mo-ta モーター

October 4, 2006

How does the tachogenerator on the back of an AC motor work?

Filed under: AC Gearmotors — mota @ 3:11 am

The tachogenerator creates a single phase, AC voltage out of the 2 wires.  The output signal of the tacho-generator motor has 2 components: voltage amplitude and frequency. The amplitude represents the speed of the motor. This amplitude is subject to other variations besides the motor speed. The primary factors are the tolerances of the winding resistance and the magnetic flux density of the magnet.  Also, as each motor heats up, the resistance will change and the magnetic flux density can also change. 

The frequency of the signal can be easily calculated by the formula: (N/60) x 12 = Hz where: N = speed of the motor in rpm, Hz = frequency of the voltage signal, the 60 is used to convert the units from minutes to seconds, and the 12 comes from the fact that the generator uses a 24 pole magnet , which will give you 12 cycles per revolution.  Temperature of the generator itself will not affect the motor speed.  Keep in mind that as the motor heats up, the torque that it produces will reduce, which will cause the motor speed to decrease, assuming a constant friction load. This is normal.

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