Machine Parts

The circuit shows in figure 1 is used to duplicate the required flux and speed for any specific load. The rheostat in series with the armature is adjusted until the voltmeter reads the same as the induced voltage at the specific load (generated voltage if the dynamo is a generator, counter emf if the dynamo is a motor. Since the I a R a drop of the dynamo as connected is very small, it can be neglected and the voltage across the armature is assumed to be the counter emf. The field rheostat is now adjusted to obtain the same speed at the specific load condition. The voltage induced in a dynamo is given by the equation E=KΦS. Inasmuch as the induced voltage as well as the speed have now be duplicated, it is apparent from the formula that the flux must now be the same as the flux at the load condition. Having duplicated the flux and the speed, the stray power is now determined by subtracting the copper loss from the power input to the armature.               

In a series motor the flux depends upon the current in the series field, which is the same current that flows through the armature. At light loads the armature current is small; therefore the flux and torque are small. An increase in the load increases both the armature current and the flux. The basic speed equation                                    S =        Vt   - IaRa   - IaRa           KФ Since the current in the series field is the armature current, the equation may be rewritten substituting Ia for Ф.                                        S =    Vt   - IaRa   - IaRa           KIa With no load connection to the motor the value of I is very small. The numerator of the speed equation is large, while the denominator is very small, indicating that the speed of   the motor   will be extremely large. In fact, the speed of the motor may reach such a value as to cause the centrifugal force to destroy the motor. It is

  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

When power is firs applied to a motor, the armature is not rotating. Under these conditions the counter Emf is zero and only factor limiting the current in the armature is the armature resistance. The armature resistance of a motor is very small. Therefore the current through the armature would be very small. Therefore the current through the armature would be very large if power were applied to the motor. This current might produce an excessive voltage drop, affecting other equipment. Additional resistance must be connected in series with the armature to limit the current .Until the motor rotation can build up the Counter Emf. The resistance is gradually cut out as the meter rotation builds up the Counter Emf. What will happen if a shunt motor is directly connected with load? The flux of the shunt motor remains constant. If a shunt motor is directly connected to the supply line, the current increases. The increases the Armature current decrease the counter emf . The