Operation Construction Classification Applications DC Motors
A DC Motor converts electrical energy into mechanical energy. Special applications where dc motors are used include: in steel mills, mines and electric trains. Its operation is based on the principle that when a current carrying conductor is placed in a magnetic field, the conductor experiences a mechanical force.
DC Motor Operation:
The direction of this force is given by Fleming's left hand rule and magnitude is given by: F = BIl newtons. It acts in tangential direction to the rotor, thus producing torque.
Working of D.C. Motor Suppose the conductors under N-pole carry currents into the plane of the paper and those under S-pole carry currents out of the plane or the paper. Since each armature conductor is carrying current and is placed in the magnetic field, mechanical force acts on it. Applying Fleming's left hand rule, force on each conductor tends to rotate the armature in anticlockwise direction. All these forces add together to produce a driving torque which sets the armature rotating. When the conductor moves from one side of a brush to the other, the current in that conductor is reversed and at the same time it comes under the influence of next pole which is of opposite polarity. Consequently, the direction of force on the conductor remains the same. part of a multipolar d.c. motor
How the Commutator Works As the rotor turns, the commutator terminals also turn and continuously reverse polarity of the current it gets from the stationary brushes attached to the battery.
Controlling Motor Direction To change the direction of rotation: Simply switch the polarity of the battery leads going to the motor (that is, switch the + and battery leads) Direction of Rotation CW + - CCW - +
Construction of DC Motor All d.c. machines have five principal components : (i)field system (ii) armature core (iii) armature winding (iv) commutator (v)brushes
Back or Counter E.M.F. 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 induced which opposes the applied voltage V. The applied voltage V has to force current through the armature against the back e.m.f. The electric work done in overcoming and causing the current to flow against Eb is converted into mechanical energy developed in the armature. shunt wound motor Therefore, energy conversion in a d.c. motor is possible only due to the production of back e.m.f. Eb.
Power Equation If Eq.(i) is multiplied by Ia,
Types of D.C. Motors (a) D.C. Series motor (b) D.C. Shunt motor (c) D.C. Compound motor: (i) Cumulative compound motor (ii) Differential compound motor (a) Shunt-wound motor in which the field winding is connected in parallel with the armature. The current flows partly through the field and partly through the armature. 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. Supply voltage, V = E b + I a R a or generated e.m.f., E b = V I a R a Supply current, I L = I a + I sh, from Kirchhoff s current law.
(b) Series-wound motor in which the field winding is connected in series with the armature. Therefore, series field winding carries the armature current. Since the current passing through a series field winding is the same as the armature current, series field windings must be designed with much fewer turns than shunt field windings for the same m.m.f. Therefore, a series field winding has a relatively small number of turns of thick wire and therefore have low resistance. Supply voltage, V = E b + I a (R a + R se ) or generated e.m.f., E b = V I a (R a + R se ) Supply current, I L = I a = I se, from Kirchhoff s current law.
(c)compound-wound motor It is a combination of series and shunt motor. The compound motor has a shunt field winding in addition to series winding so that the number of magnetic flux lines of force produced by each of its poles is the resultant of flux produced by shunt and series coils. There are two types of compound wound motor: (i) Cumulative compound, in which the series winding is so connected that the field due to it assists that due to the shunt winding. (ii) Differential compound, in which the series winding is so connected that the field due to it opposes that due to the shunt winding.
Applications of D. C. Motors Shunt Motors It is an approximately constant speed motor. It is therefore, used where the speed is required to remain almost constant from no-load condition to full loadcondition. Industrial Use : - Lathes, Small Drills, Shapers, Spinning and Weaving Machines etc. Series Motors It is a variable speed motor. Its speed is low at high torque and vice-versa. However, at light or no-load, the motor tends to attain dangerously high speed. The motor has a high starting torque. It is therefore, used where (i) Large starting torque is required like in Elevators and Electric Traction. (ii)the load is subjected to heavy fluctuations and the speed is automatically required to reduce at high torques and vice-versa. (iii) Should not operate at no-load. Industrial Use : - Electric traction, elevators, cranes, hoist, conveyor belts, rolling mills etc.
Compound Motors Applications of D. C. Motors (i)differential-compound motors are rarely used because of their poor torque characteristics. (ii)cumulative-compound motors are used where a fairly constant speed is required with irregular loads or suddenly applied heavy load. Industrial Use : - Presses, Shears, Reciprocating Machines, Drilling etc.