RPM's and Hertz

[David HK]

I have built a simple test rig consisting of a variable power supply (DC); one 18 Volt DC motor; one disc with 60 'holes' which is connected on a shaft to the motor; and an opto interruptor switch. The slotted disc breaks the light beam as it spins through the the slotted switch.

I have powered the motor with 12 Volts DC and my multi meter set on Hertz shows 1.85 KHz. Remember the disc has 60 slots. Does this mean the RPM is 1,850 or have I got my maths wrong?

The next question is:- what changes if I replace the slotted disc (60 holes) and replace it with one that has 50 holes? The voltage supplied to the motor remains unchanged at 12 Volts DC. What is the RPM in this scenario?

Confused!

David HK

[Flux]

Yes a 60 hole disc would give 1850 rpm with a frequency of 1.85kHz.

In the second case if the motor volts remains unchanged the speed doesn't change.

The frequency now becomes 50/60 x 1.85 = 1.54kHz.

Flux

[David HK]

Thanks Flux,

Let me see if my thoughts are on the right track then for a RPM meter on a wind turbine.

From a maths point of view it would therefore seem easier to equip the magnet rotor with 60 slots (if I use an opto switch). Thereafter the calculation of RPM is done on the simple basis of frequency conversion - typically a microprocessor with a display.

Thanks to all from a very wet Hong Kong.

David HK

[elvin1949]

  David HK

 "very wet Hong Kong"

 Please send me some rain. I ran a tiller down the middles in my garden this afternoon. No dust_bricks

instead. Sure is dry.

later

Elvin