Brushed DC Motor Basics VI

  Series-wound brushed DC motors have the field coil in series with the rotor, thus, their field currents become identical. So, the motor current is equal to the sum of the stator ( series ) current and the rotor current. Thus, during normal operation without a load, as the supply voltage is increased the motor current increases. This causes the stator and rotor fields to increase and the motor speed increases with a decrease in torque. But, as a load is placed on the motor, motor current will increase also causing the stator and rotor fields to increase. Now the motor speed will decrease and torque will increase to overcome the load.

 ● Great slow speed torque ( low end ) - But, as load is removed, speed increases sharply
 ● Ideally suited for heavy loads, examples: cranes and winches
 ● Poor high speed torque ( high end )
 ● More expensive than PMDC
 ● Poor speed control due to the series stator field
 ● Motor Runaway, if the series field is shorted

  Series wound brushed DC motors have great slow speed ( low end ) torque, but if the load is removed, speed will sharply increase. This makes these motors ideally suited for high-torque applications, such as cranes and winches because the current in both the stator and rotor increases under load. Some drawbacks are that they have poor high speed ( high end ) torque characteristics ,thus they must have a load connected to prevent damage under high speed conditions. They are more expensive than permanent magnet brushed DC motors, they do not have precise
speed control characteristics like permanent magnet brushed DC and shunt wound brushed DC motors, and they can potentially go into motor runaway if the series stator field is shorted. Again, this is a very dangerous condition that can lead the motor to literally brake apart.