Jim Cox's books on Electric Motors

Hello everyone.

I have most of the books in the Workshop Practice series. One of them is Jim Cox "Electric motors in the home workshop" (WPS 24).

I was recently checking which books I'm missing, and one of them is Jim Cox "Electric motors" (WPS 16). Can anyone tell me if it would be worth buying the older book, will I learn something new and useful?

P.S. I think I already have a reasonable practical understanding on the topic, e.g. I can handle a lathe VFD upgrade or a milling spindle drive via a brushless DC motor. I am not particularly interested to deeper learn the theory or math behind how the motors work, or to find out about some obscure types of motors that I probably won't ever use.

Take care

Gene

It is a useful book but I would strongly recommend ignoring the advice to short the supply with a screwdriver as a check for isolation, this will destroy the screwdriver and shower you with hot metal if it did happen to still be live.

Mike

Mike,

I thought that was the quick and messy way to discharge capacitors, they can hold a charge for a remarkably long time and give you a painful belt into the bargain. There are better ways to check for isolation.

Brian

Edited By Brian Wood on 09/11/2020 17:42:41

Bear in mind that it was published in 1988. Motor technology and affordable control systems have moved on a lot since then. No mention, for example, of VFD for three phase motors.

Rod

Not a good idea. For a start it won't properly discharge the capacitor. To a greater or lesser extent all real world capacitors suffer from dielectric absorbtion. This means that quickly shorting the capacitor and then removing the short doesn't fully discharge the capacitor. The effect can be modelled as a series R and C across the ideal capacitor. The resistor is high value compared to the ESR of the main capacitor. A short discharges the main capacitor through the ESR of the capacitor. But since the additional RC has long time constant the second capacitor doesn't get discharged by the temporary short. Once the short is removed the second capacitor partially re-charges the main capacitor over time. So you come back to the capacitor some while later and get an unexpexted shock.

Andrew

Thank you Andrew for the explanation.

When I have done this sort of thing I usually repeat the shorting at least once more to take the sting out of it.

My father was a radio ham and I once saw him hurled across his workroom having forgotten to discharge a pair of big capacitors that had been up to 500 volts or more, an experience that shocked me as a child and one he never forgot.

Regards Brian

Yes I understand the books are a bit date, especially the WPS 16 which I don't have.

The cover photo of WPS 16 has what seems to be a Hitachi J100 VFD, though.

Either way I already know enough about VFDs, having read Neil Wyatt's excellent practical article on the topic, as well as multiple forum topics on practical machinist, and last but not least, the 400-page Yaskawa V1000 manual. I got two of these drives second-hand for 100 EUR, rated 0.55 kW (heavy duty = constant torque rating, which is the relevant one in machine tools). One is going on my Hobbymat lathe (with a new 0.37 kW 4-pole motor), the other will be a spare or will be used in some future project. I'm waiting for a second-hand variac to arrive, to perform a capacitor reforming, just in case these units were kept in storage for too long, wouldn't want them to blow up on power on.

So it seems the consensus is there is not much to be gained to buy the older book if I already have the newer one?

As for shorting out caps, that's what I always did in the past, although I've found relatively recently it's not recommended. Apparently it's not so healthy for the capacitors either. I'm going to build a safe cap discharge tool, similar to this: https://www.instructables.com/Safe-Capacitor-Discharge-Tool/

--Gene

Gene,

I always use a high value resistor to discharge my capacitors, much better for the capacitor too. I use crock clips being careful to only touch one terminal with my hand at a time to avoid a shock. If you leave the resistor connected then the capacitor can't recharge itself later as mentioned by Andrew in an earlier post.

Berty

Television cathode ray tubes behave the same way. The capacitor is the aquadag coating with a glass dielectric.

Could be a bit awkward if you get a shock while carrying the tube!

John

Microwave oven capacitors used to have a high value resistor permanently soldered across the capacitor terminals. This was a good idea, as it reduces the number of microwave oven technician DEATHS, and subsequent funerals!

The presence of the resistor did not affect the operation of the capacitor in the microwave oven circuitry. From memory, which is really good on some, but not all days, the resistor had a value that was linked to five (or more -was it) time constants of the capacitor charge circuit.

Anyway, what I am alluding to is that: a permanently connected discharge resistor is a good idea! It works as expected until the discharge resistor fails, then the death trap is waiting!

A neon lamp and associated resistor (across the capacitor terminals) would be a better idea - in conjunction with the discharge resistor. I have a memory of this circuitry in valve radar transmitters, but again, the memory - after forty years is not so good.

The purpose of my post is to help reduce DEATHS!

Think of the Undertaker, he has to make a living too !

His slogan is "If you dont die, I can,t live "

I would have a hard time to find a cathode ray tube these days! Whenever I check our recycling center for something useful to salvage, all the TVs there are modern LCD flat-screens.

I'm guessing that many properly designed appliances containing high-voltage capacitors have a discharge resistor. For example my Yaskawa V1000 VFD manual says before opening the cover, wait 5 minutes after disconnecting for the capacitor to discharge.

The points mentioned about potential danger of (failed) discharge resistors are the rationale behind the status LEDs in the (supposedly) safe discharge device I linked above.

I'm gonna build one because it looks like a fun and simple project.

John

I remember the days when I would change a 26" CRT in the front room of a customers house, fortunately my car was big enough to fit the boxed tube on the back seat, does anyone recall seeing a car go past with a white thing tied to its bumper because it wasn't allowed inside the car because of the stink from it, it was the EHT tripler which powered the tube.

Richard,

I guess I'm showing my age but yes I do remember the triplers.

what about the 25kv eht shunt regulator triode in it's x-ray shield.

John

Back to the OP's question. I have both of his books and find them to be complementary. They are both a useful reference source. At the price you CAN afford both.

Andrew.

John

An engineer I worked with was working on a set with one these and he left the shielding off as he worked, the next thing an ambulance was called and he was taken to hospital with severe radiation around his mid section, although he survived he was unable to perform with his wife afterwards, I had to go and sort out the TV out and the fault turned out to be a fault in the focus circuit.

Aren't we all glad to be rid of these CRTs?

I agree the books are affordable, although assembling a complete WPS collection would end up costing about 500 EUR, at ~10 EUR per book. There are a few books that I will definitely not buy, e.g. there's one about selecting workshop machines, that, besides being dated, has reviews that say it's little more than an advertising blurb / catalog of machines available at the time of writing. I'm just mentioning that because knowing for sure that I'm not going to assemble a complete collection makes it possible to consider not buying some other books that I don't have yet.

Andrew, can you tell me what sort of information is present in WPS 16 that is not covered in the later book?

I guess I should make a decision based on this.

Selenium rectifiers...

My dad used to rent out TVs and so knew all about dealing with the nasty voltages etc.

He used to transport CRTs around in a van on top of a thick vinyl or leather faced sisal mat.

Disposal was tricky. He used to wrap a tube in thick cloth, then stood on top of the steps at the back of the shop he used to drop a well-aimed brick on the neck of the tube, then duck back behind the parapet. The brick would usually snap off without destroying the main part.

Sadly I was not allowed to watch, let alone, carry out this exciting exercise.

Neil

Hi, my elder brother and myself deliberately broke one or two on the back just beside the neck, just to see and hear the implosion, standing well back, it took two or three throws. It was surprising how tough they were, but if you just hit and busted the neck off, the implosion didn't happen and you just heard a dull sound of the air rushing in.

Regards Nick.