# DC Motor RPM Equation Confusion

• Blues_MTA
In summary, the output of a DC motor in kRPMs can be calculated using the formula kRPM = (V - RmI) / Kb, where V is voltage, Rm is terminal resistance, I is current, and Kb is the voltage constant. This equation suggests that an increase in current leads to a decrease in speed. This is due to the back emf of a DC motor, which opposes the supply voltage and reduces the current as the speed increases. Therefore, an increase in load or torque will cause a decrease in speed and an increase in current.
Blues_MTA
According to http://www.aveox.com/DC.aspx" The way to calculate the output of a DC motor in kRPMs is the formula

RPM of motor: kRPM = (V - RmI) / Kb

V= Voltage
Rm = Terminal Resistance
I = Current
Kb = Voltage Constant (V/kRPMs)

This insinuates that an increase in amperage leads to a decrease in RPMs? How is that Possible?

I know that Current:Torque and Voltage:Speed, but isn't Voltage = I*R, so an increase in current should consequently lead to an incresed voltage?

Last edited by a moderator:
You should read up on back emf of a dc motor. As rpm increases, an opposing voltage aka back emf (@ the armature terminal) will also increases. The back emf opposes the supply voltage and thus reduces the current to the dc motor. Less rpm = less back emf = more current.

Blues_MTA said:
This insinuates that an increase in amperage leads to a decrease in RPMs? How is that Possible?

Yes, the equation shows that if the current increases, all other variables remaining constant, that the speed decreases. The causality is actually the other way around however. If you increase the load (torque) then the speed decreases and this causes the current to increase.

## What is the equation for calculating DC motor RPM?

The equation for calculating DC motor RPM is: RPM = (60 x frequency) / number of poles. This equation applies to both brushed and brushless DC motors.

## What is the difference between DC motor speed and RPM?

DC motor speed refers to how fast the motor is rotating, while RPM (revolutions per minute) specifically measures the number of full rotations the motor makes in one minute.

## How do you determine the number of poles in a DC motor?

The number of poles in a DC motor can be determined by counting the number of electromagnets in the stator. Each electromagnet represents one pole.

## What factors can affect the accuracy of the DC motor RPM equation?

The accuracy of the DC motor RPM equation can be affected by factors such as the load on the motor, the voltage and current applied, and the efficiency of the motor. In real-world situations, there may also be other external factors that can impact the RPM of a DC motor.

## Can the DC motor RPM equation be used for all types of DC motors?

Yes, the DC motor RPM equation can be used for both brushed and brushless DC motors. However, for brushless motors, the number of poles refers to the number of permanent magnets in the rotor, rather than the number of electromagnets in the stator.

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