Industry Standard Discrepancies

I have issues with the EWRB and their exams full of wrong answers expected, based on false information taught.

There have been gaps and misunderstandings in my knowledge due to personal confusion between what is “in the books” and what I see in reality!.

ie: If you are ever asked to sketch a single phase cap start motor, you must have the centrifugal switch on the supply side of start winding. I guess the reason being is that little potential voltage will exist between earth and run and the start winding. Makes sense yes!

In reality having disassembled, reverse engineered , wound and fixed hundreds of 1phase motors, I have seen ONE wired like this. The common is ALWAYS on the phase the C/S and cap are ALWAYS on neutral return after start winding.

If they teach it. Why don’t they regulate it?

Another example

They always refer to thirmisters as though they where microtherms. There is a HUGE difference. A Microtherm is a small bi-metal device that will operate to open the circuit when a certain temperature is met due to over current over a length of time. Microtherms are in series with the motor windings.

Thirmisters (at least the ones I know of) normally come as three small silicon devices (one for each phase) that act in a similar function as a PT100 . They have a temperature co-efficient and are not designed to physically disconnect the supply …only as a transducer as such. Try and wire them up in series with the windings and your in a whole lot of trouble.

If I draw a single phase motor the way they are manufactured and distributed to the world (including NZ), I get ZERO marks.

If I use the word Microtherm instead of thirmister, I get ZERO marks.

This is only two examples, I have many more.

Have I unnecessarily complicated things again?, or maybe I am incorrect? Does anybody have anything to say about this?

Clayts

Extra Exciter On Generator

A quick question.

Worked on a Generator today. Was full of oil, basically electrosoled, dryed, megged, reassembled and loaded up/output checked. All OK.

Why did it have an extra permanent magnet exciter externally?

Asked boss, he told me it acts as a sort of tacho and will give a constant voltage dependant of speed and can be used as a benchmark of types for the AVR and something to do with it retaining it's residual magnetism (being permanent magnet). Didn't really get what he was saying.

Asked marine electrician. He told me that it is more suited for inductive loads!!

Could somebody give me an actual answer, that makes sense?

Clayton

In-wheel Dc Scooter Motor

Hi all,

Took on a foreigner out of personal interest a couple of days ago. An electric scooter wheel. Took it apart to discover that it was running on brush tails and consequently has damaged the comm and melted the brush gear/holder (is cast as one piece), however the interesting bit is the two brushes on negative supply side are pristine when the two damaged brush holders (and worn down brushes) are on the positive. Asked around to discover that it is general knowledge that this occurs now and then. I don't have an answer as to why though. The fields are fine and it's not on account of a stuck brush because it's two of them and they are worn down to tails. Any Ideas. I don't want to get too carried away until I know the cause.

Any insight appreciated.

Clayts

PS photo attached

AC speed control for 24VDC system

Hello there.

A few things have dawned on me lately!. I'm thinking that maybe I can grab a normal variable speed AC controller...rip out the full wave bridge rectifier, rewind a 400VAC to say a 24VAC 3 phase and worry about the current later...what do you think?. Is there any controller out there where the IGBTs or MOSFETS (entire Cct in general) is able to withstand say 100+ amps. I'm getting very desperate here.

Anybody want to bring some expertise to this, because quite fankly I'm legging this in a big way!!

Best Regards

C M

ps a little knowledge is dangerous but a whole lot of fun. )

24V AC 3 phase motor controller

Hi, I'm an apprentice motor rewinder and have rewound a 400VAC 3 phase motor to 25VAC for somebody elses variable speed drive project. I myself am very interested in EVs and would like to know more about VFDs and what advantages there may be to having done this. apart from the obvious..being able to seriesup a couple of 12V batteries supply maybe. Could anybody offer any input. My goal is a very simple electric motorbike!. I feel that servos are a little over complicated (basic is better) and I have a chip on my shoulders regarding DC..to much carbon dust does that to you.

Cheers in advance.