Skip to main content

Effect of Slip on Rotor Parameters : Part 3

Effect of Slip on Rotor Parameters
4. Effect on Rotor Power factor
       From rotor impedance, we can write the expression for the power factor of rotor at standstill and also in running condition.
       The impedance triangle on standstill condition is shown in the Fig1. From it we can write,
                        cos Φ2 = Rotor power factor on standstill
                                    = R2/Z2 =R2/√(R22+ X22)
       The impedance in running condition becomes Z2r and the corresponding impedance triangle is shown in the Fig.2. From Fig. 2 we can write,
                        cos Φ2r = Rotor power factor in running condition
                                     = R2/Z2r = R2/√(R22+ (s X2)2)
Key point : As rotor winding is inductive, the rotor p.f. is always lagging in nature.
Fig. 1
Fig. 2
5. Effect on Rotor Current
       Let                     I2 = Rotor current per phase on standstill condition 
       The magnitude of I2  depends on magnitude of E2 and impedance Z2  per phase.
                                  I2  = (E2  per phase)/(Z2  per phase) A
       Substituting expression of Z2 we get,
                                   I2 = E2 /√(R22+ X22)      A
       The equivalent rotor circuit on standstill is shown in the Fig.3. The Φ2 is the angle between E2 and I2 which determines rotor p.f. on standstill.
Fig. 3
       In the running condition, Z2 changes to Z2r while the induced e.m.f. changes to E2r. Hence the magnitude of current in the running condition is also different than on standstill. The equivalent circuit on running condition is shown in the Fig. 4.
                                 I2r = Rotor current per phase in running condition
       The value of slip depends on speed which inturn depends on load on motor hence X2r is shown variable in the equivalent circuit. From the equivalent we can write,
                                 I2r =  E2r/Z2r = (s E2)/√(R22+ (s X2)2)      
       Φ2r is the angle between E2r and I2r which decides p.f. in running condition .
Fig. 4
Key point : Putting s  = 1 in the expression obtained in running condition, the values at standstill can be obtained.
Labels:

Comments

Post a Comment

Comment Policy
We’re eager to see your comment. However, Please Keep in mind that all comments are moderated manually by our human reviewers according to our comment policy, and all the links are nofollow. Using Keywords in the name field area is forbidden. Let’s enjoy a personal and evocative conversation.

Popular posts from this blog

Practical Commutation

The e.m.f. induced in each coil of armature is alternating in nature. If load is connected, the current flowing will also be alternating. But the flow of current in a d.c. generator must be undirectional. This can be achieved by the use of commutator. When the armature conductors are under the influence of one pole they carry current in one direction whereas the current is reversed when the conductors are under the influence of other pole. This reversal of current takes place along the magnetic neutral axis. Fig. 1 Note : The reversal of current is likely to take place in short interval when a coil is short circuited by a brush so that transfer of current from one direction to other is carried out without any sparking. This process is called commutation.        Thus a process by which current in the short circuited coil is reversed while it crosses the MNA is called commutation. The time during which the coil remains short circuited is known as...
1 comment
Read more

Demagnetising and Cross Magnetizing Conductors

The conductors which are responsible for producing demagnetizing and distortion effects are shown in the Fig.1. Fig. 1        The brushes are lying along the new position of MNA which is at angle θ  from GNA. The conductors in the region AOC = BOD = 2θ  at the top and bottom of the armature are carrying current in such a direction as to send the flux in armature from right to left. Thus these conductors are in direct opposition to main field and called demagnetizing armature conductors.         The remaining armature conductors which are lying in the region AOD and BOC carry current in such a direction as to send the flux pointing vertically downwards i.e. at right angles to the main field flux. Hence these conductors are called cross magnetizing armature conductors which will cause distortion in main field flux.        These conductors are shown in the Fig. 2 Fig. 2  ...
Post a Comment
Read more

Armature Voltage Control Method or Rheostatic Control of dc motor

Speed Control of D.C. Shunt Motor (Part2)  2. Armature Voltage Control Method or Rheostatic Control        The speed is directly proportional to the voltage applied across the armature. As the supply voltage is normally constant, the voltage across the armature can be controlled by adding a variable resistance in series with the armature as shown in the Fig. 1. Fig. 1 Rheostat control of shunt motor        The field winding is excited by the normal voltage hence I sh is rated and constant in this method. Initially the reheostat position is minimum and rated voltage gets applied across the armature. So speed is also rated. For a given load, armature current is fixed. So when extra resistance is added in the armature circuit, I a remains same and there is voltage drop across the resistance added ( I a R). Hence voltage across the armature decreases, decreasing the speed below normal value. By varyi...
Post a Comment
Read more