Unit 4: Electric Motors and Solid State Devices

1. Electric Motors

An electric motor is a device that converts electrical energy into mechanical energy (rotation). It works on the principle that a current-carrying wire in a magnetic field experiences a force (the motor effect).

Basic Design

Most motors have two main parts:

• Stator: The stationary outer part, which contains magnets or electromagnets to create a magnetic field.
• Rotor: The rotating inner part, which is a coil of wire (armature) that the current flows through.

The force on the wires of the rotor creates a torque, causing it to spin.

Single-phase and Three-phase AC Motors

• Single-phase AC Motors:
  • These run on the two-wire (live/neutral) power found in homes.
  • A simple single-phase motor is *not* self-starting. The AC current creates a "pulsating" magnetic field, not a rotating one.
  • They require a special starting mechanism (like a starting winding or a capacitor) to create a second, out-of-phase field to give the motor its initial spin.
  • Examples: Fans, refrigerators, blenders.
• Three-phase AC Motors:
  • These run on 3-phase power (common in industry, see Unit 2).
  • The three phases, each offset in time, naturally create a rotating magnetic field (RMF) in the stator.
  • This RMF "drags" the rotor around with it.
  • Advantages: They are self-starting, more efficient, more powerful, and run smoother than single-phase motors.

Speed & Power of AC Motor

• Speed: The speed of an AC motor is primarily determined by two factors:
  1. The frequency (f) of the AC power supply.
  2. The number of poles (P) (pairs of N-S magnets) built into the motor.

  The Synchronous Speed (N_s) (the speed of the rotating field) is:

  Formula: N_s (in RPM) = (120 × f) / P

  Example: A 4-pole motor on a 50 Hz supply has a synchronous speed of (120 × 50) / 4 = 1500 RPM.

• Power: The mechanical power output is a product of its torque (T) and rotational speed (ω). Power is measured in Watts (W) or Horsepower (HP).

2. Solid State Devices

These are electronic components made from semiconductor materials (like silicon), which have properties between a conductor and an insulator. They have no moving parts.

Resistors, Inductors, and Capacitors (R, L, C)

Their response to DC and AC sources is critical:

Response with DC Sources

• Resistor: Obeys Ohm's Law (V=IR). The current is constant.
• Inductor (L): Initially, it opposes the change in current, so current starts at 0. After a short time, it acts as a short circuit (a plain wire) with 0V drop.
• Capacitor (C): Initially, it's empty and acts as a short circuit (current flows easily). As it charges up, the current drops to 0. When fully charged, it acts as an open circuit (blocks DC current completely).

Response with AC Sources

• Resistor: Obeys Ohm's Law (V=IR) at all times. Current is in-phase with voltage.
• Inductor (L): Creates inductive reactance (X_L = 2πfL). This opposition *increases* with frequency. Current *lags* voltage by 90°.
• Capacitor (C): Creates capacitive reactance (X_C = 1 / 2πfC). This opposition *decreases* with frequency. Current *leads* voltage by 90°.

3. Diode

A diode is the simplest solid-state device. It's a P-N junction made of semiconductor material.

[Image of a diode symbol with anode and cathode labeled]

Function: It acts as a one-way valve for electricity.

• Forward-biased: When positive voltage is applied to the P-side (Anode) and negative to the N-side (Cathode), the diode "turns on" and lets current flow easily (with a small voltage drop, ~0.7V for silicon).
• Reverse-biased: When voltage is applied in the opposite direction (positive to N-side), the diode "turns off" and blocks current flow.

4. Half-wave and Full-wave Rectifiers

A rectifier is a circuit that uses diodes to convert AC (which flows both ways) into DC (which flows one way).

Half-wave Rectifier

• Design: Uses a single diode.
• Operation:
  • During the positive half-cycle of the AC input, the diode is forward-biased and lets current pass to the load.
  • During the negative half-cycle, the diode is reverse-biased and blocks all current.
• Result: A pulsating DC output that is "on" for half the cycle and "off" for the other half. This is very inefficient.

[Image of half-wave rectifier circuit and its input/output waveforms]

Full-wave Rectifier

This is a more efficient design that uses the entire AC wave. The most common type is the Bridge Rectifier.

• Design: Uses four diodes arranged in a "bridge" or "diamond" shape.
• Operation:
  • During the positive half-cycle, one pair of diodes (e.g., D1 & D2) is forward-biased, directing current to the load.
  • During the negative half-cycle, the *other* pair of diodes (e.g., D3 & D4) is forward-biased. They also direct the current to the load *in the same direction as before*.
• Result: A pulsating DC output where the negative "humps" of the AC wave are flipped over to become positive. It is much smoother and more efficient.

[Image of full-wave bridge rectifier circuit and its input/output waveforms]

5. Filter Circuits

The output from a rectifier is pulsating DC (it pulses from 0V to a peak and back). This is not useful for most electronics, which need smooth, steady DC.

A filter circuit is used to smooth out these pulses. The simplest filter is a large capacitor (called a "smoothing capacitor") connected in parallel with the load.

[Image of a full-wave rectifier with a capacitor filter]

Operation:

1. As the pulsating DC voltage rises, the capacitor charges up to the peak voltage.
2. As the pulsating DC voltage starts to fall, the capacitor "discharges" its stored energy into the load.
3. This "fills in the gaps" between the pulses, resulting in a much steadier DC voltage with only a small variation called "ripple".

6. LED (Light Emitting Diode)

A Light Emitting Diode (LED) is a special type of diode that is designed to emit light when it is forward-biased.

Operation: When current flows through the P-N junction, electrons and "holes" (charge carriers) recombine. This recombination process releases energy in the form of photons (light).

Features:

• Very energy efficient (converts most energy to light, not heat).
• Long lifespan.
• Durable (solid-state).
• Only works in one direction (it's still a diode).