DC Motors ©Dr. B. C. Paul 2012 After slides prepared earlier by the author.

The DC Motor Poles for Field Current Armature or Rotor Stator A direct current voltage is applied in parallel to the field poles of the electromagnet and to conductors in the armature The wire loops on the armature are connected through slip rings and brushes so that current flow in the wire loops keeps reversing forcing the armature to rotate to line up with the electromagnetic field

Making the Motor Work Poles for Field Current Armature or Rotor The strength of the Magnetic Field is controlled by the flow of current to the electro-magnet Since the voltage is fixed the current through the field circuit is controlled by the resistance Most of the resistance on the field current comes from a variable resistor that can be adjusted as part of control of the motor

The Armature Current A DC voltage is applied to the armature the current is not quite V t / R a a current loop rotating in a magnetic field induces a back voltage against the applied voltage v g = Køη K is a constant for the machine ø is the magnetic flux from the poles of the electromagnet η is the rpms turned by the armature Thus i a = (V t - v g )/ R a

The Speed of a DC Motor The equations for armature current can be subbed into each other and solved for armature speed η = (V t - i a R a ) / (KØ) But Ø = ƒ (i f ) And i f is determined by changing the resistance on a variable resistor We now have a motor that we can speed control by adjusting a resistor

Uses of the DC Motor Because speed control is almost trivial the motor is popular where ever speed control is needed Mine Hoists Most of the motions on large shovels and draglines are candidates DC motor is an almost constant torque motor It can start at full torque (theoretically) It draws steady state current proportional to the torque of the assigned load.6

What does Almost Constant Torque Mean? When a DC motor starts there can be a rush of current 20 or 30 times greater than steady state operation This allows the motor full starting torque (provided the power supply system can supply the starting current) Once going the current draw is proportional to the load on the motor Since heating is a linear effect of current draw the ultimate motor rating is based on heat.