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In the previous Unit we spoke about a dynamo and generators. A dynamo is a device that converts mechanical energy into electrical energy or electrical energy into mechanical energy. In the study of the generator we have seen how mechanical energy supplied to the generator in the form of armature rotation was converted into electrical energy. Now we shall apply electrical energy to the same dynamo and investigate the conversion of electrical energy into mechanical energy. In accordance with the foregoing definition, a generator can be used as a motor or a motor can be used as a generator.

Force on a conductor. If a conductor is placed in a magnetic field and current is allowed to flow through the conductor, the conductor will tend to move. A force is exerted on the conductor because it is carrying current and it is located in a magnetic field. The force exerted on the conductor is dependent upon three factors. First a magnetic field must exist, then a conductor must be located in the magnetic field, and finally current must flow in the conductor.

The force on the conductor is zero in the absence of a magnetic field. The greater (smaller) the magnetic flux density, the greater (smaller) the force exerted on the conductor.

The length of the conductor is also a factor that determines the magnitude of the force on the conductor; the smaller the conductor, the smaller the force: the longer the conductor, the greater the force exerted on the conductor. An important point should be noted at this time. Only that part of the conductor located within the magnetic field is effective in producing a force on the conductor.

The third factor required to produce a force on the conductor is the magnetic of the current through the conductor. With no current in the conductor, the force produced would be zero. The greater the current through the conductor, the greater the force exerted on the conductor.

Shunt motor. The shunt motor is physically the same as a shunt generator. The shunt-field coil consists of many turns of fine wire and is connected in series with a field rheostat. The shunt-field circuit is connected across the line in parallel with the armature.

Because the shunt field is connected across the line, the flux in the machine will remain essentially constant. The torque developed by the motor would therefore be directly dependent upon the armature current. Increasing the armature current would increase the torque. Decreasing the armature current would decrease the torque.

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