Can a dc motor run completely on permanent magnets?

Question

a dc motor has magnets around the outside and an electromagnetic rotor in the centre, im confused as to why cant permanent magnets be used in stead of the electro copper rotor for free energy?

Answer 1

because the magnet would lose its field and you'd have to expend electricity to get the field back.

NO FREE LUNCHES.

Answer 2

A D.C. motor works on the principle of Fleming's rule; this states that a current-carrying conductor in a magnetic field results in a force leading to motion (of the conductor or the magnet, whichever is free to move).

By arranging a series of such conductors connected through a commutator, the direction of electric current is reversed in different conductor lengths, thus leading to rotation.

A set of simple permanent magnets can not produce such continuing force and hence, motion (at best, a single repelsion happens, leading to short movement of the two magnets, but not a continuing motion).

Principle of commutation is at the heart of understanding D.C. machines.

Answer 3

If you look at a dc motor, you will see brushes and commutator. What these do is reverse the current and so the polarity of the electromagnet as the motor turns.

Permanent magnets have no way to reverse the polarity so it wouldn't turn.

There are some newer dc motors that don't have brushes and commutator but they contain sensors and have electronic controls to do the same thing but without the maintenance issues of brushes etc.

Answer 4

Imagine the permanent magnent motor you describe in your question aligned on a bench so the north pole of the magnet is at the top or 12 O clock position and the south pole at the bottom or 6 O clock position. The windings in the rotor are fed DC through the brushes and the commutator ring The magnetic field set up in the rotor is approximately the same with the north pole at top or 12 O clock position and the south pole at the bottom or 6 O clock position. The 2 north poles and 2 south poles are directly facing each other. In magnetism like poles repel opposite poles attract so the rotor turns so the north pole of the winding moves away from the fixed north pole of the permanent magnet. But as it moves the copper pick ups on the commutator ring move off the brushes and the next section of the rotor winding is connected to the brush gear. The rotor magnetic field is re- instated in the same position opposite the fixed magnetic field. The repulsion happens again and again causing rotation. In other words the rotor has a number of windings that are only energised and producing magnetism when they line up with the brush gear.

If we replaced the rotor windings with a fixed magnet and its starting point was with the outer fixed north pole and the rotor north pole at the 12 O clock position the rotor would rotate 180 degrees and stop as the rotor south pole would now be opposite the fixed north pole and the rotor north pole  would be opposite the fixed south pole. The attraction between opposite poles would hold the rotor in a fixed position and no rotation would be possible.

This principle is used in the stepper motor where a fixed magnet rotor is surrounded by a number of field coils that can be switched on and off to produce incremental rotation of high accuracy.