Why cant we just use permanent magnets?
A permanent magnet kept in a magnetic field experiences torque, so why not use a magnet itself in a D.C motor, instead of using a coil?
With a fixed magnet your magnetic strength would be set and limited, correct? With the coil you can vary the strength of the field, right?
Compare to permanent magnet, electromagnet size reduction would be considerable, for producing the same field strength, and will reduced the motor size and weight, for same H.P development, with considerably increased efficiency. Speed could be increased above rated, with reduction in rated torque, by wakening the field. Refer book,'Electrical machine design', by A.K.Swahney.
Compounding is hardly possible with permanent magnet; however field current will be omitted then; but bulk size may cause some difficulty.
To generate electromagnetic field between armature. It is required to magnetise for charge flux gap which generates field.
Due to natural property of magnet, magnetization of magnet decreases with time. Thus it affects performance of DC machine with time
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