Brushes heat sometimes due to too much friction. They should not press against the commutator more than is necessary for good contact.
Ques. Give some causes for heating of armature.
Ans. Eddy currents; moisture; short circuits; unequal strength of magnetic poles; operation above rated voltage, and below normal speed.
Ques. What trouble is encountered with eddy currents?
Ans. Considerable heating of the whole of the armature results, which may even extend to the bearings.
Fig. 729.--Eck Manchester type motor. It is a very small size unit and is designed for special purposes where very little room is available. The motor occupies a space of 2-1/2" × 4-3/8" between bearings and develops 1/16 horse power at 2,000 R. P. M. The frame of this motor is made of high permeability steel so as to reduce the weight to a minimum. The armature is of the hand wound bipolar type built up of thin punchings. The armature, after being wound, is baked at high temperature for a prolonged period and then dipped while hot in insulating varnish. Pulley is one inch in diameter and takes a 1/4 inch round belt. Weight of motor 5-1/2 pounds.
Ques. How can this be overcome?
Ans. There is no remedy for eddy currents other than the purchase of a new armature, or reconstruction.
The fault may be detected by exciting the field magnets and running the machine on open circuit, with the brushes raised off the commutator for some time, when the armature will be found to be excessively heated.