Machine Parts

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

Load connected with the transformer always varries i.e. it may be sometime resistive load or inductive load or capacitive load. As a result p.f. sometime unity, sometime lagging or leading and it is not equal. We know that KW=KVA*COSθ As p.f. varries so KW varries. So on the nameplate of   the transformer KW rating is not appropriate.Again cu loss of a transformer depends on current and iron loss on voltage. Hence, total transformer loss deoends on volt-ampre(VA) and not on phase angle berween voltage and current i.e. it is independent of load power factor. That is why rating of transformer is in KVA and not in KW.

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.

Torque versus armature   current characteristics of DC motor : The torque developed   by the motor would therefore be directly dependent upon the armature current as indicated by the torque equation ,T=K ΦIa. Increasing the armature current would increase the torque and vice versa.   Shunt motor : For the shunt motor shunt field is connected accross the line, the flux in the machine will remain essentially constant . So, for shunt motor, T=KΦfIa Compound   Motor: The current in the armature must also pass through the series field winding. The series winding may be connected comulatively were the flux due the series field will aid the shunt field flux. In cumulative compound motor the series field flux is additive with the field flux. But in the differentially compound motor the series field flux is substractive with main field flux. Total flux does not remain constant in compound motor as it does in the shunt motor. Cumulative compound motor T=k(Φf+Φs)Ia

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