Machine Parts

Function of 4-Point starter :    4-Point starter is mainly used to prevent the “unscheduled stoppage”. Here the field circuit and the electromagnet are not in series. So, if any reason there is less current flow through the field coil, it will prevent the unscheduled stoppage. The four point starter provides the motor with no voltage protection.If the power fail, the motor must be disconnected from the line. If not, full line voltage will be applied to the armature without the benefit of starting resistor. When power is restored. The   holding magnet, being connected across the line, release the arm when the voltage drops below a specific value, thus protecting the motor when power is restored.

Back e.m.f.: When the motor armature rotates,the conductors also rotate and hence cut the flux.In accordance with the laws of electromagnetic induction, e.m.f.is induced in them whose direction,as found by Fleming , s Right-hand Rule,is in opposition to the applied voltage. Because of its opposite direction, it is referred to as counter e.m.f. or back e.m.f.E b.                                                                                                                                                                                Fig:1 Prove that the gross power developed by a DC motor is maximum when the back e.m.f. (E b ) equals to the half of the applied voltage(V):  Back e.m.f.E b =V t -I a R a   Where V t =applied terminal voltage,     I a =armature current,      R a =armature resistance                                                                                                                                                            T

  Interpoles are small poles fixed in between the main poles.They are wound with comparatively few heavy gauge Cu wire turns connected in series with the armature so that they carry full armature current. Their polarity , in the case of generator, is the same as that   of the main pole ahead in the direction of rotation. As their polarity is the same as that of the main pole ahead ,they induce an e.m.f in the coil which helps the reversal of current.The e.m.f induced by the compoles is known as reversing e.m.f. As interpoles carry armature current, their commutating e.m.f is proportionalto the armature current. The illustrate the manner in   which the interpole flux oppose the armature flux. The   interpoles are connected in a manner similar to the series field of a dc generator;i.e, the load current passes through the coils of the interpoles   and produces flux.Thus the greater the current   through the armature con-doctors , the greater the distortion of th

The ordinary or commercial efficiency of a transformer is given by the ratio                          Output in watts                         Input in watts Again, for all-day efficiency, ¶=Output in Kwh/input in Kwh (For 24 hours) Whereas core loss occurs throughout the day, the Cu loss occurs only when the transformers are loaded. Hence it is considered a good pracice to design such transformers so that Core loss is very low. The Cu losses are relatively less important, because they depend on the load. The performance of such a transformer should be judged by all day efficiency. Math: The energy output is 5*08*6=24 Kw=hr (operates at rated KVA)                                         ½*5*.5*12=15 Kw-hr (one-half rated KVA)   Total Output = 39 KW-hr The energy loss due to core loss is 35*24=840 W-hr=0.84 Kw-hr The copper loss is    40*(1)2=40*6=0240 Kw-hr                               ¼*40*42=120 Kw-hr =0.120 Kw-hr Total Copper energy loss = (0