CONSTRUCTION  The dc machines used for industrial electric drives have three major parts. Field system Armature and Commutator. Field system  The field system is located on the stationary part of the machine called stator and consists of main poles, interpoles and frame or yoke.  The main poles are designed to produce the magnetic flux.  The interpoles are placed in between the main poles. They are employed to improve the commutation condition.  The frame provides mechanical support to machine and also serve as a path for flux.

Armature  The armature is the rotating part (or rotor) of a dc machine.  It consists of armature core with slots and armature winding accommodated in slots.  The conversion of energy from mechanical to electrical or vice-versa takes place in armature. Commutator  The commutator is mounted on the rotor of a dc machine.  The commutator and brush arrangement works like a mechanical dual converter.  In case of generator it rectifies the induced ac to dc.  In case of motor it inverts the dc supply to ac. (In motor, the commutator reverses the current through the armature conductors to get unidirectional torque).

OUTPUT EQUATION  The output equation relates the power developed in armature to the main dimensions and speed of the machine.  The main dimensions of dc machine are the armature diameter, D and armature length, L. Power developed in armature, a = C0 D2L n The output coefficient, C0 = ∏2 B_avac x 10-3 Maximum gap density, Bg = B_av/Kf = B_av/Ѱ C0 in terms of Bg is given by, C0 = ∏2 ѰB_g ac x 10-3  Power developed by the armature, Pa is different from the rated power output P of the machine. The relationships between the two are Pa= P/ɳ for generator Pa= P for motors

CHOICE OF SPECIFIC MAGNETIC LOADING  The choice of average gap density or specific magnetic loading depends on the following Flux density in teeth Frequency of flux reversal Size of machine  Large values of flux density in teeth results in increased field mmf.  Higher values of field mmf increase the iron loss, copper loss and cost of copper.  The Bav is chosen such that the flux density at the root of the teeth does not exceed 2.2 Wb/m2.  If the frequency of flux reversals is high then iron losses in armature core and teeth would be high. Therefore we should not use a high value of flux density in the air gap of machines which have a high frequency.  It is possible to use increased values of flux density as the size of the machine increases.  As the diameter D of the machine increases, the width of the tooth also increases, permitting an increased value of gap flux density without causing saturation in the machine.  The value of Bg varies between 0.55 to 1.15 Wb/m2 and the corresponding values Of Bav are 0.4 to 0.8 Wb/m2

CHOICE OF SPECIFIC ELECTRIC LOADING  The choice of specific electric loading depends on the following 1.Temperature rise 2.Size of machine 3. Speed of machine 4. Armature reaction 5. Voltage 6. Commutation  A higher value of ac results in a high temperature rise of windings.  The temperature rise depends on the type of enclosure and cooling techniques employed in the machine.  If the speed of machine is high, the ventilation of the machine is better and therefore, greater losses can be dissipated. Thus a higher value of ac can be used for machine having high speed.  In high voltage machines, large space is required for insulation and therefore there is less space for conductors. This means that in high voltage machines, the space left for conductors is less and therefore we should use a small value of ac.  In large size machines it is easier to find space for accommodating conductors. Hence specific electric loading can be increased with increase in linear dimensions.  With high values of ac, armature reaction will be severe. To counter this, the field mmf is increased and so the cost of the machine goes high.  High values of ac worsen the commutation condition in machines. From the point of view of commutation a small value of ac is desirable. The value of ac usually lies between to amp.cond/m

choice of number of poles  The frequency should lie between 25 to 50 Hz.  The value of current per parallel path is limited to 200 amps, thus the current per brush arm should not be more than 400 amps.  Current per parallel path = Ia / p for lap winding= Ia /2 for wave winding  Current per brush arm = 2Ia / p for lap winding= Ia for wave winding where, p = number of poles  The armature mmf should not be too large. The normal values of armature mmf per pole are listed in Table below  If there are more than one choice for number of poles which satisfies the above three conditions, then choose the largest value for poles. This results in reduction in iron and copper. Output (KW)Armature mmf per pole (AT) upto or less 100 to to to to over 1500upto 12500

Pole proportions  The cross-section of the poles should be circular in order that the length of mean turn of the field winding is minimum. But circular poles cannot be laminated, hence the next best alternative is square pole section.  In a square section the width of the pole body is equal to the length of the machine. For square pole face, the pole arc (b) is equal to the length of the machine. L = b_p, for square pole section L = b, for square pole face  Usually the ratio of pole arc to pole pitch or the ratio L/ is specified. Ѱ= b/ τ = 0.64 to 0.72 L/τ = 0.45 to 1.1

ARMATURE CORE DESIGN  The armature of a dc machine consists of core and winding.  The armature core is cylindrical in shape with slots on the outer periphery of the armature.  The core is formed with circular laminations of thickness 0.5 mm.  The winding is placed on the slots in the armature core.  The design of armature core involves the design of main dimensions D & L, number of slots, slot dimensions and depth of core. Number of armature slots  The factors to be considered for selection of number of armature slots are Slot width (or pitch) Cooling of armature conductors Flux pulsations Commutation Cost  A large number of slots results in smaller slot pitch and so the width of tooth is also small. This may lead to difficulty in construction  But large number of slots will lead to less number of conductors per slot and so the cooling of armature conductors is better.

 If the air-gap reluctance per pair of pole is constant then the flux pulsations and oscillations can be avoided.  It can be proved that the air-gap reluctance is constant if the slots per pole is an integer plus 1/2.  For sparkless commutation the flux pulsations and oscillations under the interpole must be avoided. This can be achieved with large number of slots per pole.  In fact, the number of slots in the region between the tips of two adjacent poles should be at least 3.  The slots per pole should be greater than or equal to 9, for better commutation. When large number of slots are used the cost of lamination and the cost of insulation will be high.