copper losses may be expressed in watts per kilogram
2. brush losses the i2R losses in brushes are negligible because the current density is only about 0.1A/mm2 which is far less than that used in copper however this contact voltage drop between the brushes and commutator may produce significant losses the drop varies from 0.8V to 1.3V depending on type of brush the applied pressure and the brush cureent (fig 6.3)