generate **a.c.** in an alternator - Generate **d.c.** in a dynamo ## Alternators ### Structure and Function - An alternator is a type of generator that converts mechanical energy to electrical energy in the form of **alternating current** - An alternator consists of a rotating coil in a magnetic field connected to **slip rings** (also called commutator rings)

An alternator is a rotating coil in a uniform magnetic field connected to metal slip rings that produce an alternating output voltage

### Key Components - A rectangular coil that is forced to spin in a uniform magnetic field - The coil is connected to a centre-reading meter (or external circuit) by **metal brushes** that press on two **metal slip rings** - The slip rings and brushes provide a **continuous connection** between the coil and the meter - Each slip ring remains connected to the same end of the coil throughout rotation ### How Alternators Work **When the coil turns in one direction:** - The pointer deflects first one way, then the opposite way, and then back again - This is because the coil **cuts through** the magnetic field lines - A potential difference, and therefore current, is **induced** in the coil **Why the current alternates:** - The pointer deflects in both directions because the current in the circuit **repeatedly changes direction** as the coil spins - This occurs because the induced potential difference in the coil repeatedly changes its direction - This continues as long as the coil keeps turning in the same direction

The induced potential difference and current alternate because they repeatedly change direction during rotation

### Output Characteristics - The alternator produces an **alternating voltage** output - The frequency of the output matches the frequency of rotation of the coil - The voltage varies sinusoidally with time - The output alternates between positive and negative values **Alternator output waveform:** The graph below shows the typical output voltage from an alternator during one complete rotation of the coil:

- The sinusoidal waveform shows the voltage smoothly transitioning between positive and negative - Maximum voltage occurs when the coil is horizontal (90° and 270°) - Zero voltage occurs when the coil is vertical (0°, 180°, 360°) ## Dynamos ### Structure and Function - A dynamo is a **direct-current generator** - A simple dynamo is identical to an alternator except that the dynamo has a **split-ring commutator** instead of two separate slip rings

A dynamo is a rotating coil in a magnetic field connected to a split-ring commutator that produces a direct current output

### How Dynamos Work **The rotation process:** - As the coil rotates, it **cuts through** the field lines - This induces a potential difference between the ends of the coil **The role of the split-ring commutator:** - The split-ring commutator changes the connections between the coil and the brushes **every half turn** - This keeps the current leaving the dynamo in the **same direction** - The swap happens each time the coil is perpendicular to the magnetic field lines - This ensures that despite the alternating e.m.f. in the coil, the output current always flows in one direction ### Output Characteristics **Direct current properties:** - The induced potential difference does **not reverse its direction** as it does in the alternator - Instead, it varies from zero to a maximum value twice each cycle of rotation - The polarity **never changes** (positive to negative) - This means the current is always positive (or always negative)

The dynamo output is not a smooth DC – it pulsates between zero and a maximum value but stays in the same direction

**The output waveform:** - The voltage varies from zero to a peak value - It drops back to zero twice per complete rotation - The voltage remains entirely in the positive region (or negative, depending on connection) - This produces a 'bumpy' DC output rather than the smooth DC from a cell **Dynamo output waveform:** The graph below shows the typical output voltage from a dynamo during one complete rotation of the coil:

- Notice that the voltage never goes negative – it's always in the positive region - The waveform still pulsates but remains unidirectional - Maximum voltage occurs twice per rotation (90° and 270°) - The split-ring commutator "flips" the negative portion to positive ## Comparison: Alternator vs Dynamo | Feature | Alternator | Dynamo | |---------|-----------|--------| | **Commutator type** | Slip rings (two separate rings) | Split-ring commutator | | **Output current** | Alternating current (a.c.) | Direct current (d.c.) | | **Voltage direction** | Reverses periodically | Always same direction | | **Voltage waveform** | Sinusoidal (smooth wave) | Pulsating (always positive/negative) | | **Connection to coil** | Continuous to same coil end | Swaps every half rotation | | **Typical applications** | Power generation stations, bicycle dynamos with AC lights | Battery charging systems, older bicycle lights | ## Physical Principles ### E.m.f. Generation - Both devices rely on electromagnetic induction - When the coil rotates, the magnetic flux through it changes - The rate of change of flux linkage induces an e.m.f. in the coil - The magnitude of the induced e.m.f. depends on: - The speed of rotation - The strength of the magnetic field - The number of turns in the coil - The area of the coil ### Maximum E.m.f. Conditions - The e.m.f. is **maximum** when the coil cuts through the field lines when they are **parallel to the plane of the coil** - At this point, the coil is moving through the field lines at maximum rate - The normal to the coil area is perpendicular to the field lines - The e.m.f. is **zero** when the magnetic field lines are **perpendicular to the plane of the coil** - At this point, the coil moves parallel to the field lines - The rate of cutting field lines is zero - The normal to the coil area is parallel to the field lines ### Energy Conversion - Mechanical energy (rotation) is converted to electrical energy - An external force must continuously rotate the coil (e.g., steam turbine, wind turbine, hand crank) - The electrical energy output equals the mechanical work input (minus losses due to resistance and friction) - This is the fundamental principle behind power generation in power stations