7.1. General Aspects
The electric motor is a machine which converts electric energy into mechanical energy. It depends for its operation on the force which is known to exist on a conductor carrying current while situated in a magnetic field.
Construction. A D.C. motor is similar in construction to a D.C. generator. As a matter of fact any D.C. generator will run as a motor when its field and armature windings are connected to a source of direct current. The field winding produces the necessary magnetic field. The flow of current through the armature conductors produces a force which rotates the armature.
Though the essential construction of D.C. motor is identical to that of a generator, the external appearance of a motor may be somewhat different from that of a generator. This is mainly due to the fact that the frame of a generator may be partially open because it is located in relatively clean environment and only skilled operators are present in its vicinity. A motor, on the other hand, may be operating in a rather dusty environment and only unskilled operators may be working in its
vicinity. Therefore, frames of motors are to a large extent closed.
The body of D. C. mill motors is made in two halves bolted together for easy access to the field windings and inter-poles.
Applications. Because of their inherent characteristics D.C. motors find extensive application in:
(i) Steel plants
(ii) Paper mills
(iii) Textile mills
(iv) Printing presses
(vii) Excavators etc. where precise and accurate speed control over a wide range is required.
Advantages. The D.C. motors possess the following advantages:
(i) High starting torque.
(ii) Speed control over a wide range, both below and above the normal speed.
(iii) Accurate stepless speed control with constant torque.
(iv) Quick starting, stopping, reversing and accelerating.
Disadvantages. The disadvantages of D.C. motors are:
(i) High initial cost.
(ii) Increased operating and maintenance costs because of the commutators and brushgear.