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

• woodfich
In summary, the conversation discusses the behavior of a motor acting as a generator and how it differs from a load that consumes power. The direction of power flow and the construction of DC motors are mentioned as factors that contribute to the phenomenon of opposite sign readings for current and voltage.
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.

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.

Thanks for the fast and comprehensive response.

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.

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.

## What is a DC motor?

A DC motor is a type of electric motor that converts electrical energy into mechanical energy. It operates using direct current (DC) and consists of a stator (stationary part) and rotor (rotating part).

## How is input power measured for a DC motor?

Input power for a DC motor is typically measured in watts (W) or kilowatts (kW). This is the amount of electrical energy that is supplied to the motor.

## What is output power for a DC motor?

Output power for a DC motor is the amount of mechanical energy that is produced by the motor. It is typically measured in horsepower (hp) or Newton-meters (Nm).

## How do you calculate the efficiency of a DC motor?

The efficiency of a DC motor is calculated by dividing the output power by the input power and multiplying by 100%. This value represents the percentage of input power that is converted into output power by the motor.

## What factors affect the input and output power of a DC motor?

The input power of a DC motor is affected by the voltage and current supplied to it, while the output power is affected by the motor's design, load, and speed. Other factors such as friction, temperature, and winding resistance can also impact the motor's efficiency and power output.

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