What factors are considered in the selection of dc machine?

Answer 1

The initial factors are the working voltage and power level required. These set the size of the machine.

There are three types of dc machine depending on how the internal magnetic field is produced. A permanent magnet machine gives a constant excitation field and is preferred for smaller motors since no field winding is needed and the machine is therefore simpler.

A shunt-wound machine has the field winding in parallel with the armature (the rotating part) and is the commonest type of dc machine. This is used for high-power motors or generators (they are the same thing) where the speed of operation is fairly constant.

A series-wound machine has the field winding in series with the armature. This type is used in traction, to drive trams and trolley-buses. The torque produced by a dc machine is proportional to the field current and also to the armature current, so a series motor produces torque proportional to the square of the current. When starting, the motor draws a high current limited only by the resistance in circuit, so the torque is very high and suitable for starting the vehicle. A switched starting-resistor keeps the current within limits.

A standard shunt-wound motor acts as a motor when a mechanical load is applied, but if the motor is driven faster than it rotates naturally without a load, then power flows back into the supply and it becomes a generator.

A shunt-wound motor tends to behave as a capacitor because the starting current is high and it then reduces exponentially as the rotor gathers speed. If the supply is disconnected, the terminal voltage reduces exponentially as the speed runs down. In a capacitor the energy is stored in the electric field while in the dc machine it is in the kinetic energy of the rotor.