DC Motors: Design Considerations
|Power supply||DC||Battery supply possible|
|Driver electronics need||No|
|Speed accuracy||Low||Effected by load torque
Manufacture tol +-10% of no load speed
|Speed control||Voltage||Open loop or closed loop speed control Closed loop - high accuracy, but sensor need|
|Position control||High effort||Closed loop position control and sensor need|
|Overloadablity / load inertia||High||High start torque or peak load torque
Able to accelerate high load inertia
|Lifetime||Low||300...2000h (limited by wear of brushes) Depend on speed, current and brush config|
|Speed range [rpm]||1000...25000||No load speed|
|Efficiency||60...80%||At operating point of max. efficiency|
|Power vs. size||High|
|Winding thermal protection||Need||If overload (mechan.blocking) not excluded - Current limitation or thermal contacts|
|Load sensing||Easy||Current rises with load|
|Stalling permissible||No||No or only short time (winding overheat)|
|Electromag.interference EMI||Critical||Suppression means (capacitors / varistors)|
|Braking/Holding torque||Low||Detent torque
Braking improvable by winding short circuit
|Max.output power from JE [W]|
|Linear motors from JE||No|
Encoders (Sensors) can be implemented inside the motor or mounted to motor rearside.
They are needed for a precise speed control or angular position control. Typical application: Printers and scanners.There are different options:
- Optical encoders
- Hall-effect magnetic sensors
Johnson Electric offers a range of several optical encoders.
Basic principle: A slotted wheel or a black/transparent strip photo print is rotating with the motor shaft. A photo sensor provides a sine wave or pulse pattern that follows the speed of the motor.
Output signal can be analog (0...3.3Vac) or digital (0...3.3 Vdc or 0...5Vdc).
One signal or two signals phase shifted 90° (enabling detection of direction of rotation ; allows 4x resolution)
Several options for resolution. Terminology: CPR (Counts Per Revolution) and LPI (Lines Per Inch).
HC385MG with slotted encoder wheel
(CPR = 48 ; one signal, sine wave 0...3.3Vac)
NF123G with fotoprint wheel
(CPR= 32 ; two signals, sine wave 0....3Vac)