The following example will illustrate that not being conscious of the differences between how manufacturers “rate” their products can cause compromises in the selection of a small dc motor.

Example

Let’s assume that for a new design a 13 mm diameter motor will be the perfect diameter mechanically.  Looking at the Faulhaber 1336 series of motors you notice that the 6 Volt winding is “rated” for 1.75 Watts.  Comparing this with a similarly constructed motor from Vendor X, you notice that they have a 13 mm diameter motor of similar length but it is rated for 3 Watts.
 

Calculating Stall Torque of a Mechanically Commutated (Brush) DC Motors

What could possibly be subject to interpretation when it comes to Stall Torque?  It is one of the two points on the ubiquitous speed/torque curve.

There are at least 4 methods used to calculate Stall Torque in sub-fractional horsepower dc motors.  The key word here is ‘calculate’.  Unlike stepper motors where holding torque, pull in and pull out torque are measured, in small dc motors stall torque is calculated.

Here are some methods:

          Variables:

                I_O              No Load Current
                K_T            Torque Constant
                R_C            Terminal Resistance
                T_S             Stall Torque
                V           Applied Voltage
 

Friction Torque and Brush Voltage Drop Compensation Method

For Graphite Commutated Motors the Faulhaber Group, as an example, subtracts a 0.5 Volt Brush Drop from the Applied Voltage before the stall current is calculated and then the no load current is subtracted.  The Voltage drop across the brushes is real but the actual value of Voltage drop is subject to many variables beyond the scope of this paper.

          T_S = ((V  –  0.5 Volts) / R_C   –   I_O) x K_T

This conservative method can easily reduce the calculated Stall Torque by more than 10% vs. the more typical method.  Additionally, the lower the motor voltage rating, the more penalizing the calculated results.

Extrapolated Stall Torque

This technique is used particularly by Asian Motor Vendors.  From the no load speed and rated speed and torque a slope of the speed torque curve is calculated.  This is extrapolated to zero speed and the stall torque is then computed.  Any rounding of the no load speed, rated speed and rated torque generate wide variations in the extrapolated Stall Torque.  In other words, the numbers seldom match.