Magnets: The magnetic field lines form closed loops and any closed surface will have a net zero number of lines leaving the surface.
Commutator: The commutator is composed of conductive segments (called bars), usually made of copper, which represent the termination of individual coils of wire distributed around the armature.
Brushes: The brushes typically remain stationary with the motor's housing but ride (or brush) on the rotating commutator. As electrical energy is passed through the brushes and consequently through the armature a torsional force is generated as a reaction between the motor's field and the armature causing the motor's armature to turn. As the armature turns, the brushes switch to adjacent bars on the commutator.
Stator: The stator generates a stationary magnetic field that surrounds the rotor. This field is generated by either permanent magnets or electromagnetic windings.
Rotor: The rotor, also called the armature, is made up of one or more windings. When these windings are energized they produce a magnetic field. The magnetic poles of this rotor field will be attracted to the opposite poles generated by the stator, causing the rotor to turn.
TASK 2
a) AC motors are the simplest and most rugged electric motor and consists of two basic electrical assemblies: the wound stator and the rotor assembly. The induction ac motor derives its name from currents flowing in the secondary member (rotor) that are induced by alternating currents flowing in the primary member (stator). The combined electromagnetic effects of the stator and rotor currents produce the force to create rotation.
AC motors typically feature rotors, which consist of a laminated, cylindrical iron core with slots for receiving the conductors. The most common type of rotor has cast-aluminum conductors and short-circuiting end rings. This ac motor "squirrel cage" rotates when the moving magnetic field induces a current in the shorted conductors.
b) Like most motors, an AC induction motor has a fixed outer portion, called the stator and a rotor that spins inside with a carefully engineered air gap between the two.
Virtually all electrical motors use magnetic field rotation to spin their rotors. A 3-phase AC induction motor is the only type where the rotating magnetic field is created naturally in the stator because of the nature of the supply. DC motors depend either on mechanical or electronic commutation to create rotating magnetic fields. A single phase AC induction motor depends on extra electrical components to produce this rotating magnetic field.
Two sets of electromagnets are formed inside any motor. In an AC induction motor, one set of electromagnets is formed in the stator windings. The alternating nature of the supply voltage induces an Electromagnetic Force (EMF) in the rotor, thus generating another set of electromagnets
TASK 3
a) A transformer is a device that transfers from one to another through . A changing in the first circuit (the primary circuit) creates a changing magnetic field. This induces a changing voltage in the second circuit (the secondary circuit) this is called mutual induction. By adding a to the secondary circuit, one can make current flow in the transformer, thus transferring energy from one circuit to the other. In an ideal transformer, the induced voltage in the secondary winding is a fraction of the primary voltage and is given by the ratio of the number of secondary turns to the number of primary turns. By appropriate selection of the number of turns, a transformer thus allows an alternating voltage to be stepped up
Transformers come in a range of sizes from a thumbnail-sized coupling transformer hidden inside a stage to huge units weighing hundreds of tons used to interconnect portions of national . All operate with the same basic principles, though a variety of designs exist to perform specialized roles throughout home and industry.
b)
Windings: The transformer consists of two coils called windings which are wrapped around a core. The transformer operates when a source of ac voltage is connected to one of the windings and a load device is connected to the other. The winding that is connected to the source is called the primary winding. The winding that is connected to the load is called the secondary winding. The used for the windings depends upon the application, but in all cases the individual turns must be electrically insulated from each other to ensure that the current travels throughout every turn. For small power and signal transformers, in which currents are low and the potential difference between adjacent turns is small, the coils are often wound from . Larger power transformers operating at high voltages may be wound with copper rectangular strip conductors insulated by oil-impregnated paper and blocks of
Core: In practical transformers the core is made up of thin laminations of iron or silicon steel of identical dimensions, lightly insulated and tightly clamped together to ensure that there are no air gaps. The composition of a transformer core depends on such factors as voltage, current, and frequency. Size limitations and construction costs are also factors to be considered. Commonly used core materials are air, soft iron, and steel. Each of these materials is suitable for particular applications and unsuitable for others. Generally, air-core transformers are used when the voltage source has a high frequency (above 20 kHz). Iron-core transformers are usually used when the source frequency is low (below 20 kHz). A soft-iron-core transformer is very useful where the transformer must be physically small, yet efficient. The iron-core transformer provides better power transfer than does the air-core transformer.
Part D calculations
Q1. b) Power = Torque * Speed
P= 6.7 *3100 * = 2175 w
Efficiency =
= = 0.409 * 100 = 41%
Q2. a) = =
= =
= = 9.16666666667
= 9.16666666667 =
N2 = = = 264 windings
b) Efficiency = =
P = VI = 24 * 8.53 = 204.72
0.23 * 1000 = 230w
= = 0.89 * 100 = 89%
Q3. a)
f= np f= frequency (Hz) n= synchronous speed (rev/min) p= Number pairs of pulls
n= = = 1500rpm
b) 1500/100 * 3.5 = 52.5
1500 – 52.5 = 1447.5rpm