What happens if I exceed the current rating of the permanent magnet DC motor - generator?


If you are using a permanent magnet DC motor as a generator then you need to be careful not to exceed the manufactures current rating for your motor UNLESS you put a fan on it. This is because the current or "Amps" rating on the PM motor was calculated so that you could operate it under that current level without frying the armature. The armature has copper wire windings on it that create a magnetic field which pushes against the fixed magnets inside the PM motor housing, and make the shaft of the motor spin. The unfortunate byproduct of the magnet field is much heat dissipation. By looking at the image below you can see how the armature of a DC permanent magnet motor / generator gets hot when you use it. You can see that the copper wires around the armature are showing up as bright yellow. (For those of you who have not watched the movie "Predator", the image looks this way because a thermal temperature infrared IR imaging camera shows you hot and cold of an object by painting ... more
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To understand this concept, you must first understand how a bread toaster works. Heat is created from wire coils from inside of your toaster when they are hooked up to the power outlet. The wire heating element coils have a certain resistance, usually around 10 Ohms. When you push the button down on your toaster it connects the 120 Volts of electricity to your wire heating element coils as shown in the basic toaster schematic diagram above. So the amount of heat generated is calculated by Ohms law and the formula for power. OHMS LAW: V=IR (Volts = Amps X Ohms), Watts = VI = I2R = V2/R Dissipated toaster power is calculated by the formula: Watts = Voltage X Voltage / Resistance = V2/R In this case it would be (120 X 120 / 10) = 1,440 Watts The amount of current going through the heater coil elements is calculated by the formula: Amps = Volts / Ohms = V/R In this toaster case it would be 120 / 10 = 12 Amps The DC Permanent Magnet Motor has a component inside of it just like a toaster ... more
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