## Wednesday, 22 February 2012

### How to Convert RPM to Hz

As you can see on today's "Google Doodle", it is Heinrich Rudolf Hertz's birthday today.  So I thought it was a good time to post the easiest way to convert your engine's revolutions per minute to the electrical frequency. In order to convert rpm to Hz you need to know how many poles your alternator has.  This is usually labeled on the alternator tag.  Since most generators have a frequency meter I most often use this formula to see how fast the engine is rotating or how many poles the alternator has if it is not labeled.

RPM = (Fx120)/n

where:  rpm=Revolutions per minute;    F= frequency in hertz;    120= constant;    n= number of rotor poles

A diesel engine typically has 4 poles so that it can run at 1800 rpm to maintain 60 Hz.
1800= (60x120)/4

A gas engine may have 2 poles which means it must run at a speed of 3600 rpm in order to maintain 60Hz.
3600= (60x120)/4

Most gensets have a frequency meter on the control panel.  It may be a dial indicator style of one with teeth that vibrate at different frequencies.  Smaller generators usually do not have any gauges on their control panel, except for maybe an oil pressure gauge mounted directly to the block.  In this case I use my clamp on meter to read the frequency.  I have a Greenlee meter and there are some features that I do not think are very practical.  The main one is that I cannot read frequency using the voltage scale so I must use the clamp.  This means I can only read frequency when there is current flowing through the cable I am clamped to.  For my load tests I like to take a no-load reading and a full load reading but I cannot see the change in frequency until I apply some sort of load to the line.
Hope this was useful :)