Shunt-wound motor

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. 

Shunt-Wound Motors. These motors operate at approximately constant speed regardless of variations in load when connected to a constant supply voltage and with fixed field excitation. Maximum decrease in speed as load varies from no load to full load is about 10-12%. 

Adjustable-speed shunt-wound motors with field control are designed for operation over speed ranges of 3 to 1 or greater. Standard adjustable-speed motors are rated in three ways tapered horsepower, continuous, 40°C rise constant horsepower, continuous, 40°C rise and constant horsepower, 1 hr,. 50°C rise, of the next larger horsepower rating than for continuous duty. 

Shunt-Wound Motors. Shunt-wound and stabilized shunt-wound dc motors can supply both constant speed at any control setting and a wide speed range that is field-controllable (Fig. 5.118). Most shunt motors are operated from adjustable voltage power supplies and, therefore, do not need auxiliary starting provisions. 

The shunt winding can either be connected to the same power supply as the armature (self-excited) or be separately excited. Care must be taken never to open the field of a shunt-wound motor that is running unloaded. The loss of field flux causes motor speed to increase to dangerously high levels. 

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. 

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

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. 

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. 

Direct Current. Direct-current (DC) motors normally come with either an internal or external rectification system matched to the motor duty. The rectification of alternating current to direct current by solid-state electronics allows the control of motor speed by adjusting the applied voltage. Small DC motors, less than 5 hp (3700 W), are typically permanent magnet designs larger motors are shunt wound. 

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. 

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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