Characteristics of Electric DC Motors and Gearmotors

• Direct current electric motors generally are quite fast (high rotational speed), but have very low torque. In most applications they must be "geared down" to increase the Torque and reduce the rotational speed to usable levels. A gear motor or gearhead motor is simply an electric motor with the reduction gearing mounted permanently on the top.
• For a typical DC electric motor or gear motor the highest torque is obtained when the motor is stalled (prevented from turning). The highest speed occurs under no load (the motor is free to run). Between these two extremes generally the motor slows down as more load is applied, and visa versa, the motor speeds up as less load is applied. This relationship approximates as a straight line.

• The graph above represents the aproximate performance curve of a DC electric gearmotor which has a torque of 5 N*m when stalled and a free running speed of 50 RPM.
• Excercise; Using the data above and the equation P=(30/¹)*T*w from the previous lesson, Plot a Power vs Rotational Speed graph
  • Where is the power the highest?
• The location where the power is the highest is the design point for the motor. Use the Torque and Rotational Speed at this point in the design.
• Motor Testing Experiment
• Input power can be measured on an electric motor by multiplication of the electrical potential (Voltage) by the electrical current. P=V*I, P-Watts, V-Volts, I-Amperes.
  • implications - Demo - watch electrical current as we load up a motor.
  • Efficiency - Power in electrical, Power out Mechanical (efficiency often has an optimal speed)