Motor speed can be controlled by varying the current in the field windings. When the amount of current flowing through the field windings is increased, the field strength increases, but the motor slows down since a greater amount of counter emf is generated in the armature windings. When the field current is decreased, the field strength decreases, and the motor speeds up because the counter emf is reduced. A motor in which speed can be controlled is called a variable speed motor. It may be either a shunt or series motor.
In the shunt motor, speed is controlled by a rheostat in series with the field windings. [Figure 10-290] The speed depends on the amount of current that flows through the rheostat to the field windings. To increase the motor speed, the resistance in the rheostat is increased, which decreases the field current. As a result,
there is a decrease in the strength of the magnetic field and in the counter emf. This momentarily increases the armature current and the torque. The motor will then automatically speed up until the counter emf increases and causes the armature current to decrease to its former value. When this occurs, the motor will operate at a higher fixed speed than before.
To decrease the motor speed, the resistance of the rheostat is decreased. More current flows through the field windings and increases the strength of the field; then, the counter emf increases momentarily and decreases the armature current. As a result, the torque decreases and the motor slows down until the counter emf decreases to its former value; then the motor operates at a lower fixed speed than before.
In the series motor, the rheostat speed control is connected either in parallel or in series with the motor field, or in parallel with the armature. When the rheostat is set for maximum resistance, the motor speed is increased in the parallel armature connection by a decrease in current. When the rheostat resistance is maximum in the series connection, motor speed is reduced by a reduction in voltage across the motor. For above normal speed operation, the rheostat is in parallel with the series field. Part of the series field current is bypassed and the motor speeds up. [Figure 10-291]
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