Member postings for Ken Fox

The 95% tin solder was mandated for potable water plumbing because there was lead leaching out of the 50/50 solder into the drinking water so opening up the possibility of lead poisoning if someone drank enough of it. This was discovered in school fountains--the water contained detectable amounts of lead after sitting in the pipes all night; once the pipes were flushed out it was ok for the day. It's one place where the "precautionary principle makes some sense although I'm not sure anyone knows if the actual amount of lead was a hazard.

Don't forget that lead poisoning from the early tin cans soldered with lead solder was one of the things which doomed the Franklin expedition to find the northwest passage 150-160 years ago.

--That plus a refusal to learn how to live from the native Inuit

Ken

One thing which has caught my eye is that no one has mentioned the effect of current limit on stopping. If you call for a rapid slow down via a short time stop ramp the drive will require more current to get this if the load has a high inertia. If then the current limit is set too low the torque will simply not be available and the stop will take longer than expected from the ramp setting. I suggest taking a look at the current limit and setting it to substantially above the motor rating, say 150% to 200% or more. After all, before the days of VFD's an induction motor would draw several time full load current on startup or on stop by plugging or dc injection.

Ken

Here (Canada) the lead free solder was mandated for potable water plumbing. I think it's 95% tin 5% antimony. As far as I know ordinary 50/50 lead-tin is still ok for electrical and other work. I've seen "silver solder" 95% tin etc advertised but as far as I'm concerned it is deceptive advertising as real silver solder has a definite set of uses and cannot be replaced by the 95% tin stuff.

Ken

Swarf Mostly

My education in electrical engineering is the same vintage as yours.

In a rotating AC generator the major contributers to non-sinusoidal waveform, as I remember it are slot ripple which is high frequency and non sinusoidal flux distribution in the air gap which can give either a flat topped or peaked wave shape. Various games were/are played to reduce these effects such as slot skewing, shaping of the pole face and, I presume your lap and lead of stator windings although I'm not familiar with these names.

At one time I owned a small cheap and dirty AC generator which had a very large slot ripple and also an almost flat topped wave shape. About all I can say for it is that it would run a light bulb or small hand tool. I have a better one now which uses inverter technology and I'm very impressed by its wave shape. You cannot see any non sinusoidal component in it though it's probably not perfect.

In a large utility with many generators in parallel things like slot ripple tend to cancel out as the various machines have different ripples.

Ken

Russell

Thanks for that reference. There is a lot of good stuff in it and it goes into my favorites

Ken

We need some consistency in nomeclature here. Consider a 3 phase power supply because most of us are familiar with such things. You can get three voltages from it, A-B, B-C and C-A which are 120 degrees apart and/or for a 4 wire 3 phase system you can get A-N, B-N and C-N, again 120 degrees apart but the voltage amplitudes are different. You can connect a 3 phase motor to A,B,C wires and get a rotating field in the motor which will make it spin. You can take any 2 wires, which some here are calling phases but you will only get single phase power from them and they will only work on a single phase motor which only works because of mechanical and mathematical trickery. so keep in your minds the diffence between "wire" and "phase'.

Merry--- I, in turn am not sure what you mean in 2. I think we are saying much the same thing here.

When I mentioned 2 phase in point 3, I was referring to 2 sinusoidal voltages 90 degrees apart which was used many many years ago. This system can generate a rotating field inside the motor and works perfectly well. It fell out of favour because it takes a bit more copper per HP in the powerlines. In fact with a bit of mathematics you can prove that 3 phase gives the minimum copper per HP of any of them. Actually the difference is not much. To come back to your point, two phases 180 degrees apart is single phase cannot generate a rotating field.

You are quite right, the distortion is high and this brings its own consequences in terms of what voltage a rectifier will deliver for a given AC voltage. The IEEE definition of pf is W/V*I where W is watts, V is volts and I is amps, all measured by electrodynamomer instruments; sort of old technology but there it is.

My understanding of a "rotary phase converter" is a system in which you feed a 3 phase motor with single phase on any 2 of the 3 power supply wires then get 3 phase from the 3 motor terminals. Sort of a buck-she arrangement but it's better than nothing if thats all you have. "M-G set" is a more general term in which you use a separate motor to drive a generator. This term is used for any such arrangement be it 1 ph AC to 3 ph AC, 1 or 3 phase AC to DC or DC to AC, any number of phases.

As I said, it is all in the nomenclature.

Ken

A lot of interesting reading here and a lot of mythe and folklore. Here are my comments:

1/ 115/230 volt power is definitely not 2 phase. It is single phase.

2/ In polyphase power, that is, true 2 phase, 3 phase, 6 phase, 12 phase (yes it is used in special cases) the power is supplied continuously, not at 120 (or 100) pulses per second as it is in single phase. This is what distinguishes single phase from polyphase.

3/ In 2 phase the two voltages are 90 degrees apart, 3 phase 120 degrees, 6 phase 60 degrees, etc.

4/ Strictly speaking, inversion is the conversion to from DC to AC. The common inverters first generate DC then chop it into AC. Rectification goes the other way; AC to DC.

5/ The cons of low pf lagging are low HP per amp drawn, as noted elsewhere, increased voltage drop in power lines, and on the large scale loss of sychronization of the power line which means that the transmitting and receiving ends fall out of step. Also low pf lagging demagnetizes the generator so can make it fall out of step with the power system. I don't think anyone reading this should worry about it except the low HP per amp.

6/ Low pf leading can be just as bad for the power utility co. as it can cause a rise in voltage in the power line and over magnetization of the generator. Again no one reading this need be concerned

7/ The power utility co. is happy to get leading power factor because it offsets the lagging pf of most loads, but in principle at least it can be carried too far.

8/ Mention was made above of a motor just spinning, not connected to a load. This is a "sychronous condenser" and is an overexcited synchronous generator which runs at a leading pf. It is a common strategy used by power utilities to offset low pf lagging loads.

9/ To answer an earlier question, if the a full wave rectifier is used on the input to an inverter package feeding into a capacitor I would expect the power factor seen by the power system to be fairly high. Perhaps someone it a pf meter could measure this to see how far off I am.

10/ For those wanting 3 phase power an MG set is another option to the inverter. This would use a single phase motor spinning a 3 phase generator. To make it practical you would need some source of second hand, inexpensive machines.

Ken

Russell

Actually that looks somewhat like the rotating machines lab at the university where I learned such things. Some of my work was in steel plants both here (Canada), Italy and France and they use the same sort of setup for line panels, both power breakers and DC relaying. However, most of the MG sets, amplidynes, etc have been replaced by solid state power electronics in cubicles and many DC relaying functions have been replaced by PLC's. Anymore the safety people are hot to lock everyone out of such places, even those who must track down problems which actually makes it more difficult to stay safe when tracking down a problem

Anyway it's nice to talk to someone of my vintage.

Ken

Inverters generate a lot of high frequency noise in addition to to the power frequency they are intended to generate. The more robust earthing/grounding systems and the shielded cable specifications are an effort (more less effective) by the manufacturer to prevent interference in other eletrical/electronic systems you may be using. You can always ignore these specifications but don't be surprised if you get radio and television interference or erratic operation of other electronic gadgetry such as hi-fi systems, microwaves, computers etc.

Ken

Just an afterthought:

Actually at any speed right down to standstill the current is proportional torque as long as the field is kept constant which is the case with most small dc motors as they use a pemanent magnet field.

The lbft per amp and voltsper radian per second are key constants in designing a speed control

Ken

Russell

I'll assume you meant to say "current squared X winding resistance".

A handy formula to remember is "T=5250XHP/N

where T is in lbft, HPis in horses and N=RPM or

HP=TXN/5250

I guess lbft and HP date me but I'll get around to converting some day

Ken

I've done a lot of work with dc motors and control in much larger sizes so here are a few of my thoughts.

On a dc motor, voltage and no load speed are very very nearly proportional so a motor rated 250 vdc 1000 rpm could just as well be rated 125 vdc and 500 rpm. The limit on voltage at the high end is how much the commutator can stand before a flashover or how much speed the motor can stand before it flies apart. At the low end the brush dropand under load and the IR drop become significant so the volts/speed characteristic will depart from linearity but this happens at only a few volts. In your case you will be nowhere near flashover. In your case with a small motor put a test voltage on it and measure the speed at no load then calculate the voltage you need for the speed you want and use this plus make an allowance for speed drop under the load you intend to apply.

In other words for small motors you can pretty much invent your own voltage rating to get the speed you want. Of course, for the load rating (hp, watts, or current) you will have to try a few increasing loads, give it some time at each load and stop when it gets uncomfortably hot on your fingers.

Ken

I put one of these belts (Fenner) on my Atlas 618 a while ago and I was only moderately satisfied with it--it was very noisy. As a result of these postings I took a closer look to-day. First I tried reversing the direction with not much difference then I went over the drive train tightening set screws etc. and this was the magic solution. It quietened right down to little more noise than a normal V belt with little significant vibration and also it makes no difference which way it runs. I can only suspect that what vibration there is excites any loose parts to rattle and shake.

One possible problem I see is that if the v-grooves are a shade large (or the belt a shade small) the belt could ride low enough in the groove that the little nubs on the bottom could be riding on the bottom of the groove. This would certainly cause slipping at any reasonable tension and the higher the tension the higher the vibration.

Ken