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Commutated Resistance Change of DC Motors

September 30, 2021

Commutated Resistance Change of Portescap Brush DC Motors


For a one brush pair Portescap DC motor, the catalog value for resistance is specified assuming that all coils are active (that is, no coil is short circuited by the brush, which would occur if the brush were simultaneously touching two commutator segments).

The individual coil resistance can be modeled by assuming that the rotor has the same number of coils as commutator segments and that they are arranged in a fashion when non-commutated of two individual parallel branches, one branch having one more coil in series than the other. At the moment of commutation, (since one coil is effectively shorted out) the equivalent circuit becomes two parallel branches: each with the same number of coils in series.


Number of Commutator Segments

Catalog Nominal Resistance Value

Commutated Rotor Resistance Value

Individual Coil Resistance Percent Rotor Resistance Change from Non-Commutated to Commutated Commutator Resistance Factor
5 1 0.833 0.833 -16.67% 0.833
7 1 0.875 0.583 -12.50% 0.875
9 1 0.900 0.450 -10.00% 0.900
11 1 0.917 0.367 -8.33% 0.917
13 1 0.929 0.310 -7.14% 0.929

The relationship between commutated and non-commutated rotor resistance can be expressed by: RC = FC R