Direct current motors

Direct-current motors are adjustable in speed over a wide range. Further, efficiency is high over the entire speed range, unlike wound-rotor motors, in which efficiency is roughly proportional to speed. This flexibility is attained at the expense of additional complexity and cost. 

Direct-current motor fields are on the stator. The rotor is the armature. The magnetic field does not rotate like the field in ac machines. Current in the armature reacts with the stator field to produce torque. 

Direct-Current Motor Control Control for dc motors runs the gamut from simple manual line starters to elaborate regulating systems. Only the starting problems are considered here since variable-speed drives and regulating systems are discussed elsewhere. 

The attraction and repulsion between the poles on the rotor and stator cause the electromagnet to rotate. Direct-current motors usually need commutators to achieve continuous motion. 

Direct-current motors are classified as separately excited motors, series motors, shunt motors, and compound motors. The field winding of a separately excited motor is in a circuit that is energized by a separate dc source the field winding is not physically connected to the armature circuit (containing the armature winding). 

Direct current motors are most appropriately used in applications where a dc power supply is available or where a simple method of speed control is desired. The fans used in automobile heating and air conditioning systems are driven by direct-current motors. 

Variable Speed Drive. This can be accomplished by turbines, direct current motors, variable-speed motors, or slip-ring motors. With the changing speed of the driver, the fan output capacity and pressure can be varied. For capacity reductions below 50%, an outlet damper is usually added to the system. 

Shaft or brake horsepower (input) based on direct current motor ... 

This chapter gives primary attention to the commonly used general-purpose alternating current motors, although others are noted to acquaint the engineer with the variety of motors, motor enclosures, and mechanical drive turbines available and sometimes used in the chemical and petrochemical industries. Because direct-current motors are not frequently used in the process industries (other than in heavy-duty processes), they are not examined further in this chapter. 

For direct-current motors, the standard horsepower ratings are essentially the same as the previous, except the largest rating is about 8,000 hp. 

An industrial small motor is an alternating-current or direct-current motor built in either NEMA frame 42, 48, or 56 suitable for industrial use. 

An industrial direct-current motor is a medium motor of mechanical construction suitable for industrial sue under usual service conditions and has ratings and constructional and performance characteristics applying to direct current medium motors as given in Parts 10, 11, 12, and 14. 

Direct-current motors are of three general types, shunt wound, series wound, and compound wound, and are defined as follows ... 

Shunt-Wound Motor. A shunt-wound motor is a direct-current motor in which the field circuit and armature circuit are connected in parallel. 

Straight Shunt-Wound Motor. A straight shunt-wound motor is a direct-current motor in which.the field circuit is connected either in parallel with the armature circuit or to a separate source of excitation voltage. The shunt field is the only winding supplying f ield excitation. 

Speed Regulation of Direct-Current Motors. The speed regulation of a direct-current motor is the difference between the steady no-load speed and the steady rated-load speed, expressed in percent of rated-load speed. 

Important advantages of direct-current motors for machine drives are the adjustable speed over a wide range, the fact that speeds are not limited to synchronous speeds, and the variation of speed-torque characteristics [8]. 

Tapered horsepower motors develop the maximum rated horsepower at three times the minimum speed, the horsepower decreasing in direct proportion to the decrease in speed down to the horsepower rating at 150% of the minimum speed. Figure 3-12 plots characteristics of shunt, series, and compound-wound direct-current motors. 

Figure 3-12. Direct-current motor performance curves [8].

A numerically controlled filament winder has been designed and constructed. The distance between the head-stock and tail-stock is six feet. Clearance between the centers emd the body of the winder is adjustable, but a specimen several feet in diameter could presently be fabricated. The mandrel and carriage are each driven by one horsepower, direct current motors with a maximum 2000 revolutions per minute. Three phase, 480 volt alternating current is transformed to a 90 volt, direct current power supply. A gear... 

Direct current motors are used for continuous operation at constant load when fine speed adjustment and high starting torque are needed. A wide range of speed control is possible. They have some process applications with centrifugal and plunger pumps, conveyors, hoists, etc. 

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