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# calculatinng motor RPM

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#### haleemul

##### New Member
hi, i'm new to this forum.

got a question: how do i calculate the rpm of a dc motor given its volts and amps requirements?

in other words say the motor needs 12v and 0.2A to function, how do i know the rpm of the thing?? looking for an equation, i am. I know this can be done using software too.

thankx.

In order to calculate the RPM, you need to know the torque versus armature current and field current, and the load. In a shunt motor, when the back EMF equals the input voltage with no load, the RPM is constant. If you know the Back EMF versus RPM, the speed is easy to calculate. When the motor is loaded, it slows down enough that the difference between the back EMF and applied voltage can drive more current and produce more torque.

haleemul said:
hi, i'm new to this forum.

got a question: how do i calculate the rpm of a dc motor given its volts and amps requirements?

in other words say the motor needs 12v and 0.2A to function, how do i know the rpm of the thing?? looking for an equation, i am. I know this can be done using software too.

thankx.

The DC motor follows a linear equation of torque and RPM. These are also related to the voltage and current.

The easiest way is to look at the curves provided by the manufacturer. Some companies are good about this others are not. If you cannot ID the motor and/or do not have curves available to you, you can perform some no-load, and stall current tests to determine the constants that can fit the linear equation. These constants will vary a bit from motor to motor.

I do not recall the details of the tests but I have a text book around that shows them. You can probably find the same thing on the web.

Once you fit the equation, you can calculate the RPM based on the current / volts delivered. So, I can give you the general form of equation, but it won't do you any good until you can fit the constants to it for that particular motor.

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