The field of a DC motor is varied using external devices, usually field resistors. For a constant applied voltage to the field (E), as the resistance of the field (R) is lowered, the amount of current flow through the field (I_f) increases as shown by Ohm's law in Equation (6-4).

An increase in field current will cause field flux (f) to increase.Conversely, if the resistance of the field is increased, field flux will decrease. If the field flux of a DC motor is decreased, the motor speed will increase. The reduction of field strength reduces the CEMF of the motor, since fewer lines of flux are being cut by the armature conductors, as shown in Equation (6-5).

A reduction of counter EMF allows an increase in armature current as shown in Equation (6-6).

This increase in armature current causes a larger torque to be developed; the increase in armature current more than offsets the decrease in field flux as shown in Equation (6-7).

This increased torque causes the motor to increase in speed.

This increase in speed will then proportionately increase the CEMF. The speed and CEMF will continue to increase until the armature current and torque are reduced to values just large enough to supply the load at a new constant speed.

Summary

DC motor theory is summarized below.

DC Motor Theory Summary

There are two conditions necessary to produce a force on a conductor:

The conductor must be carrying current.

The conductor must be within a magnetic field.

The right-hand rule for motors states that when the forefinger is pointed in the direction of the magnetic field lines, and the center finger is pointed in the direction of current flow, the thumb will point in the direction of motion.

The function of torque in a DC motor is to provide the mechanical output to drive the piece of equipment that the DC motor is attached to.

Torque is developed in a DC motor by the armature (current-carrying conductor) being present in the motor field (magnetic field).

CEMF is developed in a DC motor by the armature (conductor) rotating (relative motion) in the field of the motor (magnetic field).

The function of the voltage that is developed in a DC motor (CEMF) opposes the applied voltage and results in the lowering of armature current.

The speed of a DC motor may be changed by using resistors to vary the field current and, therefore, the field strength.