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• draw the basic connection circuit of a series DC motor.
• describe the effects on the torque and speed with a change in current.
• describe the effects of a reduction of a load on the speed of a DC series motor.
• connect a DC series motor.
Despite the wide use of alternating current for power generation and transmission, the DC series motor is often used, such as for starter motors in automobiles and aircraft. This type of motor is also used as a traction motor because of its capability to produce a high torque with only a moderate increase in power at reduced speed.
The basic circuit for the series motor is shown in FIG. 1. The field circuit has comparatively few turns of large-diameter wire that permit it to carry the full-load current of the motor.
A series motor develops up to 500% of its full-load torque at starting. Therefore, this type of motor is used for railway installations, cranes, and other applications requiring high torque at low speed. The series motor is used in many electric locomotives, where the shaft becomes the axle for the drive wheels.
Remember that the shunt motor operates at constant speed. In a shunt motor, any increase in torque requires a proportionate increase in armature current. In a series motor, the stator field is operated below magnetic saturation, and any increase in load causes an increase of current in both the field and armature circuits. As a result, the armature flux and the field flux increase together. Because torque depends on the interaction of the armature and field fluxes, the torque increases as the square of the value of the current increases. Therefore, a series motor produces a greater torque than a shunt motor for the same increase in current. The series motor, however, shows a greater reduction in speed as mechanical load is added. A light load has little current draw, and the armature and field current are reduced.
SPEED CONTROL AND SPEED REGULATION
The speed regulation of a line-connected series motor is inherently poorer than that of a shunt motor. If the motor is running with a mechanical load and the load is suddenly reduced, the motor speed increases. The increase in speed creates more CEMF in the armature, and the circuit current is reduced. The circuit current is the same current in both the stator field and the rotor field, therefore squaring the effect of the current reduction. The torque that is produced drops off dramatically. The change in speed and torque is much greater in a series motor than the same change in load imposed on a shunt motor. If the mechanical load is removed entirely, the motor speed increases to dangerous levels, even to the point of destroying the motor by its own centrifugal force. For this reason, series motors are permanently connected to the mechanical load and should not be run with full voltage without a load.
When a mechanical load is added to the series motor, the opposite effect is true.
The motor slows with a new, higher torque requirement. As the motor slows, the CEMF in the rotor is reduced and the line current is allowed to increase. This affects both the stator field and the rotor field. With a drop in speed, the torque is increased dramatically, and the motor runs at a lower speed but with much larger torque. This gives the speed torque curve its flattened slope (see FIG. 2). Note that the series motor has more torque with lower armature current than the other DC motors.
If the maximum branch-circuit fuse size (time delay and nontime delay fuses, and inverse time breakers) for any DC motor is limited to 150% of the full-load running current of the motor, the starters used with such motors must limit the starting current to 150% of the full load current rating. Instantaneous trip breakers allow the rating to be 250% of the rated current, according to Table 430.52 of the 2011 National Electrical Code. Such starters must be equipped with an automatic, no-load release to prevent the armature from reaching dangerous speeds.
The no-load release is set to open the circuit at the armature current corresponding to the maximum speed rating.
The speed of a series motor is controlled by varying the applied voltage. A series motor controller usually is designed to start, stop, reverse, and regulate the speed.
The direction of rotation may be reversed by changing the direction of the current either in the series field or the armature (FIG. 3).
Series DC motors are rated for voltage, current, horsepower, and maximum speed.
The DC series motor has very high starting torque at very low speed (FIG. 4). This characteristic makes it ideal for traction motors. These motors are used in forklifts and diesel electric locomotives. The relative speed of the motor is controlled by adjusting the applied voltage to both the series field and the armature. The motor can be reversed by changing the direction of current in either the series field or the armature.
Select the correct answer for each of the following statements, and place the corresponding letter in the space provided.
1. The torque of a series motor ____
a. is lower in its starting value than the starting torque for a shunt motor of the same horsepower rating.
b. depends on the flux of the armature only.
c. increases directly as the square of the current increases.
d. increases with a load increase but causes less of a reduction in speed than a shunt motor for the same current increase.
2. For a series motor, ____
a. the field is operated below saturation.
b. an increase in both the armature current and the field current occurs because of an increase in load.
c. the reduction in speed due to an increase in load is greater than in the shunt motor.
d. all of the above are true.
3. Because a DC series motor has poor speed regulation, ____
a. a reduction in load causes an increase of current in both the field and armature.
b. the removal of the mechanical load causes the speed to increase without limit, resulting in the destruction of the armature.
c. it should not be connected permanently to its load.
d. it does not require speed control.
4. The speed control for a DC series motor ____
a. is accomplished using a diverter rheostat across the series field.
b. has an automatic no-field release feature included on all starters regardless of the limitations on the starting current.
c. varies with the applied voltage.
d. is all of the above.
5. A series motor controller usually is designed for ____
b. railway propulsion.
c. starting heavy loads.
d. all of the above.
6. Complete the electrical connections for a simple series motor in FIG. 5.