A question about DC motor (input power, output power)

[woodfich]

I have a question. I set up a voltmeter and and ammeter (properly connected in parallel and in series respectively) so that when I input power into a motor in the form of DC current, both the voltmeter and the ammeter read positive values.

Then, I removed the voltage source (i.e. battery). Now what I don't understand is, if I manually rotate the motor in one direction, the motor (acting as a generator), I get a positive voltage reading and a negative current reading.
If I rotate the motor (generator) in the other direction, I get a negative voltage reading and a positive current reading.

I don't understand, how can current flow against the voltage established by the generator? Can someone please explain this phenomenon to me? Thanks.

 

[uart]

Well think about the difference between a load that consumes power (a resistor or a motor for example) versus one that supplies power (a battery or generator for example).

For the resistor the voltage drop is positive at the terminal that the current enters whereas with a (dischaging) battery the current leaves from the positive terminal. The motor behaves in the same way as the resistor in this regard (because it's absorbing power) whereas the generator behaves in much the same way as a battery. It's all about the direction of power flow.

 

[woodfich]

Thanks for the fast and comprehensive response.

 

[Nishant Gosai]

Most of the DC motors take AC as an input and then uses the DC by rectifying it.
And the negative sign you're taking about is because many of the DC motors have transformer type construction inside it.
And it causes 180 degree phase difference between the current and voltage that's why current shows opposite sign.

 

[PJC01]

I can explain this phenomenon to you. When a DC motor is connected to a voltage source, it converts electrical energy into mechanical energy. This mechanical energy is used to rotate the motor's shaft and perform work. In this scenario, the voltage source is providing the input power to the motor.

However, when you remove the voltage source and manually rotate the motor, the motor now acts as a generator. This means that the mechanical energy from rotating the motor is converted into electrical energy. This electrical energy is then measured by the voltmeter and ammeter.

When you rotate the motor in one direction, the generated voltage is in the same direction as the initial voltage source, resulting in a positive reading on the voltmeter. However, the generated current is flowing in the opposite direction, resulting in a negative reading on the ammeter.

Similarly, when you rotate the motor in the other direction, the generated voltage is now in the opposite direction of the initial voltage source, resulting in a negative reading on the voltmeter. But the generated current is now flowing in the same direction as the initial current, resulting in a positive reading on the ammeter.

This phenomenon is known as back electromotive force (EMF). It occurs due to the interaction between the magnetic field of the motor's rotor and the stator windings. When the motor is rotating, it produces a magnetic field that opposes the initial magnetic field created by the voltage source, thus creating a reverse voltage. This reverse voltage is what causes the negative readings on the voltmeter and ammeter.

In summary, when a DC motor is connected to a voltage source, it acts as a motor and converts electrical energy into mechanical energy. But when the voltage source is removed and the motor is manually rotated, it acts as a generator and converts mechanical energy into electrical energy, resulting in back EMF.