Measuring Motor RPM - Unpowered

There is a very simple way for measuring motor RPM quite accurately in order to use this result in the calculation for the Kv of the motor. This method of measuring motor RPM utilizes a Multimeter rather then using a tachometer. What’s important about the multimeter is that it must have the ability to measure frequency, which most multimeter do. This is ideal when you do not own a tachometer of course, otherwise either method of obtaining the motor RPM would be more accurate enough.

Tools required:

1.  Multimeter with frequency measuring capability
2. Drill with a minimum speed of 1200 RPM to spin up motor
3. Manual to determine amount of poles within the motor

Procedure:

1.  Connect the multimeter up to 2 leads of the motor.
2.  Connect the drill to the motor shaft
3. Set the multimeter to the Frequency setting.
4. Record the Frequency of the motor

How this works:

The motor produces a frequency by alternating the voltage as the motor spins. When the motor is spinning fast enough this value can be measured by the multimeter. The AC current that is produced alternates each time 2 magnetic poles pass by a winding in the stator. If we know how many poles are within the motor, we can then convert this commutation frequency to the motor shaft RPM.

Measuring Motor RPM Formula:

RPM = ( 120 x Frequency ) / # of Motor Poles