NEIEP Mechanics Practice Exam

Which factors influence the speed of a DC motor?

1. Field strength and armature resistance

2. Armature voltage and brush pressure

3. Armature voltage and field strength

4. Load current and field winding

The correct answer is: Armature voltage and field strength

The speed of a DC motor is primarily influenced by the armature voltage and the field strength. This relationship can be understood through the fundamental principles of how a DC motor operates. When the armature voltage is increased, the current flowing through the armature windings is also increased, leading to a higher electromagnetic torque being produced. This increase in torque allows the motor to speed up. Conversely, if the voltage is decreased, the speed will consequently decrease due to a lower torque being generated. Field strength plays a critical role as well. In a typical DC motor, there is a relationship between the magnetic field created by the field windings and the speed of the motor. A stronger magnetic field (higher field strength) results in greater back EMF (Electromotive Force), which opposes the applied voltage. To maintain the same speed with a stronger field, more voltage is needed; therefore, as field strength increases, the speed decreases unless the armature voltage is adjusted to compensate. Together, the armature voltage and field strength determine the motor's speed by balancing the influences of torque and back EMF. Other factors mentioned in the options, like armature resistance or load current, do play roles in how the motor functions, but they are not direct