The 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

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Since 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

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As 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

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For 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

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Eb = 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

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The torque which is available at the motor shaft for doing useful work is known as shaft torque. It is represented by Tsh. Fig. (4.9) illustrates the concept of shaft torque. The total or gross torque Ta developed in the armature of a motor is not available at the shaft because a part of it is lost in overcoming the iron and frictional losses in the motor. Therefore, shaft torque Tsh is somewhat less than the armature torque Ta. The difference Ta – Tsh is called lost torque. Ta - Tsh =9.55 × iron and frictional losses/N For example, if the iron and frictional losses in a motor are 1600 W and the motor runs at 800 r.p.m., then, Ta - Tsh =9.55 × 1600 /800 =19.1 N-m As stated above, it is the shaft torque Tsh that produces the useful output. If the speed of the motor is N r.p.m., read more

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Torque is the turning moment of a force about an axis and is measured by the product of force (F) and radius (r) at right angle to which the force acts i.e. D.C. Motors torque T = F × r In a d.c. motor, each conductor is acted upon by a circumferential force F at a distance r, the radius of the armature (Fig. 4.8). Therefore, each conductor exerts a torque, tending to rotate the armature. The sum of the torques due to all armature conductors is known as gross or armature torque (Ta). Let in a d.c. motor r = average radius of armature in m l = effective length of each conductor in m Z = total number of armature conductors A = number of parallel paths i = current in each conductor = Ia/A B = average flux density in Wb/m2 Φ = flux per pole in Wb P = number of poles Force on each read more

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Like generators, there are three types of d.c. motors characterized by the connections of field winding in relation to the armature viz.: (i) Shunt-wound motor in which the field winding is connected in parallel with the armature [See Fig. 4.4]. The current through the shunt field winding is not the same as the armature current. Shunt field windings are designed to produce the necessary m.m.f. by means of a relatively large number of turns of wire having high resistance. Therefore, shunt field current is relatively small compared with the armature current. (ii) Series-wound motor in which the field winding is connected in series with the armature [See Fig. 4.5]. Therefore, series field winding carries the armature current. Since the current passing through a series field winding is the read more

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