Electric Motors and Domestic Circuits (Basic)

Electric Motor

An electric motor is a device that converts electrical energy into mechanical energy. It is a fundamental component in countless appliances and machines used in our daily lives, from fans and pumps to mixers and electric vehicles.

Principle of an Electric Motor

The working principle of a DC (Direct Current) electric motor is based on the magnetic effect of electric current and the force experienced by a current-carrying conductor in a magnetic field. Specifically, it uses the fact that a current-carrying coil placed in a magnetic field experiences a torque. This torque causes the coil to rotate.

As discussed in the previous section, the force on a straight current-carrying wire of length $l$ in a magnetic field $\vec{B}$ is $\vec{F} = I(\vec{l} \times \vec{B})$. For a rectangular coil in a uniform magnetic field, these forces on the sides of the coil produce a net torque that makes the coil rotate.

Construction of a Simple DC Motor

A simple DC electric motor typically consists of the following main parts:

Armature: This is the rotating part of the motor. It consists of a rectangular coil (usually with a large number of turns) wound on a soft iron core. The coil is free to rotate about an axis.
Field Magnet: This provides the magnetic field in which the armature coil rotates. It can be a permanent magnet or an electromagnet. In a simple motor, a strong permanent magnet is used.
Commutator: This is a crucial component that allows the current in the coil to be reversed periodically. In a simple DC motor, it is a split ring made of a conducting material (like copper). The ring is split into two halves (A and B) insulated from each other. Each half is connected to one end of the armature coil.
Brushes: These are stationary conducting contacts (usually made of carbon) that press against the rotating commutator. They make electrical connection between the external power supply and the armature coil through the commutator.
Axle: The armature coil, along with the commutator, is mounted on an axle about which it rotates. The mechanical energy output is delivered through this axle.

Diagram showing the parts of a simple DC electric motor

Simplified diagram showing the main parts of a DC electric motor.

Working of a Simple DC Motor

Let's consider a rectangular coil PQRS placed in the magnetic field of a permanent magnet. Suppose current $I$ flows from P to Q, then Q to R, R to S, and S to P. Let the magnetic field $\vec{B}$ be horizontal, from North to South pole.

Diagram illustrating forces and torque on a current loop in a motor

Forces on the sides of the armature coil in a magnetic field.

Sides QR and SP: These sides are parallel or anti-parallel to the magnetic field, so the force on them is zero.
Side PQ: The current is perpendicular to the magnetic field (assuming the coil is vertical initially). Using Fleming's Left-Hand Rule, if current is upwards and field is rightwards, the force on PQ is outwards (say, into the plane).
Side RS: The current is downwards, and the field is rightwards. Using Fleming's Left-Hand Rule, the force on RS is inwards (say, out of the plane).

The forces on PQ and RS are equal in magnitude but opposite in direction, and they act on different points, forming a couple. This couple produces a torque that rotates the coil in, say, the clockwise direction.

As the coil rotates, the sides PQ and RS move. When the coil reaches the vertical position, the forces are still acting, but they act along the axis of rotation, producing no torque at that instant. If the commutator were not present, the coil would just oscillate back and forth.

This is where the commutator plays its vital role. Just as the coil passes through the vertical position (where its plane is perpendicular to the field), the brushes lose contact with one commutator half and make contact with the other half. This reverses the direction of the current flowing through the coil.

With the current reversed, the force on side PQ (which is now on the right) is inwards, and the force on side RS (now on the left) is outwards. These forces continue to produce a torque in the same direction (clockwise), causing the coil to rotate continuously in that direction. The commutator reverses the current every half rotation, ensuring that the torque always acts in the same direction, resulting in continuous rotation.

In practical motors, the armature has many coils and commutator segments to produce a more uniform and powerful torque. The field magnet is often an electromagnet powered by the same source or a separate supply.

Uses of Electric Motors

Electric motors are ubiquitous. Basic motors are used in:

Household appliances: Fans, refrigerators, washing machines, mixers, vacuum cleaners.
Pumps: Water pumps for irrigation and domestic use.
Toys.
Small electric vehicles and tools.

More complex and powerful motors are used in industrial machinery, electric trains, cars, etc.

Domestic Electric Circuits

The electrical energy generated at power stations is transported over long distances through a network of cables (transmission and distribution) and finally reaches our homes through the domestic electric circuit. This circuit provides power to operate various electrical appliances.

Main Supply and Wiring

In India, the mains electric supply is typically 220 V at a frequency of 50 Hz. The electricity is usually supplied through two insulated wires:

Live Wire (or Phase Wire): This wire has a high potential, oscillating between positive and negative values. It is usually insulated with red or brown plastic coating.
Neutral Wire: This wire is usually at zero potential (ground potential). It is connected to the ground at the power station. It is usually insulated with black or light blue plastic coating.

The potential difference between the live wire and the neutral wire is the supply voltage (220 V in India).

In domestic wiring, a third wire, called the Earth Wire (or Ground Wire), is also used, especially for appliances with metallic bodies. This wire is connected to a metal plate buried deep in the earth near the house. It is usually insulated with green plastic coating.

The wiring generally passes through a main fuse or circuit breaker (MCB - Miniature Circuit Breaker), then through an electric meter (to measure energy consumption in kWh), and then to the main distribution board from which the wires are connected to various rooms and appliances.

Safety Devices

Safety is paramount in electrical circuits to prevent damage to appliances due to overcurrents or short circuits, and to protect users from electric shocks. Common safety devices include:

Fuse: As discussed earlier, a fuse is a safety device consisting of a wire with a low melting point. It is connected in series with the live wire. If the current exceeds a safe limit, the fuse wire melts, breaking the circuit. Fuses are rated according to the maximum current they can safely carry (e.g., 5 A fuse, 15 A fuse).
Circuit Breaker (MCB / ELCB / RCBO): Modern homes increasingly use Miniature Circuit Breakers (MCBs) instead of fuses. MCBs are electromagnetic switches that automatically break the circuit when the current exceeds a safe limit. They can be reset manually. Earth Leakage Circuit Breakers (ELCBs) or Residual Current Circuit Breakers (RCCBs) are designed to detect small leakage currents to the earth, which can cause shocks, and quickly break the circuit.

Fuses and circuit breakers should always be connected in the live wire so that when they operate, they disconnect the appliance from the high potential, making it safer to handle.

Parallel Connection of Appliances

In domestic circuits, all electrical appliances (lights, fans, sockets, etc.) are connected in parallel with each other across the live and neutral wires.

Diagram showing appliances connected in parallel in a domestic circuit

Appliances connected in parallel in a domestic circuit. Each appliance receives the full supply voltage.

There are several reasons for connecting appliances in parallel:

Each appliance gets the full supply voltage: In parallel, the potential difference across each appliance is the same and equal to the mains voltage (220 V). This ensures that all appliances operate at their rated voltage and power. If they were in series, the voltage would divide among them, and appliances designed for 220 V would not operate correctly.
Independent operation: Each appliance can be switched ON or OFF independently without affecting the others. In a series circuit, switching off one appliance would break the entire circuit, stopping all other appliances.
Lower total resistance and higher total current: When appliances are connected in parallel, the total resistance of the circuit decreases (reciprocal of total resistance is the sum of reciprocals). This allows the circuit to draw more current as needed when more appliances are switched on, and the total current is the sum of individual currents.
Easier fault finding: If one appliance is faulty or short-circuited, it does not affect the operation of other appliances in parallel. The issue is isolated to that specific branch.

Earthing

Earthing is a crucial safety measure, especially for appliances with metal casings, such as electric irons, refrigerators, geysers, and toasters. The metal casing of the appliance is connected to the earth wire.

Metal casing of an appliance connected to the earth wire for safety.

Normally, the earth wire does not carry any current. However, if due to faulty wiring or damaged insulation, the live wire comes into contact with the metal casing, the casing becomes live (at high potential).

If a person touches the live casing, they would provide a path for current to flow to the earth, resulting in a severe electric shock.

With earthing, if the live wire touches the metal casing, a low-resistance path is created for the current to flow directly to the earth through the earth wire. This sudden surge in current (because the total resistance of the circuit, $R_{circuit} + R_{earth}$, is very low) causes the fuse to blow or the circuit breaker (MCB) to trip, disconnecting the power supply to the appliance. This prevents the casing from remaining at a high potential, making it safe for a person to touch and averting electric shock. The earth wire provides a safety route for stray currents.

Overloading and Short Circuiting

Overloading: This occurs when too many electrical appliances (especially high-power appliances) are connected to a single circuit (or a single socket) simultaneously. This draws a total current from the mains supply that exceeds the safe limit for the wiring. Overloading can cause the wires to overheat, leading to insulation damage and potential fire hazards. It can also cause the fuse to blow or the MCB to trip.

Short Circuiting: This occurs when the live wire and the neutral wire come into direct contact, either due to damage to the insulation or a fault in an appliance. The resistance of the path between the live and neutral wires becomes very low. According to Ohm's Law ($I = V/R$), a very large current flows through the circuit instantaneously. This sudden large current is called a short-circuit current. Short circuits cause excessive heating of wires, sparking, and can lead to fires. Fuses and MCBs are designed to quickly break the circuit during a short circuit condition.

Both overloading and short circuiting are dangerous situations, and safety devices like fuses and MCBs are essential for protecting lives and property.