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How to Measure DC Motor Rotation

One method for measuring the rotation of a motor involves coupling a mechanism to the motor shaft that provides leads to a tachometer circuit. This results in mechanical loss that also prevents the motor from turning at it maximum speed. Another method can be done without making contact with the motor shaft. A Hall effect sensor is a p-type semiconductor device that induces a current in the presence of a magnetic field. By mounting a magnet on the motor shaft, a Hall sensor in proximity to the magnet will send an electrical signal to a tachometer circuit with each rotation.

Things You'll Need

DC motor driven by speed control circuit
Hall effect sensor-based motor tachometer
Magnet
Adhesive or electrical tape

Instructions

- 1
  Secure the magnet to the shaft of the DC motor. Use an adhesive or a strip of electrical tape that is long enough to wrap around the shaft at least two times. Any adhesive you use must be strong enough to hold the magnet in place on the shaft, especially while it's rotating.
- 2
  Place the Hall effect sensor over the the motor shaft. Review any documentation that came with your tachometer circuit for any specifications for properly positioning the Hall effect sensor. The sensor should be close enough to the shaft to reliably detect the presence of the magnet. Make sure that nothing will touch the any part of the motor or otherwise impede its normal performance. The motor shaft should be able to turn freely.
- 3
  Connect power to the motor tachometer. Plug the circuit into a wall outlet. If plugging in the tachometer did not activate the circuit, locate the power switch and set it to the "ON" position.
- 4
  Supply the direct current (DC) motor speed control circuit. Plug it into an outlet. If you need to flip a switch to start turning the motor, then do so now..
- 5
  Test the tachometer circuit. Check any product documentation for varying the speed to the DC motor. Once you have determined how to adjust the speed, increase and decrease the motor speed over the full range. Set the motor to turn at a certain speed and check for a readout from the tachometer display. Repeat this process for high speeds, low speeds and in between.

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