The Basement Mechanic's Guide to Testing Perpetual Motion Machines
4. Magnet Motors.
Current serious designs for PMM are seldom purely mechanical. That approach has been fruitless for so many centuries that many inventors conclude there's nothing new there to be discovered. Nearly all of those devices were variations of overbalanced wheels.
Magnet-motors are all the rage these days. But even folks who have had physics and engineering courses are often deficient in understanding of magnetic fields and magnetic materials.
Motors and generators are electromagnetic machines, and electrical engineers have had long experience with them. Moving magnets and wires moving in a magnetic field continually radiate electromagnetic fields, and these can cause interference with nearby electronic equipment such as telephone, radio and television sets. Such interfering radiation can propagate directly as fields, or can be conducted over power lines. Commercial motors are designed to minimize these problems, are housed within partially shielding enclossures, and often have capacitave-inductive filters to reduce interference with other equipment.
The experimenter's PMM magnet motor, however, is usually exposed and unshielded, without any attention given to designing it to minimize electromagnetic radiation. The output wave form is far from sinusoidal, and induction effects may modify the input wave form as well. These wave forms contain abrupt discontinuities and even sharp pulses and spikes, which simple electrical meters can't respond to properly. The radiated fields may affect the meter's circuitry directly. The output likely has a considerable phase angle between current and voltage. For these reasons, electrical meters can give false readings.
http://www.lhup.edu/~dsimanek/museum/test-pm.htm