Hamdard Institute of Engineering & Technology, Hamdard University

AC single-phase induction motors

Single-coil AC induction motors

AC induction motors usually use two or more coils to generate a rotating magnetic field, which produces

torque on the rotor. When a single coil is used, it will generate a pulsating magnetic field, which is enough

to sustain rotation, but not sufficient to start the motor from a standstill. Motors with a single coil have to

be started by using an external force.

Start capacitor AC induction motors

One way to improve on the single coil design is by using an auxiliary coil in series with a motor starting

capacitor. The auxiliary winding (coil), also called starting winding, is used to create an initial rotating

magnetic field. In order to create a rotating magnetic field, the current flowing through the main winding

must be out of phase in respect to the current flowing through the auxiliary winding. The role of the

starting capacitor is to lag the current in the auxiliary winding, bringing these two currents out of phase.

When the rotor reaches sufficient speed, the auxiliary coil is disconnected from the circuit by means of a

centrifugal switch, and the motor remains powered by a single coil creating a pulsating magnetic field.

Start/run capacitor AC induction motors

Another way to further improve on the single-coil single-phase induction motor design is to introduce an

auxiliary coil, which remains powered not only during the motor startup phase, but also during normal

operation. As opposed to an AC motor using only a motor start capacitor, which creates a pulsating

magnetic field during normal operation, AC motors using a motor start capacitor and a motor run capacitor

create a rotating magnetic field during normal operation. The function of the motor start capacitor remains

the same as in the previous case – it gets disconnected from the circuit after the rotor reaches a

predetermined speed by means of a centrifugal switch. After that point, the auxiliary winding remains

powered through a motor run capacitor.

A centrifugal switch is an electric switch that operates using the centrifugal force created from a rotating

shaft, most commonly that of an electric motor. The switch is designed to activate or de-activate as a

function of the rotational speed of the shaft.

The iron-core electromagnetic ballast consists of a magnetic core of laminated transformer steel around

which is wound a coil of insulated copper or aluminium wire use to limit the arc current. Electronic ballasts

are energy-efficient electromagnetic ballasts with added electronic circuitry which cuts off current to the

lamp filaments once the gas discharge has been established. Using this type of ballast with the same lamps

as the previous example, the ballast loss would typically be only 5% of the input power.

Hamdard Institute of Engineering & Technology, Hamdard University

Fluorescent Tube light

A fluorescent light does not have the usual glowing filament of an incandescent bulb, but instead contains a

mercury vapour that gives off ultraviolet light when ionized. The ultraviolet light makes particles that coat

the inside of the tube, and these particles glow or fluoresce.

Fluorescent starters are there to help the lamp light. When voltage is applied to the fluorescent lamp, here's

what happens:

1. The starter (simply a timed switch) allows current to flow through the filaments at the ends of the tube.

2. The current causes the starter's contacts to heat up and open, thus interrupting the flow of current.

3. Since the lighted fluorescent tube has a low resistance, the ballast now serves as a current limiter.

When you turn on a fluorescent tube, the starter is a closed switch. The filaments at the ends of the tube are

heated by electricity, and they create a cloud of electrons inside the tube. The fluorescent starter is a time-

delay switch that opens after a second or two. When it opens, the voltage across the tube allows a stream of

electrons to flow across the tube and ionize the mercury vapour.

Without the starter, a steady stream of electrons is never created between the two filaments, and the lamp

flickers. Without the ballast, the arc is a short circuit between the filaments, and this short circuit contains a

lot of current. The current either vaporizes the filaments or causes the bulb to explode.