What is the difference between an electric motor and an electric generator?

Answer 1

HOW ARE ELECTRIC GENERATORS DIFFERENT THAN MOTORS?

There is no fundamental difference between an electric motor and an electric generator or dynamo. In normal use, all motors behave as generators, and all generators behave as motors.

DC Motors act like generators because they use less electrical energy when allowed to spin fast.

DC generators act like motors because they become easier to spin when less electrical energy is drawn from their terminals.

For example, connect two small DC magnet motors together. Then if you spin the shaft of the first motor, the second motor's shaft will start spinning too. One acts as a DC generator, and the other acts as a DC motor. Or, spin the second one's shaft, and the first one will start spinning.

Another example: If you connect a small DC motor to a small battery, then an electric current will appear in the motor's coils, and the motor starts spinning. But if you spin the motor's shaft slightly faster than the normal speed, the direction of current in the circuit will reverse, and the battery starts taking in energy from the motor. The motor has become a generator, and it's recharging the battery.

Here's a good way to visualize what's happening. Imagine two pulleys connected by a rubber drive-belt. If you spin the first pulley, it pumps the belt into motion, and the second pulley starts spinning. Ask yourself this: which one is the generator? Answer: both and neither. A pulley is just a pulley. Of course we can *force* a pulley to behave like a motor: let the pulley rub against a moving belt, and the pulley will start spinning. Or we can turn it into a generator: turn the pulley, and the belt will be forced to move. In electric circuits the wires contain a circular "belt" made of electric charges (the electron-sea within the metal.) And as with the pulleys, the "belt" inside an electric circuit will slowly move along just like a leather belt. (You cannot see individual electrons, so when crowds of electrons start moving inside the wires, you cannot see any motion.)

Or pretend that you have two water pumps which are connected together with hoses. Fill the pumps and the hoses with water. Now, when you turn the first pump, the pressure inside the "water circuit" drives the second pump into motion. This shows you that all pumps are motors, and all motors are pumps. It just depends on for which purpose they're employed.

COILS AND MAGNETS

Here's a simple electrical experiment which shows what's happening with motors and generators. Get two coils of wire wrapped on hollow plastic spools. Connect their wires together. Get two powerful bar-magnets and place the end of each magnet inside each spool. Now if you jerk one magnet suddenly, the other magnet will feel a kick. What happened?

By suddenly yanking the first magnet, the first coil created a voltage because of "Lenz-Law Induction." This voltage caused all the charge in both coils to begin moving along. That created a magnetic field in the second coil, which gave the second magnet a kick. Motors and generators are based on this phenomenon.

AC MOTORS AS GENERATORS

But what about AC motors? And what about DC motors which use a "field coil" but no permanent magnets? If you spin either of these, a voltmeter will show no voltage. It's because you aren't using them correctly. To act as a generator, an AC motor needs to be connected to a power grid or to a large-value capacitor. And a DC field motor needs to be connected correctly (series-wound motors must be shorted, while parallel-wound motors must be left open.) And finally, these types of motors can only generate a voltage/current if they're spinning FAST. It takes awhile for tiny initial currents to build up. When no permanent magnets are present, and all you have are moving coils and pieces of metal, it takes current to make current. The current and voltage gradually build up because of a positive-feedback process, and this process doesn't kick in until the motor's shaft is turning faster than a certain speed.

MODERN MOTORS DISCOVERED ACCIDENTALLY

The modern DC motor was not designed as a motor. In the middle 1800s, inventor Zenobe Gramme was trying to build an electric generator which gave a very smooth output voltage. He did this by using many coils with overlapped fields. Because of it's smooth DC output, such a generator could replace large banks of batteries being used at the time. In 1873 one of his assistants accidentally connected two of his generators together. The first generator was being spun by a steam engine, and the second one unexpectedly started spinning like a motor. The "Gramme Machine" has been used ever since, under the name "DC motor" or "DC generator."