3 phase vs single phase

Oh, and no,people generally dont clean the filters, thats why they catch fire, that and abysmally crap design!

Sean, an important question that no one has asked yet:- What lathe is it?

1/2hp is suitable for a Myford sized lathe, but 3/4 is better for even an ML7 (I speak from experience). A lathe that's sized for a 2hp motor is not going to be happy with larger work or more aggressive cuts if hobbled by a 1/2hp motor.

In terms of power consumption, the losses of a 2hp single phase motor will be greater than those of a 1/2hp single phase motor or 3ph motor + VFD, but the difference is likely to be no more than 25-50W. I.e.. 3/5 of 5/8 of F-all, as one of my colleagues used to say.

The suggestions of getting a plug in power meter are your best bet. They are cheap these days (especially when Aldi or Lidl have got them on sale, but also from a number of online sellers), and can give you a good idea of just what in the house is eating the power.

Before you do anything else though, have you got an immersion heater that someone's turned on?

Sean - I think the cost and torque issues have been answered adequately already. What you need to decide is what do you want to run and how big a motor does your lathe require.

As described here, 3 phase motors are smoother running and the VFD brings benefits like soft start-stop and great speed control via a variable resistor input to the VFD to state but two.

Having both my lathe and my mill/drill are on 3 phase via a VFD, the latter an upgrade from 1 phase, so to me the choice is a no-brainer, but you pays your money and takes your choice, as the saying goes!

Bon chance as they say across the ditch.

Chris

First question might be : what power motor was on the lathe, before the last month?

Second is where the power has been used. Our leccy bills always go up in autumn/winter. But not by £85 each month.

Regarding condenser or vented tumble dryers - at least the heat rejected by the condenser type heats the property - not just rejected to the .

Jumping to unsubstantiated conclusions is often proved wrong on proper analysis of the situation.

It's always worth checking the motor current off load, this will show up a bad winding that is not running hot enough show on a visual winding inspection.

+1 for that. Rather easy to do these days without breaking the bank. This typical example on ebay is £15. The monitor can also be used to hunt down whatever is eating all the electricity in your home. Quite an interesting thing to do, some appliances are terrible: I found a hifi unit consuming 30W on standby. Pretty bad, and I found switching it on without playing anything reduced the waste to only 10W. The standby function was worse than useless!

Irrespective of the cause of his £85 electricity bill Sean's idea about replacing an oversized single phase motor with a smaller 3-phase model is still good. It's not smart to over-power equipment because of the damage it can do when something goes wrong. And a 3-phase motor performs better on a lathe than single phase. It's not smart to seriously under-power it either!

A mains monitor can used to establish how big the replacement should be. Plug the lathe in via the monitor and take a deep fast cut out of a bit of steel pipe. The monitor will tell you how much power was consumed taking the cut. By testing the lathe off load (not cutting) you can run through the various gearbox, change-gear and power-traverse combinations and find out how many watts it takes just to turn them over.

Buy a motor about 20% bigger than the maximum cutting requirement and - depending on how bad the tick-over measurement is and how long you expect to continuously run the lathe - consider paying for a higher duty cycle as well. (For hobby use a cheaper 50% duty motor should be fine: but not if it never gets a chance to cool off!)

Doesn't need to be accurate and if you can't be bothered to measure it you can get a good idea by comparing the size of your lathe with similar sized machine. I'd guess anything between 600W and 1500W would be fine.

Dave

Being a pedantic so-and-so (who me?) I wonder how it measures kilowatts per hour? That seems like a rate of change of power to me. On a more serious note, is this any better than just measuring the current with a meter? Obviously it's a lot handier, but I've got a meter so could save myself £15. It's something I've been meaning to do for ages, lots of stuff on standby in our house.

I feel trepidaceous about comment on condensing tumble driers and heading off piste again... but!

We had one for several years, it was a very simple one that drew in fresh air through an aluminium cassette with hollow fins the 'output' air went back through. The main advantage was that it didn't steam up the kitchen, but it also seemed to work faster and use less juice than a conventional drier. Cleaning and emptying water was just a matter of habit.

I suspect the 'expensive' ones are just trying to be too clever, that simple passive condenser added no energy usage and had little potential to go wrong.

Neil

Edited By Neil Wyatt on 09/12/2018 14:33:34

I agree Neil, the main problems I have seen with condensers, is lint choking in the condenser because some models are difficult to clean properly (and people dont do it anyway!) and in the earlier ones with the pumped outlet, the water cooling jet fails, or blocks with scale, which results in hugely extended drying times. all in all the enrgy saving, as shown in the links I provided, is marginal. whichever way you go about it, it takes a given amount of energy to evaporate and then condense a given amount of water.

Edited By Phil Whitley on 09/12/2018 14:58:18

I bought a cheap power meter from the lamented Maplin's. Hard to read display but very useful, and it measures volts, amps, power and power factor. Probably not particularly accurate but useful from time to time.

This is bad - now I want to take one apart.

I guess inside there's a means of measuring actual voltage and the millivolts dropped as the mains passes through a low-value shunt resistor.

From that data a microcontroller can calculate current from I=mV/R, and Watts from VI. As all microcontrollers come with a clock I suppose it periodically counts the number of watts it sees and then infers kWh from that. It could do that thousands of times per second. Not tested it but I expect kWh measurements get more realistic the longer you leave the monitor plugged in. Especially on something like a fridge which uses a lot of power in short bursts but is off most of the time.

My monitor calculates cost incorrectly because I've lost the manual and don't know how to enter the tariff!

True much the same job with can be done with an Avo - I've done it. But the way I cut into the live line to measure current was a dangerous lash up involving exposed wires and crocodile clips. Then setting the circuit up took a few minutes on each appliance and I had to read the meter and do the sums manually. I'm not good at sums.

A purpose made monitor is faster, far easier to use and - in my sorry hands - a lot safer! In theory I'm confident I can manage the risk of being zapped by one of my own electrical lash-ups. In practice I've had too many near misses due to unexpected distractions like the unexpected arrival of a cat! Now I'd rather do electricity properly than bodge it, and making the connections safely to a multimeter for repeated measurements could easily cost £15.

Dave

Edited By SillyOldDuffer on 09/12/2018 17:56:12

I checked mine with an old school General Electric moving coil clamp meter. I have two 1/2hp motors one on the mill and one on the lathe (which should have a 3/4hp) both run at about the plated 4A current on idle or working, start up is 6A on the mill and 13A (big chuck to move and not a cap start either). The compressor is 2HP with 20A start and 11.5A run and plated at 15.5A. In the case of the lathe and mill off load means no belt on the motor pulley.

So for some reason the little ones do not use less off load.

The motor draws the most current at start up. So with my Myford and its VFD, I leave it running in between if making lots of parts. I really like the variable speed the VFD give me, as well as the controlled motor start and stop. With mine the stop can be controlled and take place over a range of program time. Mine is set for either .1 or .2 seconds, cant remember now. It also allows for near instant reverse, just like a regular 3 phase set up does. But because it has a program start and stop, is not as harsh.

Given the choice of full 3phse and VFD, I have chosen VFD over the full 3phase.

I can have a micro switch and stop the motor and feed position when screw cutting internal or external threads.

Neil

Measuring the current of an AC motor is completely meaningless as a way to infer the power consumption. You need to measure the power, which is Voltage * current * Cos(power factor). the little plug in wat meters will do this. An Ammeter won't.

Pet peeve of mine, since I spent 7 1/2 years measuring the electrical output of power station generators during performance tests all around the world.

+1 to Mark R, Power Factor is important. Some VFDs have PF correction but most don't . An off-load motor consumes little power (I2R losses plus bearing and air drag) but it will have a poor PF. Most domestic meters read apparent power, not real power so even a low power load with poor powerfactor will cause it to over-read. This is a particular issue if you leave the motoer running but disengaged when not running.
A correctly sized 3ph motoer combined with a VFD that has PF correction an approriate mechanical speed reduction is the best solution. Any 3PH motor and VFD will be better than the same size 2ph motor.

On condenser driers the unvented heatpump types are the most efficent overall. They are fast and have low electrical consumption. They are also gentler on the clothes. The vented types extract heat from the house and dump it outside. When the house is being heated (or aircondtioned) this is a big energy loss. This energy loss is not included in the drier efficency calculations. The Input energy from a heatpump condenser is mostly dumped down the drain as liquid water. I'm biased, I have a heatpump dryer. Most buyers nad a lot of retaillers don't know the difference.

Robert G8RPI.

Correct do a search for models with the lowest consumption.

Going back a few years when electric was cheaper and from the big 7 an AEG worked out at 23.5p/hr whereas next best the best Bosch at 25p/hr. Even other A rated ones came in at 36p/hr. The general low budget were of the order 40p+ p/hr.

Mrs has our AEG on for hours daily, annoys me when the suns out. 4hrs a day 6 or 7 times per week adds about £23 p/m. Budget A/B to run of the order £10 p/week.

Going back 8 years i did run a test on consumption with a 3hp 3ph 400v motor, from memory:-
Transawave 5.5hp convertor on standby no load used 39p/hr, powering a 2.2kw motor used 52 p/hr.
Digital 10hp invertor standby no load 12p/hr, powering 2.2kw motor 35 p/hr.

Umm:- a domestic supply meter will be within about 2.5% worst case at any load from 6A up to 60A and any power factor from -.75 to +.75 . If it isn't it can be challenged. 'smart meters' tend to be far better.

The only supply meters that don't read true power are additional ones used on large industrial contracts where there is a power factor charge.

Well I don't disagree with Robert and Mark but how important is power factor in a home workshop?

Most of us don't know the power factor of our motors or understand what effect PF might have on our electricity bills. I guess an old-fashioned electromagnetic disk meter would read higher than it should, but a new electronic meter might be smart enough to compensate.

However, my feeling is that a home workshop uses too little energy to make power factor worth worrying about. Am I wrong?

Second question, my understanding of VFDs is that they start by converting single phase AC into DC. Wouldn't that conversion act as a buffer such that the AC supply wouldn't 'see' that the VFD output was connected to an inductive motor load? On the input side what sort of load does the DC stage of a VFD offer to the mains - I guess capacitive?

This turns out to be a another subject I don't understand adequately!

Dave

Edited By SillyOldDuffer on 09/12/2018 22:32:38

Nope! They measure power, not current. if you aren't a very large customer, your metering will measure the energy that you use and will not be materially affected by the power factor of your loads.

Edited to delete rambling on about excess load charges etc.

Edited By Mark Rand on 09/12/2018 23:29:07

The old electromechanical meters accumulated real power, and that's what you paid for, even if your power factor was awful. However, the utlility had to generate the full apparent power, while only being able to charge for a proportion of it. Electronic meters have no problem measuring volts, amps and power factor and thus can accumulate real power and apparent power. Or at least the IC chip sets I've used professionally can do so.

Correct that a VFD converts the incoming AC to a DC link voltage and then uses that to produce the PWM signals that drive the motor. In the simpler VFDs the AC-DC conversion is simply a rectifier. Cheap, but it generates awful current harmonics. In theory you're not allowed to directly connect them to the supply within the EU. Instead you need an input filter, or DC reactor, to reduce the current harmonics. Higher quality, or larger, VFDs have a power factor corrector at the front end after the rectifier and before the DC capacitor. These are essentially a boost converter, but the control loop works to force the input current to follow the input voltage, ie, sinusiodal.

Obviously a PFC looks like a resistive load. For the case of a rectifier it's more complicated as the current waveform is highly distorted, even though the voltage and current waveforms are in phase. I suppose it looks like a time varying resistor.

With the advent of the electronic meters, and especially 'smart' meters, you'd better worry about power factor as you're going to be charged for it. Not at the moment, but believe me it will be arriving, along with variable kWh pricing. Just so you don't know what you're paying for electricity at any particular point in time.

Andrew

Edited By Andrew Johnston on 09/12/2018 23:33:47