DC MOTORS

DC MOTORS

1. Categories

   • Permanent Magnet (PM)
     • -No external power source, No (I^2)R heating
     • Lighter and Smaller 'cause high field strength
     • Easily reversed by switching voltage
     • Computer Control application 'cause the linearity of its torque-speed relation.
     • SERVOMOTOR: used only in low-power aplications, with feedback
     • Can be: Brushed, Brushless or stepper motors
   • Shunt Motors
     • Armature and filed windings in parallel, powered by the same supply
     • ITotal Load= Iarmature + Ifields
     • Cosntant Speed/large range of loading
     • Lowest starting torque
     • Adjustable Speed by potentiometer
   • Series Motor
     • Armature and field windings in series
     • Iarmature = Ifields
     • High starting torque
     • High variable speed depending on load
       • Can fail when unloaded 'cause forces and speed
     • Inverse relationship between torque and speed
   • Compound Motors
     • Shunt and Series field windings
       • Load current throught armature and series winding
       • Remaining load current throught shunt windings
     • Limited speed
     • Lower torque than series
     • Doesn't change direction

2. Characteristics

   • Speed
   • Ratio of Torque
   • Dynamic Breaking
   • Regenerative Breaking

3. Selecting a Motor

   • Speed Range
   • Torque-speed variations
   • Operating Duty Cycle
   • Starting Torque
   • Power Required
   • Start and Accelerate fast enough?
     • Generate sufficiet torque to overcome friction and load torques
     • Acceleration = (Torquemotor - Torqueload)/Moment of inertia (J)
   • Maximum Speed?
     • Zero torque point determines the maximum speed
   • Operating duty cycle?
     • Ratio of time motor is on respect to total elapsed time
     • Motor in't operating continuously
     • Load requres a low duty cylce, a lower-power motor is recomended
   • Power required by load?
     • Choose a motor with adequate power based on duty cycle
   • Power source available?
     • Battery characteristics match load requirements
   • Load Inertia?
     • For fast dynamic need low motor rotor and load inertia J
       • Load inertia large, to high acceleration need to size motor
   • Load with constant speed?
     • DC Shunt motor: constat speed/ load torques
   • Position or speed control?
     • Stepper motor
       • Angular positioning and incremental motion
       • Easily rotate, control step rate
       • Open-loop control (No sensor feedback)
       • Too fast may slip, use an encoder
     • Servomotor
       • Precise position or speed profiles
       • Closed-loop control (sensor feedback)
       • Faster response
       • Process input and generates amplified voltage
   • Transmission or Gearbox?
     • Use transmission to match motor output to load requirements
     • Inertia of load is Jeff= Jload* (wload/wmotor)^2
       • wload = (M/N) w motor
         • N is turns of motor required to creare M turns of load
   • Motor Torque-speed matches to load?
     • With load line select a similar torque-speed characteristics on a motor
     • Speed controlled by small changes in voltage
   • Operating speed?
     • For a motor torque-speed and a load line characteristics are a fixed speed operating point
       • Changing by adjustig the voltaje supplied
   • Reverse?
     • Requires rotation in 2 directions
   • Size and weight restrictions?