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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Let in a d.c. motor (See Fig. 4.3), V = applied voltage Eb = back e.m.f. Ra = armature resistance Ia = armature current Since back e.m.f. Eb acts in opposition to the applied voltage V, the net voltage across the armature circuit is V- Eb. The armature current Ia is given by; Ia = (V – Eb)/Ra or V = Eb + IaRa ……………………………..(i) This is known as voltage equation of the d.c. motor. Power Equation If Eq.(i) above is multiplied by Ia throughout, we get, VIa = EbIa +I2aRa VIa= electric power supplied to armature (armature input) EbIa = power developed by armature (armature output) I2aRa = electric power wasted in armature (armature Cu loss) Thus out of the armature input, a small portion (about 5%) is wasted as a I2aRa read more

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When the armature of a d.c. motor rotates under the influence of the driving torque, the armature conductors move through the magnetic field and hence e.m.f. is induced in them as in a generator The induced e.m.f. acts in opposite direction to the applied voltage V(Lenz’s law) and in known as back or counter e.m.f. Eb. The back e.m.f.  Eb (= P Φ ZN/60 A) is always less than the applied voltage V, although this difference is small when the motor is running under normal conditions. Consider a shunt wound motor shown in Fig. (4.2). When d.c. voltage V is applied across the motor terminals, the field magnets are excited and armature conductors are supplied with current. Therefore, driving torque acts on the armature which begins to rotate. As the armature rotates, back e.m.f. Eb is read more

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