Motors compound-wound

Compound-wound dc motors have both series and shunt fields. The addition of a small series field helps provide the proper amount of no-load to fuU-load speed regulation or droop. Shunt or compound-wound motors are apphed widely to many adjustable-speed drives. They are important for drives requiring accurate speed regulation and adjustment. 

Compound-Wound Motor. A compound-wound motor is a direct-current motor which as two separate field windings. One, usually the predominating field, is... 

Compound-Wound Motors. These motors are used to drive machines that require high starting torque or in which the loads have a pulsating torque. Changes in load usually produce wide speed regulation. This motor is not suited for adjustable speed by field control. 

From no load to full load, the drop in speed of compound-wound motors is approximately 25%. Compound-wound motors are used where reasonably constant speed is required and for loads where high starting torque is needed to accelerate the drive machine. 

DC methods mostly use shunt or compound wound motors. Occasionally series wound motors are used when high torque at low speeds is required. These machines are fed with DC voltage derived from a three-phase AC source using a thyristor converter. The thyristor converter rectifies the AC into DC but with control over the magnitude of the average DC voltage. Thyristors are also called silicon controlled rectifiers . 

Compound-Wound Motors. Compound-wound motors combine both series and shunt fields (Fig. 5.120). The disadvantage of series-motor overspeeding at light loads is avoided since there is so httle current in the series field at no load that speed is determined by the shunt field alone. At higher loads, speed depends on the sum of the two fields, making speed reduction similar to that of a series motor. 

FIGURE 5.120 Schematic and speed vs. torque diagram—dc compound-wound motor. 

Open-Field Protection. DC shimt and compound-wound motors can be protected against the loss of field excitation by instaUing field-loss relays in the shunt field circuit. Larger dc motors may race dangerously with the loss of field excitation, while other motors may not race because of friction and the fact that they are small. 

One of the oldest adjustable-speed drives is the Ward-Leonard system. This consists of an ac to dc motor-generator set and a shunt or compound-wound dc motor. Speed is adjusted by changing the generator voltage. A functional equivalent of this drive uses an adjustable-voltage rectifier feeding a dc motor. This system has only one rotating machine in contrast to the three of a conventional Ward-Leonard system. 

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

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. 

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