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3 phase tachometer - anyone with a deep understanding?

I have a classic three phase tachometer generator system on my twin. The tachogenerators are driven by short rotating cables. The indicator needles have become increasingly jumpy – swinging +/- 100rpm or so. The tachogenerators were serviced 2 years ago, as was the dual tacho. I have moderate, but not complete confidence in the overhaulers. As a test, I have been able to obtain another dual tacho – this one freshly overhauled by the manufacturer. Curiously, the needle swinging is worse with the newly overhauled dual tacho. I suspect, but have not been scientific about this, that the needle swinging is worse when things are hot.

Anyone have a deep understanding of these systems that might be able to help me with any theories as to:

(a) the potential root cause or causes of the swinging needles
(b) why a freshly overhauled dual tacho just released from a reputable source appears to amplify the problem

Thanks in advance


Alan, I have no deep experience with 3-phase electric tachometers but it’s probably worth mentioning that jumpy instrument needles are very often the result of rotating drive cables that need lubrication or are routed poorly. If your system has only very short cables then this is less likely, but might still be worth checking.

Last Edited by Silvaire at 30 Jul 14:33

I thought those 3 phase tachos worked a bit like a step motor and it is normal for them to ‘jump’ from one reading to the next. They never appear to work smoothly!

EGHI, United Kingdom

Is this the 3 phase tachometer described here?

If so, there is no reason why it should indicate erratically. The slip angle of the indicating device should be a smooth function of the RPM.

I would troubleshoot it with an oscilloscope on the generator outputs, initially, and then check if the indicator has shagged bearings etc.

Shoreham EGKA, United Kingdom

Peter – exactly as described. I don’t really understand how a synchronous motor system works and was wondering if there was a fault mode to give erratic readings. For example would wiring/connector resistance in one of the phases give an erratic result? On the mechanical side, I’m comfortable that I know the sort of things that cause jerky cables, on the electrical side much less sure.

What is odd for me is that a regular cable driven eddy current speedo has a damping dashpot on the needle – but as far as I understand, there is no mechanical damping on a 3 phase tacho.

Either way, troubleshooting with a scope and checking the cable is the right thing to do in due course

The generator just generates a 3 phase waveform, proportional to the revs.

The “motor” is a 3 phase motor but with its armature held stalled. The torque on the stalled shaft will be proportional to the input frequency (and voltage – the two will scale at the generator anyway).

So a means has to be found to convert that torque into a reading. So it isn’t rigidly stalled. It is held in place by a spring, so some movement is possible, and the motor just bends the spring around.

The system should be perfectly smooth. No damping is needed if the frequency is high enough. At 2400rpm a 3 phase generator puts out 40Hz on each of the 3 phase outputs and that is a pretty high frequency. Especially as the ripple on the torque will be at 120Hz (google rectified 3 phase). No way should the indicating needle be wobbling at 40Hz, never mind 120Hz. But at lower rpm (idle) it might be, so one would need to make the armature+etc mass adequate to suppress that.

I would put a scope on the 3 phases and check the waveform is smooth in both amplitude and frequency. This is trivially done on the ground, or by driving the cable which drives the generator with an electric drill or whatever which saves you walking into the prop

Shoreham EGKA, United Kingdom

Thanks – gives me some routes for test. Regarding the mechanics, the motor in the indicator is free rotating with a magnet and drag cup assembly to drive the needle.

I have known speedometers and tachometers wobble due to the bowden cable sticking.

Darley Moor, Gamston (UK)

Yes, drive cable issues are surely the most common cause, often overlooked in the pursuit of something more complicated.

This is a video of a Baron with a similar sort of problem. There’s some digging around to be done!

[video fixed – youtube and vimeo URLs just drop directly into your post; no link needed]

Last Edited by Peter at 31 Jul 09:36
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