Losses in a D.C. Motor
Saturday, September 19th, 2009The losses occurring in a d.c. motor are the same as in a d.c. generator (i) copper losses (ii) Iron losses or magnetic losses (iii) mechanical losses As in a generator, these losses cause (a) an increase of machine temperature and (b) reduction in the efficiency of the d.c. motor. The following points may be noted: (i) Apart from armature Cu loss, field Cu loss and brush contact loss, Cu losses also occur in interpoles (commutating poles) and compensating windings. Since these windings carry armature current (Ia), Loss in interpole winding = Ia 2× Resistance of interpole winding Loss in compensating winding = Ia 2× Resistance of compensating winding (ii) Since d.c. machines (generators or motors) are generally operated at constant flux density and constant speed, the iron losses read more
If you enjoyed this post, make sure you subscribe to my RSS feed! Read more »
If you enjoyed this post, make sure you subscribe to my RSS feed! Read more »
Commutation in D.C. Motors
Friday, September 18th, 2009Since the armature of a motor is the same as that of a generator, the current from the supply line must divide and pass through the paths of the armature windings. In order to produce unidirectional force (or torque) on the armature conductors of a motor, the conductors under any pole must carry the current in the same direction at all times. This is illustrated in Fig. (4.10). In this case, the current flows away from the observer in the conductors under the N-pole and towards the observer in the conductors under the S-pole. Therefore, when a conductor moves from the influence of N-pole to that of S-pole, the direction of current in the conductor must be reversed. This is termed as commutation. The function of the commutator and the brush gear in a d.c. motor is to cause the reversal read more
If you enjoyed this post, make sure you subscribe to my RSS feed! Read more »
If you enjoyed this post, make sure you subscribe to my RSS feed! Read more »
Armature Reaction in D.C. Motors
Wednesday, September 16th, 2009As in a d.c. generator, armature reaction also occurs in a d.c. motor. This is expected because when current flows through the armature conductors of a d.c. motor, it produces flux (armature flux) which lets on the flux produced by the main poles. For a motor with the same polarity and direction of rotation as is for generator, the direction of armature reaction field is reversed. (i) In a generator, the armature current flows in the direction of the induced e.m.f. (i.e. generated e.m.f. Eg) whereas in a motor, the armature current flows against the induced e.m.f. (i.e. back e.m.f. Eg). Therefore, it should be expected that for the same direction of rotation and field polarity, the armature flux of the motor will be in the opposite direction to that of the generator. Hence instead of read more
If you enjoyed this post, make sure you subscribe to my RSS feed! Read more »
If you enjoyed this post, make sure you subscribe to my RSS feed! Read more »
Torque and Speed of a D.C. Motor
Wednesday, September 16th, 2009For any motor, the torque and speed are very important factors. When the torque increases, the speed of a motor increases and vice-versa. We have seen that for a d.c. motor; N = K (V- IaRa)/ Ф = K Eb/ Ф…………………………………………….(i) Ta α ФIa…………………………………………………………………………(ii) If the flux decreases, from Eq.(i), the motor speed increases but from Eq.(ii) the motor torque decreases. This is not possible because the increase in motor speed must be the result of increased torque. Indeed, it is so in this case. When the flux decreases read more
If you enjoyed this post, make sure you subscribe to my RSS feed! Read more »
If you enjoyed this post, make sure you subscribe to my RSS feed! Read more »
Speed of a D.C. Motor
Monday, September 14th, 2009Eb = V-IaRa But Eb=PФZN/60A PФZN/60A = V- IaRa Or N = (V- IaRa)/ Ф × 60A/ PZ Or N = K (V- IaRa)/ Ф But V- IaRa = Ea Therefore N= K Eb/ Ф Or N α Eb/ Ф Therefore, in a d.c. motor, speed is directly proportional to back e.m.f. Eb and inversely proportional to flux per pole Ф. Speed Relations If a d.c. motor has initial values of speed, flux per pole and back e.m.f. as N1 ,Ф1 and Eb1 respectively and the corresponding final values are N2 ,Ф2 and Eb2 then, N1 α Eb1/ Ф1 and N2 α Eb2/ Ф2 Therefore N2/ N1 = (Eb2/ Eb1) ×( Ф1 / Ф2) (i) For a shunt motor, flux practically remains constant so that Ф1 = Ф2. therefore N2/ N1 = Eb2/ Eb1 (ii) For a series motor, Ф α Ia prior to saturation. therefore N2/ N1 = (Eb2/ Eb1) × (Ia1/Ia2) where Ia1 = initial read more
If you enjoyed this post, make sure you subscribe
If you enjoyed this post, make sure you subscribe
No comments:
Post a Comment