Can an Electric Motor Also Be a Generator?

[lwoodlore]

Homework Statement

Both an electric motor and a generator consists of coils of wires on a rotor spinning in a magnetic field. The difference is whether you desire mechanical or electrical energy as the product. If current is generated when the rotor is made to spin by either the mechanical or electrical energy then is a motor also a generator when it is running? The choices are:

a) Yes, it will send electric energy output through the input lines and back to the source
b) It would if it weren't designed with an internal bypass circuit to prevent the problem.
c) No, the device is either a motor of a generator and both at the same time would violate energy conservation.

Explain.

I think it is (a) since it has back EMF when the electric source is stopped but that doesn't mean it is always feeding some back since electrons are going either one way or the other and not both. So I'm confused by the question...

 

[Simon Bridge]

Welcome to PF;

Is sending power the same as sending electrons?
Can't you send power with an AC current?

Your answer would need to account for why it is not (b) or (c) as well. Particularly (c).

 

[CWatters]

is a motor also a generator when it is running

You can only consider a black box to be an electrical generator if there is a net flow of electrical energy out of the box.

Option:

a) If the motor is operating as a motor it must have a load (even if that's just friction in the bearings). The load is taking mechanical energy from the motor so there has to be a net supply of energy to the motor due to conservation of energy. So its not a generator.

b) There are no such special "circuits" in DC motors. Take one apart.

c) see my comment for a).

 

[lwoodlore]

Simon - thanks for taking the time. Consider me dense, but I don't understand your comments. Could you elaborate? The teacher did not discuss whether it was a DC or AC device. Does it matter with regards to the question? Thanks - Lori

 

[Simon Bridge]

There were two questions and a bit of advice.
The questions are supposed to guide your thinking where you got confused.

I think it is (a) since it has back EMF when the electric source is stopped but that doesn't mean it is always feeding some back since electrons are going either one way or the other and not both.

Where did you get stuck?

 

[lwoodlore]

"Is sending power the same as sending electrons?
Can't you send power with an AC current?"

I'm talking about the motor being also a generator. when the motor is running is it also generating a little electricity since it is a set of coils spinning in a magnetic field? The answer seems like 'yes' but then I have to figure out where it would go since there is already a current in the coils from the power source. That is where I'm lost for an answer.

 

[Simon Bridge]

Has CWatters not answered that question?

You don't seem interested in leading questions.
How about - if you move a current carrying wire through a magnetic field, you would, indeed, get a bit more current. Where does the energy for this generated current come from?

In the case of the wires in a motor - where is the energy for the motion coming from?

 

[lwoodlore]

Wires moving in a magnetic field generate a current. In the case of a motor the power is coming from the electrical supply (e.g. wall socket). What I'm missing is since the wires already have a lot of current from the power supply the induced current from the magnets is probably low. Also, it may be trying to travel the other direction which in turn would fight the current from the power supply. If so, does it ever make it back to the input lines and power supply as indicated in choice (a)?

 

[Simon Bridge]

It is possible to build a simple motor where the voltage is applied for only half a cycle - The coil keeps moving from inertia. In that case, there would be an induced current back the other way for the other (unpowered) half the cycle, and the coil slows down.

When the voltage is applied, there is an additional electric field driving a current.
The resultant current is determined by the combination of these effects. To make a motor, you have to be sure to apply sufficient voltage to drive the current the right way or the coil won't turn. Fortunately this is easy.

Note: the current is still just in one direction.
The setup will act as a generator or a motor and not a combination of the two.

 

[CWatters]

If you stick to ordinary DC it's not possible to send current both ways down a wire at the same time. For one thing there will be a voltage drop down the wire which oposes any current flow the other way.

There are several ways you can send power both ways down a wire - perhaps using different AC frequencies for example - but that's not what the question is really about. It's about conservation of energy. Current isn't actually mentioned in the multi choice answers.