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DC Motors

Motor Selection Guide


Application requested data:

- Operating speed :  NR  = 4000 rpm(working point)

- Operating torque :    TR = 1.4 mNm (working point)

- Max. voltage supply :   Umax = 10 VDC

- Max. current supply :   Imax = 0.2 A

- Ambient temperature :   Ta = 25 °C

- Motor type :   DC coreless motor

- Max. motor length :   Lmax = 35 mm

- Max. motor diameter :   φmax = 13 mm

Step 1. Motor type and physical dimensions

Consider motor technology and physical dimensions when selecting the potential candidates from the catalog.  In this example coreless motors up to SCL12-30 series (φ= 12.5 x L = 32 mm) will be considered.


Step 2. Working point and max. continuous values

Knowing the motor working point, NR = 4000 rpm and TR = 1.4 mNm, select from datasheets all motors where working point is inside of the continuous operating area. In this example SCL12-22 (max. continuous torque 1.6 mNm) and SCL12-30 series will be considered (max. continuous torque 2.09 mNm).

Step 3. Voltage and current supply

Verify if selected motors are able to operate at the working point with the available max. current and voltage supply.


- Current absorbed at working point, IR [A]


TF = Motor friction torque [mNm], KM = Motor torque constant [mNm/A]


- Motor voltage at working point, UR [VDC]


R= Motor terminal resistance at 25°C [W], KE = Back-EMF constant [mV/rpm]


Result of calculation

SCL12-2204 IR = 0.46 A, UR = 3.1 VDC IR too high

SCL12-2222 IR = 0.18 A, UR = 7.4 VDC Ok

SCL12-3005 IR = 0.30 A, UR = 3.4 VDC IR too high

SCL12-3020 IR = 0.15 A, UR = 7.1 VDC Ok

SCL12-3043 IR = 0.14 A, UR = 10.4 VDC UR too high

In red where motor current and/or voltage exceed power supply limits.


Step 4. Motor performance analysis

Once the motors operatable at the working point have been defined performance around the working point can be calculated.


- Motor output power at working point, Pout [W]



- Motor efficiency at working point, [%]



Result of calculation

SCL12-2222 Pout = 0.59 W, = 43 % Second choice

SCL12-3020 Pout = 0.59 W, = 55 % First choice


Step 5. Thermal considerations

The described procedure is generally suitable for most of the cases. However, if more accurate calculation is required then thermal behavior at working point can be considered.


- Motor coil temperature at working point, Tc [°C]


a = 0.004 [K -1], Rth1 & Rth2  = thermal resistances [K/W]


- Motor coil resistance at working point, Rwp [W]


By replacing R in [eq. 2] with Rwp repeat steps 3 & 4 considering the motor coil temperature. For safety operations coil temperature at working point must not exceed the maximum coil temperature indicated in the catalog.


Gearhead selection

If a motor-gearhead combination is requested, before proceeding with step 2, calculate working point at motor output shaft. For gearhead selection, where life expectancy is concerned, it is important to choose a product with an output torque higher than that required by the application and to operate the motor, by selecting the appropriate reduction ratio, at a continuous speed lower than the gearhead maximum recommended input speed.


- Speed and torque calculated at motor output shaft

Nout = gearhead output speed, Nin = motor output speed (gearhead input speed),
Tout = gearhead output torque, Tin = motor output torque (gearhead input torque),
i = gearhead reduction ratio, = gearhead efficiency [%]


Note: In this catalog only standard coils are shown.  If desired, Namiki can  provide different coils to meet specific working points. Please consult our sales section for details.

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