DC MOTOR OPERATION

Types of DC Motors Summary

DC Motor Ratings

Electrical Science Volume 2 of 4
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DC MOTOR OPERATION DC Motors DC MOTOR OPERATION DC motors require special starting resistors for operation due to their unique design. A knowledge of the operation of these starting resistors is necessary to understand DC motor operation. EO 1.11 EXPLAIN why starting resistors are necessary for large DC motors. EO 1.12 LIST the four nameplate ratings for a DC motor. Starting of DC Motors At the moment a DC motor is started the armature is stationary and there is no counter EMF being generated. The only component to limit starting current is the armature resistance, which, in most DC motors is a very low value (approximately one ohm or less), as shown in Equation (6-9). (6-9) I_a E_t E_CEMF R_a In order to reduce this very high starting current, an external resistance must be placed in series with the armature during the starting period. To show why this is essential, let us consider a 10-hp motor with an armature resistance of 0.4 ohms. If the motor were supplied by a 260 VDC source, the resulting current would be as shown in Equation (6-9). I_a E_t E_CEMF R_a I_a 260VDC 0 0.4W I_a 650 amps This large current is approximately twelve times greater than actual full-load current for this motor. This high current would, in all probability, cause severe damage to the brushes, commutator, or windings. Starting resistors are usually incorporated into the motor design to limit starting current to 125 to 200 percent of full load current. ES-06 Page 12 Rev. 0

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