PMDC motor and other electrical machines.

zaveri1. Permanent magnet DC motors have a permanent magnet as the field magnet. According to laws of physics, a permanent magnet loses its magnetic property if subjected to heat. now when this PMDC motor is made to work for a long time, then doesn't the field magnet lose the magnetism ?

2. Is there any such thing as Permanent magnet DC generators and alternators ?

3. In case of an automobile alternator, the field coil receives current for excitation, from some external power source such as the battery. how is it in the case of alternators used in power plants and the engine powered generators used domestically ?

magnets loose their property when subjected to high temparatures but in pmdcs the may not reach to that particular temparature so the magnet may not loose its magnetism
in automobiles maximum pmdc are used and in alternators that were used in power plants the excitation is given from the batteries or from other sources i.e ac/dc coversion
in domestic generators due to residual magnitism some emf is induced so that voltage is developed automatically

@#-Link-Snipped-#

what do you have to say to this buddy ?

Sorry Zaveri I think @#-Link-Snipped-# is well versed in this area than me.

My two cents

Magnet looses magnetism only on heating beyond curie temperature, PMDC motors can't generate that much current on their own. PMDC can self cool itself even while running due to heat radiated outside from them

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In seperately excited DC generator the field coil is fed from a power source externally. But in the case of the Self excited DC generator the part of generated voltage is used for excitation of field coil

1. All motors are designed to deliver a rated power output (in fact designed to deliver a little higher power than the rated value). The current that would be used to deliver this power would generate heat only in the safe limits. Unless the motor is oveloaded for long intervals or forcefully driven at higher power or some fault occurs, the temperature wouldn't rise much. Thus, the permanent magnets in the motor will not lose its magnetism due to temperature rise if the motor is used the way it is designed to.

But, since the magnets are dealing with magnetic stress and strain generated by the armature and a magnet is not expected to retain the same magnetism forever, it would eventually deteriorate some day.

2. Yes, my cycle dynamo was one such alternator.

3. In large machines, sometimes a small DC generator is coupled to the same shaft as the large alternator to power its field. For dc generators, as Jeffrey said, self excited field is powered from the output of the generator itself and some kind of external power source is used for seperately excited type.

@#-Link-Snipped-# @#-Link-Snipped-#

Thanx a loT guys

thanks to @#-Link-Snipped-# , @#-Link-Snipped-# , @#-Link-Snipped-# for this insight view on PMDC.