MadSci Network: Engineering

Re: what is the difference between a DC motor and a DC generator?

Date: Sun Jul 8 01:42:44 2001
Posted By: Donald Howard, Staff, Nuclear Engineering, Retired
Area of science: Engineering
ID: 993890121.Eg

Essentially, there is no difference between a DC motor and a DC 

The little 1.5 volt motors have two "C" shaped magnets glued inside the 
case to act as the north and south poles.  The rotor contains three lobes 
that are iron with wire wrapped around them.  The lobes are connected to 
the shaft you see coming out of the motor case, and the attraction between 
the lobes and the permanent magnets makes the shaft turn.  The wire 
connects to the battery through what is called commutator - a set of 
rotary contacts that insure the proper voltage is fed to each lobe as it 
passes each magnet.  

Can this be used as a generator?  Yes.  I did an experiment with one.  I 
drilled a small hole in the eraser of a wooden pencil - just a little 
smaller than the diameter of the shaft of the motor.  I put the motor in a 
vise, stuck the pencil eraser on the motor shaft, put the other end of the 
pencil in a drill, as you suggested, and at 1500 rpm was able to generate 
about 1/4 volt.

Not 1.5 volts because, when 1.5 volts is applied to the motor it turns at 
very high speed, much greater than 1500 rpm, and these small mtors are 
very inefficient.  At least this experiment verifies the answer that there 
is no electrical difference between a DC generator and a DC motor. 

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