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why do they reverse the direction of electric motor in production line.
And got the following answer:
DC MOTORS Direct current motors are used in many industrial applications that require adjustable speed. In uses requiring quick stops, a dc motor can minimize the size of a mechanical brake or make it unnecessary. This is done by dynamic braking (motor-generated energy fed to a resistor grid), or by regenerative braking (motor-generated energy returned to the ac supply). DC motor speed can be controlled smoothly down to zero, followed immediately by acceleration in the opposite direction (without power circuit switching). Also, due to high torque-to-inertia ratio, dc motors respond quickly to controlsignal changes. Motor types DC motors in all but fractional and low integral horsepower sizes (generally below 15 hp), have wound fields and are categorized as shunt-wound, series-wound, or compound-wound motors. In fractional and low integral horsepower sizes, permanent magnets are used instead of wound fields in many motor designs. Shunt-wound motors — Stabilized shunt-wound motors are operated from adjustable voltage power supplies, Figure 1. A stabilizing winding (a small series field) helps prevent speed increases as load increases at weak field settings, Figure 2. This winding has drawbacks in reversing applications, however, because winding direction relative to the shunt winding must be reversed when armature voltage is reversed. Here, reversing contactors must be used. Where fast reversing is needed, the stabilizing winding is omitted, and the motor is designed for stable operation. Permanent-magnet motors — Operating characteristics of permanent- magnet motors are similar to those of shunt-wound types. Field flux, however, is provided by permanent magnets instead of current in a winding. Motor torque is directly proportional to armature current over the motor’s speed range. Compared with shunt-wound motors, permanent- magnet motors weigh less and are more economical to operate because no power is needed to support a field. Reversing — This operation affects the power supply and control. When the motor cannot be stopped for switching series fields before reverse operation, compound and stabilizing windings should not be used if full load torque is needed in both directions. Bi-directional operation may also affect brush adjustments. Duty rating — DC motors carry one of three ratings: • Continuous duty is applied to motors that will continuously dissipate all the heat generated by internal motor losses without exceeding rated temperature rise. • Definite time, intermittent duty motors will carry rated load for specified time without exceeding rated temperature rise. These motors must be allowed to cool to ambient before load is repeated. • Indefinite time, intermittent duty is usually associated with some RMS load of a duty-cycle operation. Peak torque — The peak torque that a dc motor delivers is limited by that load at which damaging commutation begins. Brush and commutator damage depends on sparking severity and duration. Therefore, peak torque depends on the duration and frequency of occurrence of the overload. Peak torque is often limited by the maximum current that the power supply can deliver. Motors can commutate greater loads at low speed without damage. NEMA standards specify that dc machines must deliver at least 150% rated current for one minute at any speed within rated range, but most motors exceed this requirement. Heating — The temperature of a dc motor is a function of ventilation and losses in the machine. Some losses — core, shunt-field, and brushfriction — are independent of load, and vary with speed and excitation. Several methods can predict operating temperature. The best method is to use thermal capability curves available from the manufacturer.