Microwave repair

[whisky-dave]

Well better than them not working at all ever, anyway what's the problem when you have plenty :)

>

[Chris B]

I have had two similar failures. Both times of a little synchronous motor. Easily accessed from underneath via an access panel.

No need to go anywhere near the hot bits.

This type of thing

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[Brian Reay]

They tend to be the larger motors sometimes used in washing machines. The motors used to drive turn tables in uwaves (and water pumps in washing machines and dishwashers) are small, AC, motors.

[The Natural Philosopher]

No, the worst thing is if it is rusted all to f*ck. Especalially if it was 'stainless'

[The Natural Philosopher]

I dont think so for washing machines and dishwasher. Fairly sure the ones I have taken out were brushed.

I agree with µ-wave turntables being synchs tho.

[ARW]

Indeed.

[Max Demian]

Synchronous? Is that why the turntable on my Daewoo sometimes goes one way; sometimes the other? (And no, it's not a fault.) Electric clocks sometimes had a mechanism to ensure they went the right way.

[Brian Reay]

Universal motors of the type I'm describing have brushes.

I don't recall seeing one in a dishwasher but I've not looked at many so that isn't conclusive.

[Andrew Gabriel]

Main wash pump motors that I've seen in dishwashers are all

2-phase with a run capacitor.

The waste pumps in dishwashers and washing machines are all shaded pole motors.

Drum motors in washing machines are universal motors (i.e. with brushes) when belt driven, and stepper motors (brushless) when direct drive. Drum motors need good starting torque and a very wide speed range, which the other motors above don't have.

I can still recall my parents' first washing machine, which predated any sort of electronic motor control. I'm not sure what the motor was, but it was single speed, single direction. It had a sodding great gearbox attached with a handful of large solenoids around it to do the gear changes. I can still clearly recall the loud bangs as the different solenoids engaged to change the drum speed throughout the program. When it was scrapped, I saved the mains solenoids and some other parts, which got used in a few projects afterwards. They did tend to overheat, as the washing machine was for 200V mains, and had been converted to 240V when the mains voltage changed by fitting a large autotransformer inside the case, which I hadn't kept because it was too heavy.

[Max Demian]

When did the machine date from? Was it an early automatic? My parents had a single drum followed by a twin tub, neither of which required a gearbox or speed controls I imagine as the motors drove dollies and impellers and pumps and mangles and spin driers.

[Roger Hayter]

I misread that as singular "motor" at first, and had this brief mental image of your parents' scullery with a long shaft across one side of the ceiling and a row of domestic appliances all with their own (unguarded in my image) belt drives from the ceiling. Silly, I know.

[Johnny B Good]

I can imagine the need for a 10A rated 240v supply which would require a

400VA auto transformer in this case (40v difference at 10A) which could easily weigh some 16 to 20 Lbs. Just as you described, not a lightweight item. :-)

I can understand why you didn't hang onto it since any future need to adapt a 200v appliance to 240v mains would be extremely improbable. The only potential use would be in a museum of domestic electrical appliances. Did you ever consider donating it to any such museums?

[Andrew Gabriel]

English Electric Liberator, would have been 1961 or 1962. Yes, it was automatic.

(not my parents' actual one, but identical)

You turned the red dial to show the program you wanted in the window, and then selected the number it showed on the dial on the right, which slowly turned to go through the wash cycle. There were contacts on both dials, which combined to form the wash cycle program. The red dial showed 1-3 different programs, and the right dial selected which of those it was going to do.

The only thing I ever recall going wrong with it was the clutch, which allowed the drum inertia to freewheel when it was spinning and the gearbox changed down gear for a slower speed without back-feeding the gearbox at high speed. It was a large coil spring slid over the drum shaft which gripped the shaft when driven by the gearbox, but relaxed grip if the drum was turning faster than the gearbox output. (The machine could not reverse the direction of the drum.) The spring broke after around 12 years, but a replacement was obtained and fitted by my father.

[Andrew Gabriel]

It was probably 20 years old, and not noteworthy at the time.

If it was still around now, I would either find a museum, or keep it going!

Although I vivdly recall the sounds it made, it kind of seems a shame there's probably no video or even audio history of such devices running remaining today.

I did record an older device which I still have working, a Hoover Constellation vacuum cleaner, playing with it as a frictionless puck much as I did as a child in my parents' house, although they had flatter floors than this one was.

[Roger Hayter]

My knowledge of transformer theory was never very clear, but why is it not rated for the total output, which might well be something like 10A at 200v, 2000VA?

[Andrew Gabriel]

An auto-transformer has just one tapped winding. The core only has to cope with the VA of the voltage difference. The common part of the transformer winding can be thinner too, although that depends on the no-load inductance and the harmonic component of the load.

[Graham.]

We had a Hotpoint Liberator about 15 years later, we must have had it for 10 years until the bearings gave up, and I replaced it with an identical machine from my parents-in law cellar that had little use.

The first front lording automatic I ever saw was at my next door neighbours when I was a kid. A Bendix. I was very impressed.

[Graham.]

My Grandmother had a clock built in to a large mirror perminently screwed onto the lounge wall. There was no accesable connector, but it was wired to its own fuse in the massive metal fuse-box. The clock was self-starting, but it was 50/50 if it went in the right direction. As it didn't have a second hand, if there was a power interruption, you had to wait until you could perceeve some movement of the second hand and pull the fuse breifly if it was going backwards.

[Roger Hayter]

Thanks. I must make the effort to read a fuller explanation some time so I understand it. But the point is noted.

[Johnny B Good]

====snip====

The main advantage of an auto-transformer over a conventional one with isolated primary and secondary windings is that you only need to rate it for the VA difference between input and output voltages.

The easiest way to explain this is to consider the equivalent derived from, in this case, a 40v 10A rated secondary output 240v mains transformer where the secondary winding is wired in series opposing to the 240v live connection (voltage buck configuration) so as to provide the required 200vac. Since the 40v 10A secondary is merely handling its rated current and the mains live connection can handle whatever a 13A fused outlet can supply from a ring main socket, the 240v primary is only handling the 400VA load it is already rated for (its secondary winding is only handling an opposing 40v at 10A leaving the mains to directly supply a 240v 10A loading).

A purpose designed auto-transformer in this case would consist of a single winding designed with appropriate tapping points of a common neutral (0v reference end), 200v and a 240v tapping points (with possibly other tap voltages such as 210, 220, 230 and 250 volts). Normal practice when winding such auto-transformers is to use appropriate gauges of wire between each tapping point to ensure that whichever way such a transformer is used, no more copper than is actually required at the maximum VA rating is used.

In the case of Andrew's 240v to 200v autotransformer (assumed to have a

400VA rating), the section between the hot end of the 200v winding (200v tap) and the 240v tap (the mains input side) will have thicker gauge wire designed for the 10A load (less the portion of current supplied by the 200v winding section to the load) whilst the 0 to 200 volt winding will be specified to handle a maximum current calculated from the sum voltages of the 40v winding and itself which works out at a mere 1.67A which means the 200 to 240v section only has to handle a mere 8.33A.

If we use the same auto-transformer to step up a 200v supply to 240v at the same 400VA load, those current ratings remain unchanged and this remains true even if we use the "240v tap" for the neutral connection so that we can provide a non-isolated 40v at 10A from the "200v tap" with the "0v tap" connected to the incoming 240v mains live.

Another example of this principle is to take a 120VA split primary

240/120v mains transformer with any arbitrary low voltage secondary winding(s) which will remain unused in this case and use it as a 240VA auto-transformer to step up a 120v supply to 240v or to step down from 240 to 120v by simply connecting the split primaries in series phase aiding as per the requirement to provide the secondary voltages from a 240v supply. The connection between the split primaries becomes our 120v tapping point which can be used either as the input or the output, depending on whether we're stepping the voltage up or down respectively.

In this example case, the 240VA auto-transformer rating arises out of the fact that each of the 120v windings is supplying 1A each to the load when used to step down the voltage. When used to step up the voltage, the required 2A at 120v is now split equally between the two windings, 1A to supply the 0 to 120v winding[1] and the remaining 1A becoming the load current via the other 120 to 240v winding to supply the load with 1A at

240v (240VA total output).

The difference between a conventional fully isolated voltage transformer and and an autotransformer is that the VA rating in the former case has to be at least that of the load whilst that of the latter merely has to handle the VA difference between the input and output voltages. If you only had to step up to 240v at 10A from a 230v supply, the VA rating required of the auto-transformer would only need to be 100VA rather than the former case of stepping 200v to 240v at 10A which requires a rating of 400VA. A fully isolated transformer would, in this case, need to be rated at the full 2.4KVA rating of the load.

[1] The reason for this portion of the winding requiring 1A is that the 1A load on the other winding is simply the result of a one to one voltage transformation to a secondary that happens to be connected series aiding to the incoming 120v mains so as to supply a 240v 1A load.