Assessment No 4

Unit 39

Electrical Principles

Stewart Parsons

TASK 1

a)  The direct current (DC) motor is one of the first machines devised to convert electrical power into mechanical power. Permanent magnet (PM) direct current converts electrical energy into mechanical energy through the interaction of two magnetic fields. One field is produced by a permanent magnet assembly, the other field is produced by an electrical current flowing in the motor windings. These two fields result in a torque which tends to rotate the rotor. As the rotor turns, the current in the windings is commutated to produce a continuous torque output. The stationary electromagnetic field of the motor can also be wire-wound like the armature (called a wound-field motor) or can be made up of permanent magnets (called a permanent magnet motor). We all know that

Every DC motor has six basic parts -- axle, rotor (armature), stator, commutator, field magnet(s), and brushes. In most common DC motors, the external magnetic field is produced by high-strength permanent magnets. The stator is the stationary part of the motor; this includes the motor casing, as well as two or more permanent magnet pole pieces. The rotor (together with the axle and attached commutator) rotates with respect to the stator.

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 conducting material 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 enameled magnet wire. Larger power transformers operating at high voltages may be wound with copper rectangular strip conductors insulated by oil-impregnated paper and blocks of pressboard

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.

=       =  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