ELECTRICAL EASY

DC Machines can be tested by three different methods namely Direct Method, Indirect Method and Regenerative Method. Direct Method of testing of DC Machine, also known as Brake Test (if carried out for a DC Motor) will be discussed in this post. Direct method is suitable for small DC machines. In Direct Method, the DC machine is subjected to rated load and the entire output power is wasted. The ratio of output power to the input power gives the Efficiency of DC Machine. For a DC Generator the output power is wasted in resistor. Direct Method of testing when conducted on a motor is also known as Brake Test. Brake Test of DC Motor is carried out as shown in figure below. A belt around the air cooled pulley has its end attached to the spring balance S1 and S2. Using belt tightening hand wheels H1 and H2, the load of motor is adjusted to its rated value. Assuming the spring balance to be calibrated in kilogram, then rated load on the DC motor is given as Motor ...

DC Series Motors are widely used for the purposes requiring high starting torque like in Hoist, Crane, Electric Traction etc. In this post we will discuss the reason behind the high starting torque of DC Series Motor. For any DC motor, torque is directly proportional to the flux and armature current i.e. Ƭ = KaØIa  ……………….(1) Where Ka = Constant Ø = Flux Ia = Armature current. In case of DC Series Motors, the flux produced is directly proportional to the armature current. Hence torque is directly proportional to the square of armature current. Therefore we can write as Ø = CIa, where C is some constant. Therefore, from equation (1), torque in DC Series Motor Ƭ = K(Ia) 2   where K = KaxC  ………..(2) As we know that the maximum permissible current at starting of DC Motor is limited to about 1.5 times that of rated current, thus loads requiring high starting torque such as Cranes, Hoists and Traction type loads, Series Motors are ...

As discussed in earlier post, Purpose of Interpole in DC Machine , there are two types of effect of armature mmf on the main field flux: 1)       Distortion in Main Field Flux 2)       Net reduction of Main Field Flux Due to the above two effects of armature mmf, there are many detrimental effects on the Electrical machine. Distortion of Main Field Flux gives rise to three detrimental effects: Rise in Iron Loss , Poor Commutation and Sparking in Carbon Brushes. Net reduction in Main Field Flux leads to increased cost of field winding as we need more field winding to have a fixed value of field flux. Now we will discuss each of the bad effects of armature reaction briefly but in systematic way as follow: Rise in Iron Loss: Iron loss basically depends on the maximum value of flux density in the teeth and in pole shoes. Because of armature reaction the value of flux density increases considerably over its value a...

Advantages: The series motor has the highest starting torque for a given power rating. When the motor is overloaded, the speed drops and the torque increases. While delivering high torque during an overload, the series motor draws less current and power from the source compared to a shunt or compound DC motor. The main applications where series motors are best suited are for traction applications. Torque of the series motor is proportional to square of the field current i.e. T α I 2 . Therefore Starting torque for the series motor is quite high. Because of this property series motors are widely used in electric traction and in crane application. Disadvantages: Speed regulation in the series motor is quite poor. With the increase in the load speed of the machine decreases, DC shunt motor maintains almost constant speed from no load to full load. With decrease in the speed torque of the dc series motor drops sharply. DC series motor should always requi...