Member postings for Colin Whittaker

Could be so many things ...

My first guess would be that the live and neutral have been confused and it's 9.7V on the neutral that got shorted to earth.

Second guess would be some load after the MCB that is connecting the neutral to live and thus allowing a neutral to live to earth to trip the RCCB.

In either case I'd check whether shorting the neutral to earth trips the RCCB. If it does then everything is most probably working normally.

(Do remember an RCCB compares the live and neutral currents and is normally set to trip if the currents differs by 30 mA. If the neutral to earth voltage is big enough to pass 30mA then the RCCB can trip without the need for any live power.)

By the way ... the neutral normally gets grounded at the local sub station. The further from the sub station and the more current you and your neighbours are drawing the larger will be the neutral to earth voltage.

Imperial units? Reservoir volumes are measured in acre feet. Anyone got a feel for that? And in the oil industry mud density is lbs per gallon, but, that's an American gallon not an English gallon! And when I worked on the railways the civil engineer's track drawings came in miles and chains; before the internet that caused some head scratching.

The SI units that really upset me was when Shell Oil decided the cable tensions should no longer be displayed in lbs but instead, Newtons. However the safe working loads remained in lbs leading to a race between my powers of mental arithmetic and the safe rise in cable tension when the end became stuck. (Differential pressure sticking was best fought by a quick rise to the maximum safe cable tension.)

In comparison a 1000 psi pressure test (that was normal and not terribly high) that overnight became 7MPa was relatively trauma free.

Ah memories ... Colin

I loved the article in the March edition of the MEW on the Spring Centre. Lovely drawings, good photos, text that I could follow, and even enough motivation from the piece to consider making my own.

But, what would I use a spring centre for? I couldn't imagine. And while the stories about RR are fine and apposite I think there may be little bit of wood for the trees here and an explanation of its use in tapping would be of benefit. (Yes, I had to googling.)

But I repeat. A very good article for me.

Colin

Why are AC motors more powerful than DC motors?

Well a HP is a HP is a HP. But if you want to consider the size needed for a given HP then things are not equal.

The star wars project wanted huge amounts of power for minimal weight and was using airborne DC motors/generators.

Induction motor power is limited by the rate of flux change per second. Once the steel of the stator/rotor is saturated then no more power is possible unless the frequency is increased. That's why airborne electrics work at 400Hz giving a factor of 7 to 8 reduction in weight. Why not increase the frequency still further? Well the choice of 50Hz was to keep the hysterisis losses tolerable with the steels available in the early 20th century. A few years later steels had improved and a 60Hz was possible. In the 1950s steels were available that could support 400Hz and that became the aircraft standard. I have heard of 1000Hz motors but that must around the upper limit.

DC motors use the same copper and steel but rotate independent of the supply frequency. There will still be steel limits to the speed of a DC motor but the commutator will normally have exploded before the steel overheats.

While it might be possible to build a 400Hz induction motor powered hand held drill for those of us stuck on the ground a DC universal motor (or compressed air motor) is the only way to go.

My current set up with a workshop at the bottom of the garden looking out over the Andaman sea works well here in Phuket; I have plenty of space to lose swarf and dust on the way back up to the house and 500W of forced ventilation just about handles 30 degC and 65% humidity. But my fantasies of returning to the UK and buying a house with an internal, heated and dry workshop are taking a severe drubbing. I guess I'd better keep on renewing the retirement visa.

Colin

Ian,

Let's agree that shaded pole motors don't use capacitors to produce a phase shift in a second coil in order to start the induction motor. The capacitors may relate to power factor correction or some kind of speed control. The following link details the four types of single phase induction motors that can normally be found. (The schematic shows a 4 pole shaded pole motor while the one being discussed here is a 2 pole shaded pole motor.)

"Run capacitors" is the name normally given to phase change capacitors that are not disconnected with a centrifugal switch, see Wikipedia. I suspect your 5uF capacitors are power factor correction capacitors.

Best regards, Colin

Thanks for posting the picture.

Nick, without the picture it had sounded as though there were two sets of coils. I can see now that there is just one set of coils connected in series or parallel so they both see the same phase.

The shading rings will delay the field causing it to rotate anti clockwise as viewed in the picture.

Lynne, I can't see any reason for a capacitor to be included unless it was for power factor correction but that seems a little over the top for a small cooling fan motor.

Variable speed control? Well it will almost certainly be a variable frequency drive with some way of limiting the current as the frequency is dropped. Getting this motor to work at a fixed "semi-synchronous" speed looks achievable. Variable speed operations sounds ambitious.

Nick again, I see your comment about aluminium wired squirrel cages. I had thought all squirrel cages used copper wiring to maximise the rotor volume/area available to the magnetic flux of the steel. However I see from an internet search that cast aluminium is being used. Is this common? I've never seen it and I'm frequently at the electric motor rewinders here in Phuket.

Cheers, Colin

P.S. I was taught electric motor theory by Prof. Eric Laithwaite. That may or may not have been an advantage when hypothesising on motors like this.

A shaded pole motor is an induction motor. Induction motors use a rotating magnetic field from the stator to drag the rotor around. Once it starts spinning an induction motor no longer needs a rotating field.

The simplest induction motor is the three phase where the rotating magnetic field comes from three or six windings and the 120 degree phase difference between phases.

Unfortunately the majority of us don't have three phases and some slight of hand is needed to make an induction motor work with a single phase.

The most common solution is to use one set of windings connected to the AC supply and a second set of windings connected via a phase shifting capacitor. Usually the capacitor is used just for starting and then disconnected with a centrifugal switch but sometimes the capacitor is left permanently connected.

A shaded pole motor saves on the sets of windings, it only uses one. The phase change is obtained by a copper ring circling half of each pole piece. As the flux starts to increase through the pole a current is induced in the copper ring that opposes and delays the build up of flux. This causes the magnetic field to rise first in the unshaded pole and then move to the shaded pole. It's not a high torque from rest so it probably won't work for a compressor but it does find many applications such as a fan.

However, if you have two sets of windings then this sounds like a variation on the capacitive start motor where one winding is normal but the second has a significantly higher resistivity. This higher resistivity delays the build up of the current in this coil and therefore delivers a delayed field hence a rotating field. (But please don't call it a shaded pole motor.)

SWNB = she who never bakes?

To the best of my knowledge this is how things are. A three phase motor needs a three phase power supply or a three phase inverter.

If all you have is a single phase power supply then you either,

buy a single phase 240V motor or

buy a simple inverter with your motor connected in delta mode or

buy an expensive boosting inverter with your motor as wired or

even more expensively ask, pretty please, for a proper three phase power supply for your workshop.

Theoretically you might be able to bodge your motor to run on single phase with a permanent starting capacitor but that's the kind of thing that only an old wireline field engineer would do with his back against the wall.

I remember a two page safety briefing on how to safely walk from the Crowne Plaza Hotel in Downing Street, Cambridge to the course dinner being held in Magdalene College on Magdalene Street. The secretary who gave it to me and the rest of the course was really embarrassed but apparently the corporate lawyers had decided some CYA was needed against capricious law suites following potential stumbles on cobbled streets or collisions with cyclists.

A third party contractor framed and kept the briefing! I wish I'd kept mine.

Colin

I used to work on the Railways back in the 1970s. It was the second most dangerous job in the UK after quarrying. The death rate was around 2.5 per month. Safety was a big rule book that proved you must have broken a rule to get killed/injured.

Anyway, I then moved onto the oilfields of the middle east working with radioactive sources, explosives, high pressures (in bigger volumes than you can probably imagine), heavy loads being swung overhead, poisonous gases, and driving at night when tired. Safety objectives were in force resulting in the inverse pyramid of accidents. Most accidents were major because they couldn't be hidden. Some minor accidents couldn't be hidden either. Occasionally a fool would report a near miss! With good luck and not a little common sense I survived.

Today in the oilfield there are mandatory targets for near miss reporting because the more near misses that get reported the fewer accidents that occur. No one is sure why but it seems to work.

The work is definitely safer today. The drillers with missing fingers are just about all gone. You will be fired for driving without a seat belt and being caught DUI means you have offered your resignation which will normally be accepted. If being asked to obtain a DNV certified plastic bag for carrying 200g thread protecters is the price for this safety then I guess I'll grin and bear it.

Thankfully I'm now retired and no longer obliged to report paper cuts.

Colin

John,

Referring to your name plate;

2.2 kW = 3 HP, so far so good.

The 1 probably refers to single phase.

At 240V and a pessimistic power factor of 0.85 the full load current will be 2200/(240*0.85)=10.8 A (At 480V this drops to 5.4A)

Therefore the 17.5 and 8.75 make no sense to me.

The B comes from the insulation temperature rating of 130 degC.

If the motor is running then I wouldn't try to "improve" anything. However, I can't tell whether this kind of motor uses the capacitor just for starting or continuously. The latter needs a more expensive item and may have been inadvertently down graded. A clamp on ammeter should tell you whether the capacitor is handling current all the time or, more crudely, a finger on the capacitor can should reveal whether it's getting too hot. Did I mention using an RCCB?

I wonder if the 17.5/8.75 and 160/200 refers to the starting capacitor specifications? 17.5 uF at 160V for 240 V operations and 8.75 uF and 200V for 480V operations. Nah, those values look way too small.

Good luck and be careful, Colin

John,

I too wonder about the dual voltage.

Changing the windings on an induction motor is a standard three phase trick to give you a lower starting current. The motor starts in a star configuration with each winding connected to a phase and neutral and then switches to a delta configuration with each winding connected between two phases.

However I can't see anything like that working on a single phase motor. All I can say is that I would be very surprised if the windings could be rearranged to switch from a 4 phase induction motor to a two or an eight phase induction motor and hence give you a factor of two speed change. (I daren't say impossible as I'm supposed to know things like this.)

I doubt if you'll ever get close to needing 3 HP from this motor but if you do then it will get very hot. At the same time I would also expect your power cord to get noticeably warm and most 13A plugs and sockets to get very hot and unhappy. But for home use, relatively gentle cuts, and no time pressures then almost any electrical set up will work safely, just make sure the earth is connected and, if you can afford it, use a 30mA RCCB, residual current circuit breaker.

Regards, Colin

The motor has two windings that can be connected in parallel for 230/250V and in series for 460/500V. The latter is very strange! Phase to phase at 250V is 435V so I can't see where you would find 460-500V.

Speed 1420 rpm is a little less than 50Hz per minute divided by two so it's a 4 pole induction motor. It's probably got a capacitive starter winding but it may be using a shaded pole or even a high resistance starter winding.

If the cooling fan is rotating freely and quietly and is sucking air in and then along the motor fins then it will probably run longer than you will.

Colin