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Letters and emails should contain complete name, address and daytime phone number. Letters to the Editor are submitted on the condition that Silicon Chip Publications Pty Ltd may edit and has the right to reproduce in electronic form and communicate these letters. This also applies to submissions to "Ask SILICON CHIP" and "Circuit Notebook".

Enthused over the Atwater Kent story

This is just a short note to congratulate you on the amazing article in the March 2012 issue of SILICON CHIP on Atwater Kent. The words create excellent stories of facts and history of the man and his radios, and the restored monochrome photos are truly magnificent, with their high contrast and level of detail. You do manage to keep raising the bar!

By the way, are the radios shown on pages 98/99 actually here in Australia? The photos are beautifully done. Geez, I’d like a few of those radios if I could afford them!

I’ve never before seen such a beautiful display of A-K radios in print. Great work on the part of Kevin Poulter.

Graeme Dennes,

Bunyip, Vic.

Comment: Kevin took the colour photos in question. All the radios are from a private collection in Melbourne.

Doubts raised about the Induction Motor Speed Controller

With respect to the comments on the Induction Motor Speed Controller in the Publisher’s Letter, (April 2012) and the admission that the 2kW Sinewave Inverter project published in the October 1992 to February 1993 issues was more complex than the VSD, I am still unsure why the delay.

Also, the third harmonic injection is not new, so while Mr Levido has done a great job, it is not his idea! But my main point is, with no dv/dt output filter, the earth currents will not be sinusoidal, even if the phase-to-phase current is. Therefore, what are you going to do about the motor bearing failures that will result? And what about the resulting possible Earth Leakage Breaker tripping problem?

Note that even an output sinewave filter will not eliminate nasty earth currents (as I have discovered via an internet Inverter/VSD forum).

Ray Hamono,

Bayswater Nth, Vic.

Comment: in response to your first question, the delay in producing the design was mainly due to our conservative approach.

We directed your other questions to the designer, Andrew Levido, for his comments, as follows:

The reader is correct in stating that the injection of third harmonic is not new. In fact the author first saw this technique used 25 years ago and it was not so new even then! The article made no claims that this was an original idea.

The high level dv/dt present in the output of induction motor speed controllers can cause a small currents to flow to earth through the capacitance between the stator windings and the earthed frame of the motor. These currents will be spiky, corresponding with the switching edges of the PWM, and not sinusoidal, but they will be symmetrical. For small motors such as those used with this project, the magnitude of these currents will be quite small.

There should not be any problems with domestic earth leakage circuit breakers which typically trip when there is an imbalance in current between the Active and Neutral lines. Despite these small current spikes, Active and Neutral currents will be balanced, so should not cause problems with earth leakage circuit breakers.

The reader also correctly suggests that AC drives have been implicated in early bearing failure in motors under some circumstances. Any potential on the rotor with respect to earth will force a current to flow via the path of least resistance. In some cases, the potential is high enough to break down the thin insulating coating of oil on the bearings causing pitting or frosting, leading to their premature failure.

This is more likely to occur in very large motors where the capacitance between stator and rotor is high but has reportedly been seen in motors as small as 10kW. I are not aware of it being a problem with motors in the 2kW range where the capacitance between stator and rotor is quite small.

The problem can be eliminated by either insulating the steel bearings, using ceramic bearings or by shorting the rotor to earth via some lower-resistance path. Sometimes this occurs naturally through the load, such as in water pumps. In other cases, the shaft can be grounded via conductive plastic brushes. In other cases, the bearings are packed with conductive grease. Adding a dv/dt filter between the AC drive and the motor can also help to some extent.

Given the size and usage of the motors that will be used with this project, it is unlikely that this problem will be an issue in the domestic setting.

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