Member postings for Macolm

In principle, induction motors only work at a speed a bit below the synchronous value, eg about 2800 rpm for a two pole 50Hz with theoretical sync 3000 rpm. The speed varies slightly with load, but must normally be quite near "2800 rpm" as the torque quickly falls off beyond some point.

However, in the case that the load is a fan, the torque needed falls even quicker as speed reduces, so if the motor power is reduced it can be stable at a considerably reduced speed. If the speed reduces a bit, the fan torque needed reduces more than commensurately. Hence this type of multi speed fan, but it is likely the designer will already have maximised the practicable speed range, so probably no scope to run it slower.

As Duncan Webster says you can easily check if there is a start switch. If so, and it is on the neutral side of the winding, you can rewire, and problem solved.

If it is a run capacitor, check for nearly equal winding resistances, and if so, likely simply a case of moving brown wire, but see my previous post to do it safely. If unequal, it is a case of completely reversing one winding with respect to the other.

I do suggest the halogen (filament lanp) method to reduce risk. I had assumed these would be banned from sale by now, but I find both Toolstation and Screwfix still offer replacement lamps (250W, 400W and 500W). Toolstation also seem to have the halogen. security lamp housings. For anyone who occasionally has to fault find or investigate workshop power electrics, this could be a good and cheap investment. Without a housing, it should be OK to solder on to the caps of the bulb, since if used correctly for this purpose it will never get to anything like full temperature. Of course, a suitable mounting and safe configiration is necessary.

If you can lay hands on an old fashioned halogen security lamp, best 500W or else 200W minimum (and no more than 500W), include it in series with the mains feed. If the motor (wired as is, and without mechanical load) runs, and the light does not come on at full brightness, then it is (fairly) safe to try it with the brown wire moved to the "grey" position. If it now runs the same but in reverse you can try it without the lamp in series. If at any stage the lamp comes on full, switch OFF immediately.

It may not apply to the motor in the photo, but sometimes the shaft and end plates can be swapped on the frame (ie the windings reversed with respect to everything else). The shaft then projects the other way, and thus rotation is reversed. Usually works for shaded pole motors.

Edited By Macolm on 04/07/2022 22:13:19

Thanks for providing this link -

https://www.model-engineer.co.uk/forums/postings.asp?th=133409

**LINK**

The voltages across L and C components must be in antiphase, so greater than applied supply voltage is to be expected. The current is the key measure, and in the link it is clear that while nowhere near resonance, it might well be possible to get pretty near by varying the value of the C. Assuming only the 35 ohms counts, the current in the mesh would be 7 amps at resonance, but in practice the magnetic circuit would contribute additional resistive load/losses.

It does confirm, at least to me, that caution is necessary in experimenting with capacitors to convert a three phase motor to single phase. It would very likely be start/run configuration, so has to be ensured to be safe over the whole operating envelope.

Edited By Macolm on 29/06/2022 21:05:54

I agree with the sentiment that converting the three phase main drive motor of a machine to single phase with a capacitor is a poor idea. However, for something like a coolant pump motor, it seems scarcely worth the expense of a VHD if it runs well enough. This is what I converted, and anyway it was before the availability of affordable VFDs.

I perhaps did not make it clear enough what the (mild) concern is. At resonance, the inductive and capacitive reactances are equal in magnitude but opposite in phase, so they cancel and thus there would be no phase shift of the start winding current. Thus if it were possible for resonance to happen, the motor would likely not start or would run slowly, but would draw excessive current in the start winding as well as stall current in the run winding, a recipe for rapid destruction.

I just wondered whether there is any reason to caution about experimenting with a wide range of capacitor values.

I must admit to little feel for the following, and perhaps someone with technical experience of induction motors could comment.

A capacitor in series with the stray inductance of the start winding (or a phase shifted winding as in this case) will be series resonant at some frequency. The capacitive/inductive part of the impedance falls to zero at resonance, leaving only the residual resistances (including the effects of motor operation (if it runs)). This is inevitable, but will not be a problem providing the resonant frequency is not near 50 Hz (or whatever the supply frequency is). Clearly replacing a start capacitor with the same value is fine as the designer will already have taken care of things, and indeed the start winding path must be net capacitive for the arrangement to work properly.

Is there any chance of a problem when converting a three phase motor for single phase use? I have indeed done this, and in the absence of the means to make the appropriate measurements, I merely checked that the currents in the two paths were not too dissimilar.

It may be copper chromium, usually about 1% chromium, the rest copper. It is used for spot welder electrodes as it wears much better than pure copper.

I chose a bad example, sorry about that, and will try to clarify.

The claw and post image above shows the claw fitted to the underside of a display readout head with the post lying in its working position. The post only needs a single hole to fit to the machine, and it allows some freedom of movement in the other two axes, but locates the readout head rigidly along the measurement axis.

The scale readout image above shows how this looks from the front of the readout head.

Unfortunately it is not possible to see this when it is assembled on the mill, because the post is intentionally fitted inside a tunnel through the mill Y axis casting. The spring plunger similarly fits in a hole so as to push the claw against the post.

The scale fitted image above shows how this looks from the outside, and you should be able to deduce how it is laid out. It was done like this to get it out of the way and provide some protection. It has been used successfully for more than ten years.

Another (perhaps easier to understand) example is the X axis readout head. The floating ref image above shows a spherical headed post (made from a hardened thread forming screw), fitted to the underside of the readout head. It is nipped between a fixed “anvil” and a sprung plunger, again not impeding alignment in the Y and Z axes, but having no backlash in X.

I hope this helps understanding the concept.

Edited By Macolm on 20/06/2022 14:21:19

I have taken a different approach, using self-aligning mountings with zero backlash to avoid the risk of distortion and high operating forces. Here is diagram showing an example. It also allows the scale unit to be detached by snapping out the fixing.

You can see how it would work in this case, needing a simple tapped hole for the post.

Edited By Macolm on 17/06/2022 21:28:55

Despite intuitive doubts, I used this method to free very corroded hydraulic unions on Citroen rear suspension cylinders. You can see the design on this photo of a tester made from a such a cylinder. I agree that spot welder dissipation will be lower for aluminium, but with perhaps 5000 joules available over a couple of seconds, 10 grams can be heated by more than 300C. Then taking the differential expansion as 7 parts per million per degree C, we have 2 thou per inch (to mix units!), a very useful loosening of the parts. It does indeed work.

I have had reasonable success with parts of that configuration using a portable spot welder. There needs to be access to clamp the jaws across opposite sides of the boss. After scraping to get contact, power is applied to heat it. The current only flows in the outer, so expansion tends to loosen the grip. With steel it takes maximum welder settings to get to near red heat as quickly as possible. Aluminium must obviously to be kept cooler, but expands more. If the parts move at all, it is then merely a case of working in penetration oil, and patience. The method can also facilitate oil penetration at lower temperatures, with just enough heat to result in a capillary gap.

The fast and targeted heating seems to increase the chance of success. The nearest alternative is fast heating with oxy-acetylene which is far more risky.

A point about cutting glass is to score as lightly as possible. Too heavy pressure tends to cause small sideways cracks which may result in the snap not following the marked line.

The magical thinking is that something new and different will prove better than an existing technology that has had the sharp edges knocked off over many years. Take wind generation, which is perceived as new and shiny. When was the first wind farm of recognisably modern machines? The answer is about 1978! Thus the technology is more than 40 years old, and it can be argued has just about reached its full potential.

A rule of thumb that fits the energy sector quite well starts with a successful demonstrator. Unless this leads to a satisfactory first of series within 10 years, the concept will not succeed. Try this to evaluate your favourite notion. Then remember that full infrastructure takes decades to put in place.

The world population has tripled over my lifetime, in no small part enabled by low cost energy to provide a myriad of essentials to our wellbeing. Such things as refrigeration and distribution systems, and pharmaceuticals and chemicals including fertilisers to name but two sectors. These things are the real baseload of the energy system, and it is unclear how any existing renewables can adequately support our lifestyle. On the other hand, reversion to an agrarian economy may not turn out to be popular!

Edited By Macolm on 06/04/2022 12:01:33

Edited By Macolm on 06/04/2022 12:02:16

As soon as I switched off the computer last night, I realised I got my arithmetic wrong. One mega battery a day only equates with 2 days storage, it needs 10 a day. After 11 years, bingo, 20 days storage, and we can then continue installing them at the same rate in perpetuity to replace those at the end of life.

The difficulty with storage to make good the variability of renewables is the vast amount necessary. Currently on average, the UK uses 30GW daily, which equates to about 700GWh of energy (0.7TWh). If heating and vehicles are to be powered solely by electricity, this will increase to perhaps 2TWh. For comparison, current pumped storage capacity is about 5GWh.

There can be many weeks with overall average wind power of less than 10% of capacity, and midwinter solar output is a few percent of capacity. So at least 20 days of storage would likely be required, say 40TWh. Large grid connected batteries of circa 1GWh are currently being deployed to stabilise the grid against wind power fluctuations, but 4,000 would be needed. Sounds OK? Well, it would be one a day installed for the next 11 years. The story using hydrogen is similar, many millions of 700 bar cylinders, cryogenic storage of about the same capacity as the UK current tank farm inventory, or chemical storage yet to be developed at scale.

A further thought. Crossrail will have taken 14 years (including detail planning) if completed in 2022. When it kicked off, the technologies were all available and mature, the task was fully defined, and the funding was understood. And this electrical transformation will be completed in just 13 years? Is there really a feasible plan?

The quietly hidden trigger for the current energy price escalation was forcing coal, nuclear and oil out of electricity generation in Europe. This would have been fine if there was a competent plan to ensure that feasible replacements were available promptly. The coming crisis was clear enough to see when large price spikes in wholesale electricity prices started happening in 2019 in the UK and many other counties. There was simply too little spare capacity.

Covid restrictions reduced demand and the problem seemed to disappear, but with the start of economic recovery last year it re-emerged, and now more serious due to further loss of dispatchable capacity in the interim. What had started in the electricity sector quickly affected the whole gas market. No doubt suppliers took their chance for a fast buck, but where do we go from here?

Search for Starrett C331, though perhaps only available easily in USA. It has 1mm graduations one edge, and short 0.5mm on the other, and is 300mm long so can readily be used reversed. Not cheap!

Edited By Macolm on 22/03/2022 21:19:44

Do Internet search for brand and type you want eg Renata SR44, and you should find you can get a pack of 10 at an acceptable price. Indeed be wary of ebay, but it is possible to buy genuine cheaply there.

Edited By Macolm on 08/03/2022 14:56:09

A low risk way to test a motor like this after doing the basic checks is to connect it in series with an old fashioned quartz halogen security light, assuming you can find one. A 500 watt 230V mains lamp will reach 125 ohms when hot, but start at 12 ohms or so when cold. If the motor (without mechanical load) is OK, it will run, and the lamp will not light up much. If the motor does not run, the lamp will light fully, and the current will be limited to less than 2 amps.