Member postings for Andrew Johnston

A real one, or a medic?

Andrew

Not sure where that came from? Internet 'expert' maybe.

Let's consider a standard motor, in delta, running from 240V phase to phase and designed for 50Hz. At 240V and 50Hz the difference between the applied 240V and the backemf will be sufficient for the motor to drive rated current through the windings. In simple terms winding current controls torque so the motor can provide rated power. This is known as the base speed, ie, the speed resulting from a frequency of 50Hz. So if you have a standard motor there is no reason not to run it at 50Hz.

Now if we increase the frequency the speed of the motor increases in proportion, as does the backemf. The rise of backemf with a fixed applied voltage means that the rated current cannot be reached, and the torque drops as well. But since the speed has increased the power stays pretty much constant. The limit to the upper frequency will most likely be the motor itself - too fast and bits come off! The 2-pole high speed spindle on my CNC mill will run at 400Hz (24000rpm), but it's designed to do so. For a normal motor I'd go to 75Hz or so, unless I was sure it wouldn't come apart - like manufacturers data.

Conversely if we decrease the frequency from 50Hz the backemf drops and the larger difference between the backemf and 240V applied voltage will drive more than the rated current. That's not a good idea long term due to overheating of the windings. So the applied voltage needs to be reduced in proportion to the frequency to maintain rated current. Since the current stays constant so does the torque. But the motor speed has decreased so the power has decreased.

To summarise, above base speed the motor characteristic is constant power. Below base speed it is constant torque. And there should be no problem running a standard motor at 50Hz and a bit above.

Andrew

A star-delta starter is used to start large motors without causing uneccesarily large inrush currents while the motor comes up to speed. The crucial point is that they operate at a fixed input voltage. So I agree with Mike. If the motor is designed to run at full power, at full voltage, in delta then in star the phase currents will be lower, by the square root of 3, and hence inrush currents will also be lower. Since the phase currents are lower the torque during startup, in star, will also be lower. But that seems to be the opposite of what Steviegtr is saying?

I don't know at what motor power it is generally advisable to use a star-delta starter. The biggest motor in my workshop is 5hp (horizontal mill) and that is started DOL (direct off line). The mill has a clutch so starting torque is fairly small.

Andrew

The difference in density between 1050A and 6082 is 0.37%.

Andrew

In the UK the main distribution pylons now run at 400kV, not 132kV. What is completely mind boggling is that people work on these lines while they're live.

Andrew

I don't understand why that would be? If I've got a dual voltage motor then in both star and delta the power available (from voltage/current values on the plate) is the same. In both cases the speed is also the same. Since power is torque time angular velocity, and power and angular velocity are same for delta and star, surely the torque is also the same in star and delta?

The issue of stalling a motor running at low speed is simply explained. Below base speed an induction motor runs at constant torque (*). Whereas using pulleys or gears to reduce speed has a constant power characteristic. So using the formula above running at slow speeds, through a mechanical reduction, results in an increase in torque in proportion to the decrease in speed.

Andrew

(*) Before any smartypants points it out I know VFDs can be programmed to output more than normal current at slow speeds. But it's not a good thing in the long term as it results in significantly more losses in the windings.

Thanks John. Worth re-iterating that for the motor to produce full power from the reduced voltage it needs to be configured for delta.

Not sure I agree with SoD on a couple of points. The use of high voltage for power transmission is not connected with the 3 wires needed for 3-phase distribution. It's equally valid for single phase. It's simply the application of Ohms law. For a given power transmitted the current goes down as the voltage rises. And since power dissipation is proportional to current squared as the voltage rises the power lost falls faster. Or thinner, and lighter, wire can be used. The big advantage of 3-phase over single phase is that one can transmit three times as much power for an increase of only 1.5 times the amount of cable.

The same is true for industrial power users. Although of course 3-phase has other advantages such as smoother running and easily reversible motors, and constant power if the phase currents are equal.

Not sure what SoD means when he says 3-phase is self-balancing. If you have unbalanced phase currents and a neutral then there should be a neutral current flowing. But, for instance, most electric motors don't have a neutral connection! So voltages start to go awry.The utilities go to great lengths to ensure that phase currents are balanced over a large number of individual loads. A 3-phase generator, and 3-phase transformers, are not going to be happy if the phase currents are unbalanced.

Andrew

The two most common aluminium products are 1050A and 6082, unless you're into proper engineering. As far as I'm aware 1050A is only available as sheet; it's pretty much pure aluminium and is great for sheet metalwork as it bends without cracking. It's horrid stuff to machine, warm fudge would be easier. It would seem an odd choice for letters, if bending wasn't needed. On the other hand 6082 is available in a variety of hardnesses and tempers in sheet and bar form. It machines very nicely, although it is prone to built up edges on tools.

Andrew

Should have mentioned that the reason the phase to phase voltages are higher is because the phases are time shifted with respect to each other by 120 degrees.

Andrew

In the UK electricity distribution is 3-phase and earth, no neutral. At the local sub-station transformer the primary is wired delta and the secondary is wired star. So the secondary has a 'neutral' point. This is grounded at the sub-station. The cable running down the road has three phases, the neutral (aka earth at the sub-station) and a protective earth. The sub-station transformer is arranged so that the voltage from any phase to 'neutral' is 240V, while the voltage phase to phase is 415V, ie, times the square root of 3. So a property on single phase will have an earth, neutral and live (from one of the three phases). An industrial property will have the three phases and earth. A neutral may also be supplied for any single phase loads. That's what I have at home; three phases (which are fused) a neutral (which isn't fused) and an earth. I run the house off one phase and the neutral whereas the workshop runs from 3-phase. The neutral also runs around the workshop, but none of my machine tools use it. They just take 3-phases and earth. Remember that all quoted voltages are rms aka root-mean-square.

I feel the need to go and machine a large lump of steel! If no-one has explained why a 240V in VFD cannot normally produce a 415V output by the time get back then I'll explain.

Andrew

I've re-read it and am still none the wiser as to what he's trying to say.

Andrew

The 240V output from the VFD is phase to phase, not phase to neutral.

You can run a 415V motor on a 240V output VFD, but the torque, and hence power, will be reduced in proportion. If the 415V motor is designed to run in star (most are) then by changing the arrangement to delta you can run from 240V without loss of power. Many motors have links in the connection box that make it easy to change from star to delta.

You don't need to change the frequency to make the VFD work. What your adviser may have been saying is that the output voltage needs to reduce as the frequency reduces. Otherwise you will overcurrent, and overheat, the motor. It's called a v/f curve, and is looked after by the VFD, not something the user needs to alter.

Andrew

Good grief, 5000 posts. That's an awful lot of sitting bulls. And a lot of time spent sitting at the computer instead of in the workshop.

I've finally got my steel bar to make a former for the traction engine chimneys:

Bar is 500mm long and 100mm diameter. The fixed steady just fits, by a whisker! First job is to drill a centre and then make the flat pattern from sheet steel; blank was guillotined yesterday.

Andrew

Compounds like Trefolex and Rocol RTD do indeed tend to melt as they get hot. But that's a convenience, not a key part of their performance. It means they stay where they're put until needed. Whereas oil tends to drain away. They often contain high pressure additives which helps with tapping in particular. I use Rocol RTD tapping difficult materials like stainless steel and tungsten, and larger taps, say bigger than 1/2" depending upon material. I don't think the behaviour or performance of drilling/tapping compounds has any significant bearing on turning.

There are a myriad of cutting fluids available for turning, but two basic types, soluble oils and cutting oils. Soluble oils,when diluted, are mostly water. So they're excellent at cooling, but not so good at lubrication. Conversely cutting oils are good at lubrication, but not so good at cooling.

For general machining (turning/milling/grinding) I use Castrol Hysol XF, which is a soluble oil. Provided one keeps the concentration correct I have no problems with rust or staining. I do a lot of my machining dry, so the coolant sits in the tanks for many months at a time. I've never had a problem with bacterial growth and no niffs of any sort.

Andrew

Keysteel is normally a thou or two oversize so it can be filed/machined to fit the keyways.

Andrew

A word of warning; MT4 is not common from professional or model engineering suppliers.

Andrew

Unless some of the model engineering suppliers list it you'll need to cut from 2.5mm cold rolled sheet.

Andrew

I've had a go at printing a thin wall box:

The box is nominally 25mm square and all wall thicknesses are 0.4mm, to match my nozzle. First time round without fiddling with the slicer the walls weren't printed. Should have spotted that in the slicer before printing. On the second go I set the wall thickness to 0.4mm in the slicer - and the walls were ignored. So I set it to 0.2mm and got a snotty message saying it was smaller than my nozzle setting. So I set it to 0.39mm and the message went away! Even better the walls appeared in the slicer representation. In practise the walls are 0.52mm, as measured with a groove micrometer.

Andrew

I've never had a chuck spin on it's arbor, or had the Morse taper spin in the tailstock; and I drive my drills pretty hard on an industrial lathe. I've had drills spin in the chuck, but not the tapers. If the tapers are spinning with a relatively low power lathe then I suspect the mating surfaces are dirty, bruised or the tapers are not correct.

Andrew

As well as faffing about with 3D printing experiments I also finished a batch of ~40 bolts and ~75 nuts (all 1/4" BSF) for my traction engine spectacle and front plates:

I made extra nuts as you can never have enough 1/4" BSF nuts.

Andrew