Advantage of D.C. Motors

Why, shall we ask, the industrial application of the d.c. motor is still so largely demanded notwithstanding every (erroneous) information reports that it has gone into disuse ?

• Their reduced overall dimensions permit a considerable space saving which let the manufacturer of the machines or of plants not to be conditioned by the exaggerated dimensions of circular motors.
• Mechanic stress of the system is minimum, acceleration and deceleration are very mild and progressive, which is very difficult to achieve at a certain power if inverter and a.c. Motors are used.
• Speed regulation by constant torque and power can be easily achieved, there is the possibility of a careful regulation even at lower speed. At high speed the d.c. motor once mounted for instance onto wood working machines such as the veneer cutting machines or the sheet working machines such as the unwinding-winding reels presents the characteristic of a wide speed variation once de-energized at constant power, up to three times the base speed. As a matter of fact as the primitive diameter reduces speed increases and torque linearly decreases together with the flux, thus the power will keep constant. This characteristics makes it non-replaceable by an equivalent a.c. Motor which would not have the same torque variation and therefore could not get the same performance by the end of the manufacturing cycle at higher speed.
• The elevated yield of the group converter-motor results into an electric energy saving which is much more considerable if a bidirectional converter is adopted working on four quadrants thus carrying out the energy recovery.
• By the diffusion of d.c. motors everywhere in the world spare parts can be easily get and the machine-stops reduce.
• Absence of solicitations and voltage peaks on the winding of the motor, windings will last more if compared to the asynchronous motor's. .

As a matter of fact in an a.c. Motor quick commutation causes elevated voltage gradients. As a consequence:

• Marked solicitations of the motor's insulations and necessary heavier dimensioning in order to avoid its rupture.
• Necessary use of the insulated bearings due to the rotor's inducted high frequency currents, which by their earth discharge through the bearings provoke an excessive wear.