# Finding out Kv motor velocity constant

• Trainee28
In summary, when a DC motor is put in generator mode and a resistance is connected to the wire ending, the motor will turn slower compared to when the wire is connected to nothing. This is because there will be a voltage drop in the resistor, causing the back-emf of the motor to decrease. The motor constant, Km, plays a role in both the back-emf and torque of the motor, and can be measured by adjusting the current and measuring the torque as a function of motor current.
Trainee28
Hello,

If I take a DC motor and put it in generator mode, what will happen if I connect a resistance at the wire ending of the DC motor? Will the motor turn slower compared to when the electric wire of the motor is connected to nothing?
What effect will the resistance have on the current flowing in the motor coil?

I know that in generator mode, mechanical energy is being converted to electrical energy. In this case, mechanical power = torque x angular velocity and electrical power=voltage x current.

I also know that V=RI+L(dI/dt)+E, and E=Kv.W. By neglecting the coil resistance R, is the voltage in the electrical power equation equals to E in generator mode?

Trainee28 said:
Will the motor turn slower compared to when the electric wire of the motor is connected to nothing?
Yes, because there will be a voltage drop in the resistor, thus the back-emf of the motor must also be decreased ( emf = ω * Km ). Km stands for motor-constant, and that's a better name because this motor-constant also have another role in the motor:

Torque = Km * I

So maybe it's easier to measure this Km by measuring the torque as a function of motor-current? Load the motor by some known torque and adjust the current so that the motor can just turn ( slowly ).

Last edited:

## 1. How is the Kv motor velocity constant calculated?

The Kv motor velocity constant is calculated by dividing the motor's no-load speed (in revolutions per minute or RPM) by its input voltage (in volts). The result is typically expressed in units of RPM per volt, and can be used to determine the motor's speed under various load conditions.

## 2. What does the Kv value indicate about a motor's performance?

The Kv value of a motor is an important indicator of its speed and torque characteristics. A higher Kv value indicates a motor that can spin faster under a given voltage, while a lower Kv value indicates a motor with more torque and slower speed. It is important to select a motor with an appropriate Kv value for the intended application.

## 3. How does the Kv constant affect the motor's efficiency?

The Kv constant does not directly affect the motor's efficiency. However, a motor with a higher Kv value will typically have lower torque and may require more current to achieve the same level of performance, resulting in lower overall efficiency. It is important to balance Kv value with efficiency when selecting a motor.

## 4. Can the Kv constant be changed or adjusted?

No, the Kv constant is an inherent characteristic of the motor and cannot be changed or adjusted. However, the Kv value can be varied by changing the motor's windings or other design factors. Some motors also have adjustable Kv values through the use of external devices such as electronic speed controllers.

## 5. How do I choose the right Kv value for my motor?

The appropriate Kv value for a motor depends on the specific application and desired performance. Higher Kv values are better suited for high-speed applications, while lower Kv values are better for applications requiring higher torque. It is important to consider the motor's intended use and select a Kv value that will provide the desired performance. Consulting with a motor expert or conducting thorough testing may also be helpful in choosing the right Kv value.

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